The monosynaptic stretch reflex is the only spinal reflex we know of that involves only one synapse. All others are polysynaptic. Examples include relatively simple ones, such as limb withdrawal in response to noxious stimulation, and relatively complex ones, such as the ejaculation of semen. Spinal reflexes do not exist in isolation; they are normally controlled by the brain. For example, Chapter 2 described how inhibition from the brain can prevent a person from dropping a hot casserole dish, even though the painful stimuli received by the fingers serve to cause reflexive extension of the fingers. This section will describe some general principles by which polysynaptic spinal reflexes operate. Before I begin the discussion, I should mention that the simple circuit diagrams used here (including the one you just looked at in Figure 8.6) are much too simple. Reflex circuits are typically shown as a single chain of neuron, but in reality most reflexes involve thousands of neurons. Each axon usually synapses on many neurons, and each neuron receives synapses from many different axons.
The monosynaptic stretch reflex is the only spinal reflex we know of that involves only one synapse. All others are polysynaptic. Examples include relatively simple ones, such as limb withdrawal in response to noxious stimulation, and relatively complex ones, such as the ejaculation of semen. Spinal reflexes do not exist in isolation; they are normally controlled by the brain. For example, Chapter 2 described how inhibition from the brain can prevent a person from dropping a hot casserole dish, even though the painful stimuli received by the fingers serve to cause reflexive extension of the fingers. This section will describe some general principles by which polysynaptic spinal reflexes operate. Before I begin the discussion, I should mention that the simple circuit diagrams used here (including the one you just looked at in Figure 8.6) are much too simple. Reflex circuits are typically shown as a single chain of neuron, but in reality most reflexes involve thousands of neurons. Each axon usually synapses on many neurons, and each neuron receives synapses from many different axons.
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