SPMD 201 Spinal Cord and Nerves Chapter 12

Cross Section of Spinal Cord

A cross section reveals that the spinal cord consists of a superficial white portion and a deep gray portion. The white matter consists of myelinated axons, which form nerve tracts, and the gray matter consists of neuron cell bodies, dendrites, and axons. An anterior median fissure and a posterior median sulcus are deep clefts partially separating the two halves of the cord. The white matter in each half of the spinal cord is organized into three columns, or funiculi (fū-nik′ū-lī), called the ventral (anterior), dorsal (posterior), and lateral column The central gray matter is organized into horns. Each half of the central gray matter of the spinal cord consists of a relatively thin posterior (dorsal) horn and a larger anterior (ventral) horn. Small lateral horns exist in the levels of the cord associated with the autonomic nervous system The two halves of the spinal cord are connected by gray and white commissures (figure 12.3a,b). The gray and white commissures contain axons that cross from one side of the spinal cord to the other.

Organization of Spinal Cord

All of the 31 pairs of spinal nerves, except the first pair and those in the sacrum, exit the vertebral column through intervertebral foramina located between adjacent vertebrae. The first pair of spinal nerves exit between the skull and the first cervical vertebra. The nerves of the sacrum exit from the single bone of the sacrum through the sacral foramina. Eight spinal nerve pairs exit the vertebral column in the cervical region, 12 in the thoracic region, 5 in the lumbar region, 5 in the sacral region, and 1 in the coccygeal region. For convenience, each of the spinal nerves is designated by a letter and a number. The letter indicates the region of the vertebral column from which the nerve emerges: C, cervical; T, thoracic; L, lumbar; and S, sacral.

Nerves

Bundled axons that form neural "cables" connecting the central nervous system with muscles, glands, and sense organs.

Coccygeal Plexus

The coccygeal (kok-sij′ē-ăl) plexus is a very small plexus formed from the ventral rami of spinal nerve S5 and the coccygeal nerve (Co). This small plexus supplies motor innervation to the muscles of the pelvic floor and sensory cutaneous innervation to the skin over the coccyx. The dorsal rami of the coccygeal nerves innervate some skin over the coccyx.

Withdrawl Reflex

The function of the withdrawal reflex, or flexor reflex, is to remove a limb or another body part from a painful stimulus. The sensory receptors are pain receptors (see chapter 15). Following painful stimuli, sensory neurons conduct action potentials through the dorsal root to the spinal cord, where the sensory neurons synapse with excitatory interneurons, which in turn synapse with alpha motor neurons. The alpha motor neurons stimulate muscles, usually flexor muscles, that remove the limb from the source of the painful stimulus. Collateral branches of the sensory neurons synapse with ascending fibers to the brain, providing conscious awareness of the painful stimuli.

Lumbosacral Plexus

The lumbar plexus originates from the ventral rami of spinal nerves L1-L4, and the sacral plexus originates from L4-S4. However, because of their close, overlapping relationship and similar distribution, the two plexuses are often considered together as a single lumbosacral plexus (L1-S4; figure 12.23). Four major nerves exit the lumbosacral plexus and enter the lower limb: the obturator, femoral, tibial, and common fibular (peroneal). The obturator nerve innervates the medial thigh; the femoral nerve innervates the anterior thigh; the tibial nerve innervates the posterior thigh, leg, and foot; and the common fibular nerve innervates the posterior thigh, anterior and lateral leg, and foot. Other lumbosacral nerves supply the lower back, hip, and lower abdomen.

Meninges of Spinal Cord

The most superficial and thickest membrane is the dura mater. The next deeper meningeal membrane is a very thin, wispy arachnoid (ă-rak′noyd; spiderlike—i.e., cobwebs) mater. The space between this membrane and the dura mater is the subdural space; it contains only a very small amount of serous fluid. The third, deepest meningeal layer, the pia (pī′ă; affectionate) mater is bound very tightly to the surface of the spinal cord. Holding the spinal cord in place within the thecal sac are the denticulate ligaments and the filum terminale. The space between this membrane and the dura mater is the subdural space; it contains only a very small amount of serous fluid. The dura mater around the spinal cord is separated from the periosteum of the vertebral canal by the epidural space.

