NERVOUS TISSUE 

FUNCTIONS OF THE NERVOUS SYSTEM
The nervous system functions as a Sensory organ. Receptors sense changes within and external to the body and pass that information to Integrative Centers that analyze the sensory information, store data, and make decisions based on that data. Motor impulses stimulate effectors to respond to the stimuli by initiating muscular contractions or glandular secretions.

NERVOUS SYSTEM DIVISIONS
The nervous system contains the Central Nervous System (CNS), the Peripheral Nervous System (PNS), and the Autonomic nervous system. The CNS comprises the brain and spinal cord and is connected to sensory receptors, muscles, glands by the peripheral nervous system. Peripheral Nervous System (PNS) include cranial nerves that arise in the brain and spinal nerves that arise in the spinal cord. Sensory or afferent neurons in various parts of the body bring impulses into CNS. Motor or efferent neurons send impulses from CNS to muscles and glands.

The PNS can be further subdivided into the Somatic nervous system (voluntary) and the Autonomic nervous system (involuntary). In the somatic nervous system, neurons conduct impulses from cutaneous and special sense receptors to the CNS and motor neurons conduct impulses from CNS to skeletal muscle tissue. Since the Autonomic nervous system is involuntary, sensory neurons from viscera send impulses to CNS, and impulses from CNS are sent to smooth muscles, cardiac muscles and glands.

HISTOLOGY OF NERVOUS TISSUE
Neurons differ in structure and function. Based on structure, neurons are multipolar (several dendrites and one axon), bipolar (one dendrite and one main axon), or unipolar (one process extending from main body). Most neurons in brain and spinal cord are multipolar. Unipolar neurons are always sensory. Bipolar neurons are found in retina of eye, inner ear and olfactory area of brain. Based on direction or function, neurons are sensory [(afferent) in which nerve impulses from receptors are carried to the brain or the spinal cord], interneuron or association neurons conduct impulses to other neurons (most neurons in body are this type and are found exclusively in the CNS), or motor (efferent) neurons [conduct impulses (effectors) from the brain or spinal cord to effectors (muscles or glands)].

Most neurons have a cell body, many dendrites, and usually a single axon. Dendrites conduct impulses from receptors or other neurons to the cell body. The axon conducts impulses from the neuron to the dendrites or cell body of another neuron or to an effecter organ of the body at a synapse. The axon joins the cell body at the axon hillock. The first portion of the axon is the initial segment; where nerve impulses arise (trigger zone). Nerve fiber is a general term for any neuronal process (dendrite or axon). These processes of neurons are arranged into bundles called nerves in the PNS and tracts in the CNS. Nerve bodies in the PNS form clusters called ganglia. Axonal transport can be a fast or slow natural mechanism of intracellular transport in neurons.

CLASSIFICATION OF NEUROGLIAL CELLS
Neuroglia are specialized tissue cells that support neurons, attach neurons to blood vessels, produce the myelin sheath, and carry out phagocytosis. Neuroglia are found in the CNS. Astrocytes participate in brain development and help form the blood-brain barrier. Oligodendrocytes are the most common glia cells in CNS. They produce the myelin sheath. Microglia protect the CNS by engulfing microbes. Ependymal cells are epithelial cells that produce CSF and assist in its circulation. The accepted theory has been that the neurons did all the communicating in the brain and nervous system, and that the glial cells merely nurtured the neurons. Recent research has demonstrated that glia cells communicate with one another as well as with the neurons and that the glial cells can alter the signals at the synaptic gaps. As a result, glial cells may be critical in learning and memory functions.

Neuroglial cells found in the PNS include neurolemmocytes (Schwann cells) that produce myelin sheaths around neurons in the PNS. Satellite cells support neurons in PNS.

Myelination is the process by which a myelin sheath is produced around the axons in the CNS and PNS. Those axons that are sheathed are called myelinated, those axons that aren’t are called unmyelinated. The sheath electrically insulates the axon and increases the speed of the nerve impulse conduction

REGENERATION OF NERVOUS TISSUE
At about 6 months of age, neuronal cells lose their ability to divide. Once a neuron is destroyed, it is permanently lost. Only some types of damage can be repaired. In the PNS, damage to myelinated axons and dendrites may be repaired if the cell body remains intact and if the neurolemmocytes remain active. In the CNS, injury to the brain or spinal cord is usually permanent to the neural cells.

NEUROPHYSIOLOGY
Cell membranes are usually charged or polarized. This means that there is an unequal distribution of ions on either side of the membrane. Ions pass through membranes via pores or protein channels. There are two types of channels, leakage (nongated) and gated ion channels. Leakage channels are always open. Gated channels open and close in response to some stimulus. Examples of gated channels include voltage-gated, chemically gated, mechanically gated, and light-gated. Voltage-gated ion channels in nerves and muscle plasma membranes give these cells excitability. The presence of chemically, mechanically, or light-gated ion channels in a membrane permits the appropriate stimulus to cause a graded potential.

Resting Membrane Potential
A cell that is not being stimulated to send an impulse is in a resting state. Factors that contribute to resting membrane potential include unequal distribution of ions across the plasma membrane (high concentration of sodium ions outside the cell and a high concentration of potassium inside), or a large concentration of negatively charged ions inside the cell, and the relative permeability of the plasma membrane to sodium and potassium. In a resting cell more positive ions leave the cell than enter it.

Graded Potentials
Grades potentials are produced by the opening and closing of chemically gated channels. The flow of ions through a particular channel may cause either depolarization (polarization less negative than the resting level) or hyperpolarization (more negative than the resting level), depending on the charge of the ion and the direction of flow.

