The Nervous System:

The Importance of the Nervous System

Through a series of adjustments, all systems of the body are regulated to maintain the internal environment within safe limits.

The Nervous system is an elaborate communication system that contains more than 100 billion nerve cells in the brain alone.

Memory, learning, and language are all functions of the human nervous system.

The Vertebrate Nervous System:

The nervous system has two main divisions:

1. Central Nervous System (CNS)

2. Peripheral Nervous System (PNS)

The Central Nervous System:

The CNS consists of the nerves of the brain and spinal cord.

The CNS acts as a coordinating centre for incoming and outgoing information.

The Peripheral Nervous System:

The PNS consists of nerves that carry information between the organs of the body and the CNS.

The PNS can be divided into two systems:

1. The somatic nervous system

2. The autonomic nervous system

The Somatic Nervous System:

The somatic nervous system controls the skeletal muscle, bones, and skin.

Sensory somatic nerves relay information about the environment to the CNS, while motor somatic nerves initiate an appropriate response.

The Autonomic Nervous System:The autonomic nervous system contains special motor nerves that control the integral organs of the body.

The autonomic nervous system has two divisions:

1. The sympathetic nervous system ("Fight or Flight")

2. The parasympathetic nervous system ("Rest and Digest")

Cells of the Nervous System:

The two different types of cells found in the nervous system are glial cells and neurons.

Glial cells: are nonconducting cells and are important for the structural support and metabolism of the nerve cells.

Neurons: are the functional units of the nervous system.

Neurons are then categorized into sensory neurons, interneurons, and motor neurons.

A) Sensory Neurons:- sense and relay information (or stimuli) from the environment to the CNS for processing.

- They are located in these clusters called Ganglia located outside of the spinal cord.

B) Interneurons:- link neurons within the body

- Found mainly in the brain and spinal cord, interneurons integrate and interpret the sensory information and connect neurons to outgoing motor neurons.

C) Motor Neurons:- Relay information to the muscles, organs, and glands (effectors).

All neurons contain dendrites, cell bodies, and axons.

* The dendrites receive information, either from sensory receptors or from other nerve cells.

- nerve cells contain a cell body with a nucleus

- dendrites conduct nerve impulses TOWARD the cell body

- the axon projects nerve impulses FROM the cell body

- the axon carries the nerve impulse toward other neurons or to effectors.

Many axons are covered with a coat of fatty protein called the myelin sheath, which acts as insulation for the neurons (these axons are said to be myelinated).

The myelin sheath is formed by special glial cells called Schwann cells. This prevents the loss of charged ions from the nerve cell.

The areas between the sections of myelin sheath are known as the nodes of Ranvier. Nerve impulses jump from one node to another, speeding the movement of nerve impulses.

Nerve impulses move faster along myelinated nerve fibres.

All nerve fibres found within the PNS contain an thin membrane called the neurilemma, which surrounds the axon.

The neurilemma promotes the regeneration of damaged axons.

Nerves within the brain that contain myelinated fibres and a neurilemma are called white matter, because the myelinated axons are white in appearance. Other nerve cells within the brain and spinal cord are called grey matter due to the lack of a myelin sheath and neurilemma. These nerves do not regenerate after an injury. Damage to the grey matter is usually permanent.

What happens if you accidentally touch a hot stove?The sensation of heat is detected by temperature receptors in your skin, and the nerve impulse is carried to the spinal cord. The sensory neuron passes the impulse to an interneuron, which relays the impulse to a motor neuron. The motor neuron causes the muscles in your hand to contract and the hand to pull away. All this happens in less than a second, before the information even travels to the brain.

Reflexes:Reflexes are involuntary and often unconscious.

The simplest nerve pathway is the reflex arc. Most reflexes occur without the brain coordination.

Reflex arcs have five essential components:

the receptor, the sensory neuron, the interneuron in the spinal cord, the motor neuron, and the effector.