The nervous system is an electrochemical communication

1. It receives sensory messages from the external environment.

2. It organizes information and integrates it with already stored information.

3. It uses integrated information to send out messages to muscles and glands, producing organized movement and secretions.

4. It provides the basis for conscious experience.

Neurons information

Neurons information

1. Brain is a single organinternal structure and organization are quite complex.
2. Contains about 10¹¹ neurons,
3. Mature neurons do not have the ability to go under Mitosis
4. The "normal" loss of neurons has been estimated to be about 10³ per hour,  in 75 years (660,000 hours), a loss of only 0.66 percent of the total number of neurons present at birth.
5. People of advanced age
6. less brain substance than their younger counterparts.
7. brain shrinkage with age
8. Environmental factors: accidents, drug use, can increase the rate of either cell shrinkage or cell loss.

Basic structural and functional units of the nervous system. They cannot divide by mitosis. The fully differentiated, post-mitotic cells cannot be replaced during the lifetime of the organism. As neurons die in the course of normal aging, they cannot be replaced. Therefore, the maximum number of neurons exists near the time of birth.  

Neurons respond to physical and chemical stimuli.These cells are able to produce and conduct electrochemical impulses. Electrical impulse move from one cell to another cell by the release chemical regulators.

Cell body (perikaryon):

·         “Nutrition center.”

·         Cell bodies within CNS clustered into nuclei,
and in PNS in  ganglia.

·         Dendrites:

–        Provide receptive area.

–        Transmit electrical impulses to cell body.

·         Axon:

–        Conducts impulses away from cell body.

–        Each neuron usually contains an axon

·         Synapses

–        cell body of another neuron (axo-somatic synapses),

–        contact dendrites (axo-dendritic synapses).

–        one cell's axon and another cell's axon (axo-axonic, or presynaptic, synapses). 

Neurons are Excitable cells. They vary considerably in size and shape, Important biochemical and physiological properties are common.The dendrites are the short processes of the cell body. They conduct the nerve impulse toward the cell body.Axon is the largest process of the cell body. An axon conducts impulses away from the cell body. The majority of axons within the CNS, are only a centimeter or two in length. The dendrites and axons are often referred to as nerve fibers. Within the CNS, most axons make functional connections with other neurons at junctions called synapses.

Functional Classification of Neurons

Based upon direction impulses conducted.

Sensory or afferent:

–        Conduct impulses from sensory receptors into CNS.

Motor or efferent:

–        Conduct impulses out of CNS to effector organs.

Association or interneurons:

–        Located entirely within the CNS.

–        Serve an integrative function.

Functional Classes of Neurons

 Neurons are classified in three ways:

a. Afferent neurons transmit information into the CNS from receptors at their peripheral endings.

b. Efferent neurons transmit information out of the CNS to effector cells.

c. Interneurons lie entirely within the CNS and form circuits with other interneurons or connect afferent and efferent neurons.

Glia or Glial Cells

Other cells in CNS, "glia/ glial cells" comes from "glue,”

One-tenth the size of neurons.

Number of glial cells is ten times greater than neurons in the brain (approximately 10¹¹ neurons and 10¹² glial cells),

Constitute approximately half of the overall mass of the brain.

White matter consist predominantly of glial cells and a small volume of axons.

There are three types of glia: They arise from two different embryological lines.

–        astrocytes,

–        oligodendrocytes, and

–        microglia

Myelin Sheath and Blood-Brain Barrier

Myelin Sheath

Myelin is a substance that forms the myelin sheath associated with nerve cells.

This sheath is a layer of phospholipids that increases the conductivity of the electrical messages that are sent through the cell.

Diseases such as multiple sclerosis are a result in a lack of this myelin sheath, with the resultant effect being that the conductivity of signals is much slower severely decreasing the effectiveness of the nervous system in sufferers.

Myelinated nerve fibers

A Myelinated nerve fiber is one that is surrounded by a myelin sheath.

The myelin sheath is not part of the neuron but is formed by a supporting cell.

Blood-Brain Barrier

Capillaries in brain do not have pores between adjacent endothelial cells.

–        Joined by tight junctions.

Molecules within brain capillaries moved selectively through endothelial cells by:

–        Diffusion.

–        Active transport.

–        Endocytosis.

–        Exocytosis.

The nervous system consists of a large number of neurons that are linked together to form functional conducting pathways.

When two neurons come into close proximity and functional interneuronal communication occurs, the site of such communication is referred to as a synapse.

The space between two cells is known as the synaptic cleft. 

Reflex and Reflex arc

1. Reflex is a rapid, involuntary response to a stimulus
2. A reflex arc is the pathway travelled by the nerve impulses during a reflex..
   

Most reflexes are spinal reflexes with pathways that traverse only the spinal cord. During a spinal reflex, information may be transmitted to the brain, but it is the spinal cord, and not the brain, that is responsible for the integration of sensory information and a response transmitted to motor neurons. Some reflexes are cranial reflexes with pathways through cranial nerves and the brain stem.

A reflex arc involves the following components

1. The receptor is the part of the neuron (usually a dendrite) that detects a stimulus.
2. The sensory neuron transmits the impulse to the spinal cord.
3. The integration center involves one synapse (monosynaptic reflex arc) or two or more synapses (polysynaptic reflex arc) in the gray matter of the spinal cord. In polysynaptic reflex arcs, one or more interneurons in the gray matter constitute the integration center.
4. A motor neuron transmits a nerve impulse from the spinal cord to a peripheral region.
5. An effector is a muscle or gland that receives the impulse form the motor neuron. In somatic reflexes, the effector is skeletal muscle.
6. In autonomic (visceral) reflexes, the effector is smooth or cardiac muscle, or a gland.

The brain can be damaged in a variety of ways, and depending on the areas damaged and the severity of the damage

Genetics 

 A dysfunctional hereditary gene could have been passed on to the offspring which prevented the full development of a healthy brain 

Blow 

A sufficient blow to the head can supercede the skulls defences (particularly at the temple) and can therefore allow structural damage to occur.

Lack of Blood 

Lack of blood to the brain can cause severe problems for the cells associated with the brain. A human can survive for four minutes without oxygen before the brain damage becomes so severe there is no realistic chance of survival. A stroke is an event where there is a blood shortage to the brain, which is caused by a blood clot. 

Tumours 

Cancer has been a major non-infectious disease more recognised over the last decade, and more cases of brain tumours are detected nowadays due to more sophisticated techniques. The continued growth of these cancerous cells puts pressure on the brain, which can cause a blood clot or directly cause brain damage due to the pressure of the tumour pressing against it.

Types of Brain Damage

Aphasia - A type of brain damage affecting communication capabilities in the organism. This can range from the inability to construct a sentence either in voice or on paper, to the inability to recognise speech itself. This sort of damage focuses on the frontal lobe area of the brain.

Visual Neglect - This is where the information collated on one half of the brain is rejected and therefore the sufferer can only operate with one eye, because the part of the brain receiving visual information from the other eye is not functioning properly. In some cases, sufferers may only be able to paint half a painting or eat one half of a plate of food as they are unaware of the information about the other half of the environment. 

Amnesia - Or retrograde amnesia, this sort of damage affects the memory, caused by degeneration / damage in the frontal lobe. Sufferers have memory blanks when relating to past experiences in their life

Agnosia - This unusual sort of brain damage is where sufferers still have the complete ability to see around them (unlike visual neglect), though cannot relate their surroundings in a quantifiable way, i.e. they fail to recognize a familiar surrounding, person or object, due to a malfunction in recalling past events involving the surrounding, person or object.