Transcript File

Higher Human Biology
Unit 3
Neurobiology & Communication
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
3f) Neurotransmitters-chemical transmission

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
Pre-synaptic
neuron
Synaptic cleft
Post-synaptic
neuron
Neurotransmitters
Receptors

Chemical messages are relayed across synaptic
clefts from neuron to neuron both inside and
outside of the brain by chemicals called
Neurotransmitters
Vesicle with
neurotransmitter
3g) Neurotransmitters-chemical transmission
3h) Neurotransmitters at synapses & effects

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
3i) Removal of Neurotransmitters

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

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

Synapses can filter out weak stimuli arising
from insufficient secretion of
neurotransmitters
Pre-synaptic
neuron
Vesicle with
neurotransmitter
Synaptic cleft
Post-synaptic
neuron
Neurotransmitters
Receptors
3j) More on the removal of Neurotransmitters
• 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
3k) Summation
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
3m) Plasticity
•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
3n) Neurotransmitters, mood & behaviour
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
3o) Neurotransmitters related disorders
•
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
3p) Mode of action – recreational drugs
•
•
•
•
•
•
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)
3q) Mode of action – recreational drugs
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
3r) Recreational drugs: Addiction
•
Drug addiction:
chronic disease that causes the
sufferer to compulsively seek out and use the
drug regardless of the consequences
•
The initial decision:
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
•
Drug tolerance:
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
•
Sensitisation:
is an increase in the number and sensitivity of
receptors as a results of exposure to drugs that
are antagonists and leads to addiction
•
Desensitisation:
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
Page 256
Q’s 1-4
2.Testing Your Knowledge
Page 264
Q’s 1-2
3.Testing Your Knowledge
Page 276
Q’s 1-2
4.What You Should Know
Page 277
Q1-20
5.Research Task