Stretch Reflex

The simplest reflex is the stretch reflex, in which muscles contract in response to a stretching force applied to them. The sensory receptor of this reflex is the muscle spindle, which consists of 3-10 small, specialized skeletal muscle fibers. The fibers are contractile only at their ends and are innervated by specific motor neurons called gamma motor neurons (the term gamma refers to motor neurons with small-diameter axons) originating from the spinal cord and controlling the sensitivity of the muscle spindle cells. Sensory neurons innervate the noncontractile centers of the muscle spindle cells. Axons of these sensory neurons extend to the spinal cord and synapse directly with motor neurons in the spinal cord called alpha motor neurons, which in turn innervate the muscle in which the muscle spindle is embedded. Neurons can be classified by the diameter of their axons, with alpha motor neurons having the largest diameter. As a skeletal muscle contracts, the tension on the centers of muscle spindles within the muscle decreases because the muscle spindles passively shorten as the muscle shortens. The decrease in tension in the centers of the muscle spindles causes them to be less sensitive to stretch. Sensitivity is maintained because, while alpha motor neurons are stimulating the muscle to contract, gamma motor neurons are stimulating the muscle spindles to contract

Spinal Cord

The spinal cord extends from the foramen magnum to the level of the second lumbar vertebra (figure 12.1). It is considerably shorter than the vertebral column because it does not grow as rapidly during development. The spinal cord is composed of cervical, thoracic, lumbar, and sacral segments, named according to the portion of the vertebral column from which their nerves enter and exit. The spinal cord gives rise to 31 pairs of spinal nerves, which exit the vertebral column through intervertebral and sacral foramin. The spinal cord is larger in diameter at its superior end, and it gradually decreases in diameter toward its inferior end. Two enlargements occur where nerves supplying the upper and lower limbs enter and leave the spinal cord. The numerous roots (origins) of spinal nerves extending inferiorly from the lumbosacral enlargement and conus medullaris resemble a horse's tail and are therefore called the cauda (kaw′dă; tail) equina

Branches of the Spinal Nerve

The term plexus means braid and describes the organization produced by the intermingling of the nerves. The ventral rami of different spinal nerves, called the roots, join with each other to form a plexus. These roots should not be confused with the dorsal and ventral roots from the spinal cord, which are more medial. The ventral rami of spinal nerves C1-C4 form the cervical plexus, C5-T1 form the brachial plexus, L1-L4 form the lumbar plexus, L4-S4 form the sacral plexus, and S5 and the coccygeal nerve (Co) form the coccygeal plexus.

Schwann Cells

Type of glia in the PNS, Supporting cells of the peripheral nervous system responsible for the formation of myelin.

Cervical Plexus

he cervical plexus is a relatively small plexus originating from spinal nerves C1-C4. Branches derived from this plexus innervate superficial neck structures, including several of the muscles attached to the hyoid bone. The cervical plexus innervates the skin of the neck and posterior portion of the head. An unusual part of the cervical plexus, the ansa cervicalis, is a loop between C1 and C3. Nerves to the infrahyoid muscles branch from the ansa cervicalis. One of the most important derivatives of the cervical plexus is the phrenic (fren′ik) nerve, which originates from spinal nerves C3-C5 and is derived from both the cervical and brachial plexuses. The phrenic nerves descend along each side of the neck to enter the thorax and then descend along the sides of the mediastinum to reach the diaphragm, which they innervate.

