Laboratory 08 Peripheral Nervous System Goals: Structure and function of the cranial nerves. 1. List the cranial nerves by name and number. 2. Describe the specific functions of each of the cranial nerves and also indicate if each is sensory, motor or mixed. 3. Identify the ganglia associated with the cranial nerves. 4. Discuss the location of the cranial nerve nuclei. 5. Propose how knowledge of the anatomy of cranial nerve nuclei can be used to help pinpoint damage to particular regions of the brain stem. Anatomy of the spinal cord and spinal nerves. Describe the gross anatomy of the spinal cord and spinal nerves and specify their location relative to the anatomy of the skeletal system. Identify the anatomical features seen in a cross sectional view of the spinal cord. Contrast the relative position of gray matter and white matter in the spinal cord with the corresponding arrangement of gray and white matter in the brain. Identify the dorsal root ganglia, dorsal and ventral roots, and spinal nerves. Discuss how the structures root, nerve, ramus, plexus, tract and ganglion relate to one another. List the four spinal nerve plexuses and give examples of nerves that emerge from each. 1. Spinal Cord Anatomy: Collectively, all of your vertebra form the vertebral column, colloquially referred to as the spine. The spinal cord, a thick nerve cord composed of many neurons, runs within the vertebral canal, which is a long channel formed by the vertebral foramina of adjacent vertebra. The spinal cord exists as a solid cord from the foramen magnum, where it exits the cranial cavity (continuous with the medulla oblongata) down to the level of the first and second lumbar vertebrae. The spinal cord ends at a cone shaped termination called the conus medularis The collection of individual lumbar and sacral spinal nerve roots continue past the conus medularis and are collectively referred to as the cauda equina, literally meaning horses tail, which it resembles. 101
Figure 2. Cross-sectional Anatomy of the Spinal Cord. As you can see from this image above, the spinal cord is bisected into mirror- image left and right halves (in a similar fashion to the brain) by the anterior median fissure in front and the posterior median sulcus in back. The spinal cord is also made up of both white and gray matter. The gray matter, however, is located deep within the spinal cord, surrounded by the more superficial white matter. This is in contrast to the distribution in the brain where the gray matter is superficial and the white matter is deep. The grey matter spans the connection between the left & right halves of the spinal cord as a thin strip called the gray commissure, which also surrounds the central canal. Lateral to the grey commissure, the grey matter flares out, forming two horns, the anterior and posterior horns. The horns of gray matter bisect the white matter into three distinct regions called funiculi, where axons convey sensory and motor impulses up and down the spinal cord. The anterior horn separates the anterior funiculus from the lateral funiculus, and the posterior horn separates the lateral funiculus from the posterior funiculus. 2. Spinal Nerves: The spinal nerves exit the vertebral column laterally on either side through the intervertebral foramina, which are formed when two adjacent vertebra articulate, roughly between the transverse processes. The 31 pair of spinal nerves are named for the region of the spinal column from which they arise: There are 8 cervical nerves (C1- C8), 12 thoracic (T1- T12), 5 lumbar (L1- L5), 5 Sacral (S1- S5), and 1 coccygeal (Co1). Each spinal nerve is formed by the union of two spinal nerve roots which from exit either side of the spinal cord at each vertebral junction. Arising from the posterior surface of the spinal cord is the dorsal (posterior) root. The dorsal root is where sensory nerve fibers enter the spinal cord (CNS). Arising from the anterior surface of the cord is the ventral (anterior) root, which is comprised of motor nerve fibers exiting the spinal cord. Axons from the ventral root connect to the anterior horn and axons from the dorsal root connect to the posterior horn. 102
