The Central Nervous System (CNS) is composed of the brain (housed within the cranium of the 

skull) and spinal cord (housed within the vertebral column).

A. Organization of the brain is related to its embryonic development.

1. Following gastrulation a neural plate forms from cephal to caudal, along what will 

form the dorsal surface of the animal. 

2. A neural groove forms as a crest forms on either side of the neural plate, and the 

center of the neural plate invaginates. 

3. As the groove deepens the crests move towards the midline and fuse--forming a neural 

tube. 

a) The cephal end of the tube will enlarge forming the brain. 

b) The rest of the neural tube will form the spinal cord.

B. The embryonic prosencephalon (forebrain) will develop into the following structures.

1. The Telencephalon is the most anterior region and will contain the following 

areas/structures.

a) The cerebral hemispheres (cerebrum). 

(1) The two highly invaginated cerebral hemispheres are separated by a deep 

groove called the median (or longitudinal) fissure.  

(a) An invagination is called a sulcus (sulcuses). 

(b) The mass of tissue between sulcuses is called a gyrus (gyruses). 

(2) Gray matter is external to white matter. 

(a) Gray matter is composed of unmyelinated neurons and is generally 

related to intelligence and conscious activities depending on its 

location. 

(b) White matter is composed of myelinated fibers, although the 

myelination is not by Schwann cells but oligodendroglia in the CNS. 

(c) Brain folding increases surface area, and therefore gray matter and 

intelligence. 

(d) This outer region of gray matter is called the cerebral cortex. 

(e) Brain folding is consistent within species. 

(f) Intelligence a difficult thing to measure empirically--some 

“geniuses” have had large brains, others small brains. 

(3) Brain activities associated with specific lobes (enlargements). 

(a) Frontal lobes-- primary motor cortex (control of muscles), speech 

centers, smell, memory, and associating information from other 

brain areas. 

(b) Parietal lobes-- somatosensory complex that processes touch and 

pressure stimuli and relates to other brain areas. 

(c) Temporal lobes-- hearing centers, balance centers, some language, 

and reading skills, identifying and naming objects. 

(d) Occipital lobes-- visual centers, these are stimulated in memory such 

as when remembering an event or trying to spell a word, you 

literally visualize it in the occipital lobe. 

(e) None of the lobes work in isolation, are able to communicate with 

one another.

(f) The right and left hemispheres are not mirror images, processing of 

information and lobe interactions differ. 

(i) Left side-- logical, temporal, language oriented. 22

(ii) Right side-- “gestalt” conclusion, spatial relations, abstract 

reasoning, artistic side. 

(4) Commissures are masses of white matter that connect lobes on either side 

of the brain. 

(5) Corpus callosum is the largest commissure at the base of the median 

(longitudinal) fissure. 

(6) Basal nuclei. 

(a) Are located deep within cerebral hemispheres lateral to the thalamus 

in each hemisphere. 

(b) Nuclei are masses of gray matter. 

(c) Nuclei are considered “relays” between different areas of the brain, 

i.e. are concentrations of synaptic junctions. 

(d) The basal nuclei relay information from cerebral cortex to other 

brain areas, and are involved in controlling the intensity of 

movements and our ability to do several motor activities at once, 

probably works with cerebellum.

2. The Diencephalon is deep to the cerebral hemispheres and surrounds the third 

ventricle, composed of the following structures.

a) Thalamus. 

(1) Two egg shaped masses forming the upper lateral walls of the third 

ventricle. 

(2) Connected by the intermediate mass that traverses the third ventricle. 

(3) Contains several important nuclei that relay to and from the cerebrum 

found in the thalamus-- all sensory impulses and those or the emotional 

center of the brain (limbic system) relay in the thalamus. 

(4) The thalamus plays a role in integrating and associating different parts of 

the brain in the following processes. 

(a) Sensation. 

(b) Motor activities. 

(c) Arousal (waking up). 

(d) Learning. 

(e) Memory. 

b) Hypothalamus. 

(1) Inferior to the thalamus, forming floor of third ventricle, and superior to 

brain stem. 

(2) Connects directly to pituitary gland (hypophysis) by a stalk of tissue 

known as the infundibulum. 

(3) The hypothalamus has both neural and vascular connections to the 

hypophysis and has profound control over it. 

