BIO210 Week 10

The nervous system may be divided on gross anatomical grounds into the central nervous system, consisting of the brain and spinal cord, and the peripheral nervous system, consisting of the cranial and spinal nerves. The nervous system may alternatively be split on functional grounds into the somatic and visceral (autonomic) nervous systems. Last week we dealt with the central nervous system. This week we will deal with the peripheral nervous system and the visceral/autonomic nervous system.

#	Name	Fibers	Origin	Exit Foramen	Functions
NI	Olfactory	sensory	telencephalon	cribriform plate of the ethmoid	special sense of smell
NII	Optic	sensory	diencephalon	optic foramen	special sense of vision
NIII	Oculomotor	motor	mesencephalon	superior orbital fissure	somatic motor to 4 extrinsic eye muscles; ciliary body; visceral motor to pupillary sphincter
NIV	Trochlear	motor	mesencephalon	superior orbital fissure	somatic motor to superior oblique eye muscle
NV	Trigeminal - Ophthalmic Branch	sensory	metencephalon	superior orbital fissure	general sensory for upper head, face, eye
	Trigeminal - Maxillary Branch	sensory	metencephalon	foramen rotundum	general sensory to maxillary region of face and nasal cavity
	Trigeminal Mandibular Branch	mixed	metencephalon	foramen ovale	general sensory for lower jaw and face; somatic motor to muscles of mastication
NVI	Abducens	motor	metencephalon	superior orbital fissure	somatic motor to lateral rectus eye muscle
NVII	Facial	mixed	metencephalon	internal auditory meatus to stylomastoid foramen	special sense of taste from anterior 2/3 of tongue; somatic motor to muscles of facial expression; visceral motor to submandibular, sublingual, and lacrimal glands
NVIII	Vestibulocochlear (Auditory)	sensory	metencephalon	internal auditory meatus	special senses of audition and equilibrium
NIX	Glossopharyngeal	mixed	myelencephalon	jugular foramen	general sense from pharynx; special senses of taste from posterior 1/3 of tongue; visceral sense from carotid baroreceptors and chemoreceptors; general motor to pharynx muscles; visceral motor to parotid salivary gland
NX	Vagus	mixed	myelencephalon	jugular foramen	minor special sense of taste from epiglottis; visceral sense from aortic baroreceptors and chemoreceptors; visceral sense from thoracic and abdominal viscera; somatic motor to pharynx and larynx; Visceral motor to thoracic and abdominal viscera
NXI	Spinal Accessory	motor	myelencephalon	enters via foramen magnum; exits via jugular foramen	somatic motor to pharynx, larynx, trapezius, and sternocleidomastoideus
NXII	Hypoglossal	motor	myelencephalon	hypoglossal canal	somatic motor to tongue

As indicated above, there are twelve paired cranial nerves, numbered NI through NXII. For each nerve you should know its name, its number, the brain region it originates from, and its functions. In other words, memorize the preceding table. You should also be able to recognize each nerve at its origin from the brain on preserved brains and models, and each exit route in the base of the skull.

The 32 pairs of spinal nerves carry sensory information from the periphery to the CNS and motor output of the CNS back to the periphery. The neural "circuit" between sensory input and motor output can be as simple as a spinal reflex arc, or as complex as the almost innumerable interactions required to hit a moving baseball.

We will concentrate on the anatomical nerve root pattern common to all of the spinal nerves, and on those nerves that run into the upper and lower extremities, forming the brachial and lumbosacral plexuses, respectively.

a) On the prepared spinal cord and the model horizontal section through the cord and column locate the following:

- Where are the neuron cell bodies located whose axons make up the ventral roots? Are these sensory or motor cells? Where are the neuron cell bodies located whose axons make up the dorsal roots? Are these sensory or motor cells?

- Where do the dorsal root ganglia lie in relation to the vertebrae of the spinal column?

- Each of the paired spinal nerves is formed by the fusion of the ventral and dorsal roots from that segment of the cord. The nerve travels a short distance as a common spinal nerve, then branches into a small dorsal and a large ventral ramus. What regions of the body are supplied by the dorsal rami? By the ventral rami?

