The Nervous System

The nervous system consists of the:

• Brain.
• Spinal cord.
• Peripheral nerves – connect the various parts of the body to the spinal cord and brain.

 The nervous system

The brain controls the body and senses.

 The sense organs

The functions of the nervous system:

• Initiate and/or regulate the movement of body parts by initiating and/or regulating the contraction of skeletal, cardiac, and smooth muscles.
• Regulate the secretions from glands.
• Gather information about the external environment and the status of the internal environment of the body by using senses and other mechanisms to detect pain, temperature pressure and certain chemicals.
• Maintain an appropriate state of consciousness.
• Stimulate thirst, hunger, fear, rage, and sexual behaviours appropriate for survival.

These functions of the nervous system require the rapid transmission of information from one site within the body to another.

Fibres called nerves pass from the brain and spinal cord to all parts of the body. Messages pass from the various parts of the body along the nerves to the brain. The brain sends a message back telling the different parts of the body what to do.

 

Classification of the Nervous System

The entire nervous system can be divided into the:

1. Central Nervous System (CNS) – brain and spinal cord.
2. Peripheral Nervous System (PNS) – cranial nerves and spinal nerves going to and from somatic (body) structures.
3. Autonomic Nervous System (ANS) – coordinates activity of visceral structures (smooth and cardiac muscle and glands), has elements in both the CNS and PNS and features both sensory and motor components.

The motor component of the ANS is further divided into 2 parts – a sympathetic and parasympathetic division. The sympathetic division prepares the animal to meet stress or danger by producing a combination of physiological changes that increase available fuel molecules, blood flow to muscles, and cardiac output while simultaneously decreasing digestive processes. The parasympathetic division stimulates digestion and storage of fuel molecules and acts to bring the animal to a state of rest. Therefore, the parasympathetic division is the opposite of the sympathetic division and is sometimes referred to as the rest-and-digest response.

 

Structure of Neurons

The nervous system is made up of nerve cells, called neurons, and neuroglia. Neurons are the primary functional cell of the nervous system and neuroglia are found mainly in the central nervous system where they maintain the cellular environment to support the activity of neurons.

Neurons possess the usual features of cells (nucleus, cytoplasm, various organelles, and a cytoplasmic membrane that surrounds the entire neuron), but in keeping their function of communication over long distances, they also have a few specialisations important to their functions. Each neuronal cell body gives rise to one or more nerve processes that are cytoplasmic extensions of the cell:

• Dendrites – transmit electrical signals toward the cell body.
• Axon – conducts electrical signals away from the cell body. Each neuron gives rise to only one, which usually branches.

A neuron receives impulses through its dendrites and passes them over to the next neuron through its axon. The junction between the axon of one neuron with another neuron or target cell is the synapse.

Neurons may be classified according to their form or structure – the number of nerve processes.

 

The morphological classification of neurons.

Neuron	Description
Unipolar Neuron	A unipolar neuron has only one process, called a neurite, which  extends from the cell body.  The neurite then branches to form dendritic and axonal processes.  True unipolar neurons are seen only during development.
Bipolar Neuron	Bipolar neurons have one dendrite and one axon.  These are common in sensory systems.
Pseudounipolar  Neuron	Many sensory neurons have their single dendrite and axon fused to give the appearance of a single process; this configuration is pseudounipolar.
Multipolar Neuron	Multipolar neurons have a few dendrites in addition to their single axon.  Most neurons are multipolar.

Neurons may also be classified according to their function mainly concerning the direction of the impulse they carry.

 

Functional classification of neurons.

Neuron	Function
Sensory (Afferent)  Neurons	Sensory neurons convey impulses from the body (for example, the skin, skeletal and cardiac muscle) to the central nervous system.
Motor (Efferent)  Neuron	Motor neurons transmit impulses from the central nervous system to the effectors (muscles and glands).
Interneurons  (Central) Neurons	Interneurons transmit impulses from one part of the brain or spinal cord to another. These neurons are situated between sensory and motor neurons.

 

The rapid transmission of information from one site in the body to another is possible because neurons have the property of excitability, which allows neurons to develop action potentials and rapidly propagate them along their individual cellular processes (axons). Action potentials are transmitted from one neuron to another or another type of cell at specialised junctions (synapses).