Question

explain the structure and function of a reflex arc. mention all the parts and functions of both a two-neuron and a three - neuron reflex arc

Answer 1

A reflex arc is a neural pathway in which the brain is not involved. It controls a reflex or a spontaneous reaction toa stimulus. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. A typical reflex arc comprises of a sensory neuron , spinal cord and a motor neuron.

The sensory neuron carries the impulse to the spinal cord and the motor neuron brings the instruction to the muscles. The action thus caused is called reflex action.

It is categorised into two types --

* autonomic reflex arc , affecting the inner organs , and

* somatic reflex arc , which affects the muscles.

   During a typical reflex arc , the signals travel along the following pathway--

a] The sense organs [ skin, eyes, ears, tongue ] receive the stimulus

b] Afferent nerves carry signals from the somatic receptors to the posterior horn of the spinal cord or to the brainstem.

c] The signals reach the integrating center, the point at which neurons that compose the gray matter of the spinal cord or brainstem synapse.

d] Efferent nerves carry motor nerve signals from the anterior horn to the muscles .

e] Effector muscles innervated by the efferent nerve fiber carries out the response

Thus, a reflex arc is a pathway followed by nerves which carry sensory information from the receptor to the spinal cord and then carry the response generated by the spinal cord to the effector organ/ organs during a reflex action.

When a reflex arc involves the stimulation of two or more sensory neurons and motor signals , it is called a poly-synaptic reflexe. In polysynaptic reflexes two to several synapses are involve one or more interneurons.

Knee jerk and stretch reflexes ar monosynaptic

Stimulation of pain receptors in the skin initiates a withdrawal reflex which involves several synapses with several motor neurons and results in the removal of the organism or part from the stimulus. Hence, this is an example of polysynaptic reflex.

When we withdraw our arm from a stimulus , two motor nerves are activated. In the upper arm, the nerve ennervating the biceps instructs it to contract and the nerve ennervating the triceps instructs it to expand. As a result of this the lower arm is withdrawn .

Another example can be the monosynaptic excitation of the soleus and dysynaptic inhibition of of tibialis anterior . The sensory nerves carry the impulse to the spinal cord and the interneurons in the gray matter are stimulated. The instruction is carried by the motor nerves to the muscles for suitable response. The reflex arc follows the typical pathway i.e. motor nerves to spinal cord and through the motor nerves to the muscles.