Anatomy & Physiology

Anatomy & Physiology Study Guide neurons have two or more dendrites and a single axon. These are the most communal neurons in the CNS. All the motor neurons that control skeletal muscles are multipolar neurons. The axons of multipolar neurons can be as long as those of unipolar neurons; the longest carry motor commands from the spinal cord to small muscles that move the toes. 13.5 Functional Classification Neurons are classified by purpose as sensory neurons, motor neurons, or interneurons. Sensory neurons, or afferent neurons , create the afferent division of the PNS. They transfer communication from sensory receptors to the CNS. Sensory neurons are unipolar neurons with processes, known as afferent fibers that extend between a sensory receptor and the spinal cord or brain. They collect information concerning the external or internal environment. Somatic sensory neurons observe the outside world and our position within it; visceral sensory neurons observe internal conditions and the status of other organ systems. They detect this information with receptors . Receptors are grouped as: interoceptors, exteroceptors, and proprioceptors. Interoceptors monitor the cardiovascular, respiratory, digestive, urinary, and reproductive systems and provide sensations of taste, deep pressure, and pain. Exteroceptors provide sensory information about the external environment through touch, temperature, or pressure sensations and the more complex senses of sight, smell, and hearing. Proprioceptors observe the location and action of skeletal muscles and joints. Motor neurons , or efferent neurons , form the efferent division of the PNS. These neurons carry instructions from the CNS to effectors in a peripheral tissue, organ, or organ system. Axons traveling away from the CNS are called efferent fibers . The somatic nervous system includes all the somatic motor neurons that innervate skeletal muscles. Visceral motor neurons innervate all peripheral effectors other than skeletal muscles (smooth muscle, cardiac muscle, glands, and adipose tissue throughout the body). In order to carry a message from the CNS to an effector, the signal must travel along one axon, be relayed across a synapse, and then travel along a second axon to its final destination. The axons extending from the CNS to an autonomic ganglion are referred to as preganglionic fibers ; axons connecting the ganglion cells with the peripheral effectors are known as postganglionic fibers . Interneurons , or association neurons , outnumber all other types of neurons combined. Most are located within the brain and spinal cord. Interneurons are responsible for both the distribution of sensory information and the coordination of motor activity. One or more interneurons are located between sensory neurons and motor neurons. Interneurons are involved with all higher functions, such as memory, planning, and learning.

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