(4) Includes the mamillary bodies that are relay points in olfactory pathway. 

(5) Contains many nuclei is the major visceral homeostatic control center 

related to following functions. 

(a) Autonomic control center. 

(b) Physiological response center related to emotions. 

(c) Body temperature regulation. 

(d) Satiety and thirst centers. 

(e) Circadian rhythms. 

(f) Endocrine regulation via pituitary gland.

c) Epithalamus. 

(1) Forms roof of the third ventricle. 

(2) Includes the pineal body, which secretes the hormone melatonin, which 

triggers sleep cycles. 

(3) Also includes the choroid plexus of the third ventricle-- choroid plexus a 

network of arterioles that lose blood plasma forming cerebrospinal fluid in 

the third ventricle (a choroid plexus is also found in each of the lateral 

ventricles and the fourth ventricle).

C. The mesencephalon develops into the superior part of the brain stem called the midbrain--

the midbrain includes the following structures.

1. Cerebral peduncles--large tracts of white matter that connect to the cerebral 

hemispheres. 

2. Superior cerebellar peduncles-- large tracts of white matter that connect to the 

cerebellum. 

3. Corpora quadrigemina-- four masses of tissue posterior and inferior to the pineal body 

composed of the following. 

a) Superior colliculi-- visual reflex centers such as tracking moving objects. 

b) Inferior colliculi-- auditory reflex centers such as tracking sound and startle 

reflex. 

4. Substantia nigra-- deep to cerebral peduncles, nucleus functionally linked to basal 

ganglia, secretes the neurotransmitter dopamine, degeneration of Substantia nigra 

leads to Parkinson’s disease. 

5. Red nucleus -- deep to Substantia nigra, controls motor pathways, part of a network of 

nuclei in brain stem know as the reticular formation.

D. The rhombencephalon is the inferior portion of the embryonic brain forming the remainder 

of the brain stem and the cerebellum.

1. The more superior portion of rhombencephalon is called the metencephalon and gives 

rise to the following brain structures. 

a) Pons. 

(1) Inferior to the midbrain. 

(2) Conduction pathway between higher and lower brain centers. 

(3) Respiratory center located in Pons (works with hypothalamus). 

(4) Middle cerebellar peduncles communicate with cerebellum. 

b) Cerebellum. 

(1) Dorsal to Pons and Medulla oblongata. 

(2) Integrates sensory and motor information to carry out learned motor 

activities-- is the athletic brain, acting to refine and direct all motor 

activities. 

(3) Like the cerebrum gray matter external and white matter internal, also 

enfolding. 

(4) Develops an “athletic memory” so activities do not have to be relearned. 

2. The more inferior portion of rhombencephalon is called the myelecephalon and gives 

rise to the following brain structures. 

a) Medulla oblongata. 

(1) Inferior to the Pons. 

(2) Conduction pathway between higher and lower brain centers. 

(3) Inferior cerebellar peduncles communicate with cerebellum. 

(4) Centers for many autonomic reflexes located at least in part in the medulla 

oblongata: cardio regulatory, blood pressure, respiratory, vomiting, 

coughing, etc.  

(5) About 2/3 of nerve tracts decussate (cross over) from one side of body to 

other side of brain in brainstem. 

1. The reticular formation extends throughout the brains stem and describes the many 

nuclei surrounded by white matter. 

2. The limbic system is a functional area of the brain composed of parts of the cerebrum 

and diencephalon and is the emotional center of the brain. 

3. The brain has fluid filled chambers, called ventricles, that conduct cerebrospinal fluid, 

but these will be discussed below.

F. The spinal cord is composed of ascending and descending tracts of myelinated fibers and 

neurons associated with spinal reflex arcs.

1. In the spinal cord white matter is external to the gray matter. 

2. The white matter is composed of ascending and descending tracts of nerve fibers. 

3. The gray matter controls spinal reflex arcs, which are simple systems of stimulus and 

response that do not involve the higher brain (cerebral cortex). 

a) Afferent (sensory) neurons conduct impulses via peripheral somatic nerves 

through the dorsal horns to the gray matter of the spinal cord. 

b) The afferent neuron may synapse with the following neurons in the gray matter. 

(1) Directly with an efferent (motor) neuron (discussed below). 

(2) With an associative (inter) neuron in the gray matter. 

(a) An associative neuron synapses with the following neurons. 

