Figure 1126

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Spine Healing Therapy

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Cross section of the human spinal cord. The photomicrograph shows a cross section through the lower lumbar (most likely L4-L5) level of the spinal cord stained by the Bielschowsky silver method. The spinal cord is organized into an outer part, the white matter, and an inner part, the gray matter that contains nerve cell bodies and associated nerve fibers. The gray matter of the spinal cord appears roughly in the form of a butterfly. The anterior and posterior prongs are referred to as ventral horns (VH) and dorsal horns (DH), respectively. They are connected by the gray commissure (GC). The white matter contains nerve fibers that form ascending and descending tracts. The outer surface of the spinal cord is surrounded by the pia mater. Blood vessels of the pia mater, ventral fissure (VF), and some dorsal roots of the spinal nerves are visible in the section. x5.

riphcral segment that brings information from the periphery to the cell body and a central segment that carries information from the cell body into the gray matter of the spinal cord. Because the sensory neuron conducts impulses to the CNS, it is called an afferent neuron. Impulses are generated in the terminal receptor arborization of the peripheral segment.

Afferent (Sensory) Receptors

Afferent receptors are specialized structures located at the distal tips of the peripheral processes of sensory neurons

Although receptors may have many different structures, they have one basic characteristic in common: They can initiate a nerve impulse in response to a stimulus. Receptors may be classified as

• Exteroceptors, which react to stimuli from the external environment, e.g., temperature, touch, smell, sound, or vision

• Enteroceptors, which react to stimuli from within the body, e.g., the degree of filling or stretch of the alimentary canal, bladder, and blood vessels

• Proprioceptors, which also react to stimuli from within the body, providing sensation of body position and muscle tone and movement

The simplest receptor is a bare axon, called a nonencap-sulated (free) ending. This ending is found in epithelia, in connective tissue, and in close association with hair follicles.

Most sensory nerve endings acquire connective tissue capsules or sheaths of varying complexity

Sensory nerve endings with connective tissue sheaths are called encapsulated endings. Many encapsulated endings are mechanoreceptors located in the skin and joint capsules (Krause's end bulb, Ruffini's corpuscles, Meissner's corpuscles, and Pacinian corpuscles) and are described in Chapter 14, "Integumentary System" (page 400).

Muscle spindles are encapsulated sensory endings located in skeletal muscle; they are described in Chapter 10, "Muscle Tissue" (page 260). Functionally related Golgi tendon organs are encapsulated tension receptors found at musculotendinous junctions.

Autonomic Nervous System

Although the ANS was introduced early in this chapter, it is useful here to describe some of the salient features of its organization and distribution. The ANS is classified into three divisions:

• Sympathetic division

• Parasympathetic division

• Enteric division

The ANS is that portion of the PNS that conducts impulses to smooth muscle, cardiac muscle, and glandular epithelium. These effectors are the functional units in the organs that respond to regulation by nerve tissue. The term visceral is sometimes used to refer to the ANS or its neurons, which are, therefore, called visceral efferent neurons.

Sensory neurons also leave the organs to convey impulses to the CNS. These visceral afferent neurons have the same arrangement as other sensory neurons; i.e., their cell bodies are located in sensory ganglia, and they possess long peripheral and central axons, as described above.

The main organizational difference between the efferent flow of impulses to skeletal muscle (somatic effectors) and the efferent flow to smooth muscle, cardiac muscle, and glandular epithelium (visceral effectors) is that one neuron conveys the impulses from the CNS to the somatic effector (Fig. 11.27a), whereas a chain of two neurons conveys the impulses from the CNS to the visceral effectors

SOMATIC EFFERENT (MOTOR) NEURON

skeletal muscle

SOMATIC EFFERENT (MOTOR) NEURON

skeletal muscle

VISCERAL EFFERENT (AUTONOMIC) NEURONS

presynaptic postsynaptic

VISCERAL EFFERENT (AUTONOMIC) NEURONS

presynaptic postsynaptic

blood vessel

sweat gland

VISCERAL EFFERENT (AUTONOMIC) NEURONS

presynaptic neuron

VISCERAL EFFERENT (AUTONOMIC) NEURONS

celiac ganglion postsynaptic neurons

presynaptic neuron sympathetic trunk with paravertebral ganglion splanchnic nerve containing axons of presynaptic neurons sympathetic trunk with paravertebral ganglion celiac ganglion postsynaptic neurons splanchnic nerve containing axons of presynaptic neurons

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