- How many pairs of spinal nerves are associated with each of the five regions of the vertebral column?

b) In some regions of the spinal cord the spinal nerves (ventral rami) from neighboring segments form anastomosing networks called plexuses. One principal nerve from the cervical plexus is the phrenic nerve. What muscle does this nerve (pair) innervate?

c) Locate the brachial plexus on the cat excised nervous system and models. Identify the following:

- State which muscle groups of the upper extremity are innervated by each of the above nerves.

- Note the anatomical relationship of the brachial plexus to the axillary artery.

d) Locate the lumbosacral plexus on the charts and models. Locate the following nerves where they pass through the abdominal cavity and where they run in the lower extremity.

- What muscle groups are innervated by each of these nerves? By what route does each nerve leave the abdominal or pelvic cavity?

The visceral or autonomic nervous system has traditionally been considered to be a purely motor system for "involuntary" control of visceral smooth muscle and glandular activity. More recently, it has come to be understood as a true "visceral nervous system" with both sensory and motor components. The afferent sensory fibers carry "state" information from sensors of the viscera, e.g. pulmonary stretch receptors, carotid and aortic baroreceptors. The efferent motor fibers carry "control" information back to the viscera.

We will concentrate on the visceral efferent neurons, because their structures and functions are the most clearly demonstrated. The visceral efferent system has two main divisions - sympathetic and parasympathetic. These are roughly antagonistic systems, and a balance between their activities maintains homeostasis. Review the specific sympathetic and parasympathetic actions. Review also the meanings of the terms preganglionic and postganglionic, as applied to the visceral efferent fibers.

a) The sympathetic division arises from the thoracic and lumbar spinal cord. Identify the following structures in the charts, prepared spinal cord, and models (as appropriate):

- In which horn of the spinal gray matter do sympathetic preganglionic neurons lie? By what route do their axons reach the paravertebral chain ganglia? By what route do postganglionic fibers run from the paravertebral ganglia to the spinal nerves?

- Some preganglionic fibers run up or down the paravertebral chain to synapse at another vertebral level. This characteristic makes the sympathetic division act in a very diffuse, whole body manner, in contrast to the organ specificity of the parasympathetic system. How does this anatomical pattern promote general (whole body) arousal when the sympathetic nervous system is activated?

- Some preganglionic fibers pass through the chain ganglia to synapse in one of the three collateral ganglia - the celiac, superior mesenteric, or inferior mesenteric. What is the solar plexus?

- Finally, some preganglionic fibers bybass all of the sympathetic ganglia to synapse in the adrenal medulla (discussed further in the endocrine week next semester). The adrenal medulla releases adrenalin (epinephrine) into the blood. Adrenalin mimics most of the effects of noradrenalin (norepinephrine), the neurotransmitter at post-ganglionic sympathetic nerve terminals. How might the involvement of the adrenal gland also promote the whole-body nature of sympathetic arousal?

b) The parasympathetic division arises from the brain and the sacral spinal cord, hence the name parasympathetic (around the sympathetic). The cranial preganglionic components travel in cranial nerves III, VII, IX, & X. The sacral preganglionic components travel in the pelvic nerves.

- Parasympathetic preganglionic fibers are quite long and travel to ganglia located in the target organs themselves. This makes the parasympathetic division act in a very direct and organ- specific manner, in contrast to the diffuse sympathetic action.

- The sympathetic and parasymapathetic systems are generally antagonistic and the relative degree of activation in each constitutes "autonomic balance". As a simple example, parasympathetic arousal constricts the pupils of the eyes and sympathetic arousal dilates the pupils. Preganglionic axons of the parasympathetic system travel to the eye in the oculomotor nerve (NIII) and innervate the pupillary sphincter muscle. Postganglionic sympathetic fibers from the superior cervical ganglion (the anterior end of the paravertebral chain ganglia) travel by way of the ophthalmic branch of the trigeminal nerve (NV) to the pupillar dilator muscles.

- As a routine part of an eye exam, your doctor will dilate your pupils with atropine, a parasympathetic blocker of the pupillary sphincter muscles. Why not just use adrenaline (epinephrine) which directly mimics sympathetic arousal of the dilator muscles?