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THE NERVOUS SYSTEM - THE SPINAL CORD AND SPINAL NERVES

INTRODUCTION

1. The spinal cord and brain form the central nervous system;the rest of the nervous tissue in the body is included in the peripheral nervous system. Spinal nerves communicate with the spinal cord, and cranial nerves with the brain.

ANATOMIC ORGANIZATION

The Distribution of Neurons and Axons

1. Nerve cells are found in groups with distinct anatomical boundaries. In the CNS these centershave specific functions. The gray matter of the spinal cord contains nuclei; higher centers are situated in the brain. In the PNS the cells are clustered in peripheral ganglia.

2. Sensory neurons are found in ganglia, and their afferent fibersextend into the CNS.

3. Somatic sensory neurons monitor the outside world and our position in it.

4. Visceral sensory neurons monitor internal operations.

5. The efferent fibers ofmotor neurons extend into the periphery to control peripheral structures.

6. Somatic motor neurons control skeletal muscles. Visceral motor neuronscontrol other peripheral tissues. Visceral motor control involves a neuron inside the CNS that sends a preganglionic fiber to an adjacent ganglion. After synapsing, a postganglionic fiber continues to the peripheral structure.

7. Interneurons, or association neurons, may be interposed between sensory and motor neurons.

8. Axons are bundled according to origin, destination, and/or function. In the CNS these bundles are calledtracts, and tracts are clustered together to formcolumns.

9. Pathways consist of the nuclei and tracts that link the brain with the rest of the body.

Gross Anatomy of the Spinal Cord

1. The adult spinal cord measures approximately 45cm in length.

2. In general the diameter decreases as you approach the sacral segments, but there are localized enlargements associated with the innervation of the limbs.

3. The spinal cord can be divided into 31 segments, each associated with a single pair of dorsal and ventral roots.

4. The adult spinal cord extends to the level of the first or second lumbar vertebrae. The filum terminalethat originates at the conus medullaris enters the sacral canal and blends into the coccygeal ligament .

The Spinal Meninges

1. The meninges surround, protect, and stabilize the CNS.

2. Proceeding from the verterbral column toward the spinal cord you encounter the epidural space, thedura mater, the subdural space, the arachnoid, the subarachnoid space, and the pia mater.

3. The spinal dura attaches to the coccyx via the coccygeal ligament, and to the periosteum of the skull around the foramen magnum.

4. The subarachnoid space contains cerebrospinal fluid (CSF).

5. The pia and arachnoid attach to the dura via the denticulate ligaments. The fluids between the various meningeal layers cushions shocks and provides additional stability.

6. Spinal nerves form through the union of dorsal and ventral roots between the pedicles of adjacent vertebrae. The spinal meninges are continuous with the coverings of the spinal nerves and their branches.

Sectional Anatomy of the Spinal Cord

1. Prominent landmarks in sectional view include the anterior median fissure, the posterior median sulcus, the peripheral white matterand the central gray matter that surrounds the central canal.

Gray Matter Organization

1. The posterior gray horns contain sensory neurons, the anterior hornscontain motor neurons.The lateral gray horns (where present) contain visceral motor neurons.

2. The gray commissures provide a route for communication between nuclei on opposite sides of the cord.

White Matter

1. The white matter can be divided into columns, each ofwhich contains a number of tracts (fasciculi). Ascending tracts carrysensory information toward the brain; descending tracts convey commands to motor neurons in the spinal cord.

THE PERIPHERAL NERVOUS SYSTEM

Spinal Nerves

1. There are 31 pairs of spinal nerves, 8 cervical, 12 thoracic,5 lumbar, 5 sacral, and one coccygeal

2. Peripheral nerves contain a series of connective tissue layers, the epineurium, perineurium, and endoneurium.

The Peripheral Distribution of Spinal Nerves

1. A typical spinal nerve has a gray ramus, a white ramus, a dorsal ramus, and a ventral ramus.

2. The segmental distribution of the dorsal rami can be seen in the sensory dermatomes of the skin.

Nerve Plexuses

1. Where the peripheral musculature is especially complex, several segments participate in their innervation. The intermingling of ventral roots creates a nerve plexus. There are three large plexuses: the cervical plexus, brachial plexus, and lumbosacral plexus. The latter can be further subdivided into the lumbarand sacral plexuses.

2. The major peripheral nerves studied in the chapters on the muscular system arise at these plexuses.

FUNCTIONAL INTEGRATION OF THE SPINAL CORD

AND PERIPHERAL NERVOUS SYSTEM

Spinal Reflexes

1. In spinal reflexes all of the processing events occur within the spinal cord.

2. There are monosynaptic and polysynaptic spinal reflexes. In an intersegmental reflexmore than one segment of the spinal cord is involved.

Monosynaptic Reflexes

1. The knee jerk, or patellar, reflex involves the stimulation of muscle spindles. This is an example of a stretch reflex.

2. The load reflexes also involve muscle spindles.

Polysynaptic Reflexes

1. Polysynaptic reflexes include the tendon, withdrawal, and crossed extensor reflexes.

2. The tendon reflex protects components of the muscular system from extreme stresses.

3. There are several withdrawal reflexes. The flexor reflex produces the withdrawal of the stimulated limb. During this response the antagonistic muscles are inhibited.

4. The above examples are ipsilateral reflexes. The crossed extensor reflex represents a contralateral reflex because it produces an effect on the side opposite the stimulus.

Integration with Higher Centers

1. Complex reflexes utilize pools of interneurons, they are often segmental, and they may show intersegmental effects. Reciprocal innervation coordinates muscular contractions and reverberating circuits prolong the response.Several reflexes may operateconcurrently, as long as they are not mutually antagonistic.

2. Higher centers generally provide facilitation to the neurons involved in spinal reflexes.

3. When higher centers initiate a movement, reflexes within the spinal cord may provide essential patterns of motion and reduce resistance by inhibiting antagonistic muscles and muscle groups.

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