The Human Brain : From Neurone to Nervous System

THE STRETCH REFLEX (MONOSYNAPTIC REFLEX, MYOTATIC REFLEX, KNEE JERK, JAW JERK)


  1. Reflexes are automatic responses, such as movements, initiated by stimulation of sensory receptors, which involve synaptic contacts between nerve cells.

  2. They are the basic level of regulation of muscles or glands, and the simplest reflexes are found in the spinal cord.

  3. Spinal Reflexes are automatic movements that involvea stimuus to a specific type of sensory receptor, at leas one synapse on another neurone, and an efferent, motor, pathway that activates an effector organ.

  4. The simplest reflex is the monosynaptic reflex, consisting of a muscle spindle afferent synapsing on a motoneurone that causes the muscle to contract. This is the reflex circuit that operates in the knee jerk; tapping the patellar tendon stretches the muscle spindle, whose message reaches the spinal cord and excites motoneurones innervating the quadriceps muscle.

  5. Reciprocal Innervation is the pathway that inhibits the activity of the hamstring muscles at the same time as the quadriceps contraction, and involves the addition of an inhibitory interneurone that hyperpolarises flexor motoneurones.

  6. Key Words: monosynaptic reflex, knee jerk, muscle spindle, inhibitory interneurone, receiprocal innervation






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