5:1:3 Neuronal Communication

Question 1

What are neurones

Answer 1

Specialised cells of the nervous system, which carry electrical impulses around the body. A bundle of neurones is a nerve

Question 2

What are the features of neurones

Answer 2

• 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

Question 3

What is a myelin sheath

Answer 3

• 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

Question 4

What are the main types of neuron

Answer 4

• 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

Question 5

What is the structure of motor neurones

Answer 5

• 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

Question 6

What is the structure of relay neurones

Answer 6

• Short but highly branched, which an axon and dendrites

Question 7

What is the structure of sensory neurones

Answer 7

• 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

Question 8

What is a receptor cell

Answer 8

A cell that responds to stimuli, and are transducers as they convert energy from one form into energy in an electrical impulse

Question 9

What are examples of sensory receptors in the body, and their stimulus

Answer 9

• Photoreceptors: light
• Chemoreceptors: chemicals
• Mechanoreceptors: mechanical strain/stretching

Question 10

What are pacinian corpuscles

Answer 10

• 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

Question 11

Describe the process of converting mechanical pressure into a nervous impulse in the Pacinian corpuscle

Answer 11

• 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

Question 12

What is resting potential

Answer 12

• 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

Question 13

How is the resting potential established and maintained

Answer 13

• 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

Question 14

How does the myelin sheath conduct the electrical impulses faster

Answer 14

• The phospholipid bilayers of the schwann cell which wrap around the axon
• They insulate the neurone and conduct electrical impulses

Question 15

What is an action potential

Answer 15

• 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

Question 16

What happens during the stimulus of an action potential

Answer 16

• 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

Question 17

What happens during the depolarisation of an action potential

Answer 17

• 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

Question 18

What happens during repolarisation of the action potential

Answer 18

• 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

Question 19

What happens during hyper polarisation of the action potential

Answer 19

• 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

Question 20

How does the action potential return to resting potential

Answer 20

• 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

Question 21

Describe the five stages of action potential and its graph

Answer 21

Question 22

How an impulse transmitted across a neurone

Answer 22

• The action potential triggered occurs via depolarisation and repolarisation, therefore is a discrete event, meaning that the impulse can only travel in one direction

Question 23

What is the all or nothing principle in neurones

Answer 23

• 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

Question 24

What is the refractory period in the action potential

Answer 24

• 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)

Question 25

Why is the refractory period important

Answer 25

• 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)

Question 26

What factors affect the speed of conduction of an impulse across a neuron

Answer 26

• Myelination
• Diameter of axon
• Temperature

Question 27

How does myelination affect the speed of conduction of impulses along a neurone

Answer 27

• 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

Question 28

How does diameter affect the speed of conduction along the neurone

Answer 28

• 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

Question 29

How do you say the speed of conduction along the neurone

Answer 29

• 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

Question 30

What is the structure of a synapse

Answer 30

• Presynaptic knob
• Synaptic cleft
• Postsynaptic membrane

Question 31

Describe how impulses are transmitted at synapses

Answer 31

• 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

Question 32

Describe how impulses are transmitted at cholinergic synapses

Answer 32

• 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

Question 33

How are ACh molecules broken down and recycled

Answer 33

• 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

Question 34

What is temporal summation

Answer 34

• Multiple impulses arriving within quick succession to generate an action potential

Question 35

What is spatial summation

Answer 35

• Multiple impulses arriving simultaneously at different synaptic knob is stimulating the same cell body to generate an action potential

Question 36

What are excitatory neurotransmitters

Answer 36

• Neurotransmitters that can stimulate the generation of an action potential in a postsynaptic neurone

Question 37

What are inhibitory neurotransmitters

Answer 37

• Neurotransmitters which prevent the generation of an action potential in a postsynaptic neurone