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Sensory recptors are ____ ______ because they convert one form of energy into another. They convert other forms of energy into a form of ____ energy called a ____ _____.
- energy transducers
- nerve impulse
List some examples of sensory receptors.
- Light-sensitive cells (rods and cones) in retina (light intensity and wavelengths)
- Olfactory cells lining inner surface of nasal cavity
- Taste buds in tongue, first part of oesophagus
- Pressure receptors in skin
- Sound receptors in inner ear (cochlea)
- Muscle spindles - detect length of muscle fibres.
A nerve impulse is created by altering the ____ of nerve cell membrane to ___ ions. A stimulus will cause these gated channels to ___. This creates a change in ___ ____ across membrane. Inside of cell becomes less ___ than usual, and this is called ____.
- sodium (Na+)
- potential difference
Explain what a polarised and depolarised membrane is.
- Polarised membrane is one that has potential difference (difference in charge/voltage)across it. This is resting potential.
- Depolarisation is loss of polarisation across membrane. Cell becomes less negative with respect to outside.
What is a generator potential?
- A small depolarisation caused by soium ions entering the cell in the initial stages of nerve impulse.
- If generator potential is large enough, it will trigger an action potential.
What is an action potential?
- When the nerve creates an impulse, by the membrane depolarising to a value of about +40mV. It is an all-or-nothing response.
- In the events leading up to it, membrane depolarises and reaches a threshold level, and then more sodium floods into the axon, causing an action potential.
How does a larger stimulus cause an action potential while a small one doesn't?
Because a larger stimulus (change in energy levels in environment), the more sodium gated channels will open. If enough open and enough Na+ enter the cell, an action potential will be intiated.
List/describe the 3 types of neurone
- Seonsory neurons - carry action potential from sensory receptor to CNS
- Motot neurons - carry a.p from CNS to effector such as muscle or gland
- Relay neurons - connect sensory and motor neurones.
List and explain the structure of neurones to their function.
- Often very long - transmit impulse over long distance
- Plasma membrane has many gated ion channels and sodium/potassium ion pumps
- Maintain potential difference across membrane
- Surrounded by fatty myelin sheath (actually series of Schwann cells) that insulates. And gaps in Schwann cells called nodes of Ranvier.
- Cell body that has nucleus and many mitochondria and ribosomes.
- Motor neurones have cell body in CNS, and have long axon and small dendrites.
- Sensory have cell body just outside CNS, a short axon and long dendrites.
- Dendrites connect to other neurons too.
How does the nerve cell maintain the resting potential across the membrane?
- Sodium/potassium ion pumps use ATP to pump 3 Na+ ions out for every 2 K+ in.
- Plasma membrane is also more permeable to potassium ions than to Na+ ions, some K+ diffuse out again (some K+ ion channels are open)
- Cell cytoplasm also contains large organic anions (negatively charged ions).
What is the potential difference across the cell membrane during resting potential?
In the ____ region of receptor cells, the gated channels are opened by ____ changes in evnrionment. Give eg.
- Eg. Gates in pressure receptor cells open by deformation.
How is an action potential created?
When depolarisation caused by generator potential is large enough, to reach the threshold potential. This will open nearby voltage-gated channels, which causes a large influx of Na+ ions and depolarisation reaches +40mV, which is an action potential.
An action potential is self-perpetuating. Why?
Becuase once it starts at one point in the neurone, it will continue along to the end of the neurone.
What value is the threshold potential?
Describe the stages after the potential difference hits +40mV.
- Sodium ion channels close and potassium channels open.
- K+ ions diffuse out of cell, bringing potential difference back to negative inside compared to outside - repolarisation.
- Potential difference overshoots slightly - making cell hyperpolarised.
- Orignal p.d is restored and cell returns to resting state.
After an action potential, the sodium and potassium ions are in the ____ places. To restore the conc of these ions inside and outside of cell, the ___ ___ ___ ___ must come into action, also restoring it to resting state after hyperpolarisation.
- sodium/potassium ion pumps
Why is it impossible to stimulate cell membrane to reach another action potential straight after an action potential?
- Sodium channels need time to recover, and so remain closed for a short time.
- The cell membrane is hyperpolarised, which means it is even more negative and harder to depolarise (about -80mV instead of -60mV)
- Much Na+ ions are still in cell, which reduce influx of Na+ ions if channels open
- Some K+ ion gates are still open, allowing K+ ions out of cell, cancelling the voltage change in cell.
What is the period when the neuron membrane cannot reach action potential shortly after one?
What is a local current?
- Movements of ions (Na+ in this case) along the neurone. Flow of ions is caused by an increase in conc at one point, which causes diffusion away from the region of higher conc.
- In this case, Na+ ions diffuse along the axon/dendron within the cell, from region of higher conc.
How is an action potential transmitted along neurone?
- Action potential in one area means a lot of Na+ within the cell in that area.
- Na+ diffuse sideways along the dendron/axon, away from this area.
- This causes depolarisation (reducing potential difference across membrane) further along the neurone, opening voltage-gated sodium ion channels, thus allowing action potential here too.
What cells make up the myelin sheath? And what are the gaps in these cells called?
- Schwann cells
- Nodes of Ranvier
Explain how an action potential is transmitted in myelinated neurone.
- Myelin sheath is insulating layer of fatty material - ions cannot diffuse through it.
- Ionic exchanges across membrane that cause action potential can only happen at nodes of Ranvier.
- The local currents are elongated and sodium ions diffuse along the neurone from one node to the next.
- The action potential appears to jump from one node to next.