(i) An efferent (motor) neuron (discussed below). 

(ii) An ascending tract neuron that will tell higher brain what is 

going on. 

(3) With an ascending tract neuron that will tell the higher brain what is going 

on. 

c) The afferent neuron or the associative neuron will synapse with an efferent 

(motor) neuron. 

(1) The efferent neuronal axon will exit the spinal cord via the ventral horn, 

and innervate a muscle via a peripheral somatic nerve. 

(2) The efferent neuron forms a motor unit stimulating muscle fibers to 

contract, eliciting the response. 

(3) The response does not involve commands from the higher brain--the 

response initiated in the spinal cord.

d) Example: the stretch reflex of the patellar tendon. 

(1) Doctor strikes patellar tendon, stretching the tendon initiating an impulse 

in a (actually many) stretch receptor embedded in the patellar tendon. 

(2) The stretch receptor is the afferent neuron and carries the impulse to the 

spinal cord. 

(3) The afferent neuron synapses with an associative neuron. 

(4) The associative neuron synapses with an efferent neuron and ascending 

tract neurons that will eventually go to the higher brain centers telling it 

the patella was stretched. 

(5) The efferent neuron innervates muscle fibers of the quadriceps group 

causing them to contract. 

(6) Contraction of the quadriceps group extends the lower leg, relieving the 

stretch in the patellar tendon ending the stimulus, and the reflex. 

(7) The response in no way involves the higher brain is-- completely 

coordinated by the spinal cord. 

(8) The stretch reflex probably protects us from joint and muscle damage. 25

e) More complex reflexes (cranial reflexes) involve the cerebellum or brain stem, 

but again not the higher brain (cerebral cortex). 

f) The spinal cord ends at about the first lumbar vertebra (L1). 

(1) The cord terminates in a structure called the conus medullaris. 

(2) At the conus medullaris the cord splits into numerous nerves that run 

within the vertebral foramen called the cauda equina (horse tail). 

g) The spinal cord has two areas that are thicker than the rest of the cord--the 

cervical and lumbar enlargements. 

1. The inner surface of the cranium and vertebral foramen is covered with dense irregular 

connective tissue forming the endosteum. 

2. Internal to the endosteum is a fluid filled space called the epidural space-- an 

“epidural” anesthesia is delivered into this space. 

3. Internal to the epidural space are the meninges, listed below from superficial to deep. 

a) The dura mater means “tough mother” and is the outermost meninge-- as the 

name implies it is extremely fibrous and tough. 

b) The arachnoid is deep to the dura mater and means “spider like.” 

(1) The arachnoid is much more diffuse and delicate, adhering directly to the 

dura mater. 

(2) The arachnoid has a space associated with it called the subarachnoid 

space, which contains cerebrospinal fluid (csf). 

(3) The subarachnoid space is also a vascular layer. 

(4) A “spinal” anesthesia is delivered into this space. 

(5) The arachnoid can be seen on the sheep brain as it spans the sulcuses. 

c) The innermost brain covering is the pia mater, which means “soft mother”--The 

pia mater directly adheres to brain and spinal cord tissue.

1. As mentioned in the beginning of our discussion of the CNS, the brain and spinal cord 

started out as a hollow tube. 

2. Remnants of that tube still exist in the form of the brain ventricles and central (spinal) 

canal of the spinal cord. 

a) Within the cerebral hemispheres are large C-shaped spaces called the lateral 

ventricles. 

(1) There is a single lateral ventricle within each hemisphere. 

(2) Each lateral ventricle has a choroid plexus (highly permeable arterioles 

that allow plasma to leach out of the bloodstream) that produces 

cerebrospinal fluid whose function and composition is discussed below. 

b) CSF flows from each lateral ventricle through a small canal, called the foramen 

of Monro into a narrow, centrally located chamber called the third ventricle. 

(1) The epithalamus forms the roof of the third ventricle. 

(2) The thalamus forms the walls of the third ventricle. 

(3) The hypothalamus forms the floor of the third ventricle. 

(4) It is in the midline of the brain, slightly inferior to the lateral ventricles. 

(5) The third ventricle also has a choroid plexus that also produces CSF. 

c) CSF flows through the cerebral aqueduct (Canal of Sylvius) to a still smaller 

ventricle called the fourth ventricle. 