Action Potential (Impulse)
A sequence of events that results first in depolarization and then repolarization is called action potential. Repolarization restores the resting membrane potential and allows inactivated sodium channels to revert to their resting state.

Refractory period
During the refractory period, another impulse cannot be generated at all (absolute refractory period) or can only be triggered by a suprathreshold stimulus (relative refractory period). An action potential conducts (propagates) from point to point along the membrane. The traveling action potential is a nerve impulse.

All-or-none principle
If a stimulus is strong enough to generate an action potential, the impulse travels at a constant and maximum strength for the existing conditions. A stronger stimulus will not cause a stronger impulse. All the impulses conducted on a axon are the same.

Impulse conduction
A saltatory conduction occurs when the impulse jumps from neurofibral node to another node. This phenomenon is more energy efficient and is used in quick responses. A continuous conduction is a step-by-step depolarization of adjacent areas. Propagation speed of a nerve impulse is not related to stimulus strength. Larger diameter fibers conduct impulses faster than small ones. Myelinated fibers conduct impulses faster than unmyelinated ones. Nerve fibers conduct impulses faster when warmed and slower when cooled. The intensity of a stimulus is coded in the rate of impulse production, i.e., the frequency of action potentials.

Transmission at Synapses
A synapse is the functional unit between one neuron and another or between a neuron and an effector such as a muscle or gland. At an electrical synapse, ionic current spreads directly from one cell to another through gap junctions. They are faster than chemical synapses, can synchronize the activity of a group of neurons or muscle fibers, and may allow two way transmissions of impulses. At a chemical synapse, there is only one-way information, transfer from a presynaptic neuron to a postsynaptic neuron.

Neurotransmitters
Both excitatory and inhibitory neurotransmitters are present in the CNS and PNS. The same neurotransmitter may be excitatory in some locations and inhibitory in others. Examples of neurotransmitters are acetylcholine (Ach), glutamate, aspartate, norepinephrine, epinephrine, and dopamine. Excitatory neurotransmitter is one that can depolarize or make less negative the postsynaptic neuron’s membrane. A depolarizing postsynaptic potential is called an excitatory postsynaptic potential (EPSP). An inhibitory neurotransmitter hyperpolarizes the membrane of the postsynaptic neuron, making the inside more negative and generation of nerve impulse more difficult.

An inhibitory postsynaptic potential (IPSP) neurotransmitter is removed from the synaptic cleft in three ways, diffusion, enzymatic degradation, and uptake into cells and is necessary for normal synaptic function. Certain synapses can modify the quantity of neurotransmitter released at other synapses. Presynaptic facilitation increases the amount of neurotransmitter released by a presynaptic neuron whereas presynaptic inhibition decreases the amount. Both can last for several minutes to hours and may be important in learning and memory. If several presynaptic end bulbs release their neurotransmitters at about the same time, the combined effect may generate a nerve impulse due to summation. Summation may be spatial or temporal.

Disorders associated with neurotransmitter imbalance include Alzheimer’s disease, clinical depression, epilepsy, Huntington’s disease, Parkinson’s disease, Myasthenia gravis, Schizophrenia, and possibly SIDS.

Alteration of Impulse Conduction and Synaptic Transmission
A neuron’s chemical and physical environment influences both impulse conduction and synaptic transmission. Chemical synaptic transmission may be stimulated or blocked by affecting neurotransmitter synthesis, release, removal, or the receptor site. Alkalosis, acidosis, mechanical pressure and others may all modify impulse conduction and/or synaptic transmission.

Neuronal  Pools & Circuits
Neurons in the CNS are organized into different patterns called neuronal pools. Each pool differs from all others and has its own role in regulating homeostasis. A neuronal pool may contain thousands to millions of neurons. Neuronal pools are organized into circuits which can be simple series, diverging, converging, reverberating (oscillatory), or parallel after-discharge circuits.

NERVE PATHWAYS (a route an impulse travels through the nervous system)
Reflex Arcs usually include a sensory neuron, a reflex center composed of interneurons, and a motor neuron. They are the behavioral unit of the nervous system. In Reflex Behavior, the reflexes are automatic, unconscious responses to changes. They help maintain homeostasis. For example, knee-jerk employs only two neurons and withdrawal reflexes are protective actions.

THE BRAIN & SPINAL CORD

THE MENINGES
The meninges are three coverings that run continuously around the spinal cord and brain. The outermost layer is the dura matter, the middle layer is the arachnoid, and the innermost layer is the pia matter which contains many blood vessels. Between the vertebral wall and dura matter is the epidural space which is lined with fat, blood vessels, and connective tissue (protective layer). Between the dura matter and the arachnoid is the subdural space which contains interstitial fluid. Between the arachnoid and pia matter is the subarachnoid space which contains cerebral spinal fluid. Inflammation of the meninges is called meningitis and requires immediate clinical intervention.

VENTRICLES AND CEREBRAL SPINAL FLUID (CSF)
Ventricles are interconnected cavities within the cerebral hemisphere and brain stem and are filled with CSF. The choroid plexuses in the walls of the ventricles secret CSF and it is reabsorbed into the blood through the arachnoid villi. About 500 cc of CSF are secreted daily of which about 140 cc are present at any one time. Most CSF is produced in the lateral ventricles. CSF functions in mechanical protection (floating shock absorber), in chemical protection by producing a stable ionic concentration, and in circulation for exchange of nutrients and wastes.

SPINAL CORD ANATOMY
The spinal cord is protected by the vertebral column. The meninges, cerebral spinal fluid, vertebral ligaments, and denticulate ligaments protect the spinal cord against shock and displacement.

