Classes of neurons

The human nervous system has three functional classes of neurons:

• Afferent neurons

• Efferent neurons

• Interneurons

Afferent neurons lie predominantly in the PNS (see Figure 6.1). Each has a sensory receptor activated by a particular type of stimulus, a cell body located adjacent to the spinal cord, and an axon. The peripheral axon extends from the receptor to the cell body and the central axon continues from the cell body into the spinal cord. Efferent neurons also lie predominantly in the PNS. In this case, the cell bodies are found in the CNS in the spinal cord or brainstem and the axons extend out into the periphery of the body where they innervate the effector tissues. By way of convergence, the centrally located cell bodies may receive inputs from several different regions of the brain that will influence their activity.

The third class of neurons includes the interneurons, which lie entirely within the CNS. Because the human brain and spinal cord contain well over 100 billion neurons, interneurons account for approximately 99% of all the neurons in the body taken together. Interneurons lie between afferent and

Figure 6.1 Types of neurons. Afferent neurons, which transmit impulses toward the CNS and efferent neurons, which transmit impulses away from the CNS, lie predominantly in the peripheral nervous system. Interneurons, which process sensory input and coordinate motor responses, lie entirely within the central nervous system.

Figure 6.1 Types of neurons. Afferent neurons, which transmit impulses toward the CNS and efferent neurons, which transmit impulses away from the CNS, lie predominantly in the peripheral nervous system. Interneurons, which process sensory input and coordinate motor responses, lie entirely within the central nervous system.

efferent neurons and are responsible for integrating sensory input and coordinating a motor response. In the simplest condition, interneurons process responses at the level of the spinal cord in the form of reflexes that are automatic, stereotyped responses to given stimuli. For example, stimulation of pain receptors generates action potentials in their associated afferent neurons. These impulses are transmitted to the spinal cord where the afferent neurons stimulate interneurons. The interneurons then stimulate efferent neurons that cause skeletal muscle contraction in the affected area to remove the body part from the painful stimulus. This withdrawal reflex involves comparatively few interneurons and does not require any input from higher nervous centers in the brain. On the other hand, a response to some other stimulus may involve more sophisticated neurological phenomena such as memory, motivation, judgment, and intellect. This type of response is not automatic, is clearly far more complex and may require the activity of millions of interneurons in many regions of the brain prior to stimulation of motor neurons to carry out the desired response.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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