Senses and movement

Most of the fibres that move your right hand start in the left side of your brain and cross over in the medulla.

More than five senses

We usually list five senses: taste, smell, touch, sight and hearing. Anatomists count more. The general senses include temperature, pain, proprioception (where your body parts are) and kinesthesia (how they are moving), plus visceral senses from the organs. It helps to sort receptors by the kind of stimulus they turn into a nerve signal. Chemoreceptors respond to chemicals, which covers taste and smell. Mechanoreceptors respond to physical change such as pressure, vibration and stretch, and they also serve hearing and balance. Thermoreceptors respond to heat or cold, nociceptors respond to signs of tissue damage and give rise to pain, and photoreceptors in the retina respond to light.

Taste is a good example of how science keeps updating the list. Sweet, salty, sour and bitter were long known; umami, a Japanese word usually translated as savory, was recognised as a fifth basic taste in the mid-1980s.

From skin to cortex

Follow a touch on your right hand. A mechanoreceptor in the skin changes its membrane voltage, and the sensory neuron fires. The signal enters the spinal cord through the dorsal root and climbs toward the brain. For fine touch, the pathway crosses the midline in the medulla, so the signal ends up on the left side. In the thalamus the message is passed to a third neuron, which carries it to the primary somatosensory cortex in the postcentral gyrus of the parietal lobe. That strip of cortex holds a map of the body surface, with each region of skin represented in its own patch. Vision and hearing follow their own routes, through the thalamus to the occipital and temporal lobes. Smell is the exception that skips the thalamus.

From decision to muscle

Now pick up a cup. Planning the movement involves the premotor area of the frontal lobe and the basal nuclei deep in the cerebrum. The command itself comes from the primary motor cortex in the precentral gyrus, just in front of the central sulcus. Like the sensory strip it holds a map of the body. Large neurons there send their axons down the corticospinal tract. In the medulla these fibres form two ridges called the pyramids, and at the bottom of the medulla most of them cross to the other side, at the pyramidal decussation. They then synapse on motor neurons in the anterior horn of the spinal cord, and those motor neurons make the skeletal muscle contract. The motor neuron for your big toe, for example, sits in the sacral part of the spinal cord.

Because both sensory and motor pathways cross, each side of the brain mainly works with the opposite side of the body. NINDS gives an example: a stroke in the right hemisphere can leave the left arm and leg paralysed. Movement is also checked while it happens. The cerebellum compares what the motor cortex asked for with what the muscles and joints report, and sends corrections. When you reach for a cup in the dark and still find the handle, the cerebellum is part of the reason.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.