Neurones Flashcards
What does the cell body of neurones contain?
The nucleus, surrounded by cytoplasm
Why does the nucleus of neurones have large amounts of endoplasmic reticulum and mitochondria?
They are involved in the production of neurotransmitters
What are dendrons and what do they split into?
Short extensions which come from the cell body and transmit electrical impulses towards the cell body. These extensions divide into smaller branches called dendrites
What is the axon?
The singular nerve fibre that transmits impulses away from the cell body
What are the 3 types of neurone?
Sensory, relay and motor neurone
What is the role of sensory neurones?
Transmit impulses from sensory receptor to relay neurone, motor neurone or the brain
What is the structure of the sensory neurone?
They have one dendron, which carries the impulse towards the cell body, and one axon, which carries the impulse away from the cell body
What is the role of relay neurones?
Transmit impulses between neurones
What is the structure of relay neurones?
They have many short dendrons and a short axon
What is the role of motor neurones?
Transmit impulses from a relay or sensory neurone to an effector
What is the structure of motor neurones?
They have many short dendrites going into the cell body and then one long axon which carries the impulse away from the cell body
What are Schwann cells?
Cells that produce many layers of membranes by growing around the axon many times.
What are the layers of membranes around axons known as?
Myelin sheath
What is the impact of myelinated sheaths on transmission speed?
Myelinated neurones transmit impulses much faster than non-myelinated neurones
What are nodes of Ranvier?
The small gaps between Schwann cells, which creates gaps in the myelin sheath
Why do myelinated neurones transmit impulses faster than non-myelinated neurones?
In myelinated neurones, the impulse jumps from one node of Ranvier to the next as it travels along the neurone, allowing the impulse to be transmitted much faster.
In non-myelinated neurones, the impulse transmits continuously along the axon without jumping, so is much slower
What is found at the end of axons?
Axon terminals
What do axon terminals do?
Make synaptic connections with another nerve cell or with an effector cell
What do sensory receptors do?
Convert the stimulus they are specific to into a nerve impulse
What are the four main types of sensory receptors?
Mechanoreceptor
Chemoreceptor
Thermoreceptor
Photoreceptor
What do mechanoreceptors detect?
Pressure and movement
What do chemoreceptors detect?
Chemicals
What do thermoreceptors detect?
Heat
What do photoreceptors detect?
Light
Why are sensory receptors described as transducers?
They convert a stimulus into a nervous impulse (called a generator potential)
What are Pacinian corpuscles?
Specific sensory receptors found in your skin that detect mechanical pressure
What is the Pacinian corpuscle like without stimuli?
It is in its resting state, in which the stretch-mediated sodium ion channels in the sensory neurones membrane are too narrow to allow sodium ions to pass through. The neurone of the Pacinian corpuscle has a resting potential.
What happens once pressure is applied to the Pacinian corpuscle?
The membrane of the corpuscle stretches, allowing Na+ channels to open and allow sodium ions to diffuse into the neurone
What is the result of sodium ions now being able to diffuse into the neurone?
The influx of sodium ions depolarises the membrane, resulting in a generator potential. This generator potential may create an action potential (nerve impulse), that passes along the sensory neurone towards the CNS
What is the resting potential of a neurone?
When a neurone is not transmitting an impulse, the potential difference across its membrane is known as the resting potential.
At resting potential, the outside of the membrane is more positively charged than the inside of the axon, so is said to be polarised
What is the typical resting potential value?
-70 mV
What controls the resting potential of neurones?
The movement of sodium and potassium ions across the axon membrane
At resting potential, what controls the movement of Na+ and K+ ions?
The sodium-potassium pump actively transport sodium ions out of the axon, and actively transport potassium ions into the axon.
For every 3 sodium ions pumped out, only 2 potassium ions are pumped in
At resting potential what are the state of different ion channels?
Non-voltage gated potassium ion channels are open, so K+ ions can diffuse back out of the axon.
Voltage-gated sodium ion channels are closed, so Na+ ions cannot reenter the axon. This result in a more positively charged outside than inside, with the inside relatively negative (-70 mV)
In generating an action potential, what does the energy of stimulus trigger?
The energy from the stimulus triggers some voltage-gated sodium channels to open, allowing some sodium ions to diffuse into the axon
In generating an action potential, how does positive feedback work?
As some sodium ions diffuse into the axon, it causes a change in charge which causes more sodium ion channels to open, allowing more sodium ions to diffuse in
In generating an action potential, when do the voltage gated sodium ion channels close, and then what happens?
At +40 mV, voltage gated sodium ion channels close and voltage gated potassium ion channels open
What happens once voltage gated potassium ion channels open?
Initially lots of potassium ions diffuse out of the axon down the electrochemical gradient, resulting in the inside of the axon becoming more negative than its normal resting potential (more negative than -70)
How is the resting potential restored?
Voltage-gated potassium ion channels close, and the Na-K pump moves sodium and potassium ions to restore the -70 mV resting potential
What is the period of depolarisation?
As sodium ion channels open and sodium ions move into the axon
What is the period of repolarisation?
As potassium ion channels open and sodium ion channels close, and potassium ions move out of the axon
What is hyperpolarisation?
Where initially so many potassium ions move out of the axon it results in a more negative potential than the resting potential
When is the axon repolarised?
When the axon has returned to its resting potential
What is a nerve impulse?
An action potential that starts at one end of the neurone and is propagated along the axon to the other end of the neurone
How are action potentials propagated (non-myelinated)?
The influx of sodium ions cause the opening of sodium voltage-gated channels a little further along the axon, causing a new region of depolarisation. This propagation of the depolarisation continues along the entire axon in localised circuits.
What is a refractory period?
After an action potential there is a short period of time when that part of the axon cannot be excited again, known as the refractory period. During this time, voltage-gated sodium ions remain closed
Why is a refractory period important?
It prevents the propagation of an action potential backwards along the axon, ensuring the nerve impulse is unidirectional. It also ensures that action potentials do not overlap
How are action potentials propagated in myelinated axons?
Depolarisation if the axon membrane can only occur at the nodes of Ranvier (where no myelin is present), meaning longer localised circuits arise between adjacent nodes. The action potential ‘jumps’ from node to node.
What is the process of action potentials jumping from node to node known as?
Saltatory conduction
Why does saltatory conduction allow for faster nervous transmission?
Every time channels open and ions move this takes time, so by reducing the number of times this happens increases the speed of action potential transmission
How does saltatory conduction also make the conduction of impulses more efficient?
Repolarisation uses ATP (for the Na-K pump), so by reducing the amount of repolarisation needed, it reduces the amount of ATP needed
Apart from myelination, what other two factors affect the speed at which an action potential travels?
- Axon diameter
- Temperature
How does axon diameter impact speed of action potential transmission?
The bigger the axon diameter, the faster the transmission, as there is less resistance to the flow of ions
How does temperature impact speed of action potential transmission?
The higher the temperature, the faster the transmission, as ions will diffuse faster.
However if the temperature increases too high this could cause the proteins (e.g. Na-K pump) to denature
What is the all-or-nothing principle?
If a stimulus exceeds a threshold value, the same sized action potential will always be triggered. However if the threshold is not reached, no action potential will be triggered
What will be the difference between the nervous responses to a weak stimuli and a strong stimuli that both exceed the threshold value?
In the strong stimuli, the size of the action potential will be the same however the actions potentials generated will be much more frequent (i.e. a greater number of action potentials will be generated)
