18.10: Somatic Spinal Reflexes
Somatic spinal reflexes are rapid, involuntary muscular responses to external stimuli that involve the somatic musculature and the spinal cord.
One of the most well-known somatic spinal reflexes is the stretch reflex, which is activated by the sudden stretching of a muscle. This reflex involves the activation of specialized sensory receptors called muscle spindles, which are located in the muscle tissue and detect changes in the length and speed of muscle contractions. When a muscle is suddenly stretched, the muscle spindles activate and send a signal to the spinal cord, triggering a reflexive contraction of the same muscle. This is a monosynaptic reflex, meaning a single synapse between the sensory and motor neurons, without any interneurons in between.
Another important somatic spinal reflex is the tendon reflex, which is triggered by the tension of a muscle tendon. This reflex involves the activation of specialized sensory receptors called Golgi tendon organs, which are located in the tendons connecting muscles to bones. When a muscle tendon becomes tensed during muscle contraction, the Golgi tendon organs activate and send a signal to the spinal cord, triggering a reflexive relaxation of the same muscle. This is a polysynaptic reflex, meaning that the reflex arc comprises interneurons present between the sensory and motor neurons.
In addition to the stretch and tendon reflexes, the somatic spinal reflex system also includes the polysynaptic flexor reflex, which is responsible for withdrawing a limb from a painful or damaging stimulus, and the crossed extensor reflex, which is responsible for maintaining balance and stability during movements involving the arms and legs. The flexor reflex involves activating flexor muscles in response to a painful or damaging stimulus. In contrast, the crossed extensor reflex involves activating extensor muscles on the opposite side of the body to maintain stability during the movement.