(1) The fourth ventricle is dorsal to the pons and medulla oblongata and 

ventral to the cerebellum. 

(2) The fourth ventricle also has a choroid plexus that produced CSF. 

(3) Lateral apertures in the fourth ventricle connect to the subarachnoid space 

of the cranium.

(4) Hydrocephalus occurs if the lateral apertures are too small or are blocked 

for some reason. 

(a) CSF is produced within the ventricles faster than it can escape, and 

pressure builds. 

(b) Fluid pressure causes seizures of varying degrees. 

(c) “Shunts” can be inserted to divert CSF but must be replaced as a 

child grows. 

d) CSF flows from the fourth ventricle through the lateral apertures into the 

subarachnoid space, or into the central canal of the spinal cord. 

e) CSF that enters the central canal reaches the end of the spinal cord, and 

percolates into the subarachnoid space and works its way back to the cranium. 

f) CSF is reabsorbed by a confluence of veins called the superior sagittal sinus. 

3. Cerebrospinal fluid (CSF) has the following characteristics and functions. 

a) It is derived from blood plasma but contains more Sodium and Hydrogen ions 

and less protein, Calcium, and Potassium. 

b) The CSF in and around the brain forms a liquid cushion that gives buoyancy to 

the brain tissue. 

(1) Brain tissue is notoriously lacking in connective tissue and is highly fatty. 

(2) The CSF provides a medium within which this fatty organ can “float” 

offsetting its tremendous mass, and minimizing the need for internal 

skeletal support, whose rigid structure or sharp edges could rupture brain 

tissue if a blow to the head occurred.

II. The peripheral nervous system (PNS) is composed of nerves that radiate to and from the CNS. 

A. Some important structures are associated with the PNS.

1. Sensory receptors generate impulses in response to specific stimuli, and are generally 

classified as listed below. 

a) Mechanoreceptors--respond to mechanical forces, such as touch, pressure, 

vibration, stretch, damage, etc, pain receptors (nociceptors), and receptors that 

monitor body part position (proprioceptors) usually included in this group. 

b) Chemoreceptors-- respond to chemical stimulation, such as olfactory (smell) and 

taste receptors. 

c) Photoreceptors-- respond to light stimulation, such as cones and rods of retina. 

d) Thermoreceptors-- respond to changes in temperature. 

2. Ganglia are masses of cell bodies outside the CNS, and are typically where synapses 

between neurons occur outside the CNS.

1. There are twelve pairs of cranial nerves that arise from different parts of the brain (see chart pg 27)

2. There are 31 pairs of spinal nerves named for where they attach to the spinal cord. 

a) There are 8 cervical spinal nerves, 12 thoracic, 5 lumbar, 5 sacral, and one 

coccygeal. 

b) A plexus is a complex association of spinal nerves that in turn form other 

nerves--they are named for their location (cervical plexus, brachial plexus, 

lumbar plexus, sacral plexus).

1. The Sensory Nervous System is composed of sensory or afferent neurons leading to 

the CNS. 

2. The Motor Nervous System is composed of efferent neurons taking impulses away 

from the CNS—the Motor Nervous System is further subdivided. 

a) The Somatic Nervous System (SNS) is composed of nerves that innervate 

skeletal muscle, i.e. motor nerves, and the sense organs (discussed later). 

b) The autonomic nervous system (ANS) is composed of nerves that sense and 

regulate the viscera (body organs) and related unconscious activities (including 

visceral motor responses such as vasodilatation and constriction, etc.) 

D. The ANS is further subdivided into two subsystems the Sympathetic division and the 

Parasympathetic division.

1. These two divisions of the ANS have the following characteristics in common. 

a) Their efferent pathways (impulses going away from the CNS) consist of two 

neurons, a preganglionic neuron that exits the CNS and a postganglionic neuron 

that innervates the target organ or tissue. 

b) The preganglionic neuron and the postganglionic neuron synapse at a ganglion 

outside the CNS. 

c) The table (page 28) summarizes differences in the two systems.

2. It is difficult to make broad generalizations about the two systems, as to whether one 

is stimulatory and the other inhibitory, etc. 

3. What can be said about the two is that they are antagonistic-- if they innervate the 

same organ they have opposite effects, i.e. if one vasodilates, the other will 

vasoconstrict, etc.

