Central Nervous System: The Spinal Cord

The spinal cord is a continuation of the medulla oblongata within the spinal cavity

It begins at the foramen magnum and ends as the conus terminalis, or conus medullaris (medullary cone) at the level of L1 or L2

It gives rise to 31 pairs of spinal nerves, each exits the spinal cavity via intervertebral or sacral foramina, or the sacral hiatus

Beyond the conus terminalis, spinal nerves of lumbar, sacral, and coccygeal travel within the spinal cavity as the cauda equina [horse tail] before exiting at their respective level

The spinal cavity terminates by an extension of the pia mater which anchors the spinal cord to the coccyx, the filum terminale [terminal thread]

Cross-Sectional Anatomy of the Spinal Cord

The spinal cord consists of a central area of gray matter divided into two dorsal horns, and two ventral horns.  The right and left halves are connected by the gray commissure; in the center is the central canal.

Sensory fibers enter the dorsal horn, sometimes synapse with an interneuron, and exit by way of the ventral root (motor fibers) of the spinal nerve

White matter consists of bundles of myelinated axons that run up and down the cord, to and from the brain. White matter is arranged into dorsal, lateral, and ventral columns

Peripheral Nervous System

Nerves

Three connective tissue tunics organize & compartmentalize nerve fibers to form a nerve:

1. Endoneurium: wraps each nerve fiber

2. Perineurium: wraps groups of fibers into fascicles

3. Epineurium: encloses all fascicles forming the outer tunic of the nerve; areas enclosed between fascicles, the interfascicular space, contains adipose & areolar connective tissue, and blood & lymph vessels

Spinal Nerves

Spinal nerves have two separate attachment points:

Information communicated to the CNS about changes in the environment (internal or external) so that the body can react (voluntary or involuntary) with an appropriate response

Two Classes of Senses

Seven primary odors have been identified

II. Sight (vision)

Structures of the eye are divided between the eyeball and accessory structures of the eye

Eyebrows:

thickened, hair covered, supraorbital ridges of skin; protect eye from sweat, sunlight, foreign matter, and cushions impacts to the eye & forehead; also instrumental in communication

Levator palpebrae superioris muscle

within the larger, more movable upper eyelid; functionally, it retracts the upper eyelid (the lower eyelid lacks such a muscle)

Conjunctiva [joined together]:

Lacrimal apparatus [tear]:

cleans & protects the eyeball with a supply of lacrimal fluid (tears)

the passage of tears includes the following structures:

 also known as the uvea [grape]; the middle coat of the eyeball; contains three regions: Choroid [membrane-like]: accounts for all, except the anterior 15%, of the uvea; highly vascular & highly pigmented; forms a dark chamber to absorb light

Ciliary body: apparatus to modify the shape of the lens; smooth muscles (ciliary muscles) act on suspensory ligaments which, in turn, are attached around the perimeter of the lens Iris: colored portion of the eyeball; continuous with the ciliary body; contains smooth muscle to regulate light into the eye by controlling the diameter of the pupil; it separates the anterior chamber, filled with aqueous humor, from the posterior chamber, filled with vitreous humor

Macula lutea [yellow spot]: area of high concentration of photoreceptor cells; the number of cones increases proportionally from its periphery toward the center

Fovea centralis: center of the macula lutea; contains cones only for the most detailed vision

Optic disc (blind spot): area of the retina devoid of photoreceptor cells; where optic nerve exits the eye, and also the location of vessels entering & exiting the eye

Taste (gustation)

1. Filiform [thread-like] papillae:

2. Fungiform [mushroom-like] papillae:

3. Foliate [leaf-like] papillae:

4. Circumvallate (vallate) papillae:

 Filiform [thread-like] papillae:

most numerous over surface of tongue for added texture to facilitate food manipulation; no taste buds present

Fungiform [mushroom-like] papillae:

Foliate [leaf-like] papillae:

confined to the lateral margins of the tongue; includes numerous sensitive taste buds

largest, but least numerous; confined to the back of the tongue in a “V” formation

1. Salty: salts usually contain metal ions (Na+, K+, etc.) which are important electrolytes

2. Sweet: produced by sugars which are included in high energy foods

3. Sour: produced by acids (e.g. citric acid in citrus fruits)

4. Bitter: produced by alkaloids (e.g. caffeine, nicotine, etc.); most alkaloids exist as botanical toxins rendering them unpalatable

