5:1:3 Neuronal Communication Flashcards
1
Q
What are neurones
A
Specialised cells of the nervous system, which carry electrical impulses around the body. A bundle of neurones is a nerve
2
Q
What are the features of neurones
A
- Axon: long fibre
- Cell body containing a nucleus and other organelles
- Axon terminal contains many nerve endings
- Nerve endings at the axon terminal connect to many other neurones, forming a network
3
Q
What is a myelin sheath
A
- Some axons are myelinated, and are insulated by a myelin sheath
- The myelin sheath is formed by specialised cells (schwann cells) which wrap around the axon
- There are gaps (nodes of ranvier) along the sheath so the electrical impulses can jump along the axon, allowing the impulses to move faster than in a non-myelinated sheath
4
Q
What are the main types of neuron
A
- Sensory: carry impulses from receptors to the CNS
- Relay: found only in the CNS and connect sensory and motor neurones
- Motor: carry impulses from CNS to effectors
5
Q
What is the structure of motor neurones
A
- A large cell body at one end, which lies in the spinal cord or brain
- A nucleus (in the cell body)
- Many branched dendrites extending from the cell body
6
Q
What is the structure of relay neurones
A
- Short but highly branched, which an axon and dendrites
7
Q
What is the structure of sensory neurones
A
- Cell body branches off the middle of the cell
- A long dendron that caries impulses to the cell body
- A long axon that carries impulses away from the cell body
8
Q
What is a receptor cell
A
A cell that responds to stimuli, and are transducers as they convert energy from one form into energy in an electrical impulse
9
Q
What are examples of sensory receptors in the body, and their stimulus
A
- Photoreceptors: light
- Chemoreceptors: chemicals
- Mechanoreceptors: mechanical strain/stretching
10
Q
What are pacinian corpuscles
A
- Mechanoreceptors found deep in the skin
- Found at the ends of sensory neurone axons
- Made of layers of membrane separated by gel containing Na+ ions
- They respond to changes in pressure
- When the receptors are stimulated by pressure, they establish a generator potential by movement of Na+ ions
11
Q
Describe the process of converting mechanical pressure into a nervous impulse in the Pacinian corpuscle
A
- Before pressure: sodium ion channels are too narrow, so Na+ ions remain outside the membrane and resting potential is maintained
- Pressure is applied: pacinian layers are distorted causing the sodium channels to open, and Na+ ions to enter the axon of the sensory neurone
- The influx of Na+ changes the potential of the axon, causing depolarisation of the membrane, establishing an action potential which moves the nerve impulse along the axon
12
Q
What is resting potential
A
- A resting neurone (isn’t transmitting impulses) inside the axon always has a negative electrical potential compared to the outside of the axon
- This is the resting potential
- The potential difference of the inside and outside of the axon (without an electrical impulse) is -70mV
13
Q
How is the resting potential established and maintained
A
- Carrier proteins (sodium-potassium pumps) in the membranes of neurones use ATP to actively transport 3 Na+ ions out of the axon for every 2 K+ ions they actively transport in
- Creating a larger concentration of positive ions outside the axon than inside, creating a potential difference of -70mV
- The movement of ions through the pumps establishes an electrochemical gradient
- Leakage channels allow Na+ and K+ to move across the membrane via facilitated diffusion, but are less permeable to Na+ ions, so K+ ions diffuse back down their gradient at a higher rate
14
Q
How does the myelin sheath conduct the electrical impulses faster
A
- The phospholipid bilayers of the schwann cell which wrap around the axon
- They insulate the neurone and conduct electrical impulses
15
Q
What is an action potential
A
- A brief change in the distribution of electrical charge across the cell surface membrane, caused by the rapid movement of Na+ and K+ ions across the membrane of the axon
- Several things occur during an action potential, including stimulus, depolarisation, repolarisation, hyperpolarisation, and then the return to resting potential
16
Q
What happens during the stimulus of an action potential
A
- Stimulus (electrical/chemical impulse) triggers Na+ ion channels to open, allowing Na+ ions diffuse into the neuron down an electrochemical gradient
- A large enough stimulus converts the resting potential to an action potential
17
Q
What happens during the depolarisation of an action potential
A
- When a threshold of -55mV is reached an action potential is stimulated, causing the movement of Na+ ions into the axon reducing the potential difference across the axon membrane
- This is depolarisation, and it causes the inside of the axon to be less negative on the outside
- Depolarisation triggers more channels to open, allowing more Na+ ions in to cause more depolarisation in a positive feedback loop. The action potential generated reaches +30mV
18
Q
What happens during repolarisation of the action potential
A
- After the action potential has reached +30mV, all the Na+ ion voltage gated channel proteins close to stop for the Na+ ions into the axon
- K+ ion voltage gated channel proteins in the section of the axon open, allowing K+ ions to diffuse out down their concentration gradient