III. The sensory organs are associated with the Somatic nervous system of the PNS and are interfaces 

where physical stimuli are converted to impulses for interpretation by the brain. 

A. The sense of touch is associated with numerous encapsulated and naked mechanoreceptors 

of the epidermis and dermis. 

1. Encapsulated receptors include Meisner’s, Pacinian and Ruffini’s corpuscles, and 

Krause’s end bulbs. 

2. Naked receptors include Merkel Discs, root hair plexuses, and nociceptors.

1. On the tongue are papillae that have taste buds composed of clusters of gustatory cells 

with hairs (microvilli) that protrude from their ends. 

2. The gustatory hairs have receptors that will bind food chemicals dissolved in the 

saliva, triggering a chemical cascade leading to an action potential and impulse. 

3. The impulse goes to the parietal lobe cortex and is interpreted by the brain. 

4. There are five types of gustatory cells that account for the five major tastes-- sweet, 

sour, salt, bitter, and umami (also called glutamate for the distinctive taste of 

monosodium glutamate (msg)). 

5. The sense of taste is intimately tied to that of smell, and the texture of food.

1. Olfactory cells are chemoreceptors that line the mucous membrane of the nasal cavity. 

2. Olfactory receptor proteins in the olfactory cells trigger a chemical cascade leading to 

an action potential and impulse. 

3. Smell is interpreted in the frontal lobe cortex. 

4. The olfactory lobes are in the frontal lobes of the brain, and are where the olfactory 

neurons enter the brain. 

5. Olfaction is not well understood-- it is thought that there are hundreds of different 

olfactory receptor cells, in stark contrast to the five identified in taste.  

1. The wall of the eye is composed of three tunics. 

a) The fibrous tunic protects the eye. 

(1) The cornea forms the fibrous tunic anteriorly. 29

(2) The sclera forms the fibrous tunic posterior to the cornea and is the 

“white” of the eye. 

(3) The sclera is particularly tough. 

b) The vascular tunic is highly vascular and is composed of the following 

structures moving from anterior to posterior-- iris, ciliary body (composed of 

ciliary muscle, ciliary processes, and suspensory ligaments), and choroid. 

c) The retina forms the nervous tunic-- it is where light is converted to impulses. 

2. A narrative of the pathway of light as it goes from anterior to posterior is listed below. 

a) Light first passes through the conjunctiva, a layer of epithelium that covers the 

cornea, and then through the cornea itself, where significant light refraction 

occurs. 

b) Light then enters the anterior cavity of the eyeball. 

(1) The anterior cavity is anterior to the lens of the eye. 

(2) The anterior cavity is filled with a watery fluid derived from blood plasma 

called the aqueous humor. 

(3) The aqueous humor provides a transparent support medium for the 

eyeball. 

(4) Aqueous humor is produced by blood vessels associated with the ciliary 

body, circulates throughout the entire eyeball (including the posterior 

chamber) and is reabsorbed by a radial vein the encircles the cornea called 

the canal of Schlemm. 

(5) The anterior cavity is divided into two chambers. 

(a) The anterior chamber is anterior to the iris. 

(b) The posterior chamber is posterior to the iris. 

c) The pathway of light in the anterior cavity is as follows. 

(1) Anterior chamber. 

(2) Pupil. 

(a) The pupil is the opening formed by the iris. 

(b) The iris is a large pigmented muscle with circular and radial muscles 

that control the amount of light entering the vitreous cavity. 

(3) Posterior chamber, and then to the lens. 

d) Light passes through the lens, where the lens accommodates (changes shape), 

refracting light and focusing it on the retina. 

(1) The lens is suspended in the center of the eyeball by suspensory ligaments 

(processes) of the ciliary body. 

(2) The cells of the lens are filled with highly elastic, transparent proteins. 

(3) By contracting and relaxing, the ciliary muscle, attached to the lens by 

ciliary processes and suspensory ligaments, changes the shape of the lens 

in a process called accommodation (discussed in more detail below. 

(4) Accommodation of the lens focuses light on the retina. 

e) Light exiting the lens enters the vitreous cavity. 

(1) The vitreous cavity is filled with a jelly like matrix called the vitreous 

humor. 

(2) The vitreous humor is composed of collagenous proteins and 

glycoproteins that bind tremendous amounts of water derived from 

aqueous humor. 

