Reflexes
Basic Definitions
- REFLEX ARC: basic unit of integrated reflex activity
- consists of:
- sensory receptor
- afferent neuron
- one or more synapses in the CNS
- efferent neuron
- muscle
- consists of:
- MONOSYNAPTIC REFLEXES: afferent neuron synapses directly on the efferent neuron
- POLYSYNAPTIC REFLEXES: paths branch in a complex way
- synaptic delays mean that activity in the branches with fewer synapses reaches the motor neurons first, followed by activity in the longer pathways
- a single stimulus can cause prolonged "waves" of response
- some branches can turn back on themselves, allowing activity to be terminated through the process of RECURRENT INHIBITION
- a well-known example is that of Renshaw cells, inhibitory interneurons that are activated by axon collaterals of motor neurons - once activated, a Renshaw cell inhibits the motor neuron that stimulated it, as well as other neighbouring motorneurons that innervate agonist muscles
- the stimulus that evokes a reflex (the ADEQUATE STIMULUS) is usually very precise
- e.g., the scratch reflex in a dog requires multiple touch stimuli arranged in a line, as would be produced by an insect crawling across the skin; if the touch stimuli are widely separated, or not in a line, there is no scratching (fleas take advantage of this; by jumping instead of crawling, they don't evoke the reflex!)
- the fact that reflex responses are stereotyped does not mean that they can't be modified
- reflex responses can habituate (decline in amplitude in response to repeated stimuli) or be sensitized (augmented postsynaptic responses after a a habituated stimulus is paired with a noxious stimulus)