The brain contains a lot of nerve cells or neurons. The points of communication between neurons are called synapses. The brain and the spinal cord make up the central nervous system. Extensions from the central nervous system, called nerves, make up the peripheral nervous system. The difference between a neuron and a nerve is that a nerve is a bundle of neurons within the peripheral nervous system.

There are three types of neurons:

1. Sensory neurons
   These neurons carry information from sensory organs into the central nervous system.
2. Motor neurons
   These neurons carry messages out from the central nervous system to operate muscles and glands.
3. Interneurons
   These neurons exist entirely within the central nervous system and carry messages from one set of neurons to another. They collect, organize and integrate messages from various sources.

Neurons consist of the following things:

• Cell body
  This is the widest part of the neuron. It contains the cell nucleus.
• Dendrites
  These are thin, tube-like extensions that branch extensively and function to receive input for the neuron. In interneurons and motor neurons, the dendrites extend directly from the cell body. In sensory neurons, dendrites extend from one end of the axon rather than directly from the cell body.
• Axon
  This is another thin, tube-like extension from the cell body. Its function is to carry messages to other neurons or, in the case of motor neurons, to muscle cells. Each branch of an axon with a small swelling is called an axon terminal. Axon terminals are designed to release chemical transmitter molecules onto other neurons (or onto muscle cells or glandular cells in the case of motor neurons). The axons of some neurons are surrounded by a casing called a myelin sheath. This is a fatty substance produced by supportive brain cells called glial cells.

Neurons exert their influence on other neurons and muscle cells by firing off all-or-none impulses called action potentials. In motor neurons and interneurons, action potentials are triggered at the junction between the cell body and the axon. In sensory neurons, they are triggered at the dendritic end of the axon. Each action potential produced by a given neuron is the same strength as any other action potential produced neuron and the action potential retains its full strength down the axon.

A cell membrane encloses each neuron. It is porous skin that permits certain chemicals to flow into and out of the cell while blocking others. Among the various chemicals dissolved in the intracellular and extracellular fluids are some that have electrical charges. These include soluble protein molecules (A-), which have negative charges and exist only in the intracellular fluid, and sodium ions (Na+) and chloride ions (Cl-) which are more concentrated in extracellular than intercellular fluid.

The charge (-70mv relative to the outside) across the membrane is called the resting potential. The action potential is a wave of change in the electrical charge across the membrane and it moves rapidly from one end of the axon to the other. When the action potential is ‘activated’ thousands of tiny channels that permits sodium ions to pass through open up. As a result, enough sodium moves inward to cause the electrical charge across the membrane to reverse itself and become momentarily positive relative to the outside. This sudden shift constitutes the depolarization phase of the action potential. As soon as depolarization occurs, the channels that permitted sodium to pass through close, but channels that permit potassium to pass through remain open. The electrical charge is pushed back to the resting potential and even for a short while below that. This is called the repolarization phase of the action potential. To maintain the original balance of ions across the membrane, each portion of the membrane contains a chemical mechanism, referred to as the sodium-potassium pump.

The speed at which an action potential moves down an axon is affected by two things:

1. The axon’s diameter
   The larger the diameter, the less resistance there is, thus the faster the action potential moves down an axon.
2. Presence of a myelin sheath
   If there is a myelin sheath present, the action potential moves faster down the axon.

Neurons generate action potentials are rates that are influenced by all the information that is sent to them from other neurons. The junction between each axon terminal and the cell body or dendrite of the receiving neuron is referred to as a synapse. When an action potential reaches an axon terminal, it causes the terminal to release packets of a chemical substance, called a neurotransmitter. Having too little or too many neurotransmitters can cause psychological disorders. A very narrow gap, the synaptic cleft separates the axon terminal from the membrane of the cell that it influences. The membrane of the axon terminal that abuts the cleft is the presynaptic membrane and that of the cell on the other side of the cleft in the postsynaptic membrane. Within the axon terminal are hundreds of tiny globe-like vesicles.

At an excitatory synapse, the transmitter opens sodium channels in the postsynaptic membrane. The movement of the positively charged sodium ions into the cell causes a slight depolarization of the receiving neuron which tends to increase the rate of action potentials triggered in that neuron. At an inhibitory synapse, the transmitter opens either chloride channels or potassium channels. This can cause hyperpolarization.

New-born infants have more neurons in their brains than adults do. The process of creating new neurons is referred to as neurogenesis. After a while neurons enter the last stage of their development: differentiation. During this time, neurons grow in size and increase their numbers of dendrites and axons terminals as well as the number of synapses they form. Neurons and synapses die during the development of the brain. This is called selective cell death, or apoptosis. The brain first overproduces neurons and synapses, but then just as a sculptor chisels them away.

