Spinal Nerves

There are thirty-one pairs of spinal nerves. These nerves are grouped into eight cervical, twelve thoracic, five lumbar, five sacral, and one coccygeal according to the region of the vertebral column from which they arise (fig. 8.23).

Each spinal nerve is a mixed nerve composed of sensory and motor fibers. These fibers are packaged together in the nerve, but they separate near the attachment of the nerve to the spinal cord. This produces two "roots" to each nerve. The dorsal root is composed of sensory fibers, and the ventral root is composed of motor fibers (fig. 8.24). An enlargement of the dorsal root, the dorsal root ganglion, contains the cell bodies of the sensory neurons. The motor neuron shown in figure 8.24 is a somatic motor neuron that innervates skeletal muscles; its cell body is not located in a ganglion, but instead is contained within the gray matter of the spinal cord. The cell bodies of some auto-nomic motor neurons (which innervate involuntary effectors), however, are located in ganglia outside the spinal cord (the auto-nomic system is discussed separately in chapter 9).

Reflex Arc

The functions of the sensory and motor components of a spinal nerve can be understood most easily by examining a simple reflex; that is, an unconscious motor response to a sensory stimulus. Figure 8.24 demonstrates the neural pathway involved in a reflex arc. Stimulation of sensory receptors evokes action potentials that are conducted into the spinal cord by sensory

Cervical plexus Brachial plexus

Lumbar plexus

Sacral plexus

Some peripheral nerves:

Ulnar

Median

Radial

Femoral

Cervical plexus Brachial plexus

Lumbar plexus

Sacral plexus

Median

Radial

Femoral

Cranial Nerves And Spinal Cord

Cranial nerves (12 pairs)

Cervical (8 pairs)

Thoracic (12 pairs)

Lumbar (5 pairs)

Sacral (5 pairs)

Coccygeal (1 pair)

Lateral femoral cutaneous Sciatic

Cranial nerves (12 pairs)

Cervical (8 pairs)

Thoracic (12 pairs)

Spinal nerves

Lumbar (5 pairs)

Sacral (5 pairs)

Coccygeal (1 pair)

■ Figure 8.23 Distribution of the spinal nerves. These interconnect at plexuses (shown on the left) and form specific peripheral nerves.

Association neuron (interneuron)

White matter Gray matter Spinal cord

Dorsal root ganglion

Dorsal root ganglion

Association neuron (interneuron)

White matter Gray matter Spinal cord

Spinal Cord Gray Matter Neuron

Effector (muscle)

■ Figure 8.24 A spinal reflex. This reflex involves three types of neurons: a sensory neuron, an association neuron (interneuron), and a somatic motor neuron at the spinal cord level.

Effector (muscle)

■ Figure 8.24 A spinal reflex. This reflex involves three types of neurons: a sensory neuron, an association neuron (interneuron), and a somatic motor neuron at the spinal cord level.

neurons. In the example shown, a sensory neuron synapses with an association neuron (or interneuron), which in turn synapses with a somatic motor neuron. The somatic motor neuron then conducts impulses out of the spinal cord to the muscle and stimulates a reflex contraction. Notice that the brain is not directly involved in this reflex response to sensory stimulation. Some reflex arcs are even simpler than this; in a muscle stretch reflex (the knee-jerk reflex, for example) the sensory neuron synapses directly with a motor neuron. Other reflexes are more complex, involving a number of association neurons and resulting in motor responses on both sides of the spinal cord at different levels. These skeletal muscle reflexes are described together with muscle control in chapter 12, and autonomic reflexes, involving smooth and cardiac muscle, are described in chapter 9.

Clinical Investigation Clues

Remember that Frank displayed a knee-jerk reflex despite his paralysis.

■ Why was the knee-jerk reflex present?

■ What is the most likely cause of Frank's symptoms?

Test Yourself Before You Continue

1. Define the terms dorsal root, dorsal root ganglion, ventral root, and mixed nerve.

2. Describe the neural pathways and structures involved in a reflex arc.

The Central Nervous System

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Responses

  • medhane
    How is a nerve packaged together?
    7 years ago
  • Demi
    Why are spinal nerves grouped into a plexus?
    7 years ago
  • valdemar
    What part of a sensory neuron is in the spinal nerve?
    7 years ago
  • melba brokaw
    Are lumbar plexus composed of white matter?
    7 years ago
  • Junior
    Is somatic neuron in spinal cord?
    7 years ago
  • HANNO
    What is the dorsal root of the spinal cord?
    7 years ago
  • alberto
    Where is an effector in a neural reflex?
    6 years ago
  • aapo
    Where spinal nerves interconnect?
    6 years ago
  • Asmait
    What kind of neurons are human spinal cord?
    6 years ago
  • Priscilla
    What are the spinal neuron in human?
    6 years ago

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