A physician may evaluate your nerves by testing how well you sense touch, pain or position when a limb is manipulated. This information can tell him that a functional connection exists. In some cases, he may conduct a nerve conduction velocity test to evaluate how well the nerve conducts an impulse. In this test, two small electrodes are placed a fixed distance apart from each other on the surface of the skin above a nerve. One electrode electrically stimulates the underlying nerve while the other records the corresponding electrical activity in the nerve. The recording shows the time it takes for the nerve to conduct the electrical impulse across the distance. By dividing the distance by time, the physician (or the machine) calculates the conduction velocity. The test is often performed when a conduction block or demyelinating disease (like multiple sclerosis) is suspected.
The nervous system has many types of sensory neurons. Nerve endings on one end of each neuron are encased in a special structure to sense a specific stimulus.
- Chemoreceptors sense chemicals. The olfactory bulb that monitors your sense of smell has chemoreceptors that sense odors (chemicals in the air). Taste buds have chemoreceptors to detect chemicals dissolved in liquids. Chemoreceptors in the brain also monitor the concentration of carbon dioxide in the blood and cerebrospinal fluid to help control your rate of breathing.
- Mechanoreceptors sense touch, pressure and distortion (stretch). Stretch receptors in your muscle tendons are the first link in the knee-jerk reflex.
- Photoreceptors, which sense light, are found in the retinas of your eyes.
- Thermoreceptors are free nerve endings that sense temperature, but we're not sure exactly how they do this. Changes in temperature could affect the movements of ions across the cell membrane and influence action potentials in that way.
- Nociceptors are free nerve endings that sense pain. They respond to a variety of stimuli (heat, pressure, chemicals) and sense tissue damage.
- Auditory receptors in the inner ear sense vibrations from sound waves.
Typically, a stimulus causes ionic changes in the receptor neuron's dendrites, which lead to the formation of action potentials in the receptor neurons. These action potentials travel the sensory neuron, which connects to a motor neuron (and possibly an ascending neuron) in the spinal cord. The action potential causes neurotransmitter release within the presynaptic cell. The neurotransmitter binds to the postsynaptic cell and elicits an action potential there. The action potential will travel the length of the postsynaptic cell to another synapse on the effector cell (like a muscle cell, skin, blood vessel, gland), where its neurotransmitter will cause a response in the effector cell (like a muscle contraction). Alternatively, the postsynaptic cell may be another neuron that transmits the signal to another neuron in the brain or spinal cord.
What happens when nerves are damaged or diseased? We'll find out on the next page.