Sensory impulses travel on three major pathways depending on the type of sensation and typically involve three neuron relays. Information about these paths is of crucial importance when making a neurological diagnosis.
axons, which are responsible for carrying messages of heat, cold, and pain, make connections with the cells of secondary neurons in the spinal cord. Temperature and pain fibers pass immediately to the opposite side of the spinal cord and travel upward to the thalamus.
sensations of touch, light pressure, and limb localization travel for some distance before entering the gray column of the spinal cord, completing the connection.
stimuli from muscles, joints, and bones (including the sense of position in space and vibration) travel uncrossed to the brain stem via the axon of the primary neuron.
In the medulla, connections are made with secondary neuron cells whose axons then cross to the opposite side and travel to the thalamus.
Cutting a sensory nerve will cause the total loss of sensation in the area of the body it serves. severing the spinal cord will therefore permanently anesthetize the area below the injury.
Assessment of the health of this system involves testing (with eyes closed) tactile sensation, superficial pain (such as a pinprick), vibration, and pro-prioception (subjective sense of joint position). Tactile sensation is tested by lightly touching a cotton ball to the same areas on each side of the body; the sensitivity of different areas of the body is compared.
Because pain and temperature sensations are transmitted together, it is not usually necessary to test separately for temperature sensation. A patient's sensitivity to superficial pain can be evaluated by asking the person to differentiate between the sharp and blunt ends of a broken wooden cotton swab stick or tongue depressor applied with equal intensity to both sides of the body. A safety pin should not be used because it can break the skin.
vibration and proprioception are transmitted together, and the strength of these senses are often lost together. A person's sense of vibration is evaluated by having the patient report when a low-fre quency tuning fork stops vibrating when placed against a bony prominence of the body. side-to-side comparisons are made.
A person's ability to sense body position in space can be tested by having the patient close both eyes and describe the direction in which the toes have been moved.
After all these peripheral sensations have been tested, it is also important to determine if the brain is correctly integrating these sensations. This can be tested by evaluating whether a patient can perceive how many objects have been touched when touching two sharp objects at the same time. similarly, a person should be able to tell when he or she is touched in two places on the body at the same time (with eyes closed).
sensory area Part of the cerebral cortex that receives tactile sensory information from the skin, including pressure receptors, thermo-receptors, and so on.
sensory cortex A part of the cerebral cortex parallel to the motor cortex that is responsible for awareness of body sensations such as hot or cold.
sensory deprivation The removal of a person's normal sights, sounds, and physical feelings. studies have shown that sensory deprivation can produce a variety of mental changes and a slowing of brain activity. volunteers who lay immobile in a darkened environment, wearing masks and gloves in a sound-proof room, reported feelings of unreality, concentration problems, and hallucinations.
sensory integration The process of absorbing sensory information, organizing this information in the central nervous system, and using the information to function smoothly in daily life. sensory experiences include touch, movement, body awareness, sight, sound, and the pull of gravity; as the brain organizes and interprets this information, it provides a crucial foundation for later, more complex learning and behavior. This critical function of the brain is responsible for producing a composite picture of a person's existence, so that
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