External Anatomy of the Spinal Cord
The spinal cord begins as a continuation of the medulla oblongata and terminates at about the second sacral vertebra in an adult. It is composed of thirty-one segments, each giving rise to a pair of spinal nerves. It contains the cervical and lumbar enlargements that serve as points of origin for nerves to the extremities. The tapered end of the spinal cord is the conus medullaris, from which arise the filum terminale and cauda equina. The spinal cord is divided into left and right sides by the anterior medium fissure and posterior median sulcus. A spinal tap removes CSF to diagnose pathologies and to administer drugs.

Internal Anatomy of the Spinal Cord
The gray matter is shaped like the letter H and is surrounded by white matter; gray matter is divided into horns and white matter into columns. White matter is composed of bundles of myelinated nerve fibers. In the center of the spinal cord is the central canal, which runs the length of the spinal cord and contains CSF. Parts of the spinal fluid observed in cross section are the gray commissure, central canal, anterior, posterior, and lateral gray horns, anterior, posterior and lateral white columns, and ascending and descending tracts. The spinal cord conveys sensory and motor information by way of the ascending and descending tracts, respectively.

SPINAL CORD PHYSIOLOGY

Sensory and Motor Tracts
A major function of the spinal cord is to convey nerve impulses from the periphery to the brain (via sensory tracts) and to conduct motor impulses from the brain to the periphery (via motor tracts). Sensory information travels up the spinal cord to the brain along three main routes on each side of the cord: the spinothalamic tracts and the posterior column tracts (fasciculus gracilus and fasciculus cuneatus). The spinothalamic tracts conduct impulses related to pain, temperature, touch and deep pressure. Proprioception is awareness of movements of muscles, tendons, and joints. Discriminative touch is the ability to feel exactly what part of the body is touched. Two-point discrimination is the ability to feel when two points are touched even if they are close together.

The descending tracts are corticospinal, reticulospinal, and rubrospinal tracts found in the lateral portions of the spinal cord. Motor information travels from the brain down the spinal cord to the muscles and glands along two main descending tracts, the pyramidal tracts (corticospinal tracts) and the extrapryamidal tracts (reticulospinal and rubrospinal tracts). The direct (pyramidal) tracts carry impulses for voluntary movements. The indirect (extrapyramidal) tracts carry impulses for automatic movements of voluntary muscles. Many of the fibers in the ascending and descending tracts cross over in the spinal cord or brain.

Reflexes
A second function of the spinal cord is to be an integrating center for spinal reflexes: done in the gray matter. A reflex is a fast, predictable, automatic response to changes in the environment that helps to maintain homeostasis. Reflexes may be spinal, cranial, somatic, or autonomic. A reflex arc is the simplest type of pathway; pathways are specific neuronal circuits and include at least one synapse. The components of the reflex arc are the receptor, sensory neuron, integrating center, motor neuron, and effector. Reflexes help the body maintain homeostasis by permitting the body to make exceedingly rapid adjustments to homeostatic imbalances. Somatic spinal reflexes include the stretch (myostatic) reflex, reflex neuron, integrating center, motor neuron.

BRAIN

The brain is the largest and most complex part of the nervous system. It contains nerve centers that are associated with sensations. The brain issues motor commands and carries on higher mental functions. 

Brain development
During embryonic development, brain vesicles or cavities (prosencephalon, mesencephalon, and rhombencephalon) are formed from a neural tube, which serves as forerunners of various parts of the brain. The forebrain develops into the telencephalon and the diencephalons. The midbrain develops into the metencephalon and the myelencephalon.

Five resulting cavities remain as ventricles in the mature brain. Ultimately the telencephalon develops into the cerebrum. The diencephalon develops into the epithalamus, thalamus, subthalamus, and hypothalamus, the mesencephalon develops into the midbrain, the metencephalon becomes the pons and cerebellum, and the myelencephalon becomes the medulla oblongata.

Hindbrain & Midbrain
BRAIN STEM
The brain stem extends from the base of the cerebrum to the spinal cord and consists of the midbrain, pons, and medulla oblongata. The midbrain contains reflex centers associated with eye and head movement. The pons transmits impulses between the cerebrum and other parts of the nervous system, and contains centers that help regulate the rate and depth of breathing. The medulla oblongata transmits all ascending and descending impulses, and contains several vital (heart rate, respiratory rate) and non-vital reflex centers. The reticular formation regulates muscle tone; helps maintain consciousness and awakening from sleep. 

CERUBELLUM
The cerebellum consists of two hemispheres connected by the vermis. A thin cortex surrounds the white matter of the cerebellum. The cerebellum functions primarily as a reflex center, coordinating skeletal muscle movements and maintaining equilibrium.

Forebrain
DIEENCEPHALON
The diencephalon begins where the midbrain ends and surrounds the third ventricle. Found in the diencephalons are the epithalamus, thalamus, subthalamus, and hypothalamus. The thalamus contains nuclei that that serve as relay stations for all sensory impulses to the cerebral cortex, registers conscious recognition and temperature, and plays a role in cognition and awareness. The hypothalamus regulates the autonomic nervous system, secretes a variety of regulating hormones, functions in rage and aggression, controls body temperature, regulates food and fluid intake, and establishes a diurnal sleep pattern. Memory is established in phases and is stored in both hemispheres utilizing the limbic system, which is found in the central hemispheres, and the diencephalon. The limbic system also functions in emotional aspects of behavior. The pineal gland found in the epithalamus secrets melatonin which plays a role in sleep and setting of the body’s biological clock. 