Mirror neurons reflect an individual being able to recognize when another is doing something that the self can do. Human’s mirror neurons seem to code for movement forming an action and not only for the action itself. Mirror neurons are important for imitation.

Neurons in the central nervous system are organized into nuclei and tracts. A nucleus is a cluster of cell bodies in the central nervous system and a tract is a bundle of axons that course together from one nucleus to another.

There are three categories of identifying the functions of specific brain areas:

1. Observing behavioural deficits that occur when a part of the brain is destroyed or temporarily inactivated
2. Observing behavioural effects of artificially stimulating specific parts of the brain
3. Recording changes in neural activity that occur in specific parts of the brain when an individual is engaged in a particular mental or behavioural task

Lesions in one area of the brain can also lead to changes in 0ther brain areas. Lesions are areas of damage. There are several ways of recording brain activity in humans:

There are also several ways of studying the effects of certain areas in the brain using non-human animals:

• Deliberately placing brain lesions (using surgery to destroy certain neurons)
• Stimulating specific areas of the brain (using surgery to stimulate certain neurons)
• Electrical recording of single neurons

The nervous system contains two distinct, but interacting hierarchies:

• Sensory-perceptual hierarchy
  This is involved in the data processing. It perceives sensory data about a person’s internal and external environment and it analyses those data to make decisions. The flow is from the bottom (sensory receptors) to top (perceptual centres in the brain).
• Motor-control hierarchy
  This is involved in movement. The flow is from top to bottom (brain to muscles).

The peripheral nervous system consists of the entire set of nerves, which connect the central nervous system to the body’s sensory organs, muscles and glands. Nerves are divided into two classes:

• Cranial nerves
  Nerves that project directly from the brain (12 pairs)
• Spinal nerves
  Nerves that project from the spinal cord (31 pairs)

Sensory neurons are activated at their dendritic ends by the effects of sensory stimuli (e.g: light on the eye, chemical son the tongue). They send their action potentials into the central nervous system. Somatosensation is all the sensory input that comes from the body, with the exception of specialized sensory organs of the head. The nervous system can control behaviour through motor neurons.

Motor neurons act on two broad classes of structures:

1. Skeletal muscles
   These are the muscles that are attached to bones and produce externally observable movement of the body when contracted.
2. Visceral muscles and glands
   Visceral muscles are muscles that are not connected to bones. They form walls of structures such as the heart. Glands are structures that produce secretions, such as sweat.

The peripheral nervous system consists of a skeletal portion and an autonomic portion. The skeletal muscles make up the skeletal portion and the visceral muscles and glands make up the autonomic portion. Most visceral muscles and glands receive two sets of neurons, which produce opposite effects and come from two anatomically distinct divisions of the autonomic system: sympathetic and parasympathetic. The sympathetic division responds especially to stressful stimulation helps prepare the body for fight or flight. The parasympathetic division serves regenerative, growth-promoting and energy-conserving functions.

The spinal cord has three categories of functions:

1. The spinal cord contains pathways to and from the brain
   The ascending tracts carry somatosensory information and the descending tracts carry motor control commands.
2. The spinal cord organizes simple reflex
   Simple reflexes, such as the flexion reflex do not require the brain but are operated by the spinal cord
3. The spinal cord contains pattern generators for locomotion
   The spinal cord contains networks of neurons that stimulate one another in a cyclic manner and thereby produce a burst of action potentials that wax and wane in a regular, repeating rhythm. These networks are called pattern generators.

The lower, more primitive parts of the brain are referred to as subcortical structures. The spinal cord and the brainstem are alike. They both contain ascending (sensory) and descending (motor) tracts. The brainstem also organizes some reflexes and certain species-typical behaviour patterns, but the brainstem organizes more complex and sustained reflex, such as the postural reflexes. This is a reflex that helps an animal maintain balance and vital reflexes such as breathing rate and heart rate in response to input signalling the body’s metabolic needs.

The thalamus is a relay station that connects different parts of the brain with one another. It also plays a role in the arousal of the brain as a whole. The cerebral cortex is the outer layer of the major portion of the brain. The entire folded cerebral cortex is divided into left and right hemispheres. Each hemisphere is further divided into four lobes or segments. The lobes are, from back to front, the occipital lobe, the temporal lobe, the parietal lobe and the frontal lobe.

Hormones are chemical messengers that are secreted into the blood. They are carried by the blood to all parts of the body where they act on specific target tissues. Some effects of hormones are long term or irreversible. Hormones can influence the body on an anatomical level and this can influence behaviour. There are also short term effects of hormones, such as the effects of adrenaline.

The pituitary produces hormones that stimulates the production of other hormones in other glands.

The brain consists of two hemispheres. The areas in the left are specialized for language and comparable areas in the right are specialized for nonverbal, visuospatial analysis of information. Any loss of language resulting from brain damage is called aphasia.