ABSTRACT

The simplest responses that the nervous system can make are known as reflexes. Unlike other responses of the nervous system, reflexes need little in the way of integrated nervous activity in order to take place. The components of a reflex arc mediated by the spinal cord (a spinal reflex) are as follows: a primary sensory neuron with its cell body in the dorsal root ganglion (DRG) and axonal processes to the periphery and to the spinal cord; a motor neuron residing in the ventral horn of spinal cord gray matter; and (in almost all cases) one or more interneurons between the primary sensory neuron and motor neuron (Fig. 1). There are no spinal interneurons between a primary sensory neuron and motor neuron in only one type of spinal reflex, known as the stretch reflex or myotatic reflex (see below). This reflex is therefore monosynaptic in that the only synapse occurs between the primary sensory neuron and the motor neuron in the ventral horn of spinal cord gray matter. The majority of reflexes involve one or more

Components of a reflex arc

Reflex responses involve the interaction of sensory neurons (responding to a stimulus) with motor neurons (producing a motor response). Normally these refer to spinal reflexes eliciting a somatic motor response, but brainstem reflexes and autonomic reflexes function along similar lines. The nature of the interaction may be monosynaptic (no interneuron), disynaptic (one interneuron), or polysynaptic (with several interneurons involved). Reflex responses cannot be eliminated completely by voluntary control, but can be suppressed to some extent.