13.11 Nervous Coordination Flashcards
What does the nervous system involve?
- Detection of stimuli by receptors
- Transmission of nerve impulses by neurons
- Response by effectors
Types of neurones
- Sensory neuron
- Relay neuron
- Motor neuron
Structure of (motor) neurone
- Dendrites
- Cell body
- Nucleus
- Axon
- Myelin sheath
- Nodes of Ranvier
- Schwann cell
- Terminal end branch
Function of nucelus
Found in the cell body and contains the DNA which codes for neurotransmitters
Function of dendrites
Extensions of the cytoplasm of the cell body receiving chemical signals from other neurons at the postsynaptic membrane
Function of cell body
Contains the nucleus and groups of ribosomes needed to synthesise neurotransmitters
Function of axon
Long extension of the cytoplasm that transmit impulses away from cell body towards the terminal ends
Function of myelin sheath
Formed as Schwann cells grow around the axon
provide electrical insulation
Speeds up transmission
Function of Schwann cells
Surround peripheral nerves and forms myelin sheath
K+ and Na+ ions cannot diffuse through
Function of terminal end branch
Connect to other neurons or effectors
How is resting potential established and maintained?
- High conc of Na+ on outside of neuron
- High conc of K+ on the inside of the neuron
- Many Na+/K+ pumps that move 3 NA+ out and 2K+ in
via ATP hydrolysis - Na+ voltage gated channel proteins are closed
- Na+ pumped out cannot diffuse back in
- K+ channel proteins are leaky/mainly closed
- K+ can diffuse back out
Net result= more positive on outside than inside
How a nerve impulse is transmitted?
- Resting potential maintained until membrane is stimulated
- Action potential is where the membrane reaches a threshold and becomes depolarised
- An action potential is the reversal of the resting potential
- -70mV to 40mV
Process of depolarisation
- Stimulus causes membrane to become more permeable to Na+ ions
- Na+ channel proteins are open if threshold reached
- Higher conc of Na+ outside the cell
- Na+ ions rapidly diffuse into the cell via FD
- Higher conc of Na+ inside the cell reverses action potential
- Inside is more positive
- K+ voltage-gated channel proteins remain closed
- High conc of + ions is action potential
Three types of nerve impulses
Resting potential
Depolarisation
Repolarisation
(Hyperpolarisation)
Process of repolarisation
- Once the internal potential reaches +40mV
Na+ ion voltage gated channel proteins close - K+ voltage gated channel proteins open
- More K+ ions inside than outside
- K+ ions diffuse out down a conc gradient
Process of hyperpolarisation
- K+ ion channel proteins remain open longer than needed to reach action potential
- Inside of the cell more negative to -90mV
- Sodium potassium pump restores the resting potential back to -70mV
The all or nothing principle
Above the threshold the full sized action potential is given regardless of the increase in the size of the stimulus
Refractory period
Time taken to restore the resting potential potential
What is the impulse transmission along the axon
Action potential acts as a stimulus to adjacent polarised areas of the membrane and this causes the action potential to be passed along
Saltatory conduction
Impulse travels by jumping from one Node of Ranvier to the next Node of Ranvier
To increase the rate of transmission
Factors affecting the speed of conduction of impulses
- Myelin sheath and saltatory conduction
- Temperature
- Axon diameter
How does temperature affect the rate of transmission?
Increase in temp
Increase in kinetic energy
Increase in the rate of diffusion
Increase the rate of conduction
How does axon diameter affect the rate of transmission
The larger the axon diameter, the greater the speed of conductance
Larger membrane surface area - increase in number of channel proteins
Most synapses are…
cholinergic- use acetylcholine neurotransmitter
Why are most impulses unidirectional?
From pre-SN to post-SN
Vesicles containing neurotransmitters only produced in pre-SN
AND
Neurotransmitter receptor proteins only found on membrane of the post-SN membrane.
