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Anatomy Chapter 15

Anatomy Chapter 15 - Chapter 15 The Peripheral Nervous...

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Chapter 15: The Peripheral Nervous system and Reflexes I. General Anatomy of the peripheral nervous system A. Subdivisions of the PNS. 1. The sensory (afferent) division carries sensory signals by way of afferent nerve fibers from receptors to the central nervous system CNS, it can be further subdivided into somatic and visceral divisions. a. The somatic sensory division carries signals from receptors in the skin, muscles, bones and joints. b. The visceral sensory division carries signals mainly form the viscera of the thoracic and abdominal cavities. 2. The motor (efferent) division caries motor signals by way of efferent nerve fibers from the CNS to effectors (mainly glands and muscles) It can be further subdivided into somatic and visceral divisions. a. The somatic motor division carries signals to the skeletal muscles.
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b. The visceral motor division, also known as the autonomic nervous system, caries signals to glands, cardiac muscle, and smooth muscle. It can be further divided in the sympathetic and parasympathetic divisions. 1. The sympathetic division tends to arouse the body to action. 2. The parasympathetic division tends to have a calming effect. 3. Nerve fibers of the PNS are classified according to their involvement in motor or sensory, somatic or visceral pathways. a. Mixed nerves contain both motor and sensory fibers b. Sensory nerves contain mostly sensory fibers; they are less common and include the optic and olfactory nerves. c. Motor nerves contain motor fibers. B. Anatomy of Nerves and ganglia 3. A nerve is an organ composed of multiple nerve fibers bound together by sheaths of connective tissue. a. The sheath adjacent to the neurilemma is the endoneurium, which houses blood capillaries that feed nutrients and oxygen to the nerve. b. In large nerves, fibers are bundled into fascicles and wrapped in a fibrous perineurium. c. The entire nerve is covered with a fibrous epineurium. 4. A ganglion is a cluster of neuron cell bodies enveloped in an epineurium continuous with that of a nerve.
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a. A ganglion appears as a swelling along the course of a nerve. b. The dorsal root ganglia associated with the spinal cord contain the unipolar neurons of the sensory nerve fibers that carry signals to the cord. The fiber passes through the ganglion without synapsing. c. However, in the autonomic nervous system , a preganglionic fiber enters the ganglion and in many cases synapses with another neuron. The axon of the second neuron leaves the ganglion as the postganglionic fiber.
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