Anatomy & Physiology I and II

Anatomy & Physiology Study Guide pressure or touch receptors. The flexor reflex , a representative withdrawal reflex, affects the muscles of a limb. When a muscle contracts, opposing muscles must relax to permit the movement. Thus, the contraction of a flexor muscle should trigger a stretch reflex in the extensors that would cause them to contract, opposing the movement. Crossed Extensor Reflexes The stretch, tendon, and withdrawal reflexes involve ipsilateral reflex arcs. The sensory stimulus and the motor response occur on the same side of the body. The crossed extensor reflex has a contralateral reflex arc in which the motor response occurs on the opposite side from the stimulus. The crossed extensor reflex complements the flexor reflex, and the two occur simultaneously. Reflex motor behaviors occur automatically, without instructions from higher centers. However, higher centers can have a profound effect on the performance of a reflex. Processing centers in the brain can facilitate or inhibit reflex motor patterns based in the spinal cord. 14.7 Reinforcement and Inhibition A single EPSP may not depolarize the postsynaptic neuron sufficiently to generate an action potential, but it does make that neuron more sensitive to other excitatory stimuli. Alternatively, an IPSP will make the neuron less responsive to excitatory stimulation through the process of inhibition. By stimulating excitatory or inhibitory interneurons within the brain stem or spinal cord, higher centers can adjust the sensitivity of reflexes by creating EPSPs or IPSPs at the motor neurons involved in the reflex responses. When many of the excitatory synapses are chronically active, the postsynaptic neuron can enter a state of generalized facilitation. This facilitation of reflexes can result in reinforcement, an enhancement of spinal reflexes. Other descending fibers have an inhibitory effect on spinal reflexes. In adults, stroking the sole of the foot produces a curling of the toes called a plantar reflex . Stroking an infant's foot on the side of the sole produces a fanning of the toes known as the Babinski sign . This reflex gives evidence for the ability to walk. As the nervous system continues to mature, the infant will lose this Babinski sign. If the Babinski sign remains after two years old, it often suggests a central nervous system disorder.

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