Process of synaptic transmission
- Action potential arrives
- Voltage gated Ca2+ channel opens
- Ca+ ion diffuse into the synaptic knob (presynaptic neuron) via facilitated diffusion
- Vesicles fuse with presynaptic membrane to release acetylcholine via exocytosis
- Acetylcholine diffuse down conc gradient, across cleft and bind to receptors on post-SN
- Na+ channels open allowing Na+ into postsynaptic neuron
- If membrane potential reaches threshold= membrane becomes depolarised and action potential occurs and spreads along axon membrane of post SN
- K+ channels open allowing K+ out into the cleft
enzyme acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid using water to break the ester bond
Types of neurotransmitters
Excitatory
Inhibitory
Type of inhibitory neurotransmitter
GABA
How do inhibitory neurotransmitters work
- Resting potential= hyperpolarized
- Less likely to reach threshold in the postsynaptic membrane and generate action potential
What is the neuromuscular junction?
A synapse between a motor neuron and a muscle cell
uses acetylcholine which binds to receptors
Adaptations to muscular junctions that differentiate them from synapses
- Lots of folds, increase in surface area for more enzymes
- More receptors
- Always triggers a response
Summation
Build-up of neurotransmitters within the synapse
Two types of summation
Spatial
Temporal
Process of spatial summation
- Different neurons converge at a single synapse
- Action potentials arrive from several different neurons
- Release of enough neurotransmitter to reach threshold and cause an action potential
Process of temporal summation
- Only one presynaptic neuron
- Impulses arrive in rapid succession giving a cumulative effect
- Sufficient to depolarise the post synaptic neuron
How do excitatory neurotransmitters work
Cause action potential in post SN by making resting potential LESS negative
LESS sodium ions required to reach threshold
As a neurone transmits an impulse, its rate of oxygen consumption increases.
Explain why.
- ATP required for active transport;
- Na+ (actively) moved out only at nodes in myelinated / Na+ (actively) moved out along whole length of axon in non-myelinated;
Describe how a resting potential is maintained in a neurone.
- active transport/pumping of sodium (ions across membrane);
- out of neurone/higher concentration outside;
- differential permeability to K+ and Na+;
- Membrane more permeable to K+ ions;
The potential across the membrane is reversed when an action potential is produced.
Describe how.
- Sodium ion gates / channel (proteins) open;
- Na+ (rapidly) diffuse in;
SYNAPSE:
Describe the sequence of events leading to the release of acetylcholine and its binding to the postsynaptic membrane.
- Depolarisation of presynaptic membrane;
- Ca2+ channels open and calcium ions enter (synaptic knob);
- (Calcium ions cause) synaptic vesicles move to/fuse with presynaptic membrane and release acetylcholine / neurotransmitter;
- Acetylcholine/neurotransmitter diffuses across (synaptic cleft);
- (Acetylcholine attaches) binds to receptors on the postsynaptic membrane;
- Sodium ions enter (postsynaptic neurone) leading to depolarisation;
When a nerve impulse arrives at a synapse, it causes the release of neurotransmitter from vesicles in the presynaptic knob.
Describe how.
- (Nerve impulse/depolarisation of membrane) causes Ca2+ channel (proteins) to open;
- Ca2+ enter by (facilitated) diffusion;
- Causes (synaptic) vesicles to fuse with (presynaptic) membrane;
The binding of GABA to receptors on postsynaptic membranes causes negatively charged chloride ions to enter postsynaptic neurones.
Explain how this will inhibit transmission of nerve impulses by postsynaptic neurones.
- (Inside of postsynaptic) neurone becomes more negative/hyperpolarised;
- More sodium ions required (to reach threshold) OR Not enough sodium ions enter (to reach threshold);
- For depolarisation/action potential;
Describe how the speed of the conduction could be increased in a neurone.
- Axon is myelinated;
- So shows saltatory conduction/impulses jump from node of Ranvier to node of Ranvier;
OR - Axon has a larger diameter;
- So less resistance to flow of ions;
Myelination affects the rate of conduction of a nerve impulse. Explain how.
- Impulse jumps from node of Ranvier to node of Ranvier / depolarisation only at node of Ranvier;
- Fewer jumps / depolarisations to travel length of axon;
Describe how the inhibition of acetylcholinesterase affects the action of synapses.
- Acetylcholine not broken down / stays bound to receptor;
- Na+ ions (continue to) enter / (continued) depolarisation / Na+ channels (kept) open / action potentials;
Describe what is meant by the term refractory period
- (Refractory period) limits number of impulses per second/frequency of nerve impulses;
- Maximum frequency of impulse transmission
- Period of time between threshold and resting membrane potential.
- When maximum frequency reached/exceeded, no further increase in information/ all (higher) stimuli seem the same;
