Chapter 13: The Spinal Cord, Spinal Nerves, and Spinal Reflexes

Transcription

1 Chapter 13: The Spinal Cord, Spinal Nerves, and Spinal Reflexes I. General Organization of the Nervous System, p. 422 Objectives 1. Describe the basic structural and organizational characteristics of the nervous system. Figure 13-1 The nervous system functions efficiently because of its high level of organization. In this chapter we will look at the spinal cord and spinal nerves, and their role in spinal reflexes -- the rapid, automatic responses triggered by certain specific stimuli. Spinal reflexes are controlled by the spinal cord alone. The brain is not involved. II. Gross Anatomy of the Spinal Cord, p. 423 Objectives 1. Discuss the structure and functions of the spinal cord. 2. Describe the three meningeal layers that surround the central nervous system. 3. Explain the roles of white matter and gray matter in processing and relaying sensory information and motor commands. Figure 13-2 The adult spinal cord is about 18 inches (45 cm) long and 1/2 inch (14 mm) wide. It ends between vertebrae L1 and L2. The groove on the posterior side is the posterior median sulcus. The deeper groove on the anterior side is the anterior median fissure. The amount of gray matter in any segment is related to its involvement with sensory and motor nerves of the limbs, forming enlargements of the spinal cord. The cervical enlargement is associated with nerves of the shoulders and upper limbs; the lumbar enlargement with the pelvis and lower limbs. The thin, conical part of the spinal cord below the lumbar enlargement is the conus medullaris, ending in a thin thread of fibrous tissue (the filum terminale) which attaches to the coccygeal ligament. The nerve roots which extend below the conus medullaris are the cauda equina. The spinal cord is divided into 31 segments, based on the vertebrae where each spinal nerve originates (although the relationship between the spinal segment and the vertebrae it s named for changes with age). A cervical nerve is named for the vertebra inferior to it. All other nerves are named for the vertebrae superior to

2 them. Each spinal nerve associated with a segment is divided into 2 branches -- the ventral root, which contains axons of motor neurons, and the dorsal root, which contains axons of sensory neurons. Dorsal root ganglia contain the cell bodies of sensory neurons. On each side of the spine, the dorsal and ventral roots pass out of the vertebral canal through intervertebral foramen and join together to form a spinal nerve. Because they carry both afferent (sensory) and efferent (motor) nerves, spinal nerves are mixed nerves. Spinal Meninges, p.425 Figure 13-3 The spinal cord is physically isolated from its surrounding structures. Specialized membranes called spinal meninges protect the spinal cord and carry its blood supply. The spinal meninges continue upward into the skull, where they are continuous with the cranial meninges. A viral or bacterial infection of these membranes is called meningitis. The spinal and cranial meninges consist of 3 layers: 1. the dura mater 2. the arachnoid mater 3. the pia mater 1. The Dura Mater The dura mater is the tough, fibrous outer layer of the spinal cord. Cranially, the spinal dura mater fuses with the periosteum of the occipital bone and becomes continuous with the cranial dura mater. Caudally, the spinal dura mater tapers to a dense cord of collagen fibers that joins the filum terminale to form the coccygeal ligament. The epidural space between the dura mater and the walls of the vertebral canal contains loose connective and adipose tissue. Anesthetics are injected into this area to produce a local epidural block. 2. The Arachnoid Mater The outer dura mater surrounds the middle meningeal layer, the arachnoid mater. These 2 layers (covered by simple squamous epithelia called the arachnoid membrane) may separate in histological preparations to form a slight subdural space. The space between the arachnoid mater and the pia mater (containing a network of collagen and elastin fibers called the arachnoid trabeculae) is the

3 subarachnoid space, filled with cerebrospinal fluid (CSF). CSF carries dissolved gases, nutrients and wastes around the spinal cord. A procedure called spinal tap withdraws spinal fluid from the subarachnoid space. 3. The Pia Mater The innermost meningeal layer, the pia mater, is a mesh of collagen and elastic fibers bound to the underlying neural tissue. Figure 13-4 Paired denticulate ligaments extend from the pia mater through the arachnoid to the dura mater and stabilize side-to-side movement. Blood vessels of the spinal cord run along the surface of the spinal pia mater, within the subarachnoid space. Sectional Anatomy of the Spinal Cord, p. 427 Figure 13-5 The spinal cord is divided into left and right by the anterior median fissure and the posterior median sulcus. The superficial white matter contains myelinated and unmyelinated axons. The gray matter (the butterfly shape with projections called horns) surrounding the central canal contains neuron cell bodies, neuroglia and unmyelinated axons. 1. Organization of Gray Matter Cell bodies are organized into functional groups called nuclei. Sensory nuclei are dorsal (posterior) and connect to peripheral receptors. Motor nuclei are ventral (anterior) and connect to peripheral effectors. Posterior gray horns contain somatic and visceral sensory nuclei. Anterior gray horns contain somatic motor nuclei. Lateral gray horns, found in the thoracic and lumbar segments, contain visceral motor nuclei. Axons that cross from one side of the cord to the other before reaching the gray matter are called gray commissures. The exact location of a sensory or motor nucleus within the gray matter determines which specific part of the body it controls. 2. Organization of White Matter White matter on each side of the spinal cord is divided into 3 columns (funiculi). 1. Posterior white columns lie between the posterior gray horns and the posterior median sulcus. 2. Anterior white columns lie between the anterior gray horns and the anterior median fissure. (Axons cross from one side of the spinal cord to the other in the anterior white commissure.)

