KEY AREA 3: Cells of the Nervous System and Neurotransmitters

Neurobiology & Communication Learning Intentions
KEY AREA 3 – The cells of the nervous system and neurotransmitters at synapses a) Neurons
(i) Structure & function
(ii) Myelin sheath
(iii) Glial cells b) Neurotransmitters at synapses
(i) Chemical transmission at the synapse
(ii) Receptors determining excitatory or inhibitory response
(iii) Removal of neurotransmitters
(iv) Summation c) Function of converging, diverging and reverberating neural pathways & plasticity d) Neurotransmitters, mood and behaviour e) Mode of action of recreational drugs

3a) Neurons – structure & function
Neurons are nerve cells which receive and transmit nervous impulses
There are 3 types of neurons – sensory, inter and motor
All neurons are made up of a cell body, one axon and several dendrites
The cell body of a neuron contains the nucleus and most of the cytoplasm. It is the control centre of the cell ’ s metabolism and contains ribosomes
The axon is a single nerve fibre that carries nerve impulses away from a cell body and into the next neuron
The axons of motor neurons are extremely long
The direction in which a nerve impulse travels is;-
Dendrites  Cell Body  Axon
Axons are surrounded by a myelin sheath which insulates the axon
The gaps in the myelin sheath along an axon are called nodes

3b) Neurons – structure & function

3c) Myelin Sheath
Axons are surrounded by a myelin sheath which insulates the axon
The gaps in the myelin sheath along an axon are called nodes
The presence of the myelin sheath increases the speed at which impulses can be transmitted from node to node along the axon of a neuron
Myelination continues from birth to adolescence
Responses to stimuli in the first two years of life are not as rapid or as coordinated as those of an older child or adult due to less myelination
Axon
Myelin Sheath
Node

3d) Problems with the Myelin Sheath
Certain diseases destroy the myelin sheath e.g. polio, multiple sclerosis, taysachs disease
In Multiple Sclerosis(MS) the myelin sheaths breakdown and muscle control is severely impaired

3e) Glial Cells
Glial cells play many roles: -
1. Physically support neurons
2. Produce the myelin sheath
(myelination)
3. Maintain a homestatic environment around the neurons and remove debris by phagocytosis

 A synapse is the tiny region between the axon end of one neuron and the dendrite end of the next neuron
 The plasma membranes of the two neurons at a synapse are very close to one another and are separated by a narrow space called a synaptic cleft
 The nerve cell before the synaptic cleft is called the pre-synaptic neuron
 The nerve cells after the synaptic cleft is called the post-synaptic neuron
 Neurons connect with other neurons, muscle fibres and endocrine gland cells at a synaptic cleft
 Chemical messages are relayed across synaptic clefts from neuron to neuron both inside and outside of the brain by chemicals called
Neurotransmitters
Neurotransmitters
Receptors
Pre-synaptic neuron
Vesicle with neurotransmitter
Synaptic cleft
Post-synaptic neuron

 Examples of neurotransmitters are Acetylcholine and Norepinephrine (noradrenaline)
 Neurotransmitters are stored in vesicles and released into the cleft on arrival of an impulse.
They then diffuse across the cleft and bind to the receptors on the nerve endings
 Receptors determine whether the signal is excitatory or inhibitory
 Acetylcholine released into the synaptic cleft between a motor neuron and a skeletal muscle fibre binds to receptors that have an excitatory effect on the muscle and make it contract
 Acetylcholine released into the cleft between a motor neuron and a heart muscle fibre binds with receptors that have an inhibitory effect which reduces the rate and strength of the contraction of the cardiac muscle
 Vesicles containing neurotransmitter occur on one side only of a synapse – this ensures that nerve impulses are transmitted in one direction only

 Neurotransmitters must be removed from the synaptic cleft straight after the impulse has been transmitted to prevent continuous stimulation of post-synaptic neurons
 Neurotransmitters can be removed by: -
Enzyme degradation
Re-uptake
Pre-synaptic neuron
 Enzyme degradation involves breaking down the neurotransmitter into non-active products which are reabsorbed by the pre-synaptic neuron and resynthesized into the active neurotransmitter (this requires energy)
 Some neurotransmitters undergo reuptake by being reabsorbed directly by the pre-synaptic membrane that secreted it and is stored in the vesicle ready for use
Neurotransmitters
Receptors
 Synapses can filter out weak stimuli arising from insufficient secretion of neurotransmitters
Vesicle with neurotransmitter
Synaptic cleft
Post-synaptic neuron

