Human Brain

The human brain, containing about 10 billion nerve cells, is notable the evolutionary increase in the size and complexity of the tow cerebral hemisphere of the fore brain, relative to other brain structures. The two hemispheres are connected by a band of fibers called the corpus callosum. The limbic system is associated with memory and emotion the cerebellum controls in voluntary body functions. The thalamus, hypothalamus, and pituitary gland are all parts of the fore brain. The brain stem includes the pons and medulla oblongata and connects the rest of the brain with the signal cord.

Nerve cell or neurons, are the most complex cells in the body. The cell's control this center are of two types, the shorter dendrites an the stalklike axon. Axons of the cells in the spinal cord that convey impulses to the feet may be up to 100 cm long, but in the brain an entire neuron is usually less than 0.1 cm long. Dendrites are the pathways for receiving impulses from other cells, whereas the axon is the pathway for the impulses transmitted by the cell seen here is sheathed in an insulating membrane called myelin. Such cell make up the so-called gray matter of the brain, as opposed to unmyelinated white matter. In sheathed axons, impulses travel from one node of Ranvier to the next (areas where the sheath is interrupted); in unsheathed axons, the impulses flow continuously. The mitochondria are the cell's energy sources. Chemical substances such as the neurotransmitters that carry message to other neurons are manufactured in the rough endoplasmic reticulum and carried along the axon by the smooth endoplasmic reticulum to the nerve ending, where they are package in vesicles for transmission. Such vesicles can contain more than one kind of neurotransmitter.