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What does it mean if a neuron is myelinated?
The axon is insulated by a fatty layer called the myelin sheath
"What is the myelin sheath made up of?"
The myelin sheath is made up of specialised cells called Schwann cells.
"What are the gaps between Schwann cells called?"
The gaps are known as the nodes of Ranvier
"How do electrical impulses travel in myelinated neurons?"
Electrical impulses jump from one node to the next, speeding up impulse transmission."
"What happens in non-myelinated neurons?"
The axon is not insulated by Schwann cells, and the impulse travels more slowly along the entire length of the axon
"Motor neurones: What is at one end of the motor neurone and where is it located?"
A large cell body at one end that lies within the spinal cord or brain
"Motor neurones: What extends from the cell body in motor neurones?"
Many highly-branched dendrites extending from the cell body, providing many connections with the axon terminals of other neurones
"Relay neurones: What are relay neurones like in structure?
"Short neurones with axons and highly branched dendrites
"Sensory neurones: Where is the cell body located in sensory neurones?"
The cell body branches off in the middle of the axon and has no dendrites
"Sensory neurones: What is attached to the axon terminal in sensory neurones?"
The axon terminal is attached to a receptor cell
"Sensory neurones: What is the section of the neurone that links the axon terminal with the cell body?"
The section is known as a dendron
"Sensory neurones: What section of the neurone connects the cell body with the CNS?"
axon
"How do nerve impulses pass through the nervous system?"
Stimulus → Receptor → Sensory neurone → CNS → Motor neurone → Effector.
"What is a reflex arc?"
A reflex arc is a pathway along which impulses are transmitted from a receptor to an effector without involving conscious regions of the brain.
"What does the pathway of a spinal reflex involve?"
Relay neurones located in the spinal cord."
"What is the spinal cord made up of?"
The spinal cord is made up of grey matter and white matter."
"What does grey matter contain?"
Grey matter contains the cell bodies of motor neurones along with relay neurones
"What does white matter contain?"
White matter contains long myelinated axons that carry information through the spinal cord
"What does the pathway of a cranial reflex involve?"
Relay neurones located in the brain
In a resting axon, the inside has a negative electrical potential compared to the outside.
This is due to different numbers of ions on each side of the membrane.
The membrane is polarised when there is a difference in charge across it.
The resting potential is around -70 mV, meaning the inside is 70 mV lower than the outside.
Sodium-potassium pumps actively transport 3 sodium ions out and 2 potassium ions in, using ATP.
This creates a concentration gradient for both sodium and potassium ions.
The membrane is more permeable to potassium ions than sodium ions.
Potassium ions diffuse out faster than sodium ions diffuse in.
This leaves more positive ions outside the neurone, making the inside negatively charged.
The final resting membrane potential is around -70 mV.
Voltage gated ion channels are closed at rest
Not all channels are voltage gated – some potassium ion channels remain open to maintain resting potential.
When the neurone is stimulated, a small number of sodium ion channels open.
Sodium ions move into the axon down their concentration gradient.
This reduces the potential difference across the membrane as the inside becomes less negative.
If the potential difference reaches -55 mV (threshold potential), more voltage gated sodium ion channels open.
This causes a further influx of sodium ions.
An action potential is only triggered if the threshold potential is reached.
When the membrane potential changes from -70 mV to around +30 mV, the membrane is depolarised and an action potential is generated."
About 1 millisecond after an action potential, voltage gated sodium channels close.
Voltage gated potassium channels open, allowing potassium ions to diffuse out down their concentration gradient.
Sodium-potassium pumps continue working, maintaining the potassium ion gradient.
Potassium ions moving out make the inside of the axon negatively charged again, a process called repolarisation.
The membrane briefly becomes more negative than the resting potential, known as hyperpolarisation.
The period of hyperpolarisation is called the refractory period.
During the refractory period, the membrane is unresponsive to stimulation, preventing a new action potential.
Voltage gated potassium channels close.
Sodium-potassium pumps restore the resting potential, making the membrane ready for another impulse.
"What is action potential propagation?"
The transmission of an action potential along the axon
"How does depolarisation at one site trigger the next action potential?
Sodium ions diffuse into the next section, opening voltage gated sodium channels.
"Why does the action potential only travel in one direction?"
