Reflex Arc
Reflex Arc
Many neurons work in groups called reflex arcs. There must be at least two neurons in a reflex arc, but usually there are more. The parts of a typical reflex arc are (1) a receptor, a sense organ in skin, muscle, or another organ; (2) an afferent, or sensory, neuron, which carries impulses toward the central nervous system; (3) the central nervous system, where synaptic connections are made between sensory neurons and interneurons; (4) an efferent, or motor, neuron, which makes a synaptic connection with the interneuron and carries impulses from the central nervous system; and (5) an effector, by which the animal responds to environmental changes. Examples of effectors are muscles, glands, ciliated cells, nematocysts of radiate animals, electric organs of fish, and certain pigmented cells called chromatophores.
A reflex arc in its simplest form contains only two neurons—a sensory (afferent) neuron and a motor (efferent) neuron (for example, the “kneejerk” or stretch reflex, Figure 35-11). Usually, however, interneurons are interposed between sensory and motor neurons (Figure 35-11). An interneuron may connect afferent and efferent neurons on the same side of the spinal cord or on opposite sides, or it may connect them on different levels of the spinal cord, either on the same or opposite sides.
A reflex act is a response to a stimulus acting over a reflex arc. It is involuntary, meaning that it is often not under the control of the will. For example, many vital processes of the body, such as control of breathing, heartbeat, diameter of blood vessels, and sweat gland secretion are reflex acts. Some reflex acts are innate; others are acquired through learning.
In almost any reflex act, a number of reflex arcs are involved. For instance, a single afferent neuron may make synaptic connections with many efferent neurons. In a similar way an efferent neuron may receive impulses from many afferent neurons. Afferent neurons also make connections with ascending sensory neurons, which travel in the white matter of the spinal cord, bringing information about peripheral reflexes to the brain. Reflex activity may then be modified by descending motor neurons, which impinge on the final efferent motor neurons before they leave the spinal cord for the periphery.
Many neurons work in groups called reflex arcs. There must be at least two neurons in a reflex arc, but usually there are more. The parts of a typical reflex arc are (1) a receptor, a sense organ in skin, muscle, or another organ; (2) an afferent, or sensory, neuron, which carries impulses toward the central nervous system; (3) the central nervous system, where synaptic connections are made between sensory neurons and interneurons; (4) an efferent, or motor, neuron, which makes a synaptic connection with the interneuron and carries impulses from the central nervous system; and (5) an effector, by which the animal responds to environmental changes. Examples of effectors are muscles, glands, ciliated cells, nematocysts of radiate animals, electric organs of fish, and certain pigmented cells called chromatophores.
A reflex arc in its simplest form contains only two neurons—a sensory (afferent) neuron and a motor (efferent) neuron (for example, the “kneejerk” or stretch reflex, Figure 35-11). Usually, however, interneurons are interposed between sensory and motor neurons (Figure 35-11). An interneuron may connect afferent and efferent neurons on the same side of the spinal cord or on opposite sides, or it may connect them on different levels of the spinal cord, either on the same or opposite sides.
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| Figure 35-11 The reflex arc. A, The “knee-jerk” or stretch reflex, a simple reflex arc. Sudden pressure on the patellar ligament stretches muscles in the upper leg. Impulses generated in stretch receptors are conducted along afferent (sensory) neurons to the spinal cord and relayed directly to an efferent (motor) nerve cell body. Impulses pass along efferent neurons to leg muscles (effectors), stimulating them to contract. B, Multisynaptic relfex arc. A more common reflex arc includes interneurons between the sensory and motor neuron. Tack puncture is sensed by pain receptors in the skin and the signal is conducted along afferent fibers to the spinal cord where synaptic connections are made with interneurons. Here, an interneuron is shown making connections with motor neurons on both sides of the spinal cord, such that stimulation of muscle fibers in more than one part of the body (both legs, or example) allows coordination of muscle responses to the tack puncture. |
A reflex act is a response to a stimulus acting over a reflex arc. It is involuntary, meaning that it is often not under the control of the will. For example, many vital processes of the body, such as control of breathing, heartbeat, diameter of blood vessels, and sweat gland secretion are reflex acts. Some reflex acts are innate; others are acquired through learning.
In almost any reflex act, a number of reflex arcs are involved. For instance, a single afferent neuron may make synaptic connections with many efferent neurons. In a similar way an efferent neuron may receive impulses from many afferent neurons. Afferent neurons also make connections with ascending sensory neurons, which travel in the white matter of the spinal cord, bringing information about peripheral reflexes to the brain. Reflex activity may then be modified by descending motor neurons, which impinge on the final efferent motor neurons before they leave the spinal cord for the periphery.
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