Module 5 Neuronal communication essential notes

Question 1

Describe the key features of sensory cells

Answer 1

1. Sensory cells are always specific for a particular stimulus 2. They detect changes in energy or chemicals 3. They act as transducers, converting various types of energy (or the presence of chemicals) into electrical energy 4. This is carried by protein channels sensitive to certain types of energy or chemical 5. The activated protein channels in the sensor cells starts a nerve impulse in the sensory neurone

Question 2

Give examples of sensory receptors in mammals

Answer 2

Question 3

Describe the role of the Pacinian corpuscle

Answer 3

1. The Pacinian corpuscle is responsible for the detection of pressure
2. It acts as a transducer, converting pressure changes into electrical impulses
3. Pacinian corpuscles are found in skin and joints
4. And allows the mammalian sense of touch

Question 4

Describe how a general sensory cell stimulates nerve impulses in sensory neurones

Answer 4

1. Cells have an imbalance of positive ions (eg sodium ions) across their cell membrane
2. Usually, cells are more negative inside, than outside
3. This is called polarisation
4. Chemicals and energy changes, alter the shape of ion channel proteins, changing their function, and allowing positive ions to enter the cell
5. This changes the polarisation of the cell to depolarised
6. The sensory cell is now stimulated, and in turn, this stimulates the sensory neurone

Question 5

Describe how pressure changes are detected by the Pacinian corpuscle

Answer 5

1. A Pacinian corpuscle is a sensory neurone surrounded by many layers of connective tissue
2. Pressure causes the cell membrane of the sensory neurone to stretch
3. The mechanoreceptors in the cell membrane open
4. Allowing sodium ions to enter the sensory neurone
5. The sensory neurone becomes depolarised
6. This produces a generator potential in the sensory neurone
7. Which can become a nerve impulse if the stimulus is strong enough
8. The nerve impulse moves along the sensory neurone towards the CNS

Question 6

Explain the purpose of the nervous system

Answer 6

1. Maintenance of internal conditions requires responses to internal and environmental stimuli
2. Responses require communication between sensors and effectors
3. Sensor and effector organs can be distant from each other
4. The nervous system acts as a communication pathway between sensors and effectors
5. For when responses need to be very fast

Question 7

Describe the role of the nervous system in making a response to a stimulus

Answer 7

1. An internal or environmental stimulus is detected by sensory cells/organs
2. A nerve impulse is generated in the sensory neurone
3. The nerve impulse moves towards the central nervous system (brain and spinal cord)
4. The impulse then passes on to an effector neurone
5. The effector neurone stimulates the effector cells/organ
6. A response is made by the stimulated effector

Question 8

Describe the role of the nervous system in reflex responses

Answer 8

1. Stimulus detected by sensory cells
2. Impulses pass along sensory neurone to CNS
3. Impulse passes along Relay neurone stimulates effector neurone
4. Effector neurone send impulses to effector
5. Response is made (flexor contracts to move hand away from flame)
6. (note that impulses are also sent to the brain so conscious thought can influence/override reflex response)

Question 9

Describe the structures of neurones and their respective functions

Answer 9

Question 10

Describe the functions of the three main types of neurone

Answer 10

Question 11

Describe the structure of purpose of myelination

Answer 11

1. Myelination is when specialised cells called Schwann cells are wrapped around axons and/or dendrons
2. By surrounding the axon/dendron with many layers of lipid bilayer
3. This provides electrical insulation
4. Between cells there are gaps called Nodes of Ranvier where impulses can still occur
5. As impulses can jump from node to node, this speeds up the nerve impulse

Question 12

Describe the structure of sensory neurones

Answer 12

1. Sensory neurons have long dendrons and axons
2. The cell body is located away from the main axon
3. The dendrons have many dendrites
4. The axons have many terminal branches
5. While the dendrites, dendron, cell body and part of axon are located outside the CNS
6. The terminal branches are located within the CNS

Question 13

Describe the structure of relay neurones

Answer 13

1. Contain many short dendrons, each with many dendrites
2. Contain a single short axon, with many terminal branches
3. Located entirely within the CNS

Question 14

Describe the structure of effector neurones

Answer 14

1. Sometimes also called motor neurones
2. Multiple short dendrons (each with many dendrites)
3. The cell body at one end of the neurone
4. One very long axon with many terminal branches
5. Synapsing with the effector
6. Dendrites and cell body located within CNS
7. Axon and terminal branches outside of CNS

Question 15

Summarise the nature of a nerve impulse in a neurone

Answer 15

1. Normally neurones are in a state of polarisation
2. They are more negative inside the cell than outside
3. A small area at one end of the neurone can be depolarised (more positive inside than outside)
4. This small area of depolarisation can cause depolarization in the next part of the neuron
5. While the first area returns back to being polarised
6. Thus the area of depolarisation moves along the dendron/axon until it reaches the end
7. This is a nerve impulse

Question 16

Describe how neurones maintain the resting potential

Answer 16

1. At rest sodium ions are outside the cell and most of the potassium ions are inside the cell
2. Due to the Na/K pump (active transport)
3. Voltage-gated sodium channels are closed
4. Voltage-gated potassium channels are closed
5. some K channels are open which allows some potassium ions out to diffuse out of the cell (concentration gradient)
6. The presence of some potassium ions outside, as well as sodium ions, means the outside is more positive than the inside
7. The resting potential is -65 mV