Structure of the Cerebrum
The cerebrum can be described as two lobes of cerebral hemisphere connected by the corpus callosum. Its surface is marked with ridges, grooves that increase the surface area. A sulcus is a shallow groove. Separating the hemispheres is a deep groove called a fissure.  Covering the cerebral cortex is a thin layer of gray matter, mostly composed of the neuron cell bodies. White matter is myelinated and unmyelinated nerve fibers that interconnect neurons with the nervous system and communicate with other body parts. The lobes are named after the skull bones; (frontal, parietal, temporal, and occipital, insula). 

Function of the cerebrum
The cerebrum carries out higher brain functions such as thought, reasoning, interpretation of sensory impulses, control of voluntary muscles and memory storage. The cerebral cortex has sensory, motor, and association areas. The primary motor regions are found near the central sulcus in the frontal lobe. Other areas include the motor speech area and special motor areas. Primary sensory areas are found in the occipital area (sight), temporal area (sound), frontal lobe (taste). Association areas analyze and interpret sensory impulses and provide memory, reasoning, verbalizing, judgment and emotions. One cerebral area dominates for certain intellectual functions. Left hemisphere is important for right-handed control, spoken and written language, numeric and scientific skills, and reasoning. Right  hemisphere is more important for left-handed control, musical and artistic awareness, space and pattern perception, insight, imagination, and generating images of sight, sound, touch, taste and smell.

Basal nuclei (Basal Ganglia)
These are masses of gray matter located deep within the cerebral hemispheres. They relay motor impulses originating in the cerebral cortex, and aid in controlling motor activities. 

Peripheral Nervous System (PNS)

The peripheral nervous system consists of cranial and spinal nerves that branch out from the brain and spinal cord to all body parts. It is divided into the Somatic and the Autonomic Nervous Systems. 

Cranial Nerves
Twelve pair of cranial nerves connects the brain to various body parts. Some are mixed, some sensory, some are motor. Their names indicate either their distribution or their function. They can either be somatic or autonomic.

(I) Olfactory, (II) Optic, (III) Oculomotor, (IV) Trochlear, (V) Trigeminal (Opthalmic, Maxillary and Mandibular divisions, (VI) Abducens, (VII) Facial, (VIII) Vestibulocochlear, (IX) Glossopharyngeal, (X) Vagus, (XII) Accessory (Cranial and Spinal branches), (XII) Hypoglossal 

Spinal nerves
Thirty-one pair originate in the spinal cord and provide a two-way communication system between the spinal cord and the arms, legs, neck and trunk. They are grouped according to the vertebral levels from which they arise and are numbered sequentially. They have dorsal and ventral roots. Dorsal roots contain sensory fibers and have a dorsal root ganglion. Ventral roots contain motor fibers. Just beyond its foramen, each spinal nerve divides into several branches. Most spinal nerves form plexuses that direct nerve fibers to a particular body area.

AUTONOMIC SYSTEM
A key characteristic of the autonomic system is that it functions without conscious effort, primarily controlling visceral activities that maintain homeostasis. Autonomic functions are reflexes controlled from the hypothalamus, brain stem, and spinal cord. It is divided into the sympathetic and parasympathetic divisions. Sympathetic division prepares the body for stressful and emergency situations. Parasympathetic division is most active under normal conditions. Sympathetic fibers leave the spinal cord and synapse in specific ganglia. Parasympathetic fibers begin in the brain stem and sacral region of the spinal cord and synapse in ganglia near various visceral organs or in the organs themselves.

 THE SPECIAL SENSES 

RECEPTORS AND SENSATIONS
Each type of receptor is sensitive to a distinct stimulus. Major types of receptors include chemoreceptor (sensitive to changes in chemical concentration), pain receptors (sensitive to tissue damage), thermoreceptor (sensitive to mechanical forces), photoreceptors (sensitive to light), and mechanoreceptors (sensitive to mechanical forces such as changes in pressure or movement of fluids).  

Sensory impulses
When receptors are stimulated, changes occur in their membrane potentials. Receptor potentials are transferred to nerve fibers triggering action potentials. Sensations are feelings from sensory stimulation. A particular part of the sensory cortex interprets every impulse that reaches it in the same way. The cerebral cortex projects a sensation back to the region of stimulation. Sensory adaptations are adjustments of sensory receptors to continuous stimulation. Impulses are triggered at slower and slower rates. 

OLFACTORY SENSATIONS: SMELL
The receptors for olfaction, which are bipolar neurons, are in the nasal epithelium in the superior portion of the nasal cavity. Substances to be smelled must be volatile, water-soluble, and lipid-soluble. Adaptation to odors occurs quickly, and the threshold to smell is low; only a few molecules of a substance need be present in air to be smelled. Olfactory receptors convey nerve impulses to olfactory (I) nerves, olfactory bulbs, olfactory tracts, and the cerebral cortex and limbic system. 

GUSTATORY SENSATIONS: TASTE
The gustatory receptor cells are located in taste buds. Substances to taste must be in solution in saliva. The four primary tastes are sour (mainly on the side of the tongue), salty (tip of tongue), bitter (back of tongue), and sweet (tip of tongue). The senses of smell and taste are very closely related; impaired ability to smell significantly affects one’s ability to taste. Think about the ability to taste food when you have an upper respiratory tract infection. Adaptation to taste occurs quickly; the threshold varies with the taste involved. Taste receptor cells convey nerve impulses to cranial nerves V, VII, IX, and X, the medulla, the thalamus, and the parietal lobe of the cerebral cortex. 

AUDITORY SENSATIONS AND EQUILIBRIUM
External (outer) ear collects the sound waves and passes them inwards. The outer ear consists of the auricle, external auditory canal and tympanic membrane. Ceruminous glands secrete cerumen into the external canal that helps prevent dust and foreign objects from entering the ear.

