Touch, Position, and Balance

Tactile, or touch, sensation is also referred to as somatosensation. The somatosensory system detects bodily sensations such as temperature, pressure, and pain. There are a number of different sensory receptor cells that respond to different types of stimulus energies. Some types of receptors respond to pressure, touch, and stretch, some respond to temperature, and some respond to painful stimuli.

The skin is made up of multiple layers. The epidermis is the top layer, which gives people their skin tone and provides a protective waterproof barrier. The dermis is the next layer down. This thicker layer holds sweat and oil glands, hair follicles, and blood vessels. Beneath the dermis, the hypodermis helps connect the dermis to muscles and bones. Both the epidermis and dermis hold sensory receptors. Receptors close to the surface of the skin respond to low-frequency skin vibrations, light touch, pain, temperature, and skin damage. Deeper receptors respond to stretching, pressure, and high-frequency vibrations. Each receptor is specialized to detect specific sensations. For example, thermoreceptors detect temperature changes and send signals along to spinal cord to the thalamus, which serves as the sensory relay of the brain. From there, touch information travels to the primary somatosensory cortex within the parietal lobes. Two types of sensory neurons carry information about pain sensations to the brain: fast fibers and slow fibers. Fast fibers are myelinated neurons (nerve fibers covered with an insulating sheath) that send a quick signal about acute pain to the brain. This is usually perceived as a sharp pain. This helps a person to stop engaging in the harmful behavior. Slow fibers are unmyelinated neurons that send slower signals to the brain to convey information about continued tissue damage. This is usually perceived as a dull, throbbing pain and serves as a reminder to discontinue use of a specific body part until it is healed.

Because these nerve cells transmit many different sensations, sensations such as pressure or vibration can reduce the sensation of pain. This is called the gate control theory of pain, based on the idea that new input closes the gates to the pain message.

In addition to these bottom-up processes related to pain sensation, top-down processes can also influence pain perception. Brain regions associated with pain perception, such as the amygdala, contribute to emotional responses or activation of schemas (patterns of thought or behavior) that can change the way a person perceives pain. As a result, pain perception varies widely across individuals. Two people experiencing the same painful stimulus may have different perceptions of the intensity or severity of the pain depending on their personal context.

Proprioception is the sense of one's own body position, motion, and strength. Proprioceptors, the sensory receptor cells for proprioception, receive mechanical stimuli about body positions and movements and are found in muscles, tendons, and ligaments. One example of a proprioceptor is the Golgi tendon organ, which detects the amount of stretch in tendons. Coordinated movement of any kind depends critically on the communication of proprioceptors to the spinal cord and brain. For example, picking a bag up off the floor requires contraction of the bicep muscles and relaxation of the tricep muscles in order to bend the elbow. As motor neurons activate the bicep muscles, proprioceptors called muscle spindles relay information about the stretching of the muscle to the spinal cord. Within the spinal cord, two additional signaling pathways are activated. First, motor neurons activating the bicep muscle continue to activate the muscle to cause it to contract. Second, motor neurons activating the tricep muscles are inhibited, leading to relaxation of the triceps. The heavier the bag, the greater the stretch in the biceps, leading to recruitment of additional muscle fibers to succeed in lifting the bag.

The vestibular sense uses feedback from head movements to help maintain balance and spatial orientation. Much like sound waves are converted to vibrations that move fluid in the cochlea to activate cochlear hair cells, changes in head position move fluid within the vestibular canals of the inner ear to activate hair cells. Each of three semicircular canals conveys different information about balance and body position. The posterior canal senses the head tilting left and right. The superior canal senses the head moving forward to backward (e.g., nodding "yes"). The horizontal canal senses the head moving from side to side (e.g., shaking the head "no"). Information from the vestibular system is sent primarily to the cerebellum, a region of the brain that coordinates movement, and the extraocular muscles, which coordinate eye movements. Dizziness, nausea, and vertigo may result from problems with the vestibular system. Additionally, motion sickness may occur when information the brain receives from the visual system and vestibular system do not match.