The Nervous System

The brain contains near one hundred billion cells. Each neuron has one thousand synapses each. A neuron is around 10-100µm wide. The cell body of a neuron is surrounded by dendrites and has an elongated myelinated axon leading to the axon terminals. There are three types of nerve cell the multipolar neurone we have just described, the bipolar neuron where the dendrites are also on an extended vestige and the monopolar neuron both the cell body and axon and dendrite vestige are myelinated. Glial cells help to support, nourish and modulate neurons in the brain. Astrocytes are one type of glial cell. Astrocytes serve as a medium between the neurons and the capillary’s.

Resting potential is an uneven distribution of ions across the neural membrane. The difference in electrical potential causes electro-activity across the neuron. Information is conducted much faster along myelinated axons. Sodium-potassium pumps fire at intervals across the axon and serve to propel impulses to the axon terminals. Neural information travels from dendrite to the axon terminal. The surface of a neuron is rough because it is covered with boutons. Where an axon terminal meets and exchanges neurotransmission via synaptic vesicles to a dendrite is instrumental in information processing, this is known as a synapse.

Neural tissue helps organisms interact with their environment. Afferent sensory neurons carry information into the brain. Information may be electromagnetic e.g. visual by the optical nerve and the retina is part of the brain. The right hemisphere of the brain controls the left side of the body and visual field and vice versa.  Senses may be mechanical e.g. touch and hearing and chemical e.g. taste and smell. Afferent neurons act as thermo-receptors throughout the body. Nociception or pain is processed sensory cortex. Efferent motor neurons tell the body how to respond. Vestibular information, balance and motion are coordinated in the inner ear. Proprioceptive sensory information is the relative position of bodily parts.

At the age of one month the embryonic brain has specialises into three regions the forebrain, the midbrain and the hindbrain. In an adult human, the hindbrain develops into medulla, the pons and the cerebellum. The midbrain associates with the hindbrain to form the brainstem. The brain stem is responsible for automatic survival functions e.g. homeostasis. The medulla drives heart beat and breathing. The cerebellum orchestrates coordination and balance. The embryonic forebrain matures into the diencephalon and the cerebrum. The diencephalon consists of the thalamus, the hypothalamus and the epithalamiums. The thalamus relays messages and may be thought of the brains switch board for the cerebrum. The hypothalamus regulates the pituitary gland (therefore serves to regulate the sleep-wake cycle), hunger, thirst, and pleasure.

The cerebrum constitutes the cerebral cortex, white matter and basal nuclei; the cerebellum is made up of many lobes. The frontal lobe functions for decision making and speech. The partial lobe processes sensory information. The temporal lobe processes audio information and the occipital lobe, positioned at the posterior of the cerebrum, process visual information. The right and left hemispheres of the cerebrum are specialised. The left specialises in symbolic thinking, detailed learning and lateral thinking. The right side deals with spatial perception, context and metaphor.