4 3. The lateral white columns are made up of the white matter between anterior and posterior columns on each side. Each column contains bundles of axons (tracts or fasciculi) which relay the same type of information in the same direction. Ascending tracts carry information to the brain. Descending tracts carry motor commands to the spinal cord. Key The spinal cord has a narrow central canal surrounded by gray matter containing sensory and motor nuclei. Sensory nuclei are dorsal; motor nuclei are ventral. The gray matter is covered by a thick layer of white matter consisting of ascending and descending axons. These axons are organized in columns that contain axon bundles with specific functions. Because the spinal cord is so highly organized, it is often possible to predict the results of injuries to localized areas. III. Spinal Nerves, p. 429 Objectives 1. Describe the major components of a spinal nerve. 2. Relate the distribution pattern of spinal nerves to the regions they innervate. Figure 13-6 Every segment of the spinal cord is connected to a pair of spinal nerves, each of which is surrounded by a series of 3 connective tissue layers that support structures and contain blood vessels. 1. The epineurium or outer layer consists of a dense network of collagen fibers. 2. The perineurium or middle layer divides the nerve into compartments containing bundles of axons (fascicles). 3. The endoneurium or inner layer surrounds individual axons. Spinal nerves branch and interconnect to form peripheral nerves, which are also surrounded by connective tissue sheaths. Peripheral Distribution of Spinal Nerves, p. 430 Figure 13-7 A typical spinal nerve (e.g. a thoracic nerve) forms just lateral to the intervertebral foramen, where the dorsal and ventral roots unite. From there, the spinal nerve branches and forms pathways to its final destination. The first branch from the spinal nerve carries visceral motor fibers to a sympathetic ganglion of the autonomic nervous system. The preganglionic branch

5 (which has myelinated axons) is called the white ramus. The postganglionic fibers (which innervate smooth muscles, glands and organs) form sympathetic nerves. The unmyelinated nerves that return from the sympathetic ganglion to rejoin the spinal nerve are the gray ramus. Together, the gray ramus and white ramus are the communicating branches or rami communicantes. The dorsal ramus contains the somatic and visceral motor fibers that innervate the back. The large ventral ramus innervates ventrolateral structures and limbs. Figure 13-8 In addition to motor impulses, the dorsal, ventral and white rami also carry sensory information: The bilateral region of skin monitored by a specific pair of spinal nerves is called a dermatome. Regional loss of sensory or motor function (due to trauma or compression) is called peripheral neuropathy. Key Each peripheral nerve provides sensory and/or motor innervation to specific structures. Nerve Plexuses, p. 431 Figure 13-9 Skeletal muscles of the neck and limbs are controlled by nerves formed from blended fibers of the ventral rami of adjacent spinal nerves. These complex, interwoven networks of nerve fibers are called nerve plexuses. The ventral rami form 4 major plexuses: cervical, brachial, lumbar and sacral. Figure The Cervical Plexus - The cervical plexus includes ventral rami of spinal nerves C1-C5. - The cervical plexus innervates the neck, thoracic cavity and diaphragmatic muscles. - The major nerve of the cervical plexus is the phrenic nerve, which controls the diaphragm. Table 13-1 summarizes the structures of the cervical plexus. 2. The Brachial Plexus - The brachial plexus includes ventral rami of spinal nerves C5-T1. - The brachial plexus innervates the pectoral girdle and upper limbs. - The nerves that form the brachial plexus originate from the superior, middle and inferior trunks (large bundles of axons from several spinal nerves) and the lateral, medial and posterior cords (smaller branches that originate at trunks). - The major nerves of the brachial plexus include the musculocutaneous

