Unit 8- Nervous Coordination Flashcards
What is a neurone and what does it do?
A neurone is one nerve cell. Neurones carry electrical impulses called action potentials.
Structure of a neurone
Role of the dendrites?
Where the action potential starts. Intermediate neurone synapse here. Carry action potentials to the cell body.
Role of the cell body?
Contains the cell’s organelles. Neurotransmitters are made here.
Role of the axon?
Long fibre which carries action potentials away from the cell body to the terminal.
Role of the Myelin Sheath?
Made of Schwann cells, with membranes rich in myelin. Speeds up the conduction of the action potential.
Role of the node of Ranvier?
Gaps between Schwann cells where there is no Myelin Sheath.
Role of the axon terminal?
The end of the axon. Effectors synapse here.
How is a resting potential maintained?
The sodium potassium pump actively transports 3 sodium ions out of the neurone and 2 potassium ions into the neurone.
The membrane is more permeable to potassium ions than sodium ions.
There is a higher concentration of potassium ions inside and higher concentration of sodium ions outside the neurone.
The membrane is polarised, there is more positive charge outside the cell than inside creating a resting potential of about -70mV.
Action potential graph
Steps of the formation of an action potential?
When the neurone is stimulated the voltage gated sodium ion channels open.
Sodium ions diffuse into the axon down their concentration gradient causing depolarisation.
More voltage gated sodium ion channels open and if threshold is reached, the neurone depolarises to about +40mV.
The sodium ion channels then close preventing sodium ions entering the axon and potassium ion channels open.
Potassium ions diffuse out causing the neurone to become too negative. This is called hyperpolarisation.
Potassium ion channels close and the sodium potassium pump returns the neurone back to resting potential.
What is the refractory period?
The time during which no new action potentials can be generated. This is due to the sodium ion channels staying closed, so no more sodium ions can diffuse in.
Where is the refractory period on the graph?
Over the whole thing
What are the 3 reasons why refractory periods are important?
- So action potentials move in one direction
- Produces discrete impulses (separate)
- Limits the maximum frequency of action potentials
What is the all or nothing principle?
An action potential is only produced if the threshold is reached. All - if threshold is reached you get an action potential which is always the same size or nothing - if threshold is not reached there will be no action potential.
How does the brain stimulate the strength of a stimulus?
The stronger the stimulus the greater the frequency of action potentials.
Propagation of an action potential on an Unmyelinated neurone?
- An action potential occurs at a section of the axon membrane
- The membrane adjacently in front of the action potential detects the depolarisation and adjacent voltage gated sodium ion channels open, also forming an action potential.
- The action potential moves forward. The refractory period behind the action potential prevents the impulse being conducted backwards.
Propagation of an action potential along a myelinated neurone?
- The myelin insulates the axon (preventing ion movements)
- Depolarisations only take place at the nodes of Ranvier so the action potentials ‘jump’ from node to node. This is known as saltatory conduction.
- Therefore there are less depolarisations along the whole length of the axon membrane.
What are 3 ways you can speed up the conduction of action potnetials?
- Myelination allows saltatory conduction
- A higher temperature increases the kinetic energy of ions so they diffuse faster through the membrane
- A greater axon diameter reduces resistance to ion flow.
Diagram of a pre-synaptic and post-synaptic neurone?
Role of the synaptic vesicle?
Contains neurotransmitters
Role of the calcium ion channel?
Takes Ca2+ into the presynaptic neurone which causes the synaptic vesicle to fuse with the presynaptic membrane
Role of the mitochondria?
Provides the energy/ATP for the re-uptake and reformation of neurotransmitter
Role of the synaptic cleft?
Space between neurones which neurotransmitter diffuses across
Role of the neurotransmitter receptor?
Neurotransmitter binds to these, to either cause or inhibit action potentials in post synaptic neurone
Role of the neurotransmitter re-uptake pump?
Allows neurotransmitter to be reabsorbed back into the presynaptic neurone by active transport preventing further action potentials.
Steps of the transmission of an impulse across a Cholinergic synapse?
- an action potential depolarises the pre-synaptic membrane, voltage gated calcium ion channels open and calcium ions diffuse in
- calcium ions cause synaptic vesicles to fuse with the pre-synaptic membrane and release Acetylcholine into the synaptic cleft
- acetylcholine diffuses across the synaptic cleft and binds to receptors on the post-synaptic membrane
- sodium ion channels open and sodium ions diffuse into the post-synaptic neurone, depolarising the post-synaptic membrane causing an action potential (if the threshold is reached)
- acetylcholinesterase hydrolyse the acetylcholine. the sodium ion channels close and no more action potentials are generated
- the products of hydrolysis are taken back into the pre-synaptic neurone via active transport using the neurotransmitter re-uptake pump.
What can summation be used for?
To increase the neurotransmitter in the synaptic cleft to increase the depolarisation of the post-synaptic membrane, so threshold is reached.
What is spatial summation?
More than one pre-synaptic neurone connected to one post-synaptic neurone.
What is temporal summation?
Repeated action potentials arrive in short time from one presynaptic neurone to provide enough neurotransmitter to reach threshold in the post synaptic neurone.
Synapses have unidirectionality, what is this?
Action potentials travel in one direction across the synapse, from pre-synaptic neurone to post-synaptic neurone.
Explain how synapses have unidirectionality?
-Neurotransmitter only released from/store in the presynaptic neurone
-Receptors only on post-synaptic membrane
Describe the effect of chloride ion channels opening and explain how this leads to synapse inhibition?
Chloride ions diffuse in and make the post-synaptic membrane more negative (hyper polarised). More sodium ions need to diffuse in to reach threshold for an action potential.
Diagram of a neuromuscular junction?
Describe how the action potential crosses the neuromuscular junction?
- nerve impulses depolarise the presynaptic membrane and calcium ion channels open
- calcium ions diffuse into the presynaptic neurone causing synaptic vesicles to fuse with the presynaptic membrane
- acetylcholine is released into the synaptic cleft and diffuses across
- acetylcholine binds receptors on sarcolemma
- sodium ion channels open and sodium ions diffuse into muscle fibre causing depolarisation of the sarcolemma
Similarity between a neuromuscular junction and a cholinergic synapse?
Both use acetylcholine as the neurotransmitter which is broken down by acetylcholinesterase
Differences between a neuromuscular junction and a cholinergic synapse?
Cholinergic synapse:
-neurone to neurone
-can be excitatory or inhibitory
-no cleft in post synaptic membrane
-less ACh receptos on post synaptic membrane
Neuromuscular junction:
-neurone to muscle fibre
-only excitatory
-contains clefts in muscle fibre membrane which store acetylcholinesterase
-more ACh receptors on muscle fibre
Definition of a motor unit?
All the neuromuscular junctions from a single motor neurone. When a motor unit is activated, all of the connected muscle fibres contract at the same time.
Defintion of an electrochemical gradient?
A concentration gradient of ions. The amount of diffusion depends not only on the concentration of ions but also the difference in electrical charge (how positive or negative it is).
