Review of Key Concepts
The Spinal Cord (p. 482)
- The spinal cord conducts signals up and down the body, contains central pattern generators that control locomotion, and mediates many reflexes.
- The spinal cord occupies the vertebral canal from vertebrae C1 to L1. A bundle of nerve roots called the cauda equina occupies the vertebral canal from C2 to S5.
- The cord is divided into cervical, thoracic, lumbar, and sacral regions, named for the levels of the vertebral column through which the spinal nerves emerge. The portion served by each spinal nerve is called a segment of the cord.
- Cervical and lumbar enlargements are wide points in the cord marking the emergence of nerves that control the limbs.
- The spinal cord is enclosed in three fibrous meninges. From superficial to deep, these are the dura mater, arachnoid mater, and pia mater. An epidural space exists between the dura mater and vertebral bone, and a subarachnoid space between the arachnoid and pia mater.
- The pia mater issues periodic denticulate ligaments that anchor it to the dura, and continues inferiorly as a coccygeal ligament that anchors the cord to vertebra L2.
- In cross section, the spinal cord exhibits a central H-shaped core of gray matter surrounded by white matter. The gray matter contains the somas, dendrites, and synapses while the white matter consists of nerve fibers (axons).
- The dorsal horn of the gray matter receives afferent (sensory) nerve fibers from the dorsal root of the spinal nerve. The ventral horn contains the somas that give rise to the efferent (motor) nerve fibers of the ventral root of the nerve. A lateral horn in the thoracic and lumbar regions contains somas of the sympathetic neurons.
- The white matter is divided into dorsal, lateral, and ventral columns on each side of the cord. Each column consists of one of more tracts, or bundles of nerve fibers. The nerve fibers in a given tract are similar in origin, destination, and function.
- Ascending tracts carry sensory information up the cord to the brain. Their names and functions are listed in table 13.1.
- From receptor to cerebral cortex, sensory signals typically travel through three neurons (first- through third-order) and cross over (decussate) from one side of the body to the other in the spinal cord or brainstem. Thus, the right cerebral cortex receives sensory input from the left side of the body (from the neck down) and vice versa.
- Descending tracts carry motor commands from the brain downward. Their names and functions are also listed in table 13.1.
- Motor signals typically begin in an upper motor neuron in the cerebral cortex and travel to a lower motor neuron in the brainstem or spinal cord. The latter neuron's axon leaves the CNS in a cranial or spinal nerve leading to a muscle.
The Spinal Nerves (p. 490)
- A nerve is a cordlike organ composed of nerve fibers (axons) and connective tissue.
- Each nerve fiber is enclosed in its own fibrous sleeve called an endoneurium. Nerve fibers are bundled in groups called fascicles separated from each other by a perineurium. A fibrous epineurium covers the entire nerve.
- Nerve fibers are classified as afferent or efferent depending on the direction
Saladin: Anatomy & I 13. The Spinal Cord, Spinal I Text I © The McGraw-Hill
Physiology: The Unity of Nerves, and Somatic Companies, 2003
Form and Function, Third Reflexes Edition
Chapter 13 The Spinal Cord, Spinal Nerves, and Somatic Reflexes 511
of signal conduction, somatic or visceral depending on the types of organs they innervate, and special or general depending on the locations of the organs they innervate (table 13.2).
- A sensory nerve is composed of afferent fibers only, a motor nerve of efferent fibers only, and a mixed nerve is composed of both. Most nerves are mixed.
- A ganglion is a swelling along the course of a nerve containing the cell bodies of the peripheral neurons.
- There are 31 pairs of spinal nerves, which enter and leave the spinal cord and emerge mainly through the intervertebral foramina. Within the vertebral canal, each branches into a dorsal root which carries sensory signals to the dorsal horn of the spinal cord, and a ventral root which receives motor signals from the ventral horn. The dorsal root has a swelling, the dorsal root ganglion, containing unipolar neurons of somatic sensory neurons.
- Distal to the intervertebral foramen, each spinal nerve branches into a dorsal ramus, ventral ramus, and meningeal branch.
- The ventral ramus gives rise to intercostal nerves in the thoracic region and nerve plexuses in all other regions.
The nerve plexuses are weblike networks adjacent to the vertebral column: the cervical, brachial, lumbar, sacral, and coccygeal plexus. The nerves arising from each are described in tables 13.3 through 13.6.
Somatic Reflexes (p. 503)
- A reflex is a quick, involuntary, stereotyped reaction of a gland or muscle to a stimulus.
- Somatic (spinal) reflexes are responses of skeletal muscles. The nerve signals in a somatic reflex travel by way of a reflex arc from a receptor, via an afferent neuron to the spinal cord or brainstem, sometimes through interneurons in the CNS, then via an efferent neuron to a skeletal muscle.
- Many somatic reflexes are initiated by proprioceptors, organs that monitor the position and movements of body parts.
- Muscle spindles are proprioceptors embedded in the skeletal muscles that respond to stretching of the muscle. They are composed of modified intrafusal muscle fibers, primary and secondary afferent nerve fibers, and 7 motor neurons, all enclosed in a fibrous sheath.
- The stretch reflex is the tendency of a muscle to contract when it is stretched, as in the patellar tendon (knee-jerk) reflex. Stretch reflexes smooth joint actions and maintain equilibrium and posture. Many stretch reflexes travel via monosynaptic pathways so there is minimal synaptic delay and a very quick response.
- A stretch reflex is often accompanied by reciprocal inhibition, a reflex that prevents an antagonistic muscle from contracting and interfering with the reflex action.
- The flexor reflex is the withdrawal of a limb from an injurious stimulus, as in pulling back from a hot stove. It employs a polysynaptic reflex arc that produces a sustained response in the muscle.
- The crossed extensor reflex is contraction of the extensors on one side of the body when the flexors are contracted on the other side. It shifts the body weight so that one does not fall over.
- The Golgi tendon reflex is the inhibition of a muscle contraction that occurs when its tendon is excessively stretched. Stretching stimulates a receptor in the tendon called a Golgi tendon organ. The reflex prevents tendon injuries and helps to distribute workload across a muscle.
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