The Neurones Types of neurones

Introduction

NS is made up of millions of nerve cells called neurones

Neurones : transmit nerve impulses to other nerve cells, glands or muscles

Types of neurones

Afferent neurones

Efferent neurones

Interneurones

Types of neurones

Afferent neurones

Carry sensory information from receptor cells to the brain and spinal cord

Efferent neurones

Carry information from the brain or spinal cord to the effectors, that is muscles or gland cells

Interneurones

Transmit nerve impulses between the various parts of the brain and spinal cord

Transmit nerve impulses between the afferent neurones and efferent neurones

Interneurones

The Parts of a typical neurones

Has four distinct parts;

a) the dendrites

b) the cell body

c) the axon

d) the synaptic terminals (synaptic knobs)

Dendrites

Fibres that receive information or signals from other neurones or external environment and conduct them towards the cell body

Dendrites of afferent neurones produce signals in

response to specific stimuli from the environment, such as pressure, light or heat

Cell body

Has nucleus and other organelles which are found in cells

Cell body integrates the signals and coordinates the metabolic activities

Axon

A long, thin fibre extends outward from the cell body

The axon conducts the nerve impulses away from the cell body

The axons are usually bundled together into nerves

Axon

Some axons are insulated by a thick coat of material called a myelin sheath

Myelin sheath : protects and insulates the axons, and helps to speed up the transmission of the nerve impulses

As a nerve impulse travels along an axon, it jumps from one node of Ranvier to the next. This speeds up the transmission of nerve impulses

Synaptic terminals

Located at the far end of each axon

The synaptic terminals are swellings at the branched ends of the axons

The synaptic terminals then transmit these signals to the muscle cells, glands cells or the dendrites of another neurone

Transmission of information along the neurone

Transmission of information along the neurone is through electrical signals known as nerve impulses

An impulse is a wave of positive charges that

travel along the axon to the synaptic terminal

A neurone will not transmit an impulse unless the stimulation is strong enough

Once the magnitude or size of the stimulation reaches a sufficient level, a full-sized impulse is generated to travel the entire length of the axon

The transmission pathway of information

Consists of several steps

The transmission pathway of information from the receptors to the effectors is summarised in the next slide

The Transmission of information across synapses

Synapse : site where two neurones, or a neurone and an effector cell communicate

Beyond the synaptic terminal is a narrow space known as the synaptic cleft that separates the synaptic terminal from the dendrite of a receiving neurone, a muscle cell or a gland cell

The electrical signals carrying a message must be transmitted across the synaptic cleft to an adjacent cell

Space between presynaptic membrane and postsynaptic membrane = synaptic cleft

The transmission of nerve impulses across the synaptic cleft occurs via a chemical process

Chemical process = requires actions of the chemical substances known as neurotransmitters

The transmission of information across a synapse

Involves the conversion of electrical signals into chemical signals in the form of neurotransmittters

The reconversion of the chemical signals into electrical signals at the postsynaptic membrane

The transmission of nerve impulses across synapses is an active process which requires energy

Functions of synapses

Controlling and integrating the nerve impulses transmitted by the stimulated receptors

Facilitating the transmission of nerve impulses in one direction

Neurotransmitters

Examples :

a) acetycholine

b) noradrenaline

c) serotonin

d) dopamine

It is rapidly broken down by enzymes OR

Taken up again by synaptic terminal and recycled

Exercise

a) Identify the type of neurone shown.

b) Name the parts labelled A, B, C, D, E, F and G.

c) State the function of A.

d) The presence of E can speed up the transmission of nerve impulse. Explain how it is able to do so.

Voluntary action and Involuntary action

Introduction

- PNS has two main functions:

a) It transmits signals to the CNS for processing

b) It transmits responses from the CNS to the rest of the body

These responses are either voluntary or involuntary actions

Voluntary action and Involuntary action

The cerebrum is responsible for all voluntary

actions while the medulla oblongata controls involuntary actions

Voluntary actions

Actions that are under conscious control

Examples: walking, talking

Voluntary control of the skeletal muscles is governed by the cerebral cortex of the cerebrum

Information that reaches the cerebral cortex or the conscious level results in a perception of the external environment

Involuntary actions

Actions that involve skeletal muscles allow an immediate action that does not required conscious effort

Example: If a finger is pricked with a sharp pin,

the reaction is to pull the finger away immediately without having to think about it

Involuntary actions

In such circumstances when the responses to stimuli are involuntary, there are called reflexes

Reflexes are rapid, automatic or involuntary

responses to stimuli

The nerve pathway involved in a reflex action is called reflex arc

The Importance of Reflexes

Reflexes : provide means for an immediate

withdrawal from dangerous stimuli

Coordinate rapid automatic responses without the need for instructions from the brain.

Reflexes

Automatic response : reflex involves the brain; example : opening and closing of the pupil of the eye

Other example : knee-jerk reflex/ patellar reflex

This type of reflex involves the simplest neural circuit. Why?

Involuntary actions which involve smooth muscles, cardiac muscles or glands

The autonomic nervous system

a) controls involuntary actions involving the glands, the cardiac muscles of the heart and the smooth muscles of the internal organs such as the intestines

b) connects the medulla oblongata and hypothalamus with the internal organs and regulates the internal body processes that require no conscious effort

Does involuntary action involve the cerebral cortex?

Autonomic nervous system permits vital functions such as heartbeats and blood circulation to continue even during states of unconsciousness such as sleeping or fainting when voluntary actions have stopped

The Autonomic Nervous System

a) The sympathetic division

b) The parasympathetic division

These divisions work together, usually with one activating and the other inhibiting the actions of the internal organs

Both systems are connected to the same organs in the body and act in opposition to maintain homeostasis

Sympathetic Division

Prepares the body for stressful situations (Fight or flight)

Increases the pulse rate, heartbeat rate, blood pressure and breathing rate

Slows down the digestive system so that more blood is available to carry oxygen to the vital organs such as the brain, heart and muscles

Parasympathetic Division

Prepares body during ordinary situations or

brings on the response associated with a relaxed state

_________ pulse rate, blood pressure, and breathing rate.

Stimulates the digestive system to continue breaking down food