(3) The vitreous humor is formed as an embryo and lasts your entire lifetime. 

(4) The vitreous humor supports the eye internally and presses the retina 

against the choroid-- this is important because the retina is loosely bound 

to the choroid. 30

(5) Intraocular pressure is important--too little and the retina can detach 

easily, too much and retinal damage and blood supply will be affected 

leading to macular degeneration. 

f) Light strikes the fovea centralis of the macula lutea of the retina (described 

below). 

a) There are two types of photoreceptors in the retina. 

(1) Cones. 

(a) Cones require high intensity light, and are responsible for our day 

vision, visual acuity, and color vision. 

(b) There are three types of cones-- red cones, blue cones, and green 

cones. 

(2) Rods are sensitive to even low levels of light and responsible for our night 

vision. 

b) The lens accommodates to focus light at a specific region of the retina-- the 

fovea centralis of the macula lutea. 

(1) The macula lutea is an area of the retina that has an extremely high 

concentration of cones, and no rods. 

(2) Within the macula lutea is a small (0.4mm) depression where cone 

concentration is at its greatest. 

c) As one radiates away from the macula lutea towards the periphery of the retina, 

cone concentration decreases, and rod concentration increases. 

d) The cones and rods work with other neurons called bipolar cells to trigger action 

potentials in ganglion cells to trigger impulses. 

(1) Cones, rods, and bipolar cells do not generate impulses, but work together 

to inhibit ganglion cells in the absence of light. 

(2) Photopigments within rods and cones change shape in response to 

absorption of specific wavelengths and intensity of light. 

(3) These shape changes lead to a cascade of events that eventually remove 

inhibition of ganglion cells causing them to depolarize and conduct 

impulses.

e) Impulses are conducted through the optic nerve to the occipital lobe cortex and 

interpreted as vision. 

(1) Impulses from the medial retina cross over to the other side of the brain 

through the optic chiasma. 

(2) Impulses from the lateral retina are conducted to the occipital lobe on the 

same side of the brain. 

(3) The right occipital lobe, for example, receives impulses from the lateral 

retina of the right eye, and the medial retina of the left eye. 

(4) Question: what visual fields are processed by the right occipital lobe? 

(5) This visual pathway is unique to Primates and possibly the 

Megachiroptera. 

(6) Where the optic nerve attaches to the retina there are no rods nor cones, 

hence no vision. 

(7) The blind spots are not noticeable for the following reasons. 

(a) Our overlapping (stereoscopic) vision covers the gap in the field of 

vision of the other eye. 

(b) We keep our eyes moving so we constantly see our surroundings 

from slightly different visual fields. 31

(c) Even if one eye is closed and stationary, it is not noticeable because 

the brain will “fill in” the blind spot for us, with memory from a 

previous visual field.  

a) Images are focused at the fovea centralis of the macula lutea via light refraction 

by the cornea and the lens. 

(1) The cornea’s shape is fixed, as is its position, so it acts as a fixed convex 

lens. 

(2) The lens has a fixed position, but it can change shape in a process called 

accommodation of the lens-- it therefore acts as a variable convex lens, 

allowing one to focus objects both near and far. 

b) Lens accommodation is controlled by the ciliary body and lens elasticity. 

(1) The ciliary body is composed of the suspensory ligaments, which attach 

the lens to ciliary processes of the ciliary muscle. 

(2) Like the lens, the ciliary muscle is highly elastic. 

(3) The ciliary body attaches to the lens around its periphery. 

c) Accommodation for near vision. 

(1) Light waves from objects near the eye are strongly refracted and focused 

on the fovea centralis of the retina-- this requires a strong (thick) convex 

lens. 

(2) When the muscle fibers of the ciliary processes contract they bunch up, 

and the diameter between the ciliary processes decreases, putting “slack” 

in the suspensory ligaments. 

(3) This allows the elastic lens to assume its natural thickened (almost 

circular), convex shape. 

(4) A thick (more convex) lens refracts light more strongly allowing objects 

close to the eye to be focused at the fovea centralis. 

d) Accommodation for distant vision. 

(1) Light waves from objects distant from the eye require only minimal 

refraction by the lens to be focused on the fovea centralis of the retina-- 

this requires a weak (thin) convex lens. 

