Unit 6 - Organisms respond to changes in their internal and external environments
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Summary nervous co-ordination and muscles
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Unit 6 - Organisms respond to changes in their internal and external environments
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AQA
Summary notes for AQA A-level Biology nervous co-ordination and muscles topic. Includes clear information on nervous impulses (action potentials, refractory periods), synapses, skeletal muscle, contraction and relaxation of muscles and neuromuscular junctions. Summarised from class notes and the of...
Unit 6 - Organisms respond to changes in their internal and external environments
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Nervous coordination and muscles
Neurones and nervous co-ordination
Nervous system
• Uses nerve cells to pass electrical impulses along their length
• Stimulate target cells by secreting neurotransmitters which leads to rapid
communication
• Responses are often short lived and localised
Hormonal system
• Produces hormones transported in the blood plasma to the target cells
• The target cells have specific receptors on their membranes and the change in hormone
concentration stimulates them
• Slower and less specific communication
• Long lasting and widespread response
Neurones
Nerve cells that are specialised to carry electrochemical charges called nerve impulses.
The impulse moves from dendrites to the axon.
Sensory neurones transmit nerve impulses from a receptor to an intermediate or motor
neurone.
Motor neurones transmit nerve impulses from an intermediate or relay neurone to an
effector, such as a gland or a muscle.
Intermediate or relay neurones transmit impulses between neurones, for example, from
sensory to motor neurones.
Neurones are classified by how many
fibres are attached to the cell body.
Multipolar - many fibres, e.g. motor
neurones
Unipolar - one fibre, e.g. sensory
neurones
Bipolar - two fibres, e.g. intermediate
neurones
, Motor neurones:
Structure Function
Dendrons extensions of the cell body which divide into smaller fibres called dendrites
which carry nerve impulses towards the cell body.
Dendrites highly branched to increase number of synapses with other neurones so
many action potentials can be received
Axon long fibre that carries nerve impulses away from the cell body
Long to increase speed of conductance
Schwann cells surround the axon, protecting it and providing electrical insulation. Carry out
phagocytosis and are involved in nerve regeneration. They wrap around the
axon many times forming layers of membranes
Myelin sheath forms a covering to the axon and is made up of the Schwann cell
membranes, which are rich in a lipid known as myelin. Speed up
conductance
Nodes of ranvier gap between Schwann cells with no myelin sheath
Mitocondria to produce ATP for active transport
The nerve impulse
The nerve impulse is a self propagating wave of electrical activity that travels along the
axon membrane. Its a temporary reversal of the electrical potential difference across the
axon membrane - the reversal is between the resting potential and the action potential
Resting potential
The inside of an axon is negatively charged and is known as the resting potential. The
potential difference in charge is because the outside of the neurone has more positive
charge than inside.
Membrane is not
The sodium potassium pump pumps Membrane is permeable to
permeable to Na+ so
3 sodium ions out of the cell and 2 K+ so these ions diffuse out
these ions cant diffuse
potassium ions into the cell. This of the membrane by
into the membrane by
results in a net loss of positive charge facilitated diffusion
facilitated diffusion
, The movement of ions e.g. Na+ and K+ is controlled by:
• Phospholipid bilayer of the axon plasma membrane prevents diffusion of these ions
• Channel proteins which are gated and allow facilitated diffusion at specific times, or
undated and allow constant diffusion of the ions
• Sodium potassium pump that actively transports the ions out/into the axon
Action potential
• When a stimulus is detected by a receptor in the nervous system its energy causes a
temporary reversal of the charges either side of the membrane
• Negative charge becomes a positive charge and is known as the action potential. This
part of the axon is said to be depolarised
• This occurs as the channels change shape and open/close depending on the voltage.
They are called voltage gated channels
Process:
Stage 1: resting potential
At resting potential potassium voltage-gated channels are partially open but the sodium
voltage-gated channels are closed.
Sodium gated channels: closed and active
Potassium gated channels: partially open
Stage 2: membrane depolarisation
An action potential starts to form due to a stimulus:
• neurotransmitter from another neurone
• or a generator potential from a receptor cell
This causes some sodium voltage-gated channels in the axon membrane to open and
therefore sodium ions diffuse into the axon through these channels along their
electrochemical gradient. Being positively charged, they trigger a reversal in the potential
difference across the membrane. As sodium ions enter the axon, more sodium ion
channels open, causing an even greater influx of sodium ions
There is a threshold membrane potential that must be reached before an action potential.
• Small stimuli mean that only a few Na+ channels open and an action potential is not
produced.
• Larger stimuli means that enough ion channels open to reach the threshold membrane
potential. This causes more channels to open and more sodium ions will enter. This is
an example of positive feedback
Sodium gated channels: open
Potassium gated channels: partially open
Stage 3: membrane repolarisation
Once the action potential has been established, the voltage gates on the sodium ion
channels close and the voltage gates on the potassium ion channels open fully.
With more potassium voltage-gated channels now open, more potassium ions diffuse out
(facilitated diffusion), starting repolarisation of the axon.
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