5.1.3 - Neuronal Communication Flashcards
What are neurones?
- specialised cells of the nervous system
- their role is to transmit electrical impulses rapidly around the body so that the organism can respond to changes in the internal and external environment (stimuli)
- a bundle of neurones in known as a nerve
What are the key features of a neurone?
- cell body
- dendrons + dendrites
- axons + axon terminals
What is the cell body of a neurone?
- contains the nucleus surrounded by the cytoplasm
* within the cytoplasm there are lots of mitochondria and endoplasmic reticulum for the production of neurotransmitters
What are dendrons?
- short extensions that come from the cell body
- these extensions divide into smaller branches known as dendrites
- dendrites allow neurones to connect to many other neurones/receptor cells and receive impulses from them, forming a network for easy communication
What is the axon?
- a singlular, elongated nerve fibre that transmit impulses away from the cell body
- cylindrical in shape, with a very narrow region of cytoplasm (~1um)
- axon terminals are found at the end of neurones and connect to other neurones or effector cells
What are myelinated neurones?
- neurones which are insulated with a myelin sheath with small, uninsulated sections along its length (nodes of Ranvier)
- this insulation makes the transmission of electrical impulses much faster (up to 100m/s in myelinated neurones whereas in myelinated neurones ~ 1m/s)
What is the myelin sheath made up of?
- many layers of plasma membrane produced by Schwann cells
* the Schwann cells grow around the axon many times, creating lots of layers of membrane
What are nodes of Ranvier?
- between 2 Schwann cells, there is a gap (approx 2-3um) called the node of Ranvier
- these gaps in the myelin sheath allows the electrical impulse to ‘jump’ from one node to the next as it travels along the neurone
- as a result, the electrical impulse travels much faster
What is a sensory neurone?
Transmit impulses from a sensory receptor to a relay neurone, motor neurone or brain
Key features:
• one dendron (carries impulse towards cell body)
• cell body in the middle of the neurone
• one axon (carries impulse away from cell body)
What are relay neurones?
Transmit impulses between neurones
Have lots of short, highly branched axons and dendrons
What are motor neurones?
Transmit impulses from a relay/sensory neurone to an effector (eg muscle or gland)
Key features:
• a large cell body at one end
• lots of highly-branched dendrons extending from the cell body
• one long axon branching from the cell body
What are sensory receptor cells?
A cell that responds to a stimulus
2 key features:
• act as a transducer - convert energy from one form (such as light, heat, sound) into an electrical impulse in a sensory neurone
• will only respond to a specific stimulus
What are the main types of sensory receptors?
MECHANORECEPTOR
• stimulus = pressure and movement
• example of receptor = Pancinian corpuscle (detects pressure)
•example of sensory organ = skin
CHEMORECEPTOR
• stimulus = chemicals
• example of receptor = olfactory receptor (detects smells)
•example of sensory organ = nose
THERMORECEPTOR
• stimulus = heat
• example of receptor = end-bulbs of Krause
•example of sensory organ = tongue
PHOTORECEPTOR
• stimulus = light
• example of receptor = cone cell (detects different light wavelengths)
•example of sensory organ = eye
What are Pacinian corpuscles?
- a type of mechanoreceptor found deep in the skin (most abundant in the fingers and soles of feet, also found in joints, tendons, ligaments)
- detect mechanical pressure
- a corpuscle is made of many layers of connective tissue with a gel (containing Na+) separating each layer
- at the centre of a corpuscle is a sensory neurone ending
- the sensory neurone in a corpuscle has stretch-mediated sodium channels
How does a Pacinian corpuscle convert mechanical pressure into a nervous impulse?
1) in the resting state, the stretch-mediated sodium ion channels in the sensory-neurones membrane are too narrow to allow Na+ to pass through them. The neurone has a resting potential.
2) when pressure is applied to the Pacinian corpuscle, it changes shape. This causes the membrane surrounding the neurone to stretch
3) when the membrane stretches, the Na+ channels present widen. Na+ can now diffuse into the sensory neurone.
4) the influx of Na+ depolarises the sensory neurone. This results in a generator potential
5) in turn, the generator potential creates an action potential that passes along the sensory neurone.
What is a resting potential?
The potential difference of a neurone that is not transmitting an impulse.
The outside of the membrane is more positively charged than the inside of the embrace
The pd across the membrane is approx -70mV
What are the 2 factors that contribute to establishing and maintaining the resting potential?
- active transport of sodium ions and potassium ions
* differential membrane permeability
How does active transport of Na+ and K+ contribute to the establishment and maintenance of a resting potential?
- carrier proteins called sodium-potassium pumps are present in the membrane of neurones
- these pumps use ATP to actively transport 3Na+ ions out of the axon for every 2K+ ions actively transported into the axon
- there are more positive ions outside the axon than there are inside the axon and an electrochemical gradient is established
How does differential membrane permeability contribute to the establishment and maintenance of a resting potential?
- due to active transport, there is a high concentration of Na+ outside the axon and a high concentration of K+ inside the axon
- the membrane is more permeable to potassium ions because most of the voltage-gated Na+ channels are closed whereas many potassium channels are open
- this means that K+ can diffuse back out of the cell, but Na+ can’t diffuse back into the cell
- therefore, there are more positive ions outside the cell than inside, creating a resting potential of approx -70mV
What is an action potential?
