A-level Biology - 3.4.3 Nerve Impulses and Synaptic Transmission
Sensory neurones transmit nerve impulses from sensory receptors towards the central nervous system (CNS). They detect stimuli such as light, sound, or pressure and relay this information for processing.
State the function of sensory neurones
Carriers nerve impulses from receptor towards intermediate neurones within CNS
Key Terms
State the function of sensory neurones
Carriers nerve impulses from receptor towards intermediate neurones within CNS
State the function of motor neurones
Carries nerve impulses away from the CNS towards an effector
State the function of intermediate neurones
Connect sensory to motor neurones
Where are intermediate neurones found?
Within spinal cord
(have numerous short dendrites)
Describe the structure and function of the axon
Single long fibre that carriers nerve impulses
Describe the structure and function of dendrites
Small extensions of cell body which carry impulses toward cell body
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| Term | Definition |
|---|---|
State the function of sensory neurones | Carriers nerve impulses from receptor towards intermediate neurones within CNS |
State the function of motor neurones | Carries nerve impulses away from the CNS towards an effector |
State the function of intermediate neurones | Connect sensory to motor neurones |
Where are intermediate neurones found? | Within spinal cord (have numerous short dendrites) |
Describe the structure and function of the axon | Single long fibre that carriers nerve impulses |
Describe the structure and function of dendrites | Small extensions of cell body which carry impulses toward cell body |
Describe the structure and function of the cell body | Contains a nucleus and large number of endoplasmic reticulum Produces neurotransmitters |
Describe the structure of the myelin sheath | Covers the axon Made up of membranes of Schwann cells Membranes are rich in the lipid myelin |
Describe the structure and function of Schwann cells | Surround and wrap around axon, providing protection and electrical insulation |
Describe the structure of Nodes of Ranvier | Small gaps between adjacent Schwann cell Sodium ion channels are concentrated at the nodes |
When is the sympathetic system activated? | In times of stress |
What is the sympathetic system responsible for? | For increasing heart rate and ventilation and pupil dilation |
When is the parasympathetic system active? | Most active in relaxed states |
What is the parasympathetic system responsible for? | Responsible for decreasing heart rate and ventilation rate and pupil constriction |
What does the parasympathetic system enable? | Enables everyday tasks to be completed (digest food, fight infections, etc.) |
Describe and explain the charge of a neurone’s resting state | In neurone’s resting state, outside of membrane is positively charged compared to inside ∵ more positive ions outside cell than inside (resting potential = about -70 mv) ∴ membrane = polarised Difference in charge across it |
State how resting potential is created and maintained | By sodium-potassium pumps & potassium ion channels |
Describe how the resting potential is created and maintained by sodium-potassium pumps and potassium ion channels | Sodium-potassium pump uses active transport to move 3 Na+ out of neurone for every 2 K+ ions moved in ATP needed to do this Membrane isn’t permeable to Na+ = can’t diffuse back Creates sodium ion electrochemical gradient ∵ more Na+ outside cell than inside Membrane is permeable to K+ = diffuse back out though K+ channels, down their concentration gradient facilitated diffusion Makes outside of cell positively charged compared to inside |
Neurone cell membranes become ________ when they’re stimulated | Depolarised |
If a stimulus is big enough, it triggers rapid change in ___ | p.d. |
Name the 5 stages of how an action potential occurs i.e. how neurone cell membranes become depolarised when they’re stimulated | Stimulus Depolarisation Repolarisation Hyperpolarisation Resting potential |
Action Potentials Describe the stage stimulus | Stimulus excites neurones cell membrane = Na+ channels to open, making membrane more permeable to Na+ Na+ then diffuse (down electrochemical gradient) into neurone, making it less negative |
Action Potentials Describe the stage depolarisation | Once threshold has been met (around -55mv), more Na+ channels open = more Na+ to diffuse in rapidly |
Action Potentials Describe the stage repolarisation | (At around +30mV) Na+ channels close and the K+ channels open (Na+ channels have to close or membrane will remain depolarised) Membrane is now more permeable to K+ = K+ diffuse out of neurone down K+ conc. gradient Gets membrane back to its resting potential |