Middle Ear (Tympanic cavity) is a small, air-filled cavity in the temporal bone that is lined by epithelium. The middle ear consists of auditory (Eustachian) tube, auditory ossicles (malleus, incus, and stapes), and the oval and round windows. 

The Internal (inner) ear (labyrinth) contains two main divisions; an outer bony labyrinth that encloses an inner membranous labyrinth. The bony labyrinth is a series of cavities named on the basis of shape; semicircular canals and the vestibule (contain receptors for equilibrium), and the cochlea which contains receptors for hearing. The inner ear is lined with periosteum and contains a fluid called perilymph which is chemically similar to CSF. The fluid surrounds the membranous labyrinth. The membranous labyrinth is a series of sacs and tubes inside of and having the same general shape as the bony labyrinth. It is lined with epithelium and contains a fluid called endolymph, which is chemically similar to interstitial fluid. 

The vestibule is the oval central portion containing two sacs: utricle and the saccule. 

Semicircular canals and Cochlea
The anterior and posterior canals are oriented vertically and the lateral one is oriented horizontally. One end of each canal enlarges into a swelling called an ampulla. The portions of the membranous labyrinth that lie inside the semicircular canals are called the semicircular ducts. The cochlea is divided into three channels. The channel above the bony portion is the scala vestibuli, which ends at the oval window. The channel below is the scala tympani, which ends at the round window. Both contain perilymph. The third channel is the cochlear duct (scala media). Membranes separate the cochlear duct from the other two channels. Resting on the basilar membrane is the spiral organ (Organ of Corti), the organ of hearing. Hair cells of the spiral organ are easily damaged by continual exposure to high intensity sounds and may degenerate, producing deafness. Projecting over and in contact with the hair cells is the tectorial membrane, a delicate and flexible gelatinous membrane. 

Sound Waves result from alternate compression and decompression of air molecules. Normal range of hearing is between 1000 and 2000 Hertz (cycles/second). The frequency of a sound wave is its pitch; the greater the intensity (size) of the vibration, the louder the sound as measured in decibels.  

Events of Hearing
The auricle directs the sound waves into the external auditory canal. Sound waves strike the tympanic membrane, causing it to vibrate back and forth. The vibration conducts from the tympanic membrane through the ossicles. The malleus, connected to the eardrum, moves, causing the incus and the stapes to move back and forth, pushing the membrane of the oval window in and out. The movement of the oval window sets up fluid pressure waves in the perilymph of the scala vestibuli. Pressure waves in the scala vestibuli are transmitted to the scala tympani and eventually the round window, causing it to bulge inward into the middle ear. As the pressure waves deform the walls of the scala vestibuli and scala tympani, they push the vestibular membrane back and forth causing increasing and decreasing pressure of the endolymph inside the cochlear duct. The pressure fluctuations of the endolymph move the basilar membrane slightly, moving the hair cells of the spiral organ against the tectorial membrane; the bending of the hairs produces receptor potentials that lead to the generation of nerve impulses in cochlear nerve fibers. Pressure changes in the scala tympani cause the round window to bulge outward into the middle ear.  

Differences in pitch cause specific regions of the basilar membrane to vibrate more intensely than others. High-frequency sounds result in vibration near base of cochlea. Low-pitch sounds vibrate at apex of cochlea. Hair cells convert a mechanical force into an electrical signal; hair cells release a neurotransmitter, which initiates nerve impulses. Nerve impulses from the cochlear branch of the vestibulocochlear (VIII) nerve pass to the cochlear nuclei in the medulla. Most impulses then cross to the opposite side and then travel to the midbrain, to the thalamus, and finally to the auditory area of the temporal lobe of the cerebral cortex. 

Physiology of equilibrium
Static equilibrium refers to the maintenance of the position of the body (mainly the head) relative to the force of gravity. The maculae of the utricle and the saccule are the receptors for equilibrium. Dynamic equilibrium is the maintenance of body position (mainly the head) in response to sudden movements, such as rotation, acceleration, and deceleration. The cristae in the ampulla of the semicircular ducts are the primary sense organs of dynamic equilibrium. 

VISUAL SENSATIONS

Accessory Structures of the Eye include eyebrows, eyelids, eyelashes, lacrimal apparatus, and extrinsic eye muscles (superior, inferior, lateral and medial rectus and the superior and inferior oblique move the eyeballs, usually in concert with each other). The conjunctiva is a thin mucous membrane that lines the inner aspect of the eyelids and is reflected onto the anterior surface of the eyeball. The lacrimal apparatus consists of structures that produce and drain tears. “Watery” eyes occur when the normal drainage for the lacrimal glands is overwhelmed or obstructed. Tears contain lysozyme which has antibacterial properties. 

Anatomy of the Eyeball
The eye is composed of three layers. The fibrous tunic is the outer coat of the eyeball: divided into posterior sclera and anterior cornea. Junction of the sclera and cornea: opening known as the scleral venous sinus or canal of Schlemm. The sclera, “white’ of the eye is a white coat of dense fibrous tissue covers the entire eyeball except the most anterior portion, gives the eyeball its shape, protects the inner parts. The posterior area is pierced by the optic nerve (II). The cornea is a nonvascular, transparent, fibrous coat through which the iris can be seen; acts in refraction of light; contains many nerve fibers with low pain thresholds. Corneal transplants are the most common organ transplant.

