18.10
Somatic Spinal Reflexes
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Q1: What is the stretch reflex and how does it work?
The stretch reflex is a quick monosynaptic reflex that contracts a muscle when suddenly stretched. Muscle spindles, sensory receptors in muscle tissue, detect changes in muscle length and directly activate motor neurons that innervate the stretched muscle, causing immediate contraction without interneuron involvement.
Q2: How does the tendon reflex protect muscles and tendons?
The tendon reflex is a polysynaptic reflex that regulates muscle tension and prevents tendon damage. Golgi tendon organs in tendons detect excessive tension and activate inhibitory interneurons to relax the contracted muscle, while also activating excitatory motor neurons to contract antagonistic muscles for balance and stability.
Q3: What is the difference between monosynaptic and polysynaptic reflexes?
Monosynaptic reflexes involve a single synapse directly connecting sensory and motor neurons without interneurons, like the stretch reflex. Polysynaptic reflexes include interneurons between sensory and motor neurons, allowing more complex responses such as the tendon reflex and flexor reflex that require coordinated muscle activation.
Q4: What happens during a flexor or withdrawal reflex?
The flexor reflex is a polysynaptic response that withdraws a threatened body part from painful stimuli. Pain receptors detect the stimulus and trigger motor neurons that contract flexor muscles, pulling the limb away from danger through a multi-neuron reflex arc involving the spinal cord.
Q5: How does the crossed-extensor reflex maintain balance during withdrawal?
The crossed-extensor reflex accompanies the flexor reflex by activating extensor muscles in the opposite limb. This simultaneous contraction of extensors on the contralateral side provides stability and support, allowing the body to maintain balance while withdrawing from a painful stimulus without falling.
Q6: Where are Golgi tendon organs located and what do they detect?
Golgi tendon organs are sensory receptors located in tendons connecting muscles to bones. They detect tension built during muscle contraction and send signals to the spinal cord, triggering reflexive relaxation of the muscle to prevent excessive force and potential tendon damage.
Q7: Why are somatic spinal reflexes considered involuntary responses?
Somatic spinal reflexes are rapid, involuntary muscular responses because they bypass higher brain centers and are processed directly by the spinal cord. This direct pathway allows immediate protective reactions to external stimuli without requiring conscious thought, decision-making, or voluntary control.