the sensory neuron synapse with motor neuron in a monosynaptic reflex.
which type of neuron does the sensory neuron synapse with in a monosynaptic reflex?
motor neuron
the simplest reflex is the "monosynaptic reflex" or short-loop reflex, such as the monosynaptic stretch response. in this example, ia afferent neurons are activated by muscle spindles when they deform due to the stretching of the muscle. in the spinal cord, these afferent neurons synapse directly onto alpha motor neurons that regulate the contraction of the same muscle. thus, any stretching of a muscle automatically signals a reflexive contraction of that muscle, without any central control. as the name and the description implies, monosynaptic reflexes depend on a single synaptic connection between an afferent sensory neuron and efferent motor neuron. in general the actions of monosynaptic reflexes are fixed and cannot be controlled or influenced by intention or instruction. however, there is some evidence to suggest that the gain or magnitude of these reflexes can be adjusted by context and experience.
motor control
reflexes can be very simple, as in the monosynaptic reflex, which only contains one synapse, or more complicated, as in the polysynaptic reflex, which involves more than one synapse. the knee jerk reflex is a common example of a monosynaptic reflex when one is looking at the quadriceps motor response of kicking your leg out. it can also be used as an example of a polysynaptic reflex when looking at the involvement of inhibitory interneurons to relax the hamstrings. the complexity of the reflex can be estimated by examining the time delay, or latency, between the electrical stimulation of the sensory neuron and the corresponding motor response, as measured by emg (electromyography). most reflexes can be categorized in one of three groups depending on the latency of emg response. the short-latency reflex (slr) is the fastest (~40-50 ms) and involves a mono-synaptic pathway. the medium-latency reflex (mlr) utilizes interneurons within the spinal cord and is typically ~80-90 ms. the long-latency reflex (llr) is ~120-140 ms, suggesting that it is mediated by additional supraspinal input from the brain.
cutaneous reflex in human locomotion