THE REFLEX

Here we shall examine, in general terms how a precise circuit of interconnected neurons produce a simple behavior. We shall pay attention on the reflexes mechanisms to illustrate the two basic principles of neural functioning first put forth by Ramon y Cajal: dynamic polarization and connectional specificity.

Reflexes represent the simplest forms of behavior.

A reflex is an involuntary and relatively stereotyped response to a specific sensory stimulus. Two features of the sensory stimulus are particularly important in shaping the reflex response. First, the precise location of the stimulus determines in a fixed way the particular muscle that contract to produce the reflex response. Second the strength of the stimulus determines the amplitude of the response. Reflex therefore are graded behaviors.


Spinal reflexes are those in which the sensory stimuli arise from receptors in muscles, joints and skin, and in which the neural circuitry responsible for the motor response is entirely contained within the spinal cord. Although the neural circuits that mediate spinal reflexes are relatively simple, descending influences from higher brain centers often use these spinal circuits to generate more complex behavior. Brian stem reflexes, such as gagging and vestibulo-ocular reflex, follow basically similar rules.

First let's examine the neural circuitry of one spinal reflex: the stretch reflex. This is the simplest reflex known; it depends only in the monosynaptic connection between primary afferent fibers from muscle spindles and motor neurons innervating the same muscle.

The muscle spindle contains specialized elements that sense muscle length and velocity of length change. The spindle is innervated by large myelinated afferent fibers known as type Ia afferent fibers. The cell bodies of these neurons are clustered near the spinal cord in the dorsal root ganglia. They are an example of a bipolar cell: one branch of the cell's axon goes out to the muscle and the other runs into the spinal cord. In the spinal cord the Ia afferent fibers make monosynaptic excitatory connections to alpha motor neurons innervating the same muscle from which they arise and motor neurons innervating synergistic muscles . They also inhibits motor neurons controlling antagonistic muscles through an inhibitory interneuron. Half the neurons in the brain are inhibitory. They release neurotransmitters that hyperpolarize the membrane potential of the postsynaptic cell, thus reducing the likelihood of firing. The muscle spindle is sensitive to stretch so that when the muscle is stretched the Ia afferent fibers increase the firing rate. This leads to the contraction of the same muscle and its synergists and relaxation of the antagonist muscle. The reflex therefore tends to counteract the stretch, enhancing the spring like properties of the muscles.



The knee jerk reflex is a well known example of stretch reflex. Tapping the knee cap (patella) pulls on the tendon of the quadriceps femoris, which is an extensor muscle that extends the lower leg. When the muscle stretches in response to the pull of the tendon, information regarding this change in the muscle is conveyed by the afferent sensory neurons to the spinal cord and the central nervous system. In the spinal cord the sensory neurons act directly on motor neurons that contract the quadriceps. By the same token, they act indirectly, through inhibitory interneurons, to inhibit motor neurons that contract the antagonist muscle, the hamstring. The sensory neurons also end in projection interneurons that transmit information about the local neural activity to higher regions of the brain concerned with movement, The stretch reflex plays a central role in the maintenance of balance. It is called a monosynaptic reflex because it depends only on the simple connection between primary afferent fibers from muscle spindles and motor neurons innervating the same muscle. In other spinal reflexes such as those produced by cutaneous stimuli, one or more interneurons may be interposed between the primary afferent fibers and the motor neurons.


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