Brain Micro Structure

The brain is composed of nerve cells (or neurons), glial cells, and nutrient blood vessels. Neurons transmit nerve impulses, and glial cells, which account for half of the brain’s weight, form the supporting structure of the brain.

Each multipolar neuron has a soma, or cell body, from which extends a single large process, or axon, along which impulses are transmitted to other neurons when the soma is sufficiently excited by incoming impulses. Axons of functionally similar neurons are frequently grouped as more or less discrete fiber tracts. The somas from which they arise are grouped as nuclei, or ganglia. The soma also has shorter processes, called dendrites, whose principal function is to receive incoming nerve impulses. The area at the dendrites where the terminal branch of an axon comes into contact with another neuron to another across synapses. When the combined interplay of many incoming impulses reaches a certain threshold, the soma depolarizes and sends a nerve action potential down its axon to other nerve cells and their processes.

The term afferent and efferent describe nerve tracts as going toward or away from the cerebral cortex or other higher brain structures. The corticospinal nerve tract from cortex to spinal cord is efferent, with somas of the cells of origin in the cortex, their axons traveling downward to the cord, away from the highest center. The terms sensory and afferent are used synonymously, as are motor and efferent.

The exposed living brain in a soft mass of glistening grayish white tissue which numerous small, bright red arteries and bluish veins are visible. The brain is protected by the skull and three membranes, called meninges. When the brain is “fixed” with chemicals. It becomes firm and can be sliced. Apparently to nerve endings sensitive to pain exist within the brain. Large blood vessels supplying the brain and certain sensory nerve attached to the brain stem are pain sensitive.