Action Potentials Describe the stage hyperpolarisation | K+ channels are slow to close so there’s a slight overshoot where too many K+ diffuse out of neurone Causes p.d. to become more negative than resting potential |
Action Potentials Describe the stage resting potential | Ion channels are reset Sodium-potassium pump returns membrane to its resting potential & maintains until membrane's excited by another stimulus |
Explain why after an action potential, the neurone cell membrane can't be excited again straight away | ∵ ion channels are recovering & can't be made to open Na+ channels are closed during repolarisation and K+ are closed during hyperpolarisation |
Describe how an action potential moves along a neurone | When action potential occurs, some Na+ that enter neurone diffuse sideways Causes Na+ channels in next region of neurone to open and Na+ ions diffuse into that part Causes wave of depolarisation to travel along neurone Wave move away from parts of membrane in refractory period ∵ these parts can't fire an action potential |
What is the refractory period? | When ion channels are recovering and can't be opened |
What does the refractory period act as? | Acts as a time delay between 1 action potential and the next |
Name 3 things the refractory period ensures | Action potentials don't overlap But pass along discrete (separate) impulses Limit to frequency of which nerve impulses can be transmitted Action potentials are unidirectional (only travel in 1 direction) |
Describe the all-or-nothing nature of action potentials | Once threshold is reached, action potential will always fire with same change in voltage No matter how big stimulus is If threshold isn't reached, action potential won't fire |
What does a bigger stimulus cause? | Causes action potentials to fire more frequently DOESN'T = bigger action potential |
Name 3 factors that affect the speed of conduction of action potentials | Temperature Axon Diameter Myelination |
Describe and explain how temperature affects the speed of conduction of action potentials | Speed of conduction increases as temp. increase ∵ ions diffuse faster Speed only increases up to 40°C After proteins, denature & speed decreases Active transport is used for sodium-potassium pump & enzymes are used |
Describe how axon diameter affects the speed of conduction of action potentials | Bigger diameter = faster the conduction of action potentials |
Explain why action potentials are conducted quicker along axons with bigger diameters | ∵ there's less resistance to flow of ions in cytoplasm of bigger axon ∵ less leakage of ions from axon With less resistance, depolarisation reaches other parts of neurone cell membrane quicker |
In myelinated neurone, where does depolarisation only occur? | At nodes of Ranvier |
Describe and explain how myelination increases the speed of conduction of action potentials | Neurone's cytoplasm conducts enough electrical charge to depolarise the next node ∴ impulse jumps from node to node Called saltatory conduction & it's very fast In non-myelinated neurone, impulse travels as wave along whole length of axon membrane (Get depolarisation along whole length of membrane) Slower than saltatory conduction |
What is a synapse? | Junction between 2 neurones or between neurone and effector cell |
What is a synaptic cleft? | Tiny gap between cells at synapse |
What is the synaptic knob? | The presynaptic neurone that has a swelling |
Synaptic knob contains synaptic vesicles that are filled with ______ | neurotransmitters |
Explain how synapses make sure impulses are unidirectional | (Vesicles containing) neurotransmitter only in presynaptic membrane/neurone Receptors are only on postsynaptic membranes |
Describe what happens briefly when an action potential reaches the end of a neurone | Causes neurotransmitters to be released into synaptic cleft Diffuse across to postsynaptic membrane & bind to specific receptors Can trigger action potential & cause muscle contraction or hormone secretion |
Describe what happens in the synapse to stop a response from keep happening | Neurotransmitters are removed from cleft Taken back into presynaptic neurone or broken down by enzymes |
What is a cholinergic synapse? | Synapses that use acetylcholine (neurotransmitter - ACh) |
Describe how a nerve impulse is transmitted across a cholinergic synapse | Arrival of action potential (at synaptic knob) causes calcium ion channels to open and Ca2+ to enter synaptic knob Influx of Ca2+ causes synaptic vesicles to fuse with presynaptic membrane Releases ACh from vesicles into synaptic cleft (exocytosis) ACh diffuse across synaptic cleft and bind to specific cholinergic receptors on postsynaptic membrane Na+ channels in postsynaptic neurone to open Influx of Na+ causes depolarisation & generates action potential in postsynaptic neurone (if threshold reached) AChE (acetylcholinesterase) hydrolyses ACh into choline and ethanoic acid (acetyl) which diffuses back into presynaptic neurone Removed so that its response doesn't keep happening Sodium channels close ATP is released by mitochondria to recombine choline and ethanoic acid to form ACh Stored in synaptic vesicles for future use |
