Lecture 14: The Spinal Cord M/O Chapters 16 69. Describe the relationship(s) between the following structures: root, nerve, ramus, plexus, tract, nucleus, and ganglion. 70. Trace the path of information through all the anatomical structures involved in a response to any stimuli. 71. List the four spinal nerve plexuses and give examples of nerves that emerge from each. 72. Describe the concept of dermatomes and explain their clinical significance. The nervous system coordinates responses to sensory stimuli. This happens when afferent pathways deliver sensory information to the CNS, which processes the information, then coordinates a response that is carried out by the motor division of the nervous system. The information processing happens in the spinal cord and the brain. In today s lecture, we ll look at the role of the spinal cord. In the next lecture, we ll look at the role of the brain. Anatomy of the spinal cord Nervous system pathways Check out this chart outlining the functional components of the PNS. Somatic (outer tube) Ends in the CNS Sensory (afferent) Somatic sensory (SS) From skin and skeletal muscle, plus special senses Visceral (inner tube) Visceral sensory (VS) From guts (touch, pain) and blood vessels Starts in the CNS Motor (efferent) Somatic motor (SM) Skeletal muscles Visceral motor (VM) Autonomic nervous system! Mapping neural pathways 1. Sensory stimulus activates sensory receptors A. Sensory receptors can be SS or VS B. Both types send information to the CNS via AFFERENT pathways. 2. Sensory information is processed by the CNS, which is the BIG INTEGRATOR. A. Information often enters the spinal cord, then travels to the brain where it is processed, then an action is warranted. B. Depending on WHAT the action IS, the CNS will activate EFFERENT pathways. 3. Motor neurons send the message to DO SOMETHING. A. The action can be SM or VM B. Both types cause something to happen via EFFERENT pathways. 72
Simple neural pathways: The patellar reflex 1. Sensory stimulus: Whack the patellar ligament 2. Sensory information is delivered to the CNS via AFFERENT pathways. A. Stretch receptors in the quadriceps muscle group are activated by the WHACK B. The receptors cause SS neurons to FIRE a message that travels toward the CNS C. The axons of SS neurons travel TOGETHER, in a NERVE. D. The cell body of the SS neuron is found in the dorsal root ganglion of the spinal cord! E. This SS neuron synapses on another neuron INSIDE the spinal cord. 3. A cell body of a somatic motor neuron is activated in the ventral gray horn. A. The axon of this cell body passes all the way through the body and synapses on the EFFECTOR. This is the efferent pathway. B. The SM axons pass through a spinal nerve and out a dorsal/ventral ramus. C. Often these SM axons become part of a named nerve. In the case of the patellar reflex, they leave the spinal cord in nerves L2-L5. 4. The SM neuron synapses with a skeletal muscle (the quadriceps group), and initiates contraction of the muscle fibers. 5. The quadriceps muscle contracts, causing the leg extend. This is the ACTION carried out by the EFFECTOR. Complex neural pathways 1. Sensory stimulus can come from many sources. 2. All this afferent information enters the spinal cord (CNS) at once. 3. The CNS must process the information. This is incredibly complex and fantastic. There are very few simple pathways. Most neurons have many many axons and many many dendrites, meaning there are millions of POSSIBLE synapses. 4. When we add brain anatomy into this picture, things will become gloriously complex! Named peripheral nerves: Spinal nerves Spinal nerves exit the spinal cord through the interveretbral foramina formed between two vertebrae. The spinal nerves are named according to the vertebrae they exit from. 1. Spinal nerves C1-C7 exit through the intervertebral foramen found ABOVE the vertebra with the same name. This means that spinal nerve C2 exits throuhg the intervertebra foramen between C1 and C2. 2. Spinal nerve C8 exits BELOW vertebra C7 (which is above T1). 3. That means that the next intervertebral foramen is located between T1 and T2. Spinal nerve T1 exits through the space BELOW vertebra T1. 4. This pattern continues the rest of the way down the vertebral column. Spinal nerves then split into dorsal and ventral rami. Dorsal rami of the spinal nerves innervate back and neck muscles. They are smaller and pretty simply organized. 73
