The cells of the nervous system and neurotransmitters at

synapsesKey Area 3.4

Learning Intention

• Name and describe the components of a neuron

Cells of the Nervous System

• The nervous system consists of a complex network of nerve cells called neurons which receive and transmit electrical signals (nerve impulses) and glial cells which support and maintain the neurons

Video

Structure of a Neuron

node

Cell Body

• The cell body contains the nucleus and most of the cytoplasm. It is the control centre of cell metabolism

Cell body

Dendrites

• Dendrites are nerve fibres which receive nerve impulses and pass them towards a cell body.

Axon

• An axon is a single nerve fibre that carries nerve impulses away from a cell body.

Axon

The direction in which a nerve impulse travels is always:

dendritescell bodyaxon

DENDRITE

CELL BODY

AXON

interneurone

Learning Intention

• Compare the myelin content of adults and infants

Myelin

• Myelin is the fatty tissue that insulates an axon

• The presence of myelin greatly increases the speed at which impulses can be transmitted

Myelination

• Myelination is the process by which myelin develops round axon fibres

• Myelination is not complete at birth and so over the first two years of life many more neurons are myelinated

• As a result responses to stimuli in the first two years of life are not as rapid as there are slower impulses

Diseases of the myelin sheath

• Certain diseases destroy the myelin

sheath causing a loss of co-ordination.

• Examples of these are poliomyelitis &

Multiple Sclerosis.

• The myelin sheath becomes damaged or

destroyed.

• MS: Episodic attacks, and/or Progressive

loss of coordination over time.

• Polio: Asymmetric weakness or paralysis

of body parts, commonly limbs.

The role of Glial Cells• Glial cells do not transmit nerve impulses.

• Glial cells (some types) are responsible for

myelination.

• They lay down tightly packed layers of

plasma membrane around an axon.

• Other glial cells act as a support for

neurons. E.g. Producing chemicals the

neurons need to function.

Myelination Summary

• Axons are surrounded by a myelin sheath which insulates the axon and increases the speed of impulse conduction.

• Myelination continues from birth to adolescence. As a result responses to stimuli in the first two years of life are not as rapid or coordinated as those of an older child or adult.

• Certain diseases destroy the myelin sheath causing a loss of coordination.

Types of Neuron

Types of Neuron

• Sensory neurons, carry impulses into the Central Nervous System (CNS) from sense organs

• Interneurons, found in the CNS where they connect with other neurons

• Motor neurons, carry impulses out from the CNS to effectors such as muscles and glands

Learning Intention

• Describe the structure of a synapse and movement of neurotransmitter

Synapses

• The tiny gap between an axon ending of one neuron and the dendrite of the next neuron in the pathway is called a synapse

• The plasma membranes of the two neurons are very close but are separated by a space called the synaptic cleft

Video

Synapses

• The nerve cell before the synaptic cleft is called the presynaptic neuron

• The nerve cell after the synaptic cleft is called the postsynaptic neuron

• As well as connecting to other each, neurons can also connect with muscle fibers and endocrine glands

synaptic cleft

Neurotransmitter

• Impulses are relayed across synaptic clefts by chemicals called neurotransmitters.

• There are many neurotransmitters passed on at the synapse. Two examples are:

ACETYLCHOLINE

NORADRENALINE

Neurotransmitter

• Neurotransmitters are stored in vesicles at each presynaptic terminal. When a nerve impulse arrives, the vesicles fuse with the membrane and release neurotransmitter into the cleft

• The neurotransmitters then diffuse across the cleft and bind to receptors on the postsynaptic membrane

Neurotransmitter

• The vesicles containing neurotransmitters occur on one side of the synapse only.

• This means nerve impulses travel in one direction only.

Video 2

Video 1

Learning Intentions

• Explain the need for and methods of removal of neurotransmitters

Removal of Neurotransmitter

• Between impulses the transmitter molecules are rapidly removed from the synaptic cleft to prevent continuous stimulation of post-synaptic neurones

• There are 2 types of removal:

Re-uptake

Enzyme degradation

Re-uptake

• Noradrenaline undergoes reuptake by being reabsorbed directly into the presynaptic membrane that secreted it and is stored in a vesicle ready for use

Enzyme Degradation

• Acetylcholine is broken down by an enzyme into non active products which are then reabsorbed by the presynaptic neuron and resynthesised into active neurotransmitter

Removal and Summation Summary

• Synapses can filter out weak stimuli arising from insufficient secretion of neurotransmitters.

• Summation of a series of weak stimuli can trigger enough neurotransmitter to fire an impulse.

• Neurotransmitters must be removed from the synaptic cleft to prevent continuous stimulation of post synaptic neurones.

• Neurotransmitters are removed by enzymes or re-uptake.

Learning Intention

• State the difference between excitatory and inhibitory signals

Excitatory and Inhibitory Signals

• The type of alteration to a postsynaptic membrane that occurs following the binding of a neurotransmitter depends on the type of receptor present

• The signal generated is determined by the receptor and may be either excitatory or inhibitory

Excitatory and Inhibitory Signals

• Acetylcholine released into the cleft between a motor neuron and a skeletal muscle fibre binds to receptors that have an excitatory effect and make the muscle fibres contract eg peristalsis

• Acetylcholine released into a cleft between a motor neuron and a heart muscle fibre instead binds with receptors which have an inhibitory effect. This reduces the rate and strength of contraction of heart muscle

Neurotransmitter Summary

• Neurotransmitters relay messages from nerve to nerves within and outwith the brain.

• Neurons connect with other neurons, muscles fibres and endocrine glands at a synaptic cleft.