 The vascular tunic is the middle layer and is composed of three portions; the choroid absorbs light rays so they are not reflected and scattered within the eyeball; it also provides nutrients to the posterior surface of the retina. The ciliary body consists of the ciliary processes and ciliary muscle. The processes consist of folds on the internal surface of the ciliary body where the epithelial lining cells secrete aqueous humor. The muscle is a smooth muscle that alters the shape of the lens for near or far vision. The iris is the colored portion seen through the cornea and consists of circular iris and radial iris smooth muscle fibers arranged to form a doughnut-shaped structure. The black hole in the center of the iris is the pupil, the area through which light enters the eyeball. The function of the iris is to regulate the amount of light entering the posterior cavity of the eyeball.

The third and inner coat is the retina (nervous tunic), lines the posterior three-quarters of the eyeball. Its primary function is image formation. It consists of a pigmented epithelium (nonvisual portion) and a neural portion (visual portion). The pigmented epithelium aids the choroid in absorbing stray light rays. The macula lutea is in the exact center of the posterior portion of the retina to the visual axis of the eye. The fovea centralis is found in a depression in the macula lutea.

The neural portion contains three zones of neurons: photoreceptor neuron, bipolar neurons, and ganglion neurons. Photoreceptor neurons are called rods or cones because of their outer segments. Rods are specialized for black-and-white vision in dim light: allow us to discriminate between different shades of dark and light and permit us to see shapes and movement. Cones are specialized for color vision and sharpness of vision in bright light; most densely concentrated in the central fovea, a small depression in the macula lutea. The fovea is the area of sharpest vision because of the high concentration of cones. Rods are absent from the fovea. A detached retina is often due to trauma of the head, but may be reattached by laser surgery. 

The nonvascular lens is located just behind the pupil and the iris. Its function is to fine-tune light rays for clear vision. Loss of transparency is a cataract and is usually found with aging. The interior of the eyeball is a large space divided into two cavities by the lens: anterior cavity and posterior (vitreous) cavity. The anterior cavity is divided into anterior chamber and posterior chamber. The anterior cavity is filled with aqueous humor (AH) that is constantly being secreted by the ciliary processes behind the iris. The aqueous humor flows forward from the posterior chamber to the anterior chamber and drains into the scleral venous sinus and then into the blood. AH is replaced about every 90 minutes. Intraocular pressure is produced mainly by the aqueous humor. Excessive intraocular pressure is called glaucoma.  

The posterior cavity (vitreous chamber) lies between the retina and the lens and is filled with a gel like substance called vitreous humor (VH). VH contributes to intraocular pressure, prevents the eyeball from collapsing, and holds the retina flush against the internal portions of the eyeball. VH is formed during embryonic development and is not replaced during life. 

Image Formation
Image formation on the retina involves refraction of light rays by the cornea and lens, accommodation of the lens, and constriction of the pupil. The bending of light rays at the interface of two different media is called refraction; the anterior and posterior surfaces of the cornea and of the lens refract entering light rays so that they come into exact focus on the retina. Images are focused upside-down and right to left reversal on the retina; the images undergo a mirror reversal in the brain. Abnormalities of refraction are due to improper shape of the eyeball or to irregularities in the surface of the lens or cornea. Accommodation is an increase in the curvature of the lens, initiated by ciliary muscle contraction, which allows the lens to focus on near objects. To focus on far objects, the lens flattens out and the ciliary muscles relax. Constriction of the pupil means narrowing the diameter of the hole through which light enters the eye; this occurs simultaneously with accommodation of the lens and prevents light rays from entering the eye through the periphery of the lens. In convergence, the eyeballs move medially by action of the extrinsic eye muscles. 

Physiology of Vision
The first step in vision transduction is the absorption of light by photo-pigments on rods (@ 100 million) and cones (@ 3 million) which causes the photopigments to decompose. Photopigments are colored proteins that undergo structural changes upon light absorption.

The single type of photopigment in rods is rhodopsin. There are three types of photopigments in cones (RGB = red, green, blue). Bleaching and regeneration of the photopigments accounts for much but not all of the sensitivity change during light and dark adaptation. Once receptor potentials develop in rods and cones, they release neurotransmitters that induce graded potentials in bipolar cells and horizontal cells. 

Visual Pathway
Horizontal cells transmit inhibitory signals to bipolar cells; bipolar or amacrine cells transmit excitatory signals to ganglion cells, which depolarize and initiate nerve impulses.

Impulses from ganglion cells are conveyed through the retina to the optic (II) nerve, through the optic chiasma and the optic tract, to the thalamus, and finally to the cortex (occipital lobes). Stereoscopic vision: perceives height, width, and depth of vision. 

DISORDERS OF SPECIAL SENSES
Glaucoma is abnormally high intraocular pressure, due to a buildup of aqueous humor inside the eyeball, which destroys neurons of the retina. It is usually seen in the elderly. In cataracts the lens becomes cloudy, opaque or yellow. Specks are floaters in the aqueous humor. 

Deafness is significant or total hearing loss. Causes can be sensorineural (damage or destruction of nerve), conduction or mechanical. Otitis media refers to an acute infection of the middle ear, primarily by bacteria. Motion Sickness is a functional disorder precipitated by repetitive angular, linear, or vertical motion and characterized by nausea and vomiting. Preventative measures are more effective. 

THE ENDOCRINE SYSTEM 

ENDOCRINE GLANDS
The body contains two types of glands; exocrine and endocrine glands. Exocrine glands (sudoriferous, sebaceous, and digestive) secrete their products through ducts into body cavities or onto body surfaces. Endocrine glands secrete their products (hormones) into extracellular spaces around the secretary cell. The secretion diffuses into capillaries and is carried away by the blood to a target tissue.

