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
