The basic unit of integrated reflex activity is the reflex arc. This arc consists of a sense organ, an afferent neuron, and one or more synapses within a central integrating station, an efferent neuron, and an effector. In mammals, the connection between afferent and efferent somatic neurons is generally in the brain or spinal cord. The afferent neurons enter via the dorsal roots or cranial nerves and have their cell bodies in the dorsal root ganglia or in the homologous ganglia on the cranial nerves. The efferent fibers leave via the ventral roots or corresponding motor cranial nerves. The principle that in the spinal cord the dorsal roots are sensory and the ventral roots are motors is known as the Bell–Magnesia law.
Monosynaptic reflexes: the stretch reflex
When a skeletal muscle with an intact nerve supply is stretched, it contracts. This response is called the stretch reflex. The stimulus that initiates the reflex is the stretch of the muscle, and the response is a contraction of the muscle being stretched. The sense organ is a small encapsulated spindle-like or fusiform-shaped structure called the muscle spindle, located within the fleshy part of the muscle. The impulses originating from the spindle are transmitted to the CNS by fast sensory fibers that pass directly to the motor neurons which supply the same muscle. The neurotransmitter at the central synapse is glutamate. The stretch reflex is the best known and studied monosynaptic reflex and is typified by the knee jerk reflex
Structure of muscle spindles
Each muscle spindle has three essential elements: (1) a group of specialized intrafusal muscle fibers with contractile polar ends and a no contractile center, (2) large-diameter militated afferent nerves (types Ia and II) originating in the central portion of the intramural fibers, and (3) small diameter militated efferent nerves supplying the polar contractile regions of the intramural fibers. It is important to understand the relationship of these elements to each other and to the muscle itself to appreciate the role of this sense organ in signaling changes in the length of the muscle in which it is located. Changes in muscle length are associated with changes in joint angle; thus muscle spindles provide information on the position.
When a stretch reflex occurs, the muscles that antagonize the action of the muscle involved (antagonists) relax. This phenomenon is said to be due to reciprocal innervation. Impulses in the Ia fibers from the muscle spindles of the protagonist muscle cause postsynaptic inhibition of the motor neurons of the antagonists. The pathway mediating this effect is synaptic.
Inverse stretch reflex
Up to a point, the harder a muscle is stretched, the stronger is the reflex contraction. However, when the tension becomes great enough, contraction suddenly ceases and the muscle relaxes. This relaxation in response to strong stretch is called the inverse stretch reflex or autogenic inhibition.
Polysynaptic reflexes: the withdrawal reflex
Polysynaptic reflex paths branch in a complex fashion. The number of synapses in each of their branches varies. Because of the synaptic delay at each synapse, activity in the branches with fewer synapses reaches the motor neurons first, followed by activity in the longer pathways. This causes prolonged bombardment of the motor neurons from a single stimulus and consequently prolonged responses. Furthermore, some of the branch pathways turn back on themselves, permitting activity to reverberate until it becomes unable to cause a propagated Trans synaptic response and dies out. Such reverberating circuits are common in the brain and spinal cord.
The withdrawal reflex is a typical polysynaptic reflex that occurs in response to usually painful stimulation of the skin or subcutaneous tissues and muscle. The response is flexor muscle contraction and inhibition of extensor muscles so that the body part stimulated is flexed and withdrawn from the stimulus.
The stimulus that triggers a reflex is generally very precise. This stimulus is called the adequate stimulus for the particular reflex. A dramatic example is the scratch reflex in the dog. This spinal reflex is adequately stimulated by multiple linear touch stimuli such as those produced by an insect crawling across the skin. The response is vigorous scratching of the area stimulated. If the multiple touch stimuli are widely separated or not in a line, the adequate stimulus is not produced and no scratching occurs. Fleas crawl, but they also jump from place to place. This jumping separates the touch stimuli so that an adequate stimulus for the scratch reflex is not produced. It is doubtful if the flea population would survive long without the ability to jump.
Final common path
The motor neurons that supply the extradural fibers in skeletal muscles are the efferent side of many reflex arcs. All neural influences affecting muscular contraction ultimately funnel through them to the muscles, and they are therefore called the final common paths. Numerous inputs converge on them. Indeed, the surface of the average motor neuron and its dendrites accommodates about 10,000 synaptic knobs. At least five inputs go from the same spinal segment to a typical spinal motor neuron.
A muscle spindle is a group of specialized intramural muscle fibers with contractile polar ends and a no contractile center that is located in parallel to the extradural muscle fibers and is innervated by types Ia and II afferent fibers and γ-motor neurons. Muscle stretch activates the muscle spindle to initiate reflex contraction of the extradural muscle fibers in the same muscle (stretch reflex).