• Neurotransmitters are stored in vesicles and released into the cleft on arrival of an impulse. They diffuse across the cleft and bind to receptors on nerve endings.

• The receptor determines whether the signal is excitatory or inhibitory.

Learning Intention

• Describe the filtering of weak stimuli and summation

Weak Stimuli

• A nerve impulse is only transmitted across a synapse if there is release of enough neurotransmitter

• A critical number of neurotransmitter molecules is needed (threshold) to affect a sufficient number of receptors on the postsynaptic membrane

• This means weak stimuli are filtered out because not enough transmitter molecules reach the next neurone

Summation

• A series of weak stimuli can combine to trigger enough neurotransmitter to fire an impulse in the post-synaptic neuron, a process known as summation.

Learning Intentions

• Describe the role of endorphins

• Describe the role of dopamine and the reward pathway

Endorphins

• Endorphins are neurotransmitters that stimulate neurones involved in reducing the intensity of pain

• Increased levels are also connected with euphoric feelings, appetite modulation and release of sex hormones.

Endorphins

• Endorphin production increases in response to:

severe injury

prolonged and continuous exercise

stress

certain foods

Dopamine and Reward Pathway

• The reward pathway involves neurones which secrete or respond to the neurotransmitter dopamine

• Dopamine induces the feeling of pleasure and reinforces particular behaviours

• The neurons of the reward pathway are located in the mid-brain below the cortex, and link to the areas at the base of the cortex and in the frontal areas of the cortex.

• The reward pathway is activated on engagement of beneficial behaviours, egeating when hungry

Dopamine and Reward Pathway

Endorphins Summary

• Endorphins are neurotransmitters that stimulate neurones involved in reducing the intensity of pain.

• Increased levels are also connected with euphoric feelings, appetite modulation and release of sex hormones.

• Endorphin production increases in response to severe injury, prolonged and continuous exercise, stress and certain foods.

Dopamine Summary

• The reward pathway involves neurones which secrete or respond to the neurotransmitter dopamine.

• The reward pathway is activated on engagement of beneficial behaviours, egeating when hungry.

• Dopamine induces the feeling of pleasure and reinforces particular behaviours.

Learning Intentions

• Discuss the causes, symptoms and treatment of neurotransmitter disorders

• Describe the mode of action of agonist, antagonist and inhibitor drugs

Neurotransmitter Disorders

• Many drugs used to treat neurotransmitter related disorders are similar to neurotransmitters.

• Neurotransmitter disorders include

Alzheimer’s disease

Parkinson’s disease

Schizophrenia

Agonists and Antagonists

• Agonists bind to and stimulate receptors mimicking the neurotransmitter and triggering a normal cellular response

• Antagonists bind to specific receptors blocking the action of the neurotransmitter.

Inhibitors

• Other drugs inhibit the enzymes (e.g. cholinesterase) which degrade neurotransmitters (e.g. acetylcholine)

• Or inhibit re-absorption of the neurotransmitter (e.g. noradrenalin)

Neurotransmitter Disorder Summary

• Many drugs used to treat neurotransmitter related disorders are similar to neurotransmitters.

• Agonists bind to and stimulate receptors mimicking the neurotransmitter. Antagonists bind to specific receptors blocking the action of the neurotransmitter.

• Other drugs inhibit the enzymes which degrade neurotransmitters or inhibit re-uptake.

Learning Intention

• Describe the mode of action of recreational drugs

Recreational Drugs

• Many recreational drugs affect neurotransmission in the reward circuit of the brain.

• They cause changes in neurochemistry leading to:

changes in mood

cognition

perception

behaviour

Recreational Drugs

• Recreational drugs interact with neurotransmitters in different ways, they can:

stimulate the release of neurotransmitters

imitate the action (agonists)

block their binding to receptors (antagonists)

inhibit their re-uptake

inhibit their breakdown by enzymes

Recreational Drugs Summary

• Recretionals drugs can also act as agonists or antagonists.

• They affect neurotransmission at synpases in the brain altering an individual’s mood, cognition, perception and behaviour.

• Many recreational drugs affect neurotransmission in the reward pathway of the brain.

• Drug additction is caused by repeated use of drugs that act as antagonists.

• Drug tolerance is caused by repeated use of drugs that act as agonists

Learning Intention

• Explain drug desensitisation and sensitisation

Drug Addiction

• Drug addiction can be defined as a chronic disease that causes the sufferer to compulsively seek out and use the drug regardless of the consequences

Drug Desensitisation

• Repeated use of a drug that acts as an agonistresults in neurotransmitters (egthose that promote dopamine release) being repeatedly stimulated and increased feelings of wellbeing and euphoria

Drug Desensitisation

• The nervous system compenstates for overstimulation of these receptors by reducing their number and those receptors left become less sensitive to the agonist drug

• A larger dose is needed to stimulate the reduced number of less sensitive receptors in order to gain the original effect – this is called tolerance

Tolerance

Drug Sensitisation

• Repeated use of a drug that acts as an antagonist by blocking neuroreceptorsprevents normal neurotransmitter from acting on them.

Drug Sensitisation

• The nervous system compensates for the reduced stimulation of the receptors by increasing their number and the receptors become more sensitive to the antagonist drug

Addiction

• Sensitisation results in other psychological changes, which transform ordinary sensations of ‘wanting’ into addiction

Remember

• Desensitisation is a decrease in the number and sensitivity of receptors as a result of exposure to drugs that are agonists and leads to drug tolerance.

• Sensitisation is an increase in the number and sensitivity of neurotransmitter receptors as a result of exposure to drugs that are antagonists and leads to addiction