The previous section of membrane is hyperpolarised during the refractory period
"What is the wave of depolarisation?"
The continuous transmission of action potentials along the axon.
"How does the refractory period help propagation?"
It prevents the action potential from moving backwards.
"What is the all-or-nothing principle?"
An impulse is only transmitted if the stimulus increases the membrane potential above the threshold potential.
"How does the brain detect stimulus size?"
By the frequency of action potentials transmitted along the neurone.
"What happens with a small stimulus?
It may only generate one action potential.
"What happens with a large stimulus?"
It may generate several action potentials in a row.
"How can medications prevent impulse transmission?"
By binding to sodium ion channels and preventing them from opening.
"What does preventing sodium ion channels from opening do?"
It prevents the influx of sodium ions when an axon is stimulated.
"What is the result of preventing sodium ion influx?"
It prevents membrane depolarisation, and an action potential cannot be generated.
"Can electrical impulses jump across the synaptic cleft?
No, electrical impulses cannot jump across the synaptic cleft.
"What happens when an action potential arrives at the presynaptic neurone?"
The membrane depolarises, causing voltage gated calcium ion channels to open.
"How do calcium ions contribute to neurotransmitter release?
Calcium ions diffuse into the synaptic knob, causing vesicles to fuse with the presynaptic membrane and release neurotransmitters.
"What happens when neurotransmitters bind to receptors on the postsynaptic membrane?"
Sodium ion channels open, allowing sodium ions to diffuse into the postsynaptic cell.
"What determines if an action potential is generated in the postsynaptic neurone?"
Whether the threshold potential is reached, depending on the number of neurotransmitter molecules binding to receptors
"How does the frequency of action potentials affect neurotransmitter release?
Many action potentials release more neurotransmitter via exocytosis
"What happens when a large amount of neurotransmitter is released?"
More sodium ion channels open, allowing a large influx of sodium ions, increasing the likelihood of threshold potential being reached.
"What does unidirectionality of impulse transmission mean at synapses?"
Impulses can only pass in one direction because neurotransmitter is released on one side and receptors are on the other.
"What is divergence of nerve impulses?"
One neurone can connect to several others at a synapse, allowing nerve signals to be sent in multiple directions.
"What is amplification of nerve signals by summation?"
When multiple impulses combine to overcome the insufficient release of neurotransmitters and generate an action potential.
"Why might a single impulse not generate an action potential?"
A single impulse may release only a small amount of neurotransmitter, opening few sodium ion channels and not reaching threshold potential.
"What is summation?"
The process where multiple impulses combine to generate an action potential in the postsynaptic neurone
"What is synaptic convergence?"
When several presynaptic neurones converge to meet a single postsynaptic neurone
"What happens when many action potentials arrive at a postsynaptic knob in quick succession?"
They can generate an action potential by summation due to the cumulative effect of the impulses.
"What is the function of the circular muscles in the eye?"
The circular muscles contract to constrict the pupil.
"What is the function of the radial muscles in the eye?
The radial muscles contract to dilate the pupil
"What is nicotine and how does it affect synapses?"
Nicotine is the addictive chemical in tobacco. It mimics acetylcholine by binding to nicotinic receptors on postsynaptic neurones, triggering an action potential
"What happens to nicotinic receptors after nicotine binds to them?"
After stimulation by nicotine, nicotinic receptors become unresponsive to further stimulation for a prolonged period.
"What effect does nicotine have on dopamine?"
Nicotine stimulates the release of dopamine from the pleasure centres of the brain, reinforcing the behaviour of smoking
"What are the effects of nicotine on the circulatory system?"
Nicotine increases heart rate and blood pressure, raising the risk of circulatory problems and lung cancer.
"What is lidocaine and how does it work?"
a local anaesthetic that blocks voltage-gated sodium channels, preventing an action potential from being generated in postsynaptic neurones
"How is lidocaine used in medicine?"
used as a local anaesthetic in dental procedures and to regulate heart rhythms in people with irregular heartbeats.
"What is cobra venom and how does it affect synapses?"
binds to acetylcholine receptors on postsynaptic membranes, preventing sodium ion influx and causing muscle paralysis.
