Cutaneous.html

Cutaneous

Cutaneous sensation is part of the somatosensory system. Cutaneous receptors have a variety of nerve-fiber endings designed to respond to different types of energy produced in the environment. When these nerve endings are activated, you are able to feel changes in temperature, irritants, or mechanical stimuli that press, touch, or cause pain (also known as nociception). The different types of receptors are distributed throughout the skin; the distribution is not even and results in some areas having greater sensitivity to particular stimuli.

After the cutaneous receptors transduce the stimuli, the sensory information is sent up to the brain via the spinal cord. The nerve fibers, which are called peripheral nerves, travel along two major pathways. One of the pathways, the medial lemniscus pathway, consists of large fibers that carry proprioceptive and touch information. The other pathway, the spinothalamic pathway, consists of smaller fibers that carry temperature and pain information. Both pathways travel up the spinal cord to the brainstem, synapse in the ventrolateral nucleus of the thalamus, and travel to the somatosensory cortex in the parietal lobe of the brain. Note that "soma" means "body"—thus, "somatosensory" means "body sensation."

Once the signals reach the somatosensory cortex in the brain, they synapse in the primary somatosensory-receiving area (S1) and are then sent to the smaller secondary receiving area (S2). You may recall that this is similar to what happens to sensory information in both the visual and auditory systems. The information goes to a primary processing area, and then it projects to a secondary area. As with the other senses we have discussed, there is a topographical organization to the brain's receiving areas. However, the somatosensory receptive areas are not organized exactly the way our body is organized.

Auditory.html

Auditory

The receptors for hearing are deep inside the ear and are called hair cells. The auditory waves go through several steps and are transmitted by several different structures before they reach the hair cells.

Sound waves are funneled through the auditory canal (the outer opening in the ear) to the tympanic membrane, which is also known as the eardrum. When the sound waves enter the auditory canal, they cause the tympanic membrane to vibrate, and it is the tympanic membrane's vibrations that cause the tiniest bones in the body to move. These bones are in the middle ear and are collectively called the ossicles. The individual names of the ossicles are the hammer, the anvil, and the stirrup. When the stirrup starts to vibrate, it makes a small membrane move. This membrane is stretched across the oval window, and the movement of this membrane transfers the auditory impulses to an inner ear structure called the cochlea.

The sound wave energy is transmitted throughout the fluid-filled cochlea, causing a membrane called the basilar membrane to vibrate. When the hair cells are pressed up against the tectorial membrane, the auditory receptors are activated and transduction occurs. The hair cells are part of the auditory receptor, and there are between fifteen thousand and twenty thousand auditory receptors in the human ear. Transduction is the process of an auditory receptor converting a physical stimulus into an electrical impulse. The sound wave that is conveyed through the ear by a series of movements ends up displacing the cilia on top of the hair cells. When the hair cells are displaced, ion channels are opened and this results in a burst of electrical signals.

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