nervous control Flashcards
Give 3 functions of the nervous system
- Communication
- Coordination
- To detect changes in the external and internal environments, evaluate this information and make appropriate responses
Compare the Nervous and Endocrine systems [5]
NERVOUS
Electrical impulses
Impulses pass via neurones
Fast transmission
Short-lived effect
Localised effect
ENDOCRINE
Chemical message
Chemicals move in bloodstream
Slow transmission
Long-lived effect
Widespread effect
Neurone
Nerves are bundles of neurones
Give 6 parts of the structure of a neurone
Cell Body
Dendrites
Axon
Schwann cells
Myelin sheath
Nodes of Ranvie
Cell body function [2]
- contains a nucleus, large amounts of mitochondria and rough endoplasmic reticulum
- is associated with production of proteins and is a neurotransmitter
Dendrites function
Multiple small fibres that carry nerve impulses towards the cell body
Axon function
A single long fibre that carries nerve impulses away from the cell body
Schwann cells
Are individual cells that surround the axon by wrapping around many times, protecting it and providing electrical insulation
Myelin sheath [3]
- Forms the covering of axon
- Made of membranes of the Schwann cells.
- Rich in a lipid known as myelin
Nodes of Ranvier
Gaps between adjacent Schwann cells where there is no myelin sheath.
The Nerve Impulse [4]
- Mainly involves the ions Na+, K+
The impulse is determined by… - the potential difference across the membrane
- the concentration of the ions in and around the neurone
- the movement of ions across the axon membrane
What is Resting Potential? [2]
- The inside of the axon membrane has a relatively more negative charge of -65mV, or resting potential
- The neurone is polarised
Maintaining Resting Potential [6]
- Our bodies use a lot of energy to keep resting neurones polarised
At the sodium-potassium pump in the neurone membrane:
- 3 Na+ move out for every 2 K+ that move in
- ATPase allows the hydrolysis of ATP
- 1 ATP is converted to ADP & Pi, releasing energy
- The membrane also contains many permanently open potassium ion channels, and only a few sodium ion channels
- This means that the membrane is 50x more permeable to K+ than Na+ so some K+ move out, down their concentration gradient
Action Potential [4]
- An action potential refers to when an impulse passes along a neurone
- A receptor that is stimulated sufficiently (creates a large enough generator potential) will cause a reversal of charges across the membrane
- The charge alters from -65mV to +45mV
- The membrane is depolarised.
An action potential is an all or nothing response…
Meaning unless the threshold value is reached the impulse will not occur
Action potential are the same size so size of stimulus depends on… [2]
- More impulses in a time = larger stimulus
- Neurones can have different threshold values. The brain can interpret the type of neurone and therefore the size of stimulus
When as impulse is received…
Depolarisation
Repolarisation
Overshoot or hyperpolarisation
Back to resting potential
Depolarisation [5]
- This is the start of the action potential
- The initial stimulus causes some of the voltage-gated Na+ channels in the membrane to open
- Sodium ions enter the axon, diffusing down their concentration gradient
- This causes more voltage-gated Na+ channels to open and more sodium ions to enter
- The relative charge of the axon goes from -65mV to +45mV
Repolarisation [6]
- This is the continued process of action potential
- A potential of +45mV causes the sodium ion voltage-gated channels to shut
- The voltage-gated potassium ion channels open
- Potassium ions move down their concentration gradient out of the axon
- The potential difference across the membrane drops
- Charge goes from +45mV to -65 mV
Hyperpolarisation [4]
- Movement of the potassium ions cause hyperpolarisation, where the membrane overshoots the resting potential of -65mV
- The potential difference reaches -80mV
- The voltage-gated potassium ion channels shut (meaning both channels are shut)
- This point is also known as the refractory period when no further action potentials can take place
The Refractory Period [2]
- This is a short period of time that occurs when the neurone cannot be stimulated as it recovers from the previous stimulus
- The voltage-gated sodium ion channels can’t open
Why is the refractory period important? [3]
- Ensures impulses can only travel in one direction
- Allows separate impulses
- Limits the strength of impulse
When are the voltage-gated sodium ion channels open and closed?