(2) When the muscle fibers of the ciliary processes relax, the highly elastic 

ciliary process assume their normal shape, which is to lie flat against the 

inner wall of the eye-- this increases the diameter between the ciliary 

processes, putting tension in the suspensory ligaments that stretches (and 

flattens) the lens. 

(3) With the suspensory ligaments stretching the lens around the periphery, 

the lens flattens into a thinner, less convex shape. 

(4) A thin (less convex) lens refracts light less strongly allowing objects 

distant from the eye to be focused at the fovea centralis. 

e) At first glance accommodation seems counterintuitive, but it makes sense when 

you consider real life experiences. 

(1) The eye fatigues when reading or looking at objects close to the eye-- this 

is because the ciliary muscle is contracting to thicken the lens, and it tires. 

(2) Looking at distant objects (more than twenty feet) is easy on the eye 

because stretching of the lens is caused by relaxation of the ciliary muscle. 

a) Emmetropia-- normal vision. 

b) Myopia-- nearsightedness. 

(1) Caused by elongated cornea or elongated eyeball. 

(2) Corrected by concave lens. 32

c) Hyperopia (hypermetropia)-- farsightedness. 

(1) Caused by shortened cornea or shortened eyeball. 

(2) Corrected by convex lens. 

d) Presbyopia-- loss of accommodation due to aging. 

(1) Cause traditionally attributed to a loss of lens elasticity due to aging. 

(2) Recent evidence shows that as we age, the suspensory ligament 

attachment sites move progressively anterior on the lens, which may affect 

accommodative capacity. 

(3) The cause of the suspensory ligament changes is unknown. 

(4) Typical onset around 42 years of age. 

(5) Corrected by a convex lens. 

e) Astigmatism-- misshapen cornea that abnormally refracts light. 

f) Lasik surgery 

(1) The procedure-- tip of cornea cut and peeled back, laser removes some 

cornea tissue, corneal flap replaced, cornea heals. 

(2) Corrects myopia or hyperopia, but not both in the same eye. 

(3) Most problems occur if a fold develops in corneal flap when replaced after 

surgery, or scarring of cornea from infection.

a) Are lubricating and antiseptic. 

b) Produced by lacrimal glands superolateral to eyeball within orbit of eye. 

c) Tears wash across eye int

1. The pathway of sound is described below. 

a) Sound waves are captured by the auricle or pinna and enter the external auditory 

meatus of the temporal bone. 

(1) The external auditory meatus contains hairs to keep things out of the ear. 

(2) The external auditory meatus is lined by specialized apocrine sweat glands 

that secrete a waxy material, cerumen (ear wax) to protect the middle ear. 

b) Sound waves vibrate the tympanic membrane (tympanum). 

(1) From pinna to tympanum forms the outer ear. 

(2) The tympanum is a composed of elastic connective tissue lined by skin 

externally and mucosa internally. 

c) The tympanum vibrates the ossicles of the middle ear--they are, in order, the 

malleus, incus, and stapes. 

(1) The malleus incus and stapes are the smallest bones in the body. 

(2) They are connected to one another and the tympanum by connective 

tissue. 

(3) The middle ear is an air filled chamber within the temporal bone. 

(4) The Eustachian tube is an opening that leads to the nasopharynx. 

(a) The Eustachian tube allows air to move freely in and out of the 

middle ear in response to changes in atmospheric pressure. 

(b) If the middle ear lacked an opening to the outside, air would expand 

within the middle ear when one went up in altitude and would break 

the tympanum. 

(c) The Eustachian tube is easily plugged by mucous and can cause pain 

or muffled sound as the air within the middle ear expands, stretching 

the tympanum. 

(d) When pressure builds sufficiently the mucous plug is forced open 

stabilizing pressure within the middle ear, causing the ears to “pop.” 

d) The stapes connects to a thin layer of connective tissue called the oval window. 

(1) The oval window is an interface between the middle ear and inner ear. 

(2) The inner ear is a bony, fluid filled labyrinth within the temporal bone. 

(a) This bony labyrinth forms a spiral tube within the bone. 

(b) The fluid is called perilymph. 

(c) Suspended within the perilymph is an organ called the cochlea, 

forming part of a “membranous labyrinth” within the “bony 

labyrinth”-- the cochlea mimics the pathway of the bony labyrinth. 