The change in the potential difference across the neurone membrane when stimulated
Caused when the potential difference across the membrane is approx -50mV (threshold potential)
How is an action potential established?
1) resting potential
• some K+ channels open
• Na+ voltage-gated ion channels are closed
2) stimulus trigger
• a stimulus causes some Na+ channels to open, making the membrane more permeable to Na+
• Na+ diffuses into the axon, down the electrochemical gradient
• inside of the neurone is less negative
3) depolarisation
•if the potential difference reaches -50mV (threshold potential), more voltage gated Na+ channels open so more Na+ diffuse into the axon
• this is an example of positive feedback
4) action potential
• pd reaches +40mV
• voltage gated Na+ channels close and voltage-gated K+ channels open
• Na+ can’t enter the axon but membrane is more permeable to K+
5) repolarisation
•K+ diffuse out of the axon, down electrochemical gradient
•charge is reduced, resulting in the inside of the axon becoming more negative than the outside
6) hyperpolarisation
•K+ moving out of axon results in axon becoming more negative than its normal resting state
• voltage-gated K+ channels now close
• sodium-potassium pump causes Na+ to move out of cell and K+ to move into cell
• axon returns to resting potential
How does current flow in a neurone?
- a stimulus triggers an action potential in the first region of an action membrane, so it becomes depolarised
- this acts as a stimulus for the next part of the axon; Na+ channels open, there is an influx of Na+ and the section is depolarised
- the positive Na+ are attracted to the negative charge ahead, so diffuse down the concentration gradient along the axon (causing a current), triggering the depolarisation of the next section
- the previous section of the axon is in the repolarisation stage and therefore unresponsive. this means that the action potentials are discrete events and the impulse only travels in one direction
How is an action potential propagated across a non-myelinated neurone?
1) resting potential
• some K+ channels open
• Na+ voltage-gated ion channels are closed
• high conc Na+ on outside
2) stimulus
• stimulus causes influx of Na+ ions (voltage gated Na+ channels open) - action potential
• section of membrane is depolarised
3)
• localised electrical circuits act as a stimulus for the next section of membrane, triggering an action potential
• behind new section of depolarisation, voltage-gated Na+ channels close and K+ ones open
• K+ diffuse out of membrane - repolarisation + hyperpolarisation
4)
• action potential propagated the same way across whole neurone.
• after repolarisation and hyperpolarisation, neurone returns to resting potential
What is the refractory period?
- a short period of time after an action potential where the axon cant be excited again
- voltage gated sodium ion channels remain closed, preventing the propagation of the axion potential backwards
- also makes sure action potentials don’t overlap and occur as discrete impulses
- the length of the refractory period is key in determining the max frequency at which impulses can be transmitted along neurones
What is the all-or-nothing principle?
- if the stimulus is very weak, the sensory neurone won’t be sufficiently depolarised to trigger an action potential because the threshold potential wont be reached
- if the threshold potential is reached, an action potential will be triggered. the strength of the stimulus doesn’t affect the strength of the action potential
- however, a larger stimulus will produce more frequent action potentials
What is the all-or-nothing principle?
- if the stimulus is very weak, the sensory neurone won’t be sufficiently depolarised to trigger an action potential because the threshold potential wont be reached
- if the threshold potential is reached, an action potential will be triggered. the strength of the stimulus doesn’t affect the strength of the action potnetial
What is saltatory conduction?
- occurs in myelinated neurones
- myelin sheath insulated the membrane, so no Na+/K+ can diffuse across it and it can’t be depolarised
- action potentials can only occur at the nodes of Ranvier
- action potentials ‘jump’ from one node to the next, allowing the impulse to travel much faster (opening channels takes time - less channels, less time wasted)
- more energy efficient - repolarisation requires ATP in sodium pump
What factors affect the speed of transmission in a neurone?
Myelination
Diameter
• bigger the d, faster the impulse
• less resistance to the flow of ions in the cytoplasm
Temperature
• higher the temp, faster the impulse
• ions diffuse faster at higher temps
• however, above 40C, proteins denature so ions cannot diffuse across membrane
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What is a synapse?
A junction where 2 neurones meet
Key features: • synaptic cleft • presynaptic neurone • postsynaptic neurone • synaptic knob • synaptic vesicles • neurotransmitter receptors
What is the synaptic cleft?
gap which separates axon from one neurone and the dendrite of another
approx 20-30nm
what is the presynaptic neurone?
neurone along which the impulse has arrived at the synapse
What is the postsynaptic neurone?
neurone that receives the neurotransmitter
what is the synaptic knob?
the swollen end of the presynaptic neurone
contains many endoplasmic reticulum and mitochondria to manufacture neurotransmitters
what are synaptic vesicles?
vesicles containing neurotransmitters
fuse with the presynaptic membrane and release contents into synaptic cleft (exocytosis)
What are neurotransmitter receptors?
receptor molecules which the neurotransmitter binds to in the postsynaptic membrane