3. Cranial Nerves: The first two cranial nerves originate in the cerebrum but the rest of the cranial nerves originate from various brain stem structures. The region where the cell bodies of many neurons of a given cranial nerve are located tends to be in a particular region and forms a column shape. A cranial nerve nucleus is this column of neuron cell bodies. The functions of the various nerves can be tested to determine if there has been damage to the nuclei of particular cranial nerves. Name Sensory(S)/Motor(M)/Both(B) Action I. Olfactory S Sense of smell II. Optic S Vision III. Oculomotor M Eye movement and pupil size IV. Trochlear M Eye movement V. Trigeminal B Muscles of mastication and facial sensation. VI. Abducens M Lateral eye movement VII. Facial B Facial muscles VIII. Vestibulocochler B Hearing and balance IX. Glossopharyngeal B Tongue sensations X Vagus B Tongue, larynx, lungs, heart and GI tract XI. Accessory M Muscles which move the head and shoulders XII. Hypoglossal M Tongue muscles On Occasion Our Trusty Truck Acts Funny Very Good Vehicle Anyhow Sensory, Motor or Both Some Say Marry Money, But My Brother Believes (its) Bad Business (to) Marry Money 103
Figure 2. Human Cranial Nerves. 4. Peripheral Nerves: Once the spinal nerves have exited the spinal cord, some of them will coalesce to form networks of interconnected nerve fibers called plexuses. There are four major nerve plexuses, the cervical plexus made up of nerve fibers from C1- C5, the brachial plexus made up of nerve fibers from C4- T1, the lumbar plexus made up of nerve fibers from L1- L4, and the sacral plexus made up of nerve fibers from L4- Co1. The major peripheral nerves of the body arise out of these plexuses. The origin of selected prominant nerves is explored more below. The most important nerve arising out of the cervical plexus is the phrenic nerve. With contributions from spinal nerves C3- C5, the phrenic nerve supplies motor input to the diaphragm, which is the 104
primary muscle that facilitates breathing. The brachial plexus gives rise to the various nerves which enervate the arms, including the median, ulnar and radial nerves. These three nerves supply motor output to the various muscles of the upper & lower arms as well as sensory input from the various cutaneous receptors located in the arms. The largest nerve arising from the lumbar plexus is the femoral nerve, which provides motor impulses to the muscles of the anterior & upper thighs as well as sensory information from the cutaneous receptors along the medial aspect of the legs. The sciatic nerve is the most prominent nerve to emerge from the sacral plexus. The largest and longest nerve in the body, the sciatic nerve diverges just above the popliteal region into the tibial nerve and the common fibular nerve, tributaries of which are responsible for nearly all the motor and sensory enervation of the lower leg. Injury to the sciatic nerve can cause complete or partial paralysis of the leg, depending on where the injury occurs (closer to the hip or closer to the knee). Another common homeostatic imbalance involving the sciatic nerve is sciatica. This condition results from injury to or pressure on the sciatic nerve and is characterized by pain shooting down the pathway of the nerve, tingling & numbness in the leg & foot as well as a condition called footdrop (muscle weakness in the lower leg causing the foot to dangle). 5. Reflexes : A reflex is a quick, involuntary, stereotyped reaction of glands or muscle to sensory stimulation. These automatic responses occur without our intent or often even our awareness. Reflexes can be hardwired or learned. Hardwired reflexes are usually protective in nature, preventing some sort of damage to the body. Some examples of protective reflexes include the cough reflex in response to foreign matter entering the larynx or trachea, the gag reflex induced by bitter tastes (which are often toxic in nature), the blink and corneal reflexes which protect the eye from damage (either physical or damage to the receptors from intense light). Stretch reflexes involving skeletal muscles are protective in nature as well. Receptors in muscles and tendons monitor the degree of stretch on a muscle and the amount of tension generated by contraction. A stretch reflex occurs when a muscle is stretched, causing the muscle to contract reflexively. The knee- jerk reflex is a common stretch reflex which you ve probably encountered at the doctor s office during a physical examination. Reflexes are facilitated by very simple nerve pathways called reflex arcs. Reflex arcs are composed of at least 4 (and sometimes 5) parts: 1. Sensory receptor: Detects the physical stimulus that generates the reflex and converts it to a nerve impulse. 2. Sensory neuron: Conveys the nerve impulse from the sensory receptor to the spinal cord (CNS). 