6 nerve (of the lateral cord), the median nerve (of lateral and medial cords), the ulnar nerve (of the medial cord), axillary nerve (of the posterior cord) and the radial nerve (of the posterior cord). Table 13-2 summarizes the structures of the brachial plexus. Figure & 4. The Lumbar and Sacral Plexuses - The lumbar plexus and sacral plexus innervate the pelvic girdle and lower limbs. - The lumbar plexus includes ventral rami of spinal nerves T12-L4. - The major nerves of the lumbar plexus include the genitofemoral nerve, the lateral femoral cutaneous nerve, and the femoral nerve. - The sacral plexus includes ventral rami of spinal nerves L4-S4. - The major nerves of the sacral plexus include the pudendal nerve and the sciatic nerve. - The sciatic nerve has 2 branches: the fibular nerve and the tibial nerve. Table 13-3 summarizes the structures of the lumbar and sacral plexuses. IV. Principles of Functional Organization, p. 437 Objectives 1. Discuss the significance of neuronal pools and describe the major patterns of interaction among neurons within and among these pools. 2. Describe the steps in a neural reflex. 3. Classify the types of reflexes and explain the functions of each. Sensory neurons, motor neurons and interneurons interconnect to produce both simple and complex reflexes (automatic responses coordinated entirely within the spinal cord). Sensory neurons (about 10 million of them) deliver information to the CNS. Motor neurons (about 1/2 million) deliver commands to peripheral effectors. Interneurons (about 20 billion) interpret, plan and coordinate incoming and outgoing signals. Neuronal Pools, p. 438 Interneurons are organized into functional groups of interconnected neurons called neuronal pools, each with a limited number of input sources and output destinations. An entire neuronal pool may stimulate or depress activity in other parts of the brain or spinal cord. Figure There are 5 patterns of neuronal interactions (or neural circuits) found within

7 neuronal pools: 1. Divergence spreads stimulation to many neurons or neuronal pools in the CNS. 2. Convergence brings input from many sources to a single neuron. 3. Serial processing moves information in a single line. 4. Parallel processing moves the same information along several paths simultaneously. 5. Reverberation is a positive feedback mechanism which continues to function until actively inhibited. An Introduction to Reflexes, p. 439 Reflexes (rapid, automatic responses to specific stimuli) are the basic building blocks of neural function. A particular neural reflex always produces the same motor response. The Reflex Arc The wiring of a single reflex, beginning at a receptor and ending at a peripheral effector, is a reflex arc. A reflex response generally removes or opposes the original stimulus (negative feedback). Figure There are 5 steps in a simple neural reflex (e.g. the stretch reflex): Step 1: The arrival of a stimulus and activation of a receptor (physical or chemical changes). Step 2: The activation of a sensory neuron (graded depolarization). Step 3: Information processing by postsynaptic cell (triggered by neurotransmitters). Step 4: The activation of a motor neuron (action potential). Step 5: The response of a peripheral effector (triggered by neurotransmitters). Classification of Reflexes Reflexes may be classified in 4 ways: by early development, the type of motor response, the complexity of the neural circuit, and the site of information processing: 1. How the reflex was developed: Innate reflexes are the basic neural reflexes a person is born with. Acquired reflexes are rapid, automatic, learned motor patterns. 2. The nature of the resulting motor response: Somatic reflexes provide involuntary control of the nervous system. a. superficial reflexes of the skin or mucous membranes b. stretch reflexes (deep tendon reflexes) such as the patellar reflex Visceral reflexes (autonomic reflexes) control systems other than the muscular system.

8 3. The complexity of the neural circuit involved: In a monosynaptic reflex, a sensory neuron synapses directly onto a motor neuron. A polysynaptic reflex has at least 1 interneuron between the sensory neuron and the motor neuron. 4. The site of information processing: In spinal reflexes, processing occurs in the spinal cord. In cranial reflexes, processing occurs in the brain. V. Spinal Reflexes, p. 441 Objectives 1. Distinguish among the types of motor responses produced by various reflexes. 2. Explain how reflexes interact to produce complex behaviors. Spinal reflexes increase in complexity from monosynaptic reflexes to polysynaptic reflexes to intersegmental reflex arcs (reflexes in which many segments interact to produce a highly variable motor response). Monosynaptic Reflexes, p. 441 Figure Monosynaptic reflexes have the least delay between sensory input and motor output. The best known monosynaptic reflex is the stretch reflex (e.g. the patellar reflex) which is completed within msec. Figure The receptors in stretch reflexes are called muscle spindles. Muscle spindles are bundles of small, specialized intrafusal muscle fibers (innervated by sensory and motor neurons) surrounded by extrafusal muscle fibers (which maintain tone and contract the muscle). The central sensory region of the intrafusal fibers is wound with dendrites of sensory neurons. The axon of a sensory neuron enters the CNS in a dorsal root and synapses onto motor neurons (gamma motor neurons) in the anterior gray horn of the spinal cord. Completing the reflex arc, axons of the motor neurons (gamma efferents) synapse back onto the intrafusal fibers. Many stretch reflexes are postural reflexes (reflexes that maintain normal upright posture). A stretched muscle responds by contracting, automatically maintaining balance. Gamma efferents are also important in voluntary muscle contractions, allowing the CNS to adjust the sensitivity of muscle spindles.