Neurotransmitters can be both and _____ | Neurotransmitters can be both excitatory and inhibitory |
What are excitatory neurotransmitters? | Neurotransmitters that depolarise postsynaptic membrane Making it fire action potential if threshold is reached |
State where is acetylcholine is excitatory | At cholinergic synapse in CNS |
State what (excitatory) acetylcholine does at the cholinergic synapses in CNS | Binds to cholinergic receptors = causes action potential in postsynaptic membrane and neuromuscular junctions |
What are inhibitory neurotransmitters? | Hyperpolarise postsynaptic membrane (make pd more negative) Preventing it from firing action potential |
State where is acetylcholine is inhibitory | At cholinergic synapse in heart |
State what (inhibitory) acetylcholine does at the cholinergic synapses in the heart | Binds to receptors = causes potassium ion channels to open on postsynaptic membrane = hyperpolarising it |
What is summation? | The effect of neurotransmitter released from many neurones is added together |
What does summation allow? | Allows synapses to accurately process info, finely turning the response |
Name 2 types of summation | Temporal summation Spatial summation |
What is temporal summation? | Where 2 or more nerve impulses arrive in quick succession from same presynaptic neurone |
Why does temporal summation make an action potential more likely? | ∵ more neurotransmitters is released into synaptic cleft |
Describe how spatial summation results in an action potential | Sometimes many neurones connect to 1 neurone Small amount of neurotransmitter released from each neurone = enough altogether to reach threshold in postsynaptic neurone & trigger action potential |
What is a neuromuscular junction? | Is synapse between motor neurone and muscle cell |
What neurotransmitter is used in a neuromuscular junction? | ACh Binds to cholinergic receptors (called nicotinic cholinergic receptors) Work same way as cholinergic synapse |
Name 3 differences between neuromuscular junctions and cholinergic synapses | Postsynaptic membrane has lots of folds that form clefts Clefts stores acetylcholinesterase (AChE) Postsynaptic membrane has more receptors than other synapses ACh is always excitatory at neuromuscular junction When motor neurone fires action potential = triggers response in muscle cell Isn't the case for synapse between 2 neurones |
Name or describe 5 types of drugs that affect synaptic transmission | Agonists Antagonists Some inhibit enzymes that break down neurotransmitters Some stimulate the release of neurotransmitter from presynaptic neurone so more receptors are activated or vice versa |
Drugs Affect Synaptic Transmission What are agonists? | Have same shape as neurotransmitters = mimic their action at receptors Means more receptors are activated e.g. nicotine mimics ACh so binds to nicotine cholinergic receptors in brain |
Drugs Affect Synaptic Transmission What are antagonists? | Block receptors so they can't be activated by neurotransmitters Fewer receptors can be activated e.g. curare blocks effects of ACh by blocking nicotinic cholinergic receptors at neuromuscular junctions So muscle cells can't be stimulated = paralysed |
Drugs Affect Synaptic Transmission Describe the benefit of drugs that inhibit enzymes that break down neurotransmitters | ∴ there's more neurotransmitters in synaptic cleft to bind to receptors and they're there for longer e.g. nerve gases stop ACh from being broken down = loss of muscle control |
An action potential is generated at the cell body of the motor neurone. Explain how this action potential passes along the motor neurone to the neuromuscular junction. (3) | Depolarisation of axon membrane/influx of Na+ establishes local currents Change permeability to Na+/open Na+ gates of adjoining region Adjoining region depolarises/influx of Na+ |
In an investigation, the higher the concentration of sucrose in a rat's mouth, the higher the frequency of nerve impulses from each taste receptor to the brain. If rats are given very high concentrations of sucrose solution to drink, the refractory period makes it impossible for information about the differences in concentration to reach the brain. Explain why. (2) | (Refractory period) leads to separate nerve impulses OR (Refractory period) limits frequency of nerve impulses When maximum frequency reached, no further increase in information/all (higher) concentrations of sucrose seem the same |
The rate of ATP consumption in a non-myelinated neurone is greater than that of a myelinated neurone when conducting electrical impulses at the same frequency. Explain why. (2) | Greater entry of sodium ions Active transport |