Nerve Plexuses Ventral rami, on the other hand often combine and branch extensively in a nerve plexus, which is a braided interweaving of different ventral rami from the spinal nerves. Emerging from these plexuses comes various named peripheral nerves, many of which we will be learning in lab. There are four peripheral nerve plexuses: 1. Cervical plexus (one on each side) A. Consists of anterior rami from C1-C4 B. The phrenic nerve is a branch of the cervical plexus that innervates the diaphragm, enabling respiration. 2. Brachial plexus A. Anterior rami from C5-T1 (including C8!) B. Branches into 5 named nerves that innervate the superior limb i. Axillary nerve ii. Median nerve iii. Musculocutaneous nerve iv. Radial nerve v. Ulnar nerve 3. Lumbar plexus A. Anterior rami from L1-L4 B. 2 primary branches i. Femoral nerve ii. Obturator nerve 4. Sacral plexus A. Anterior rami from L4-S4 B. Gives rise to the largest nerve in the body: the sciatic nerve C. Superior to popliteal fossa (distal, posterior femur), sciatic splits into two nerves: i. Tibial nerve ii. Common fibular nerve Dermatome man Branches of each spinal nerve (except C1) innervate the SKIN. The map of dermatome man illustrates which spinal nerve innervates which area of the skin. Referred pain Because spinal nerves branch and innervate different structures, often damage to a visceral organ innervated by a certain spinal nerve, will refer pain to the skin! For example, the appendix is innervated by T10. T10 also supplies the area of skin around the belly button. If the appendix is damaged, the sensation of pain can be referred to the belly button! 74
Lab 14: Spinal Cord and Peripheral Nerve Plexuses Reading: M&O Ch. 16, 18 Part 1: Spinal cord Find the following structures in the isolated spinal cord. DRAW the spinal cord and indicate all required structures. 1. Regional features (see M&O Fig. 16.1) A. cervical enlargement B. lumbar enlargement C. conus medullaris D. filum terminale E. cauda equina 2. General features (see M&O 16.2) A. anterior median fissure B. dorsal roots C. dorsal root ganglia D. ventral roots E. spinal nerves F. dorsal rami G. ventral rami H. pia mater I. arachnoid J. dura mater Part 2: Peripheral Nerve Plexuses Find all the following nerves in the cadavers. For each nerve you should know the major muscles and/or skin area innervated. Letters in parentheses give the spinal nerve ventral rami of origin for each of the nerves. For example, (C1-C5) indicates the 1st to 5th cervical spinal nerves contribute to the named peripheral nerve. 1. Cervical Plexus (C1 - C5) (see M&O Fig. 16.8) A. phrenic nerve (C3 - C5) 2. Brachial Plexus (C5 - T1) (see M&O Fig. 16.9 and Table 16.3) A. musculocutaneous nerve (C5 - C7) B. median nerve (C6 - T1) C. ulnar nerve (C8 - T1) D. radial nerve (C5 - T1) E. axillary nerve (C5, C6) 3. Lumbosacral Plexus (T12 - S4) (see M&O Fig. 16.10 and 16.11) A. femoral nerve (L2 - L4) B. obturator nerve (L2-L4) 4. Sacral Plexus (L4-S4) A. sciatic nerve (L4 - S3) i. tibial nerve ii. common fibular nerve Part 4: Practice with pathways 1. Choose any sensory stimulus AND motor response. Map out the entire pathway. Simple pathways are best! Be sure to use required structures in your map! 75
External Brain 14: Spinal Cord 69. Describe the relationship(s) between the following structures: root, nerve, ramus, plexus, tract, nucleus, and ganglion. 70. Trace the path of information through all the anatomical structures involved in a response to any stimuli. 71. List the four spinal nerve plexuses and give examples of nerves that emerge from each. 72. Describe the concept of dermatomes and explain their clinical significance. 1. Draw a spinal cord cross section and label the following structures: A. Central canal B. Dorsal root C. Dorsal root ganglia D. Dorsal (posterior) gray horn E. Ventral (anterior) gray horn F. Lateral gray horn G. Ventral root H. Spinal nerve I. Dorsal ramus J. Ventral ramus K. Anterior median fissure L. Posterior mediun sulcus M. Posterior white funiculus (column) N. Lateral white funiculus (column) O. Anterior white funiculus (column) 2. Trace the path of somatic motor information from the spinal cord to the periphery. 3. Trace the path of somatic sensory information from the periphery into the spinal cord. 4. What is another name for the visceral motor system? 5. Name one or more muscles that are innervated by each of the bold nerves. 6. What is the evolutionary advantage for this bizarre braiding of spinal nerves that occurs at the plexuses? 7. What is the clinical significance of referred pain? 8. What is a spinal tap? At what level would you expect this procedure to be minimally risky? Explain. 9. What kind of information is carried by a spinal nerve? (SS, SM, VS, VM) 10. What are the functions of the meninges?severing which nerve will drastically compromise your ability to breathe? 11. What sensory and/or motor deficits might you see in the case of a badly herniated intervertebral disk between C7 and T1? How about L2 and L3? 76