- This is repolarisation and it is a negative feedback loop
19
Q
What happens during hyper polarisation of the action potential
A
- K+ ion channels are slow to close, meaning that too many K+ ions diffuse out of the neuron, causing a short period of hyperpolarisation
- This means that the potential difference across the axon membrane becomes more negative than the normal resting potential
20
Q
How does the action potential return to resting potential
A
- After hyperpolarisation, the K+ ion voltage gated channel proteins close, and the sodium potassium pump restores the resting potential
- Na+ ion channel proteins become responsive to depolarisation again
21
Q
Describe the five stages of action potential and its graph
A
22
Q
How an impulse transmitted across a neurone
A
- The action potential triggered occurs via depolarisation and repolarisation, therefore is a discrete event, meaning that the impulse can only travel in one direction
23
Q
What is the all or nothing principle in neurones
A
- When receptors are stimulated they are depolarised
- If the stimulus is weak or below a certain threshold, the receptor cells won’t be sufficiently depolarised and an impulse won’t be sent
- If the stimulus is strong enough to increase the receptor potential above the threshold potential an impulse is sent
- Therefore threshold levels in receptors of an increase with continued stimulation, so greater stimuli are required along sensory neurons
24
Q
What is the refractory period in the action potential
A
- After the action potential, there is a period of time were both Na+ ion and K+ ion channels are closed, causing the axon membrane to be in a period of recovery (unresponsive)
25
Q
Why is the refractory period important
A
- Insures action potentials are discrete events
- Insures action potentials are generated a head of the original action potential (therefore can only travel in one direction)
26
Q
What factors affect the speed of conduction of an impulse across a neuron
A
- Myelination
- Diameter of axon
- Temperature
27
Q
How does myelination affect the speed of conduction of impulses along a neurone
A
- The presence of myelin increases the speed at which action potentials can travel along the neuron
- Depolarisation can’t occur in sections of the axon that are surrounded by myelin sheaths. Therefore, they only occur at the nodes of Ranvier
- This means the action potentials jump from one node to the next in saltatory conduction
28
Q
How does diameter affect the speed of conduction along the neurone
A
- Impulses are conducted at higher speeds along neurons with thicker axons due to them having axon membranes with greater surface area
- This increased surface area increases the rate of diffusion of Na+ and K+ ions through protein channels
- Also axons with greater diameter have a greater volume of cytoplasm, reducing the electrical resistance of the axon
29
Q
How do you say the speed of conduction along the neurone
A
- Cold conditions can slow down the conduction of nerve impulses, due to there being less kinetic energy available for facilitated diffusion of Na+ and K+ ions
- Hot conditions do the opposite
30
Q
What is the structure of a synapse
A
- Presynaptic knob
- Synaptic cleft
- Postsynaptic membrane
31
Q
Describe how impulses are transmitted at synapses
A
- When an electrical impulse arrives at the end of an accident on the presynaptic neurone/knob neurotransmitters are released from versicles at the presynaptic membrane
- The neurotransmitters diffuse across the synaptic cleft, and temporarily bind with receptor molecules on the postsynaptic membrane
- This stimulates, the postsynaptic neuron to generate an electrical impulse travels down the axon of the postsynaptic neuron
- Neurotransmitters are destroyed or recycled
32
Q
Describe how impulses are transmitted at cholinergic synapses
A
- Arrival of an action potential at the presynaptic membrane causes depolarisation of the membrane, stimulating voltage gated Ca2+ ion channel proteins to open
- Ca2+ ions diffuse down their electrochemical gradient into the synaptic knob, stimulating acetylcholine containing vesicles to fuse with the presynaptic membrane
- The ACh molecules are released via exocytosis, and diffuse across the synaptic cleft to temporarily bind to the cholinergic receptors in the postsynaptic membrane
- This causes Na+ ion channels to open, and Na+ ions to diffuse down their chemical gradient causing depolarisation of the postsynaptic membrane
- ACh is then broken down and recycled
33
Q
How are ACh molecules broken down and recycled
A
- Enzyme ACh esterase catalyses the hydrolysis of the ACh molecules into acetate and choline
- The choline is absorbed back into the presynaptic membrane and reacts with acetyl CoA to form ACh
34
Q
What is temporal summation
A
- Multiple impulses arriving within quick succession to generate an action potential
35
Q
What is spatial summation
A
- Multiple impulses arriving simultaneously at different synaptic knob is stimulating the same cell body to generate an action potential
36
Q
What are excitatory neurotransmitters
A
- Neurotransmitters that can stimulate the generation of an action potential in a postsynaptic neurone
37
Q
What are inhibitory neurotransmitters
A
- Neurotransmitters which prevent the generation of an action potential in a postsynaptic neurone