3. Interneuron: Some reflexes have an interneuron which connects the sensory neuron to the motor neuron. More complex reflexes make additional connections distal to the sensory neuron, facilitated by the presence of interneurons. 4. Motor neuron: Conveys the nerve impulse to the effector. 5. Effector: The structure which generates the reflexive response. Effectors are very often muscles or glands. In the knee- jerk reflex, the sensory receptor is a muscle spindle (stretch receptor) in the muscles of the quadriceps group. Striking the patellar tendon with a hammer stretches the muscle, stimulating the stretch receptor. The action potential generated by the receptor travels to the spinal cord via the sensory axon, and junctions to the motor neuron (as well as an interneuron which splits the pathway), which enervates the quadriceps group, causing it to contract (generating the kicking motion). A secondary response is to the initial stimulus is to cause the antagonistic muscles to relax (a 105
phenomenon called reciprocal inhibition), ensuring that the shortening of the quadriceps muscles will not be antagonized by other muscles. 6. Selected Vocabulary: Funiculus A small bundle of nerve fibers enclosed in a tubulus sheath. Commissure A bundle of nerve fibers that connect one part to another. Root The initial segment of a nerve leaving the spinal cord. Nerve A combination of nerve fibers from the dorsal and ventral roots of the spinal cord carrying sensory, motor and autonomic signals between the spinal cord and the periphery of the body. Nerves are found in the peripheral nervous system. Ramus A branch of a nerve. Plexus A network of interconnecting nerves. Tract A combination of nerve fibers in the central nervous system. Ganglion A nodule that contains the cell bodies of neurons. 7. Identification List: Spinal cord: 1. Spinal nerve groups a. Cervical b. Thoracic c. Lumbar d. Sacral e. Coccygeal 2. Enlargements a. Cervical b. Lumbar c. Conus medularis (Cauda equina) 3. Spinal nerves a. Dorsal root b. Dorsal root ganglion c. Ventral root Cranial Nerves: 1. Olfactory 2. Optic 3. Oculomotor 4. Trochlear 5. Trigeminal 6. Abducens d. Sympathetic trunk/chain 4. Cross- sectional anatomy a. White columns: 1. Anterior funiculus 2. Lateral funiculus 3. Posterior funiculus 4. Posterior median sulcus 5. Anterior median fissure b. Grey matter: 1. Posterior (dorsal) horn 2. Anterior (ventral) horn 3. Grey commissure c. Central canal 7. Facial 8. Vestibulocochlear 9. Glossopharyngeal 10. Vegas 11. Accessory 12. Hypoglossal Peripheral Nerves: 1. Brachial nerve 2. Median nerve 3. Ulnar nerve 4. Radial nerve 5. Femoral nerve 6. Sciatic nerve 7. Tibial nerve 8. Common fibular nerve 106
Attributions: Cranial Nerve Flowchart courtesy of Teri Bidle, Hagerstown Community College through HAPS Educator Teaching Tips. Figure 1. http://commons.wikimedia.org/wiki/file:medulla_spinalis_- _Section_- _English.svg 107
THIS PAGE LEFT INTENTIONALLY BLANK 108
Laboratory 8 PNS Name Section 1. Fill in the blank with the name of the cranial nerve based on the given functions. First Six Cranial Nerves Smell: Sight: Touch & Chewing: Extrinsic Eye Muscles (three): (Hint, Remember OAT) Second Six Cranial Nerves Taste & Facial Muscles: Hearing & Equilibrium: Taste, Swallowing, Parotid Gland: Enervates Thoracic Viscera: Neck & Shoulder Muscles: Tongue (Swallowing & Speech): 109
2. Directions: Using your diagrams, notes and textbooks, determine the spinal nerve or cranial nerve that is damaged based on the symptoms of the individual. 1. The patient is deaf. 2. The pupil in one eye is dilated due to decreased tone of the constrictor muscles of the iris. 3. An individual has taken a hard blow to the face (e.g. as a result of falling on concrete) and has lost some sense of smell. 4. An individual has diabetes and blood vessels in the retina are damaged, what nerve would be sending less sensory impulses? 5. Due to surgical error, the patient has lost most sensory perception on one side of the face and difficulty chewing. 6. Individual cannot move most muscles of the face. 7. Paralysis of the superior oblique muscle results due to damage of this nerve. The individual s one eye rotates outward. 8. The patient complains of decreased sense of taste (3 cranial nerves). 9. The individual cannot abduct (move the eye laterally to see something to the side) one eye due to damage to this nerve. 10. Stimulation of this nerve in the region of the neck decreases the heart rate. 11. One shoulder droops. 12. The patient has Bell s palsy. 110
3. Label the structures from your list on the following picture: 111
THIS PAGE LEFT INTENTIONALLY BLANK 112