(3) The cochlea is filled with fluid called endolymph, and contains a structure 

called the Organ of Corti. 

(a) The Organ of Corti runs the length of the cochlea. 

(b) It is the organ responsible for converting vibrations to impulses. 

(c) The Organ of Corti contains mechanoreceptors called hair cells. 

(d) Hair cells run between the tectorial and basilar membranes of the 

Organ of Corti. 

e) The oval window vibrates perilymph (pressure generated is offset by another 

connective tissue interface between the bony labyrinth and the middle ear called 

the round window). 

f) Perilymph vibrates endolymph within the Organ of Corti. 

g) The vibrating endolymph causes a shearing action between the basilar and 

tectorial membranes, bending microvilli on the hair cells that run between the 

two membranes. 

h) The shearing action on the hair cells triggers an action potential and impulse. 

i) The impulse is carried by the cochlear nerve to the temporal cortex where 

impulses are interpreted as sound.

a) The bony and membranous labyrinths describe in hearing are continuous with 

the bony and membranous labyrinths of the vestibular apparatus. 

b) A small tube leads from the cochlea that expands into two large sac-like 

structures. 

(1) The first is the saccule. 

(2) The second is the utricle. 

(3) The saccule and utricle are filled with endolymph, and surrounded by 

perilymph. 

(4) The saccule and utricle are responsible for our sense of static equilibrium, 

i.e. stationary head position-- knowing whether our head is cocked to the 

side or upside down etc. 

(a) Disc like clusters of hair cells form structures called maculae-- 

stereocilia of the hair cells protrude into the endolymph. 

(b) The stereocilia of the hair cells are embedded in a gelatinous mass 

called an otolithic membrane. 

(c) Embedded in the endolymphic surface of the otolithic membrane are 

crystals of calcium carbonate called otoliths. 

(d) Depending on head position and maculae involved, gravity pulls on 

the otoliths, which causes the otolithic membrane to shift, bending 

the stereocilia of the hair cells initiation an action potential and 

impulse. 

(e) The impulse is carried via the vestibular nerve to the temporal cortex 

for interpretation of head position. 

c) From the utricle three semicircular canals follow their bony labyrinth, and are 

responsible for our sense of dynamic equilibrium (head movement, especially 

spinning). 

(1) The semicircular canals are filled with endolymph and are surrounded by 

perilymph. 

(2) The semicircular canals are found in three roughly perpendicular planes. 

(3) Hair cells form rings called cristae around the ampullae (enlargements) of 

the semicircular canals. 

(4) Stereocilia of the hair cells protrude into the endolymph. 

(5) The stereocilia are embedded in a gelatinous matrix called the cupula. 

(6) When the head moves the inertia of the endolymph bends the cupula and 

stereocilia within, initiating an action potential and impulse in the hair 

cells. 

(7) The impulses are carried by the vestibular nerve to the temporal cortex, 

which interprets the impulses as head movement. 

(8) If you keep spinning and stop, the endolymph will keep moving--again 

bending the cupula and producing impulses and the sensation of 

movement, even though you are now stationary. 

(a) This can be confusing to the brain causing dizziness and nausea. 

(b) In fact, there is an interesting reflex with the eye muscles called 

nystagmus that makes the eyes move involuntarily when the 

semicircular canals are signaling movement after the movement has 

stopped-- look at people’s eyes as they get off a spinning type ride in 

an amusement park the next time you are there.

IV. Endocrine System uses chemicals called hormones to communicate, or exert some effect, within 

the body.

A. Secretory organs are considered endocrine or exocrine organs (some, like the pancreas, is 

both). 

1. Endocrine organs secrete hormones. 

a) Hormones are chemicals that are secreted into the bloodstream and exert some 

effect on another tissue or organ. 

b) Many hormones are neuroactive, i.e. neurotransmitters secreted into the blood 

rather than into a synapse. 

c) Hormones bind to receptor proteins in the cell membranes of target tissues to 

trigger a chemical cascade. 

(1) The cascade may be like that described with acetylcholine and the 

neuromuscular junction, triggering changes in the cell membrane. 

(2) Other cascades may lead to activation of a gene in the nuclear DNA or 

have still other effects.  

2. Exocrine organs secrete products into ducts (ducts lead to the outside of the body, 

although it may be via the digestive tract, auditory meatus, etc.