Asking grade-school children to name the five senses usually results in the rapid recall of the first four: smell, taste, sight, and hearing. The last one – touch – takes a second or two for many to remember, but fingers are wiggled and palms touched, and the sense of touch, last but not least, is named.
Certainly not last in importance to scientists, “touch” encompasses the widest distribution of sensory pathways throughout the body, including the senses of pain, temperature, vibration, and body position. An easy way to remember this concept is to simply replace the word “touch” with “somatosensation.”
For neuropsychologists and neuroscientists, understanding the somatosensory system and how it reacts and responds to damage offers them valuable information on the brain’s recuperative abilities. The ability of the brain to recuperate or recover from damage is called brain “plasticity.” (for more info, see Neuroplasticity)
Scientists devote significant time and research on brain plasticity because of its relevance to many of today’s neurological diseases and disorders.
The Nervous System
The central nervous system consists of the brain and spinal cord. This system is the control center for the body’s entire sensory functioning system. The spinal cord connects to the brain at the base of the brain via the brain stem. The brain and spinal cord are both protected by a surrounding bony covering – the skull for the brain, and the vertebral column for the spinal cord.
The peripheral nervous system is a nerve network radiating from the brain and spinal cord to parts of the head and body. Peripheral areas of the body include the skin surface and skeletal muscles.
The autonomic nervous system consists of nerves inside the brain and spinal cord, and nerves located on both sides of the spinal cord, and nerves reaching into the internal organs, smooth muscles, and glands. This nerve system regulates “involuntary activity.”
How the Somatosensory System Works
Sensory information, such as heat or cold or pain, travels along sensory pathways to the central nervous system. The sensory data penetrates through sensory receptors, specialized neurons or cells found in the skin and other tissues. These neurons detect internal or external changes in the environment, and convert the energy of the sensations to electrical signals that spread throughout the nervous system.
Some sensations, such as reflexes, stop at the spinal cord where a synapse transmits the information to motor neurons, and the appropriate muscles are automatically activated or enervated. But most sensory information goes on to the brain, which perceives or processes the information.
Receptors for the somatosensory system are modified nerve endings of sensory neurons that have axons (see sidebar called “Nerves”) that run from the point of reception straight to the spinal cord. Receptors can also be bare nerve endings, or a nerve ending enclosed in another structure, such as a hair follicle. Sensory receptors are specialized, meaning that certain receptors pick up on certain stimuli or data.
The brain recognizes the exact type of sensor activated because each sensor travels a designated “path” to carry its specific sensory information to the right area of the brain.
Types of Sensory Receptors or Cells
Mechanoreceptors – Sensory receptors that pick up changes in pressure or movement.
Nociceptors – Sensory receptors that respond to pain stimuli.
Thermoceptors – Sensory receptors that respond to changes in temperature.
Scientists seem to talk in their own language, using words like somatosensation, plasticity or axon.
Another term that sounds foreign to many ears is “transduction,” but its meaning is quite simple, and important in describing how the sensory process works.
The process of sensory receptors converting external stimuli like heat or pressure into sensory impulses within the body is called transduction. Once transduction occurs, bursts of nerve impulses are sent to the central nervous system, entering the spinal cord, and then sent to the brain through one of several different sensory pathways:
- Through a pathway called the dorsal column medial lemniscal system, or simply called the dorsal column tract located at the back of the spinal cord. Touch, vibration, and pressure impulses travel through this pathway.
- Through a region of the spinal cord called the substantia gelatinosa, which then fires the neurons up to the brain in one of three different pathways known collectively as the spinothalmic tracts. These tracts carry impulses of pain and temperature.
As neurons travel to the central nervous system, some are myelinated – yet another fancy name for something that can be thought of as “insulation.” This wrapping around the axons of some neurons makes nerve impulses travel faster than those neurons that are not myelinated. For example, the dorsal column tract pathway contains myelinated neurons, and so is often called a “neural pathway.”
Because the myelination or insulation is white, neural pathways are also collectively known as “white matter.”
The body contains an intricate network of nerves. Specialized cells called neurons form nerves or bundles of threadlike fiber. Nerves differ in size and reach every body part, including the body’s organs. Thick nerves radiate from the brain and spinal cord, while thinner nerves spread to the body’s tissues.
Each neuron carries an electrical signal, which it passes along to other neurons, creating an electrical messaging system throughout the body. A neuron passes an electrical signal along to other neurons by releasing chemicals called neurotransmitters. Gaps between the neurons are called synapses. A fiber extending from the neuron called an axon carries the transmitters to the nerve ending, where the neurotransmitters cross the synapse to dendrites, or short web-like projections of the receiving neuron.
Every second, the senses send the brain large amounts of information, and the brain reciprocates by sending out millions of signals to all areas of the body. Nerve fibers carry up to 300 nerve signals per second; the fastest nerve signals move at around 265 miles her hour, traveling from toe to brain in less than 1/100 of a second.
The Somatosensory Cortex
So what happens when the sensory inputs travel via sensory and neural pathways to the brain? Amazingly, one area of the brain that extends from approximately one ear to the other ear across the cortex in a strip has a complete map of the entire body. Called the somatosensory strip or S1, this strip of cortex is sectioned into areas representing every part of the body, from the eyes, ears, nose, teeth, and jaws, to the arms, elbows, hands, each finger, down to each toe.
The parts of the body that have the highest number of receptor cells, such as the hands and fingers, and the lips, have proportionately more real estate on this S1 strip than other areas that are less sensitive, such as the elbows or thighs.
And if one area of the S1 gets damaged, such as the area for the shoulder, then reduced sensitivity will result in that area.
Scientists now also know that other areas of the brain are involved with the sense of touch, or somatosensation. But these areas are secondary to the S1, which receives the primary inputs and then sends outputs or signals to other areas of the brain.
What is Phantom Limb Pain?
During the Civil War, a veteran who had his legs amputated asked someone to rub his leg. He claimed he had a cramp in the leg, and massaging it would help. A physician hearing the veteran ask for this leg massage coined the term “phantom limb” pain.
Scientists have a couple of theories regarding this strange phenomena. One is that nerves remain in the stump, sending signals to the brain that still interprets the signals as applying to the entire leg.
Another theory states that areas of the brain’s cortex known as S1 or the somatosensation strip change or reorganize so that adjacent areas take over and start to produce sensations in areas of the body that no longer exist.
For example, studies were done in 1980 on primates that removed digits from the monkeys while they were infants. After they matured, the monkeys S1 cortex areas were examined, and researchers found that the cortical areas that would have received sensory inputs from the amputated digits had been taken over or absorbed into the areas reserved for the other digits. And the cortical areas for these adjacent digits were in fact larger than they were in monkeys without the missing digits.