Review of Medical Physiology - William F. Ganong 2002

Functions of the Nervous System
Reflexes
Main Properties of Reflexes

From the characteristics of monosynaptic and Polysynaptic reflexes described above, it is clear that reflex activity is stereotyped in terms of both stimulus and response; a specific stimulus elicits a specific response.

Adequate Stimulus

A stimulus that triggers a reflex response is typically appropriate, i.e., adequate for that specific reflex. A striking example is the scratch reflex in dogs. This spinal reflex is elicited by an adequate stimulus—multiple linear touches occurring, for instance, as an insect crawls across the Skin. The response is vigorous scratching of the irritated area. (In this case, the precision with which the scratching limb reaches the site of the stimulus is a good example of sign posture). However, if multiple tactile stimuli occur at a considerable distance apart or are not arranged linearly, the stimulus is inadequate, and the scratch reflex is not elicited. Fleas crawl, but they can also jump from place to place. These jumps result in wide spacing between tactile stimuli, preventing the scratch reflex from occurring. It is doubtful whether a flea population could survive if it were not capable of jumping locomotion.

Final Common Path

Motor Neurons innervating extrafusal Skeletal Muscle fibers constitute the efferent limb of numerous reflex arcs. All neural influences that cause Muscle contraction ultimately converge upon them and act on the Muscles through them. This is why motor neurons are referred to as the final common path. A large number of Neural Pathways converge on motor neurons; for instance, approximately 10,000 synaptic boutons are located On the surface of an average motor neuron and its dendrites. At least five inputs reach the motor neuron from the same spinal segment in which it is located. In addition, there are excitatory and inhibitory influences that typically pass through interneurons, as well as inputs from other Spinal Cord segments and from the Brain via descending pathways. All these pathways converge on the final common paths and determine their activity.

Central Excitatory and Inhibitory States

The spread of ascending and descending subthreshold influences through the spinal cord, originating from excitatory stimulation, was described above. Direct and presynaptic inhibitory effects are also widespread; they are predominantly transient. However, prolonged changes in excitability also occur in the spinal cord, likely resulting from The activity of reverberating circuits or the prolonged action of synaptic Transmitters. The terms central excitatory state and central inhibitory state are used to describe prolonged states in which excitatory influences outweigh inhibitory ones, and vice versa. When the central excitatory state becomes pronounced, excitatory impulses irradiate not only to many somatic Regions of the spinal cord but also to autonomic regions. In chronic paraplegic patients, for example, a mild noxious stimulus can trigger, In addition to a prolonged extensor withdrawal response in all four limbs, urination, defecation, sweating, and fluctuations in Blood pressure (mass reflex).

Habituation and Sensitization of Reflex Responses

The fact that reflex responses are stereotyped does not mean they cannot be modified by experience. Examples include habituation and sensitization, which are discussed in relation to synaptic function in Chapter 4, and concerning Learning and Memory processes in Chapter 16.