COMPARISON OF NERVOUS AND ENDOCRINE SYSTEMS
Together the nervous and endocrine systems coordinate all body systems. The nervous system controls through nerve impulses conducted along axons of neurons. The endocrine system releases hormones which are delivered to tissues throughout body by blood. Certain parts of the nervous system stimulate or inhibit the release of hormones and hormones may promote or inhibit nerve impulses. The nervous system causes muscular contraction or glandular secretion, the endocrine system alters metabolic activities, regulates growth and development, and guides the reproductive process. Nerve impulses are generally much faster but the responses are briefer than hormones which are slower in response timebut last longer.

HORMONES
Hormones only affect specific target cells that have receptors to recognize a given hormone. Down-regulation occurs when the number of receptors decreases, thereby decreasing the responsiveness of the target cell. Up-regulation occurs when hormone is deficient and makes target tissue more receptive. 

Types of hormones
Circulating hormones (endocrine) are hormones that pass into the blood to act on distant target cells, thus may linger for minutes or hours. Local hormones usually are inactivated quickly. For example paracrines act on neighboring cells and autocrines act on the same cell that secreted them. 

Chemical classification
Most hormones are either steroids or nonsteroids. Steroids are lipids that are derived from cholesterol; examples include the sex hormones and aldosterone. Biogenic amines are very simple molecules derived from amino acids; examples include the thyroid hormones (T₃ & T₄), epinephrine and norepinephrine (catecholamines). Peptides (short chains of amino acids) and proteins (long chains of amino acids) include thyroid stimulating hormone, antidiuretic hormone, insulin, glucagon, human growth hormone and others. Eicosanoids were recently discovered hormone and include prostaglandins and leukotrienes. Water soluble hormones circulate in free form in the blood; lipid-soluble  steroid and thyroid hormones are carried attached to transport proteins.

MECHANISMS OF HORMONE ACTION
The mechanism depends on both the hormone and the target cell. (Insulin stimulates synthesis of glycogen in the liver and synthesis of triglycerides in adipose tissue. Lipid-soluble hormones diffuse through the cell membrane into a cell. In a target cell, the hormone binds to and activates receptors within the cytosol or nucleus altering gene expression. Water-soluble hormones activate plasma membrane receptors. First messenger is hormone that activates receptor on plasma membrane which activates G-protein. A second messenger relays the message inside of the cell. Cyclic AMP (cAMP) is the best known second messenger. 

Hormonal interactions depend on the hormone’s concentration, abundance of receptors, influences by other hormones. In a permissive effect, the action of some hormones requires recent stimulation by other hormones. In some cases there may be a synergistic effect or antagonistic effect. 

Control of Hormone Secretion
Most hormones are released on short bursts, with little or no release between busts. Regulation maintains homeostasis and prevents over or underproduction. Hormone secretion is controlled by signals from the nervous system, by chemical changes in the blood, and by other hormones. Most control is through negative feedback systems.

HYPOTHALAMUS AND PITUITARY GLAND
The hypothalamus is the major integrating link between the nervous and endocrine systems. The hypothalamus and the pituitary gland regulate virtually all aspects of growth, development, metabolism, and homeostasis. 

Pituitary gland consists of an anterior pituitary gland and posterior pituitary gland. Hormones of the anterior pituitary gland are controlled by releasing or inhibiting hormones produced by the hypothalamus. There are five cell types; somatotrophs which produce human growth hormone (hGH), lactotrophs which produce prolactin (PRL), corticotrophs that secrete ACTH and melanocyte stimulating hormone, thyrotrophs secrete thyroid-stimulating hormone (TSH), and gonadotrophs secrete follicle-stimulating hormone (FSH) and leutinizing hormone (LH). 

Hormones of Anterior Pituitary Gland

• GH (somatotropin) stimulates body growth, has many effects on metabolism and is controlled by growth hormone inhibiting hormone-somatostatin (GHIH) and growth hormone releasing hormone (GHRH). Disorders associated with improper levels of hGH are pituitary dwarfism, giantism and acromegaly.
• TSH regulates thyroid gland activities (T₃ - T₄ production) and is controlled by TRH (thyroid releasing hormone).
• FSH regulates activities of the ovaries and testes and is controlled by (GnRH) gonadotropin releasing hormone.
• LH regulates activities of the ovaries and testes and is controlled by (GnRH) gonadotropin releasing hormone.
• Prolactin (PRL) helps initiate milk secretion and is controlled by PIH (prolactin inhibitory hormone) and PRH (prolactin releasing hormone).
• Melanocyte-stimulating hormone increases skin pigmentation and is controlled by melanocyte-releasing hormone and melanocyte-inhibiting hormone.
• ACTH regulates activities of the adrenal cortex and is controlled by CRH (corticotrophin releasing hormone).

Posterior Pituitary Gland
The posterior pituitary gland does not synthesize hormone, but it does store two hormones made in the hypothalamus. Oxytocin (OT) stimulates contraction of the uterus and ejection of milk from the breasts. OT secretion is controlled by uterine distention and nursing. Synthetic OT is often given to induce labor.  

The other hormone is Antidiuretic hormone (ADH). ADH stimulates water reabsorption by the kidneys and arteriolar constriction. The effect of ADH is to decrease urine volume and conserve body water. ADH is controlled by osmotic pressure of the blood. A disorder with secretion of ADH is Diabetes insipidus which is a result in a hyposecretion of ADH which causes excretion of large amounts of dilute urine.

