1. A nerve is an organ composed of multiple nerve fibers bound together by sheaths of connective tissue.

2. The sheath adjacent to the neurilemma is the endoneurium, which houses blood capillaries that feed nutrients and oxygen to the nerve.

3. In large nerves, fibers are bundled into fascicles, and wrapped in a fibrous perineurium.

4. The entire nerve is covered with a fibrous epineurium.

1. Fibers are classified for the direction in which signals are transmitted (afferent and efferent), the types of organs they innervate (somatic and visceral), and for how widespread or local the distribution of innervated organs (general or special).

2. Mixed nerves contain both motor and sensory fibers. Sensory nerves (optic and olfactory) contain mostly sensory fibers. Motor nerves contain motor fibers.

1. A ganglion is a cluster of nerve cell bodies outside the CNS.

1. The cranial nerves emerge from the base of the brain and lead to muscles and sense organs located in the head and neck for the most part. There are twelve pairs.

2. Each of the twelve pairs of cranial nerves is illustrated and described in Table 15.2, p. 509-515; Transps. 271-282.

1. Each spinal nerve branches into a dorsal root and a ventral root. The dorsal root ganglion is occupied by cell bodies from afferent neurons. The convergence of dorsal and ventral roots forms the spinal nerve.

2. The cauda equina is formed by the roots arising from segments L2 to Cx of the spinal cord.

1. After emerging from the vertebral column, the spinal nerve divides into a dorsal ramus, and ventral ramus, and a small meningeal branch that leads to the meninges and vertebral column.

2. The dorsal ramus innervates the muscles and joints of the spine and the skin of the back.

3. The ventral ramus innervates the ventral and lateral skin and muscles of the trunk, plus gives rise to nerves leading to the extremities.

1. The ventral rami merge to form nerve plexuses in all areas except the thoracic region. These nerve plexuses are shown and described in Tables 15.3 - 15.6, p. 520-523; Transps. 286-289.

1. Each spinal nerve except C1 receives sensory input from a specific area of skin, called a dermatome.

1. Reflexes are quick, involuntary, stereotyped reactions of peripheral effectors to stimulation.

2. Reflexes require stimulation, are quick, involuntary, and stereotyped.

3. A spinal reflex is made up of a reflex arc, including somatic receptors, afferent nerve fibers, interneurons, efferent nerve fibers, and skeletal muscles.

1. The muscle spindle is a stretch receptor located in muscle. It is a cigar-shaped affair containing 3-12 modified muscle fibers, wrapped in a fibrous capsule.

1. When a muscle is stretched, it contracts to maintain tone. This is the stretch (myotatic) reflex.

2. Stretch reflexes involve specific muscles and sometimes feed back to a set of synergists and antagonists. These reflexes are important in coordinating vigorous and precise movements.

3. The tendon reflex (knee jerk) is an example of a monosynaptic reflex arc.

1. For reflexes like the knee jerk to work, there must simultaneously be reciprocal inhibition of antagonistic muscles.

1. Flexor reflexes are important when a limb must be pulled away from harm. These types of reflexes involve a polysynaptic reflex arc.

1. To lift one foot quickly to avoid injury (flexor reflex) the body must maintain its balance. This is the function of the crossed extensor reflex with involves the cerebellum and the spinal.

2. The flexor reflex is ipsilateral; the crossed extensor reflex is contralateral.

3. An intersegmental reflex arc occurs when the nerve signal produces an output from a different segment than the one that received the input.

1. Golgi tendon organs are proprioceptors located at the junction of a muscle and its tendon.

2. Golgi tendon organs produce an inhibitory response called the Golgi tendon reflex when muscle contracts too tightly. This prevents damage to the tendon.

1. The visceral reflexes are mediated by the autonomic nervous system (ANS), which has two branches (sympathetic and parasympathetic).

2. Its target organs are glands, cardiac muscle, and smooth muscle; it operates to maintain homeostasis.

3. Control over the ANS is, for the most part, involuntary.

4. The ANS differs structurally from the somatic nervous system in that there are two neurons leading from the ANS to the effector: a preganglionic neuron and a postganglionic neuron.

1. The sympathetic branch prepares the body for "fight or flight" situations.

2. The parasympathetic branch functions to maintain normal operating conditions ("resting and digesting").

1. The sympathetic division is also called the thoracolumbar division because of the spinal nerves it employs.

2. Paravertebral ganglia are close to the vertebral column. Preganglionic neurons are short, while postganglionic neurons, traveling to their effector, are long.

3. When one preganglionic neuron fires, it can excite multiple postganglionic fibers that lead to different target organs (mass activation).

4. Nerve fibers leave the paravertebral ganglia by spinal, sympathetic, and splanchnic nerves.

1. The pyramid-shaped adrenal glands lie atop each kidney and consist of a glandular adrenal cortex surrounding an adrenal medulla made of modified sympathetic neurons.

2. When stimulated, the adrenal medulla produces catecholamines (as hormones) that complement the action of sympathetic postganglionic neurotransmitters.

1. The parasympathetic division is also referred to as the craniosacral division because its fibers travel in some cranial and sacral nerves.

2. The parasympathetic ganglion (terminal ganglion) lies in or near the target organs.

3. The parasympathetic fibers use the oculomotor, facial, glossopharyngeal, and vagus nerves.

4. The parasympathetic system uses long preganglionic and short postganglionic fibers.

1. The autonomic nervous system has cholinergic fibers that secrete ACh, and adrenergic fibers, that secrete norepinephrine (NE). Preganglionic fibers of both divisions are cholinergic, as are the postaganglionic fibers of the parasympathetic branch. Postganglionic fibers of the sympathetic branch are usually adrenergic.

2. Cholinergic Receptors (p. 540)

3. Adrenergic Receptors (p. 540)

1. Both divisions have nerves leading to most of the visceral organs (dual innervation).

2. The sympathetic and parasympathetic branches may have antagonistic effects or cooperative effects.

1. Control of organ function can be achieved without dual innervation. Impulses from sympathetic fibers can increase vasoconstriction, while cessation of impulses slows contractions.

1. Control of the ANS is accomplished by several levels of the CNS.

2. Cerebral Control (p. 543)

3. Hypothalamic Control (p. 543)

4. Brainstem Control (p. 544)

5. Spinal Control (p. 544)