Reciprical innervation

Reciprocal innervation is a phenomenon that reinforces the efficiency of the withdrawal reflex. Collateral axons of sensory neurons that carry action potentials from pain receptors synapse with inhibitory interneurons in the dorsal horn of the spinal cord. The inhibitory interneurons synapse with and inhibit alpha motor neurons of extensor (antagonist) muscles. When the withdrawal reflex is initiated, flexor muscles contract and reciprocal innervation causes the extensor muscles to relax. This reduces the resistance to movement that the extensor muscles would otherwise generate. Reciprocal innervation is also involved in the stretch reflex. When the stretch reflex causes a muscle to contract, reciprocal innervation causes opposing muscles to relax. In the patellar reflex, for example, the quadriceps femoris muscle contracts and the hamstring muscles relax.

Interactions with Spinal Cord Reflexes

Reflexes do not operate as isolated entities. Rather, because of divergent and convergent pathways, their activities are integrated with the functions of the nervous system as a whole. Diverging branches of the sensory neurons or interneurons in a reflex arc send action potentials along ascending nerve tracts to the brain. A pain stimulus, for example, not only initiates a withdrawal reflex, causing you to remove the affected body part from the painful stimulus, but also enables you to perceive the pain as a result of action potentials sent to your brain.

Golgi Tendon Reflex

The Golgi tendon reflex prevents contracting muscles from applying excessive tension to tendons. Golgi tendon organs are encapsulated nerve endings that have at their ends numerous branches with small swellings adjacent to bundles of collagen fibers in tendons. Golgi tendon organs are located near the muscle-tendon junction. As a muscle contracts, the attached tendons stretch, resulting in increased tension in the tendon. The increased tension stimulates action potentials in the sensory neurons from the Golgi tendon organs. The sensory neurons of the Golgi tendon organs pass through the dorsal root to the spinal cord and enter the posterior gray matter, where they branch and synapse with inhibitory interneurons. The interneurons synapse with alpha motor neurons that innervate the muscle to which the Golgi tendon organ is attached. Applying a great amount of tension to the tendon stimulates the sensory neurons of the Golgi tendon organs

Reflexes Basics

The basic functional unit of the nervous system is the reflex arc because it is the smallest, simplest portion capable of receiving a stimulus and producing a response. Therefore, it is possible to learn much about nervous system functions by examining how reflex arcs receive stimuli and produce responses. The reflex arc generally has five basic components: (1) a sensory receptor, (2) a sensory neuron, (3) an interneuron, (4) a motor neuron, and (5) an effector organ. The simplest reflex arcs do not involve interneurons. A reflex is an automatic response to a stimulus produced by a reflex arc. It occurs without conscious thought. Action potentials initiated in sensory receptors are transmitted along the axons of sensory neurons to the CNS, where the axons usually synapse with interneurons.

Brachial Plexus

The brachial plexus originates from spinal nerves C5-T1 (figure 12.17). The five ventral rami that constitute the brachial plexus join to form three trunks, which separate into six divisions and then join again to create three cords (posterior, lateral, and medial) from which five branches, or nerves of the upper limb, emerge. The five major nerves emerging from the brachial plexus to supply the upper limb are the axillary, radial, musculocutaneous, ulnar, and median nerves. The axillary nerve innervates part of the shoulder; the radial nerve innervates the posterior arm, forearm, and hand; the musculocutaneous nerve innervates the anterior arm; and the ulnar and median nerves innervate the anterior forearm and hand. Smaller nerves from the brachial plexus innervate the shoulder and pectoral muscles. Because of this anatomical organization, the entire upper limb can be anesthesized by injecting an anesthetic near the brachial plexus between the neck and the shoulder posterior to the clavicle. This is called brachial anesthesia.

Crossed Extensor Reflex

The crossed extensor reflex is another reflex associated with the withdrawal reflex. Interneurons that stimulate alpha motor neurons, resulting in withdrawal of a limb, have collateral axons that extend through the white commissure to the opposite side of the spinal cord and synapse with alpha motor neurons that innervate extensor muscles in the opposite side of the body. When a withdrawal reflex is initiated in one lower limb, the crossed extensor reflex causes extension of the opposite lower limb.