Myelinated neurones causes a ____ conduction. What does this mean.
- Saltatory (meaning jump in Latin) conduction
- Means "jumping conduction" - it refers to the way that the action potential appears to jump from one node to another, because the rest of the neurone membrane is covered by the myelin sheath.
What is the advantage of myelinated neurone?
- The transmission of action potential is faster than in a non-myelinated one. (120m/s) [compared to 2-20m/s)
- This is because of the saltatory conduction, which means the action potentials only occur at certain points and "jumps" along.
How do voltage-gated channels open?
When the potential/voltage around channel changes, this causes the protein channel to change shape, allowing ions through.
The synapse we need to know is called a ____ synapese. What is it?
- Cholinergic synapse.
- They are synapses that use acetylcholine as the neurotransmitter.
The presynaptic action potential causes the ___ of a chemical (____) that diffuses across the ____ ____ (__nm wide) and ____ a new action potential in the _____ neurone.
- neurotransmitter / transmitter substance
- synaptic cleft (20nm wide)
A chemical that diffuses across the synaptic cleft to transmit a signal (action potential) to the postsynaptic neurone.
The presynaptic neurone ends in a swelling called the ____ ____. Explain its specialised features.
- synaptic knob
- Many mitochondria - active process of moving vesicle containing neurotransmitter and creating acetlycholine/neurotransmitter.
- Many smooth endoplasmic reticulum - to create/synthesise/ and package neurotransmitter
- Vesicles of acetylcholine
- Volage-gated calcium ion channels in membrane
The postsynaptic membrane contains ___ sodium ion channels that ___ to neurotransmitter. How does it work?
- Channels consist of 5 polypeptide molecules. 2 of these have special receptor site that is specific to neurotransmitter (eg. acetylcholine). When it binds to complementary site, the sodium ion is stimulated to open.
Describe brifely the stages involved in the transmission of an impulse across the synaptic cleft.
- Action potential arrives at synaptic knob
- Voltage-gated calcium ion channels open and they diffuse in.
- Ca2+ cause exocytosis of vesicles with neurotransmitter (vesicle move and fuses etc)
- Acetylecholine (transmitter) diffuse across cleft, and bind to receptor sites on Na+ channels in postsynaptic membrane.
- Na+ diffuse into postsynaptic neurone.
- Generator potential or excitatory postsynaptic potential (EPSP) is created.
- If sufficient generator potentials combine, then it will reach threshold potential and action potential is created.
What enzyme is found in the synaptic cleft, and what is its purpose?
- This hydrolyses acetylcholine to ethanoic acid and choline. This stops transmission of signals so they don't continue to cause action potential in postsynaptic membrane.
- Ethanoic acid and choline diffuse back into synaptic knob (recycled) and recombined to acetylcholine using ATP - then stored in vesicle for future use.
Why does the presynaptic neurone end in a synaptic knob?
- Because it provides a larger surface area.
- This allows more vesicles of acetylcholine to be released, also allowing the presynaptic neurone to stimulate more than one postsynaptic neurone.
An action potential is an all or nothing response. What does this mean?
- Refers to fact that a neurone either conducts an action potential or it does not. Once it starts, it will conduct it all along its length.
- All action potentials are of the same magnitude, +40mV.
List the different roles of synapses in the nervous system. (7)
- onverge to one postsynaptic neurone - allow signals from different parts of nervous system to create same response, like different stimuli for danger.
- Diverge - allow one signal to be transmitted to several parts of n.system. Eg. in a reflex arc - one post.s.neurone elicits response, while the other informs brain.
- Ensure that signals transmitted in one direction - only knob contains vesicles of acetylcholine.
- Filter out unwanted low-level signals - if low level stimulus causes a.p in pre.s.n. it is unlikely to pass across synapse because several vesicles of n.t. needed to pass it on.
- Low-level stimuli can be aplified by summation.
- Acclimatisation - after repeated stimulation, synapse runs out of vesicles with n.t aand it is fatigued. NS no longer responds to the stimulus. eg. getting used to background noise. ALSO avoid overstimulation of an effector, which could damage it.
- Creation of specific pathways - wide range of messages and complex brain activity.
List 2 types of summation.
- Temporal summation: one action potential cannot stimulate response in postsynaptic neurone, but a series of them from one neurone can. So, many small ESPSs to create one large potential.
- Spatial summation: Several presynaptic neurones may each contribute to producing an action potential in postsynaptic neurone.
What is the significance of the frequency of impulse transmission?
- A higher frequency of action potential/signals means a more intense stimulus. (indicator of intensity of stimulus)
- One a.p is always same change in voltage, so it is the frquency that informs the intensity.
- A strong stimulus cause sensory receptor to produce more generator potentials, thus more action potentials.
Around __-___ of ___ neurones in verterbrates are myelinated. The rest and that of ___ are not.
Compare structure of myelinated and non-myelinated neurones.
- Myelinated: Schwann cells tightly wrapped around neurone, so sheath consists of several layers of membrane and thin cytoplasm. Nodes of Ranvier at intervals of 1-3mm, and each node around 2-3 micrometres.
- Non-Myelinated: Still associated with Schwann cells, but several neurones may be surrounded by one loosely wrapped Schwann cell. So a.p moves along in a wave rather than jumping from node to node.
What types of neurones tend to be myelinated?
- Sensory receptors to the CNS or motor neurones from CNS to effector.
- They tend to be very long and need to travel fast in order to create a rapid response.
- Non-myelinated tend to be shorter and responsible for things in the brain and also for breathing and digestion, which need speed less ergently.