Neurons and Synaptic Transmission

Question 1

briefly describe what a neuron is and what it does

4

Answer 1

neurons transmit information from one neuron to the next

the dendrites on a neuron receive information from sensory receptors or from other neurons

information is then passed to the cell body and then the axon

information travels down the axon in the form of an electrical signal known as an action potential (aka an electrical impulse)

Question 2

neurons

4

Answer 2

neurons are cells that are specialised to carry neural information throughout the body

3 types of neuron; sensory, relay and motor

made up of a cell body, dendrites and an axon

the length of a neuron can vary from a few millimetres to one metre

Question 3

dendrites

2

Answer 3

at one end of the neuron, connected to the cell body

receive signals from other neurons or sensory receptors

Question 4

cell body

2

Answer 4

the control centre of the neuron

from the cell body, impulses are carried along the axon where they are then terminated at the axon terminal

Question 5

myelin sheath

2

Answer 5

in many nerves there is an insulating layer around the axon called the myelin sheath

it allows nerve impulses to transmit more rapidly along the axon, so if it is damaged then impulses slow down

Question 6

sensory neurons

4

Answer 6

carry nerve impulses from sensory receptors to the CNS (brain and spinal cord)

found in areas like the eyes, ears, tongue and skin (sensory receptors)

convert information from sensory receptors into neural impulses which travel to the brain and are translated into sensations of heat, pain, etc which enables the organism to react appropriately

not all sensory information travels to the brain, some neurons terminate in the spinal cord which allows reflex actions to occur quickly without the delay of having to send impulses to the brain

Question 7

relay neurons

3

Answer 7

most common type of neuron in the CNS

lie wholly within the brain and spinal cord

allows sensory and motor neurons to communicate with eachother

Question 8

motor neurons

5

Answer 8

form synapses with muscles and control their contractions

located in the CNS but project their axons outside the CNS and either directly or indirectly control muscles

release neurotransmitters when stimulated that bind to receptors on the muscle and trigger a response which leads to muscle movement

the axon of a motor neuron fires and the muscle is has formed a synapse with contracts and the strength of muscle contraction depends on the rate that the axons fires

muscle relaxation is caused by inhibition of the motor neuron

Question 9

what is a neurotransmitter?

1

Answer 9

chemical substances and messengers that transmit nerve impulses across a synapse

Question 10

what is a synapse?

2

Answer 10

the conjunction or gap between the end of the axon of one neuron and the dendrite or cell body of another neuron

includes the end of the presynaptic neuron, the membrane of the postsynaptic neuron and the gap in between

Question 11

define synaptic transmission

2

Answer 11

the process in which a nerve impulse passes across the synaptic cleft from one neuron (the presynaptic neuron) to another (the postsynaptic neuron)

takes only a fraction of a second

Question 12

describe the process of synaptic transmission up until activation of the receptors

6

Answer 12

an action potential reaches the end of an axon and needs to be transferred to another neuron so it must cross the synapse between the presynaptic neuron and the postsynaptic neuron

the physical gap between the pre and post synaptic cell membranes is called the synaptic gap

synaptic vesicles are sacs at the end of an axon of a nerve cell that contain neurotransmitters that assist in the transfer of an electrical impulse

the action potential reaches the synaptic vesicles and cause them to release their neurotransmitters through a process called exocytosis

the neurotransmitter diffuses across the gap/synapse between the cells and binds to specialised receptors on the surface of the cell

these receptors recognise the neurotransmitter and are activated by it

Question 13

describe the rest of the process of synaptic transmission

5

Answer 13

once activated, the receptor molecules produce either excitatory or inhibitory effects on the post synaptic neuron

these effects are eventually terminated by a process called reuptake in which the neurotransmitter is taken up and reabsorbed by the presynaptic neuron, it is stored and made available for later release

the speed of reuptake determines how prolonged the effects of the neurotransmitter will be — the quicker it is taken up by the presynaptic neuron, the shorter effects are on the postsynaptic neuron

some antidepressants prolong the effect of a neurotransmitter by inhibiting the reuptake process, leaving the neurotransmitter in the synapse for longer

neurotransmitters can also effectively be turned off after stimulating the postsynaptic neuron — enzymes produced by the body make the neurotransmitter ineffective

Question 14

neurotransmitters

2

Answer 14

neurotransmitters are chemical messengers that carry signals across the synaptic gap to the receptors on the postsynaptic cell via process of synaptic transmission

2 types of neurotransmitter; excitatory and inhibitory

Question 15

excitatory neurotransmitters

3

Answer 15

increase the likelihood that an excitatory, stimulating signal is sent to the postsynaptic cell

the cell is then more likely to fire

examples include acetylcholine and noradrenaline

Question 16

inhibitory neurotransmitters

3

Answer 16

decrease the likelihood of a neuron firing

generally responsible for calming the mind and body, inducing sleep and filtering out unnecessary excitatory signals

examples include serotonin and GABA

Question 17

EPSP

2

Answer 17

an excitatory neurotransmitter binding with the a postsynaptic receptor causes an electrical change in the membrane of that cell

resulting in an excitatory postsynaptic potential (EPSP), meaning the postsynaptic cell is more likely to fire

Question 18

IPSP

2

Answer 18

an inhibitory neurotransmitter binding with a postsynaptic receptor results in an inhibitory postsynaptic potential (IPSP)

meaning the cell is less likely to fire

Question 19

summation of EPSPs and IPSPs

3

Answer 19

a nerve cell can simultaneously receive EPSPs and IPSPs but the likelihood of a cell firing is determined by adding up the excitatory and inhibitory synaptic input

the summation determines whether or not a cell fires

for example, if there are more EPSPs a cell is more likely to fire and vice versa

Question 20

2 ways to increase the strength of EPSPs

2

Answer 20

spatial summation — a large number of EPSPs are generated at many different synapses on the same postsynaptic neuron at the same time

temporal summation — a large number of EPSPs are generated at the same synapse by a series of high frequency action potentials on the presynaptic neuron

Question 21

EPSPs and IPSPs: the rate at which a cell fires

3

Answer 21

the rate at which a cell fires is determined by what goes on in the synapses

if excitatory synapses are more active then the cell fires at a higher rate

if inhibitory synapses are more action then the cell fires at a much lower rate, if at all