SS2 Third Term Biology Lesson Note – The Nervous System II

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WEEK 10

TOPIC: THE NERVOUS SYSTEM II

CONTENT:

1. The Peripheral nervous system
2. Structure and functions of a neurone
3. Reflex and voluntary actions

SUB-TOPIC 1. THE PERIPHERAL NERVOUS SYSTEM (PNS)

The peripheral nervous system links the CNS with the body’s receptors and effectors in mammals.  When receptors pick up impulses of change in the environment, messages are sent to the CNS which integrates the information and sends appropriate messages to the effectors accordingly.

The peripheral nerves are of two types;

1. the spinal nerves connected to the spinal cord and
2. the cranial nerves, connected to the brain.

The spinal nerves serve the receptors and effectors in the other body parts.

The cranial nerves are associated chiefly with the receptors and effectors in the head, while

The PNS consists of:

• The Somatic nervous system (SNS) and
• The Autonomic nervous system (ANS).

1. Somatic Nervous System

The nerves of the SNS principally serve the parts of the body which take part in responses to external stimuli (e.g. sense organs, limb muscles and glands) and voluntary activities. Nerve fibres without synapses extend from the brain through the spinal cord to the skeletal muscles. The motor neurones stimulate the effectors. The SNS also controls the emptying of the bladder and the opening of the anal sphincters.

1. Autonomic Nervous System

The ANS is concerned with control of the bodies involuntary activities e.g. heartbeat, movements of the gut and secretion of sweat.

The ANS consists of two parts;

1. The Sympathetic and
2. The Parasympathetic systems.

Both contain nerve fibres serving structures over which the body has little or no voluntary control. In both cases nerve fibres from the brain or spinal cord pass into the organs concerned. Along the course of each pathway there is a complex set of synapses forming a ganglion.

1. In the sympathetic system, the ganglia lie alongside the vertebrae close to the spinal cord.
2. In the parasympathetic system, the ganglia are embedded in the wall of the effector itself. The effects produced by the two systems generally oppose one another (antagonistic).

Thus, if the sympathetic system causes a certain muscle to contract, the parasympathetic system relaxes it.

The following is a diagrammatic representation of the autonomic nervous system showing its connections with the central nervous system and its effects on some internal organs.

The Neurone

The neurone is the basic structural unit of the nervous system.  It consists of a cell body and protoplasmic processes called nerve fibres which are tied up in bundles called nerves. It is specialized for transmitting electric impulses. Mature neurones have lost their ability to regenerate.

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