THYROID GLAND
Thyroid gland consists of thyroid follicles which secrete hormones T₃ and T₄ and parafollicular cells which secrete calcitonin. Thyroid hormones are synthesized from iodine and tyrosine. Thyroid hormones regulate the rate of metabolism, growth and development, and the reactivity of the nervous system. Secretion is controlled by the level of iodine in the thyroid gland. Cretinism, myxedema, Graves’ disease and goiter are disorders associated with the thyroid gland. Calcitonin lowers the blood level of calcium. Secretion is controlled by calcium levels in the blood.

PARATHYROID GLANDS
Parathyroid glands are embedded in the posterior surfaces of the thyroid gland. They secrete parathyroid hormone (PTH) which regulates the homeostasis of calcium and phosphate by increasing the blood calcium level and decreasing blood phosphate level. Secretion is controlled by blood calcium levels. Tetany and Osteitis fibrosa are disorders associated with the parathyroid glands. Tetany results from a deficiency of calcium caused by hypothyroidism. Osteitis fibrosa is characterized by demineralized, weakened, and deformed bones resulting from hyperthyroidism.

ADRENAL GLANDS
The adrenal glands lie on top of each kidney and consist of an outer cortex and an inner medulla. Complete loss of adrenocortical hormones leads to death due to dehydration and electrolyte imbalance within days to a week. In the Adrenal Cortex are the zona glomerulosa, zona fasciculata, and zona reticularis. Cortical secretions include mineralcorticoids, glucocorticoids and gonadalcorticoids. Mineralcorticoids (Aldosterone) increase sodium and water reabsorption and decrease potassium reabsorption. Secretion is controlled by the renin-angiotensin pathway and the blood levels of potassium. Hypersecretion of aldosterone leads to muscular paralysis and hypertension. 

Glucocorticoids (cortisol) promote normal organic metabolism, help resist stress and serves as anti-inflammatory substance. Secretion is controlled by CRH and ACTH from the anterior pituitary. Disorders of glucocorticoids production include Addison’s disease (hyposecretion of glucocorticoids and aldosterone) and Cushing’s disease (hypersecretion of cortisol and cortisone).

Gonadalcorticoids secreted by the adrenal glands usually have minimal effects. Excessive production results in virilism. 

Adrenal Medulla
The adrenal medulla consists of hormone-producing cells, called chromaffin cells, which surround large blood-filled sinuses. Medullary secretions are epinephrine and norepinephrine. Hormones are released under stress by direct innervation from the autonomic nervous system and mimic sympathetic responses. They help the body reduce stress but are not essential for life.

PANCREAS
The pancreas produces exocrine (for digestion) and endocrine (hormones) secretions. Endocrine portion consists of pancreatic islets (islets of Langerhans) which are divided into four types of cells. Alpha cells secrete the hormone glucagon which increases blood glucose levels. Secretion is stimulated by low blood glucose level. Beta cells secrete the hormone insulin. Insulin decreases blood glucose levels and is stimulated by a high blood glucose. Disorders associated with Beta cell endocrine hormone secretion include diabetes mellitus and hyperinsulinism.

Delta cells secrete growth hormone inhibiting hormone (GHIH) which acts as a paracrine to inhibit the secretion of insulin and glucagon. F-cells secrete pancreatic polypeptide, which regulates the release of pancreatic digestive enzymes.

REPRODUCTIVE GLANDS
Ovaries produce estrogen and progesterone which are related to development and maintenance of female sexual characteristics, reproductive cycle, pregnancy, lactation and normal reproductive functions. Produce also inhibin and relaxin. These will be covered in extensively in the reproductive unit. Testes produce testosterone which are related to development and maintenance of male sexual characteristics and normal reproductive functions. Also produced is inhibin.

PINEAL GLAND
Attached to roof of third ventricle inside the brain is the pineal gland. It consists of pinealocytes, neuroglial cells and post ganglionic sympathetic fibers. The pineal gland secretes melatonin in a diurnal rhythm linked to the dark-light cycle. Seasonal affective disorder (SAD) is thought to be due to over-production of melatonin.

THYMUS GLAND AND DIGESTIVE GLANDS WILL BE COVERED LATER.

Hyposecretion and Hypersecretion
Inadequate release of hormones from the endocrine glands results in imbalances in the homeostasis which leads to disease. Too little hormone being released is known as hyposecretion whereas too much hormone released is called hypersecretion. The following table is a partial list of endocrine glands and the disease states that occur in hypo- and hypersectrtion.
 

STRESS AND THE GENERAL ADAPTATION SYNDROME
Homeostatic mechanisms attempt to counteract the everyday stresses of living. If successful, the body maintains normal limits of chemistry, temperature and pressure. If stress is extreme, unusual, or long-lasting, these mechanisms may not be sufficient, which triggers the general adaptation syndrome. Stressors are the stimuli that produce the general adaptation syndrome. Include heat, cold, operations, poisons, infections, fever, and strong emotional responses. Stressors stimulate the hypothalamus via an immediate alarm reaction. They are slower to start, but longer lasting, resistance reaction.  

Alarm Reaction
This alarm reaction is also called the fight-or-flight reaction that increases circulation, promotes catabolism for energy production, and decrease nonessential activities.  

Resistance reaction is initiated by regulating hormones of the hypothalamus (CRH, GHRH, and TRH). They are long term reactions and accelerate catabolism for energy to counteract stress. Exhaustion can result from changes during alarm and resistance reactions. If stress causing exhaustion is too great, it may lead to death. 

Stress and disease
Stress can lead to diseases such as gastritis, ulcerative colitis, irritable bowel syndrome, peptic ulcers, hypertension, asthma, rheumatoid arthritis, migraine headaches, anxiety and depression. One can also develop a chronic disease or dying prematurely if stressors are not reduced.