Voltage-gated sodium ion channels:
OPEN = depolarisation
SHUT = polarisation, repolarisation, hyperpolarisation
Voltage-gated potassium ion channels:
OPEN = repolarisation
SHUT = polarisation, depolarisation, hyperpolarisation
Propagation of an Impulse in Unmyelinated Neurones
An action potential in one part of a neurone will stimulate an action potential in the next section
Propagation of an Impulse in Myelinated Neurones [3]
- The myelin sheath in made of lipid, an insulator
- The action potential cannot move through the sheath
- Saltatory conduction (jumping) takes place
Saltatory Conduction
This results in the localised circuit forming between the nodes of Ranvier
Give 3 factors affecting the speed of transmission
- Temperature
- Diameter of Neurone
- Myelination
Temperature [2]
temperature increases
- greater kinetic energy, faster transmission
temperature exceeds optimum
- proteins in the membrane denature, slower transmission
Diameter [2]
Larger diameter = faster transmission
- It is easier to maintain concentration gradients in wider neurones than narrower
- There is less ‘leakage’ of ions
Myelination [3]
- Myelinated neurones conduct nerve impulses faster than non-myelinated neurones
- because the action potential ‘jumps’ between the Schwann cells of the myelin sheath
- and only needs to be conducted at the nodes of Ranvier
Synaptic Transmission [7]
- The action potential causes Ca2+ to diffuse into the synaptic knob
- This causes vesicles of neurotransmitter to fuse with presynaptic membrane and, release the neurotransmitter into the synaptic cleft
- Neurotransmitter diffuses across and attaches to receptors on the post-synaptic membrane
- Ion channels open, causing depolarisation in post synaptic neurone
- If depolarisation above threshold value then action potential is stimulated along the post synaptic axon
- Neurotransmitter must be removed to prevent continual stimulation
- The neurotransmitter, diffuses back, is broken down and is recycled
NEUROTRANSMITTER = acetylcholine
1 - Calcium Channels Opening [3]
- The incoming action potential causes depolarisation in the synaptic knob
- This causes calcium ion channels to open
- Calcium ions (Ca2+) diffuse into the synaptic knob
2 - Neurotransmitter Release [2]
- The influx of calcium ions causes synaptic vesicles to FUSE with the presynaptic membrane
- This releases neurotransmitter into the cleft
3 - Sodium Channels [3]
- Neurotransmitter (acetylcholine) diffuses across the synaptic cleft
- Neurotransmitter (acetylcholine) binds to the receptor site on the
sodium ion channels, found on the post-synaptic neurone membrane - Sodium ion channels open
4/5 - New Action Potential [2]
- Depolarisation inside the postsynaptic neurone must be above a threshold value
- If the threshold is reached a new action potential is sent along the axon of the post- synaptic neurone
6 - Acetylcholinesterase [4]
- Attachment of the acetylcholine to the receptor will keep the sodium ion channels open
- This would mean an action potential would be continually stimulated
- Acetylcholinesterase is a hydrolytic enzyme
- Breaks up acetylcholine (the neurotransmitter) into acetyl (ethanoic acid) and choline
7 - Remaking Acetylcholine [5]
- ATP released by mitochondria is used to recombine acetyl (ethanoic acid) and choline thus recycling the acetylcholine
- This is stored in synaptic vesicles for future use
- More acetylcholine can be made at the SER
- Sodium ion channels close in the absence of acetylcholine at their receptor sites
- The synapse is now ready to be used again
Features of Synapses
Unidirectionality
Inhibition
Summation
Unidirectionality [2]
- The impulse can only be sent from the presynaptic neurone to the postsynaptic neurone
- This is due to the position of neurotransmitter vesicles and sodium ion channels
Inhibition [5]
- Some neurotransmitters stimulate chloride ion channels on the postsynaptic membrane to open and also potassium ion channels to open
- Chloride ions (Cl-) diffuse INto the postsynaptic neurone
- Potassium ions diffuse out
- This hyperpolarises the neurone
- This make it harder to achieve a action potential
Types of Summation
- Summation
- Spatial Summation
- Temporal Summation
Summation [3]
- Low frequency action potentials often release insufficient amounts of neurotransmitter to exceed the threshold in the postsynaptic neurone
- Summation allows action potentials to be generated
- This enables a build up of neurotransmitter in the synapse
Spatial Summation [2]
- A number of different presynaptic neurones share the same synaptic cleft
- Together they can release enough neurotransmitter to create an action potential
(multiple neurones)
Temporal Summation [2]
- A single presynaptic neurone releases neurotransmitter many times over a short period
- If the total amount of neurotransmitter exceeds the threshold value an action potential is sent
(1 neurone)