• Neurotransmitters (NT’s) pass across a synaptic cleft
• Vesicles have approx 10,000 molecules of NT’s
• Impulse generated if a minimum no. (threshold) of NT’s are released
• Impulses only travel in 1 direction
• NT’s are removed by enzymes once impulse has been generated
• Excess NT’s reabsorbed & resynthesised by pre-synaptic neurone. This requires energy from mitochondria

Synapses can filter out weak stimuli arising from insufficient secretion of neurotransmitters
Summation is when a series of weak stimuli can trigger enough neurotransmitter to fire an impulse

3l) Converging, diverging & reverberating neural pathways
Converging
Impulses from several sources meet at a common point
(concentrating impulses)
Example: Rods & Cones in eye
Diverging
The impulse divides
(so it can be transmitted to several destinations)
Example: Temperature control – muscles, arterioles, sweat glands)
Reverberating
Later neurons possess axons that form synapses with earlier neurons so impulses can be recycled
Example: breathing

• Neurons undergo change throughout a person’s life time
• The ability of brain cells to become altered as a result of new environmental experiences is called plasticity of response
• Plasticity of response is created when new neural pathways to form during:
- early development of the brain
- learning of new skills
- response to brain injury
- suppress reflexes or responses to sensory impulses

Endorphins are neurotransmitters that stimulate neurons involved in reducing the intensity of pain – they act as natural painkillers
Increased levels of endorphins 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 physical and emotional stress certain foodstuff (e.g. chocolate)
Increased levels of endorphins may bring about:regulation of appetite release of sex hormones feelings of euphoria
Dopamine induces the feeling of pleasure and reinforces particular behavior in the reward pathway e.g eating when hungry

• Neurotransmitter related disorders include Alzheimer ’ s disease and Parkinson ’ s disease
• Alzheimer ’ s disease (AD) is caused by the loss of brain cells that synthesise acetylcholine
• Parkinson ’ s disease (PD) is caused by loss of the dopamine synthesizing neurons
• Treatment of neurotransmitter-related disorders involves
Agonists
Antagonists
Inhibitors
• 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 reuptake
• AD is treated by using cholinesterase inhibitors
• PD is treated by use of L-dopa, Agonists that mimic dopamine

• People may choose to alter their state of consciousness by using recreational drugs (some legal, some illegal)
• Recreational drugs affect the transmission of nerve impulses in the reward circuit of the brain. This alteration on the person ’ s neurochemistry may lead to changes in:-
• Mood (happy, more confident, or more aggressive)
• Cognition thinking (the person becomes poorer at problem solving & decision making)
• Perception (the person misinterprets stimuli – colours, sounds, sense of time altered)
• Behaviour (person is able to stay awake for longer & talk about themselves endlessly)

Recreational drugs may: -
- stimulate the release of natural neurotransmitters
- imitate the action of neurotransmitters (agonists)
- block action of the neurotransmitter by binding with receptors (antagonists)
- inhibit their re-uptake of a neurotransmitter
- inhibit the breakdown of a neurotransmitter by enzymatic degradation

• Drug addiction:
• The initial decision:
• Drug tolerance:
• Sensitisation:
• Desensitisation: chronic disease that causes the sufferer to compulsively seek out and use the drug regardless of the consequences voluntary, but subsequent changes that take place in the person ’ s brain soon override selfcontrol – making the person incapable of resisting the overpowering urge to take more of the drug when their reaction to an addictive drug is found to have decreased intensity compared with previous times, even though the concentration of the drug has remained unaltered – a larger dose is now required to bring about the original effect is an increase in the number and sensitivity of receptors as a results of exposure to drugs that are antagonists and leads to addiction is a decrease in the number and sensitivity of receptors as a results of exposure to drugs that are agonists and leads to drug tolerance

Neurobiology & Communication Questions
KEY AREA 3 – The cells of the nervous system and neurotransmitters at synapses
1.Testing Your Knowledge
2.Testing Your Knowledge
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Q’s 1-4
Q’s 1-2
3.Testing Your Knowledge
4.What You Should Know
5.Research Task
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Q’s 1-2
Q1-20