Question 17

Describe the events that result in depolarisation of the axon

Answer 17

1. A depolarisation of +40 mV causes closed voltage-gated Na channels to open
2. Sodium ions diffuse into the cell (electrochemical gradient)
3. There are now more positive ions inside the axon than outside
4. Causing depolarisation of the axon membrane
5. This causes even more sodium channels to open, so even more sodium enters the cell (positive feedback)
6. When the polarisation reaches +40 mV, an action potential is reached

Question 18

Explain why depolarisation stops, and how repolarisation occurs

Answer 18

1. An action potential causes open Voltage-gated sodium channels close
2. An action potential causes open Voltage-gated potassium channels open
3. K ions diffuse out of cell due to chemical and electrical gradients
4. This causes repolarisation of the axon
5. (The presence of higher than resting levels of positive ions outside of cell causes hyperpolarisation)
6. Hyperpolarisation is when the cell reaches -70 mV inside (more negative than the normal -65 mV)

Question 19

Describe how the resting potential is restored

Answer 19

1. The voltage-gated sodium channels are closed (refractory period)
2. Potassium ions diffuse back into cell via the (always open) K channels
3. The Na/K pump (which never stopped working) relocates the ions to their original sides of the axon membrane by active transport
4. This restores the resting potential, and the original locations of the ions

Question 20

Describe how an action potential is propagated along a neurone

Answer 20

1. Action potential in one region of the axon
2. Causes voltage gated sodium channels in the surrounding areas to open
3. Which causes depolarisation in those areas (depolarisation is not caused in the previous region, as
4. those voltage gated sodium channels are still in their refractory period)
5. While the action potential develops in the next area, the initially depolarised area repolarises
6. Thus, the action potential apparently ‘moves’ along the axon, only ever, forwards

Question 21

Describe the effect of myelination on the propagation of the action potential

Answer 21

1. The Schwann cells wrap around the axons of some neurones creating the myelin sheat
2. This electrically insulates those areas
3. Leaving some uninsulated regions at regular intervals called Nodes of Ranvier
4. Action potentials can only occur at the nodes
5. As the action potential ‘jump’ from node to node, it can progress along the axon much faster than in an unmyelinated axon
6. Nerve impulses travel much faster along myelinated neurones, this is called saltatory conduction

Question 22

Describe how action potentials are ‘all or nothing’

Answer 22

1. If the stimulus is strong enough
2. Enough sodium channels will be opened to cause an action potential
3. Action potentials are always the same size
4. If the stimulus is not strong enough, initial depolarisation will not cause an action potential
5. The depolarisation must cross the threshold potential
6. Therefore there are action potentials or there aren’t, there is no intermediate

Question 23

Explain how stimuli of different intensities are perceived

Answer 23

1. Actions potentials are ‘all or nothing’ therefore they cannot be of different sizes
2. However, stronger stimuli cause more frequent action impulses
3. And less intense stimuli result in a lower frequency of impulses
4. This also affects summation and control at synapses

Question 24

Describe the structure of a synapse

Answer 24

1. The synapse is where the terminal branches of one neurone meet the dendrites of another
2. The impulse moves along the presynaptic neurone
3. It reaches the end of the presynaptic neurone known as the synaptic knob
4. Neurotransmitter diffuses across the gap between the neurones known as the synaptic cleft
5. The neurotransmitter bind receptors on the postsynaptic neurone, causing a new action potential

Question 25

Describe events that occur at a synapse in the presynaptic neurone

Answer 25

1. An action potential arrives at the axon terminal
2. This causes voltage-gated calcium channels to open
3. Calcium ions diffuse into the synaptic knob, causing synaptic vesicles to fuse with the presynaptic membrane
4. Neurotransmitter (e.g. acetylcholine) is released into synaptic cleft
5. Neurotransmitter diffuses across the synaptic cleft

Question 26

Describe how neurotransmitter in the synaptic cleft causes an action potential in the postsynaptic neurone

Answer 26

1. Neurotransmitter diffuses (slower than action potential) across synaptic cleft and binds to neurotransmitter receptors on the postsynaptic membrane
2. Receptors are complementary to the neurotransmitter, and binding causes a change in their shape, causing sodium channels to open
3. Sodium ions diffuse into post-synaptic neurone, causing depolarisation and starting an action potential
4. neurotransmitter in the synaptic cleft is broken down by enzymes (eg acetylcholinesterase) on the postsynaptic neurone, to terminate the signal
5. The broken down neurotransmitter is taken up by the presynaptic neurone and recombined using ATP (active process)

Question 27

Describe how synapses can be excitatory or inhibitory

Answer 27

1. Excitatory synapses release neurotransmitter that result in the sodium channels on the postsynaptic neurone opening
2. This results in depolarisation and stimulation of more action potentials in the postsynaptic neurone
3. Inhibitory synapses are where neurotransmitters released cause chloride ion channels on the postsynaptic neurone to open
4. This causes negative ions to enter to postsynaptic neurone, causing hyperpolarisation, and inhibiting action potentials

Question 28

State the function of synapses

Answer 28

1. Ensure that impulses move in one direction only
2. Allow impulses from one presynaptic neurone to stimulate impulses in many postsynaptic neurones
3. Allow impulses in many presynaptic neurones to converge at single postsynaptic neurones
4. Allow ignoring of low-level stimuli by summation

Question 29

Describe the two types of summation

Answer 29

1. Summation is when a certain frequency of presynaptic impulses are required to generate a single postsynaptic impulse
2. There are two types, temporal and spatial summation
3. Temporal summation is when a high frequency of presynaptic impulses from a single neurone generates enough neurotransmitter to start a postsynaptic impulse
4. Spatial summation, is when impulses from numerous presynaptic neurones collectively produce enough neurotransmitter to start a postsynaptic impulse