"What is the effect of cobra venom on the body?
paralysis of muscles, including those that control breathing, leading to death. Small amounts can be used as a muscle relaxant in asthma attacks
"What is L-dopa and how is it used to treat Parkinson's disease?"
L-dopa is converted into dopamine in the brain, increasing dopamine levels and helping to control movement in people with Parkinson's disease.
"Why can't dopamine be directly given to Parkinson's patients?
cannot cross the blood-brain barrier, so L-dopa is used to increase dopamine levels in the brain.
"What is MDMA and how does it affect serotonin?"
inhibits serotonin reuptake and triggers the release of more serotonin, leading to increased serotonin levels in the brain
"What effects does MDMA have on people?"
can cause euphoria, enhanced touch, and bodily sensations, and affects mood, anxiety, and sleep.
"What controls the amount of light that enters the eye?"
The muscles of the iris
"What is the function of the ciliary muscles in the eye?"
The ciliary muscles change the shape of the lens to focus light from objects at different distances.
"Where is light focused in the retina?"
fovea
"Where are rod cells primarily located in the retina?”
around the outer retina.
"What is the function of rod cells in the eye?"
sensitive to light intensity and detect the presence and brightness of light, contributing to black-and-white vision
"Where are cone cells mostly found in the retina?"
Cone cells are mostly found grouped together in the fovea.
"What is the function of cone cells in the eye?"
sensitive to different wavelengths of visible light and detect colour
"What types of cone cells exist in the retina?"
There are red-sensitive, green-sensitive, and blue-sensitive cone cells
"Where does the optic nerve leave the eye?"
The optic nerve leaves the eye from the blind spot.
"What is the blind spot in the eye?"
a region at the back of the eye with no photoreceptors
"What happens when light falls on light-sensitive pigments in photoreceptors?"
Light-sensitive pigments are bleached when light falls on them.
"What is the light-sensitive pigment found in rod cells?"
rhodopsin
"What happens when light hits rhodopsin?"
it breaks apart into retinal and opsin which is known as bleaching
"What happens when the light-sensitive pigments are bleached?"
This leads to the closure of sodium ion channels in the rod cell
prevents sodium ions from entering the rod cell, leading to a more negative interior and hyperpolarisation
The hyperpolarised rod cell stops releasing an inhibitory neurotransmitter, allowing an action potential to be generated in the neighbouring bipolar neurone.
"How do nerve impulses travel from the photoreceptors to the brain?"
Nerve impulses travel along a bipolar neurone to the optic nerve, which carries information to the brain.
"How do rod cells pass information to the optic nerve?"
Rod cells initiate action potentials in neighbouring bipolar neurones when they are hyperpolarised, rather than when they are depolarised.
"What happens inside rod cells in the dark?"
In the dark, sodium ions are actively pumped out of rod cells, generating a concentration gradient. Sodium ions diffuse back into the rod cell through sodium channels, causing depolarisation.
"What is the charge of the inside of the rod cell in the dark?"
slightly negative
it releases an inhibitory neurotransmitter which prevents the generation of an action potential in the neighbouring bipolar neurone."
"What is habituation?"
Habituation is the process where an animal learns not to respond to a stimulus if it is repeated many times with no negative outcome
"What happens during habituation at the synapse?"
Fewer calcium ions move into the presynaptic neurone on arrival of a nerve impulse, resulting in less neurotransmitter release. This causes fewer sodium ion channels to open, preventing threshold potential from being reached and preventing a response to the stimulus.
"What are the components of the human nervous system?
The human nervous system consists of the central nervous system (CNS) – the brain and spinal cord,
the peripheral nervous system (PNS) – the parts of the nervous system that extend to the rest of the body.
What are glial cells and what is their function?
Glial cells support and protect neurons. They do not conduct electrical signals but are essential for the proper functioning of the nervous system.
What are the functions of Astrocytes?
Astrocytes support neurons, regulate nutrients, and maintain the blood-brain barrier
What is the role of Oligodendrocytes?
Oligodendrocytes form the myelin sheath in the central nervous system.
What is the function of Schwann Cells?
Schwann cells form the myelin sheath in the peripheral nervous system.
What are the functions of Microglia?
Microglia act as immune cells, removing debris and pathogens in the nervous system.
What do Ependymal Cells do?
Ependymal cells line the brain ventricles and produce cerebrospinal fluid.