5. Umami [delicious]: The most recent taste sensation described be the Japanese identifying a “savory” or “meaty” flavor. Produced by free glutamates commonly found in fermented foods (e.g. cheeses), mushrooms, and meats. The sensation is intensified in the presence of sodium; hence, MSG (monosodium glutamate) is popularly used in cooking to enhance flavor

Structures of the Outer Ear

1.Auricle (not pinna in humans):

2. External auditory canal

3. Tympanic membrane

External auditory canal

Tympanic membrane (eardrum)

Also known as the tympanic cavity, the middle ear is an air filled cavity within the petrous portion of the temporal bone; structures within include:

Auditory ossicles: malleus [hammer], incus [anvil], & stapes [stirrup]; the ossicles conduct vibrations from the eardrum to the inner ear

Complex hollow spaces within the temporal bone; contains three regions:

Cochlea [snail]

Vestibule [entrance chamber]

Semicircular Canals (superior, posterior, & horizontal)

Membranous Labyrinth

1. Vestibular nerve: from the vestibule for equilibrium

2. Cochlear nerve: from the cochlea for hearing

      1. The heart is enclosed in a double- 

      walled sac called the pericardium.

2. The parietal pericardium consists of a tough fibrous layer of dense connective tissue with a thin, smooth, moist serous layer.

3. The serous layer turns in at the base of the heart, forming the visceral pericardium covering the heart surface.

4. Between the parietal and visceral pericardia is a space called the pericardial cavity. It contains pericardial fluid that lubricates the membranes and allows the heart to beat almost without friction.

is made up of a layer of endothelium overlying a thin layer of areolar tissue. It forms the smooth inner lining of the chambers and valves and is continuous with the endothelium of the blood vessels.

1. The first branches off the aorta are the left and right coronary arteries.

2. The left coronary artery supplies blood to the left side of the heart and divides into the anterior interventricular and circumflex arteries.

3. The right coronary artery supplies blood to the right side of the heart and divides into the marginal and posterior interventricular arteries.

4. A myocardial infarction (heart attack) occurs when any of the coronary arteries become occluded and a portion of the heart muscle dies from lack of blood flow.

5. Venous drainage refers to the route by which blood leaves an organ; blood leaving capillaries collects in small veins (venules) that merge to form larger veins.

6. The great cardiac vein drains the anterior aspect of the heart, the middle cardiac vein that collects blood from the posterior aspect of the heart, and the small cardiac vein drains the right side of the heart.

7. All cardiac veins drain into the coronary sinus which empties into the right atrium.

8. In most of the body, arterial blood flow is greater when the ventricles are contracting than when they relax. In the coronary arteries, however, flow is greater when the ventricles relax.

1. The usual route of blood flow around the body is heart to arteries to arterioles to capillaries to venules, then veins, and back to the heart.

2. An artery is any vessel that carries blood away from the heart. A vein is any vessel that carries blood toward the heart.

3. Usually, blood flows through only one capillary bed before returning to the heart. An exception is a portal system in which blood flows through two separate capillary beds on its return to the heart. Portal systems are found between the hypothalamus and anterior pituitary, in the kidneys, and between the intestines and liver.

4. Anastomoses are places where two or more vessels merge without including a capillary (e.g. vein to vein, artery to artery, or artery to vein).

1. The tunica externa (tunica adventitia) is the outermost layer, made up of loose connective tissue. It anchors the vessels in place and provides passage for the vasa vasorum that supply blood to the vessel wall.

2. The tunica media, or middle layer, is the thickest layer of the vessel, made up mostly of smooth muscle. It is responsible for vasoconstriction and vasodilation.

3. The tunica interna (tunica intima) consists of endothelium overlying a basement membrane and a sparse layer of fibroconnective tissue. It provides a smooth surrounding through which blood passes.

Anatomy of the Pulmonary Circuit

A. The pulmonary circuit begins with the pulmonary trunk that branches into the right and left pulmonary arteries, each of which leads to a lung.

B. In the lung, pulmonary arteries branch into lobar arteries that carry blood to each lobe of a lung. These arteries further subdivide and lead to capillary beds that surround each alveolus.

C. After leaving the alveolar capillaries, pulmonary blood, now carrying oxygen, flows into venules and veins, ultimately leading to the two pulmonary veins that leave the lungs and drain into the left atrium.

Hepatic Portal System

The hepatic portal vein is formed from the union of the superior mesenteric and gastrosplenic vein.

The superior mesenteric vein drains the entire small intestine and the proximal portions of the large intestine.