9 Polysynaptic Reflexes, p. 443 Polysynaptic reflexes are more complicated than monosynaptic reflexes. Their interneurons can control more than one muscle group, and can produce either EPSPs (excitatory) or IPSPs (inhibitory). 1. The Tendon Reflex The tendon reflex generally prevents skeletal muscles from developing enough tension to tear or break the tendon. The sensory receptors for this reflex are different from muscle spindles or proprioceptors, but have not been specifically identified. Figure Withdrawal Reflexes Withdrawal reflexes move a body part away from a stimulus such as pain or pressure. For example: The flexor reflex pulls your hand away from a hot stove. For this to work automatically, the stretch reflex that normally tenses the antagonistic (extensor) muscle must be inhibited by interneurons in the spinal cord (reciprocal inhibition). The strength and extent of a withdrawal response depends on the intensity and location of the stimulus. Figure Crossed Extensor Reflexes Stretch, tendon and withdrawal reflexes are ipsilateral reflex arcs, occurring on the same side of the body as the stimulus. A crossed extensor reflex occurs on the side opposite the stimulus (contralateral reflex arc). The crossed extensor reflex occurs simultaneously and in coordination with a flexor reflex. E.g. when the flexor reflex causes one leg to pull up, the crossed extensor reflex straightens the other leg to receive the body s weight, using reverberating circuits to maintain the response. All polysynaptic reflexes share 5 general characteristics: 1. They involve pools of neurons. 2. They are intersegmental in distribution. 3. They involve reciprocal inhibition. 4. They have reverberating circuits which prolong the reflexive motor response. 5. Several reflexes may cooperate to produce a coordinated, controlled response. VI. Integration and Control of Spinal Reflexes, p. 445 Objective

10 1. Explain how higher centers control and modify reflex responses. Although reflex behaviors occur automatically, processing centers in the brain can facilitate or inhibit reflex motor patterns based in the spinal cord. Voluntary Movements and Reflex Motor Patterns, p. 445 Automatic reflexes can be activated as needed by the brain, using relatively few nerve impulses to control complex motor functions. A simple mental command can trigger complex motions such as walking, running or jumping without much conscious effort. The higher centers of the brain modulate or build on lower, reflexive motor patterns. Key Reflexes are rapid, automatic responses to stimuli that buy time for the planning and execution of more complex responses that are often consciously directed. The fastest reflexes are somatic motor reflexes that (1) involve myelinated axons, (2) involve only one segment of the spinal cord or one nucleus of the brain, and (3) are monosynaptic. Reinforcement and Inhibition, p. 445 By stimulating excitatory or inhibitory neurons within the brain stem or spinal cord, higher centers can adjust the sensitivity of reflexes by creating EPSPs or IPSPs at the motor neurons involved in reflex responses. Facilitated postsynaptic neurons enhance or reinforce spinal reflexes, usually making them too strong to suppress consciously. Other nerves can suppress or inhibit spinal reflexes. A healthy adult will respond to a stimulus on the sole of the foot by curling the toes (the plantar reflex or negative Babinski reflex). The opposite response (fanning the toes or the Babinski sign), while normal in infants, may indicate CNS damage in adults. SUMMARY In Chapter 13 we learned about: - the general organization of the nervous system (CNS and PNS) - the gross anatomy of the spinal cord: (enlargements, dorsal and ventral roots, filum terminale, conus medullaris) - afferent (sensory) and efferent (motor) fibers - the structures and functions of spinal meninges and cerebrospinal fluid (CSF) - the gray matter and horns of the spinal cord - the white matter and columns (tracts) of the spinal cord - the 3 layers in spinal nerves - the distribution (rami) of spinal nerves (white, gray, dorsal and ventral) - the 4 major nerve plexuses

11 (cervical, brachial, lumbar and sacral) - neuronal pools and neural circuit patterns (divergence, convergence, serial, parallel, reverberation) - reflexes and reflex arcs - classifications of reflexes: - innate and acquired - somatic and visceral - monosynaptic, polysynaptic and intersegmental - cranial and spinal - characteristics of monosynaptic reflexes (stretch reflex, postural reflex, muscle spindles) - characteristics of polysynaptic reflexes (tendon, withdrawal, flexor and crossed extensor reflexes) - interactions of the brain with spinal reflexes - facilitation (reinforcement) and inhibition of spinal reflexes