Senses to Live By

posted: 05/15/12
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In the course of evolution, much of the ancestral brain that was devoted to olfactory stimuli has been transformed in mammals into other brain structures. Ultimately this has led to a substantial increase in brain size. In particular, the mammalian brain is distinguished by having a highly developed neocortex — an 1/8-inch- (3-mm-) thick, intricately folded layer of nerve cells devoted in part to interpreting sensory stimuli and initiating motor activity. This gray matter overlays the paleocortex, the primitive vertebrate brain, which controls vital functions such as mating and respiration.

The size and weight of a mammalian brain is not a true measure of intelligence — whales and elephants clearly have larger brains than man or any of the other primates. The amount of convolutions, however, is linked to intelligence, there being a great many more neuronal connections possible in a heavily convoluted surface area. For example, the neocortex of a chimpanzee is highly convoluted compared with that of a hedgehog, which is quite smooth.

Sensory stimuli are relayed by neurons to the neocortex as speedy electrochemical impulses. The endocrine system, moderated by the pituitary gland at the base of the brain, also sends messages to the sensory areas, but these are slower, longer-lasting and transmitted through the blood via chemicals called hormones.

The size of the areas devoted to motor activity and to receiving sensory stimuli vary from species to species, reflecting their relative importance to the mammal. The areas corresponding to touch (See Smell, Taste and Touch) and motor activity are located side by side in the motor cortex, an area that is expanded in mammals such as primates, which depend on fine motor control. Also highly developed in mammals is the cerebellum, the section of the brain that coordinates motion, balance and posture.

Most mammals have excellent eyesight (See Eyes to See With), with all kinds of information — from shape and color to distance and motion — being simultaneously interpreted in various specialized areas of the visual cortex. Hearing (See Ears for Hearing), another highly developed sense in mammals, is of particular importance to the 20 percent that rely on it instead of vision to find food and defend themselves. Sound is interpreted in the right and left halves of the auditory cortex, located near the ears.

The neurons transmitting information about smell (See Smell, Taste and Touch) synapse first in the olfactory lobe. Then the information is relayed to the neocortex — only in smell is the neocortex the first stop in the neurosensory pathway — before traveling to several brain centers, including the limbic system, a part of the brain that plays a key role in memory. Taste (See Smell, Taste and Touch), an associated sense, is first detected by buds on the tongue and then transmitted to the brainstem. En route, neurons veer off to the limbic system as well.

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