Neuronal Communication

Question 1

What is a sensory neuron’s function?

Answer 1

Transmit nervous impulses from receptors to the CNS.

Question 2

What is a motor neuron’s function?

Answer 2

Transmit nerve impulses from the CNS to effectors.

Question 3

What is a relay neuron’s function?

Answer 3

Transmit nervous impulses between sensory neurons and motor neurons.

Question 4

What does it mean for a cell to be at resting potential?

Answer 4

When a nervous system receptor is in resting state there’s a difference in charge between the inside and outside of the cell. This is generated by ion pumps and channels. This means there’s a voltage across the membrane.

Question 5

What happens to a cell at resting potential when a stimulus is detected?

Answer 5

The cell membrane is excited and becomes more permeable, allowing more sodium ion channels to open to move Na+ ions into the cell- altering the potential difference. The change in potential difference due to a stimulus is called the generator potential.

Question 6

What happens when a bigger stimulus excites the membrane?

Answer 6

It will excite more, causing a bigger movement of ions and a bigger change in potential difference - so a bigger generator potential is produced. If the generator potential is big enough it’ll trigger an action potential (nervous impulse) along a neuron. An action potential is only triggered if the generator potential reaches a certain level called the threshold level.

Question 7

What happens if the stimulus is too weak?

Answer 7

The generator potential won’t reach the threshold, so there’s no action potential.

Question 8

What are the function of dendrites and dendrons?

Answer 8

Carry nerve impulses towards the cell body.

Question 9

What do axons do?

Answer 9

Carry nervous impulses away from the cell body.

Question 10

How do sensory and motor neuron’s structure differ?

Answer 10

> Sensory have short dendrites with one long dendron to carry nervous impulses from receptor cells to the cell body but motor have many short dendrites that carry nerve impulses from the CNS to the cell body.
Sensory have one short axon that carries impulses from the cell body to the CNS whereas, motor have one long axon that carries nervous impulses from the cell body to effector cells.

Question 11

What is the structure of a relay neuron?

Answer 11

Have many short dendrites that carry nerve impulses from sensory neurons to the cell body, and many short axons that carry nerve impulses from the cell body to motor neurons.

Question 12

What is potential difference across the membrane when a neuron is in a resting state?

Answer 12

The outside of the membrane is positively charged compared to the inside. This is because there are more positive ions outside the cell than inside.

Question 13

What is does mean that the membrane is polarised?

Answer 13

There’s a difference in charge, the voltage across the membrane when it’s at rest is called the resting potential - it’s about -70mV.

Question 14

How is the resting potential is maintained?

Answer 14

The resting potential is created and maintained by sodium-potassium pumps and potassium ion channels in a neuron’s membrane, the sodium-potassium pump moves 3 Na+ out of the neuron for every 2 K+ moved in

Question 15

What does it mean that there is an electrochemical gradient across the membrane?

Answer 15

The sodium-potassium pump moves moves sodium ions out but the membrane isn’t permeable to Na+, so they can’t diffuse back in creating a sodium ion electrochemical gradient because there are more positive sodium ions outside the cell than inside. It also moves K+ into the neuron but the membrane is permeable to them so they diffuse back out by faciliated diffusion through K+ channels.

Question 16

What does it mean that the membrane becomes more permeable to sodium when it is excited?

Answer 16

More sodium ions diffuse into the neuron down the sodium ion electrochemical gradient, making the inside of the neuron less negative.

Question 17

What happens at the depolarisation stage of nerve cells being stimulated?

Answer 17

If the potential difference reaches the threshold, voltage- gated sodium ion channels open. More Na+ diffuse into the neuron, this is positive feedback.

Question 18

How does the action potential cause a wave of depolarisation?

Answer 18

1) When an action potential happens, some of the sodium ions that enter the neuron diffuse sideways.
2) This causes sodium ion channels in the next region of the neuron to open and sodium ions diffuse into that part.
3) This causes a wave of depolarisation to travel along the neuron.
4) The wave moves away from the parts of the membrane in the refractory period because these parts can’t fire an action potential.

Question 19

What happens in the repolarisation stage of an action potential?

Answer 19

At a potential difference of around +30mV the sodium ion channels close and voltage-gated potassium ion channels open. The membrane is more permeable to potassium so K+ diffuse out of the neuron down the K+ conc. gradient. This starts to get the membrane back to its resting potential. This is negative feedback.

Question 20

What is hyperpolarisation?

Answer 20

K+ channels are slow to close so there’s a slight ‘overshoot’ where too many K+ diffuse out of the neuron. The potential difference becomes more negative than the resting potential.

Question 21

What is the refractory period?

Answer 21

The neuron cell membrane after an action potential can’t be excited again straight away. This is because the ion channels are recovering and can’t be made to open (Na+ channels are closed during repolarisation and K+ channels are closed during hyperpolarisation).

Question 22

What does a bigger impulse affect?

Answer 22

Once the threshold is reached, an action potential will always fire with the same change in voltage, no matter how big the stimulus is but it will cause more frequent impulses.

Question 23

Explain the ‘all-or-nothing’ theory?

Answer 23

Once the threshold is reached, an action potential will always fire with the same change in voltage but if the threshold isn’t reached, an action potential won’t fire.

Question 24

What does neurons being myelinated mean?

Answer 24

They have a myelin sheath, which is an electrical insulator. In the peripheral nervous system the myselin sheath is made of Schwann cells.

Question 25

Describe the nodes of Ranvier?

Answer 25

Between the Schwann cells are tiny patches of bare membrane called the nodes of Ranvier. Sodium ion channels are concentrated at the nodes. In a myelinated neuron, depolarisation only happens at the nodes of Ranvier.

Question 26

What is saltatory conduction?

Answer 26

The neuron’s cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse ‘jumps’ from node to node. Saltatory conduction is really fast.

Question 27

What is different in a non-myelinated neuron?

Answer 27

The impulse in a non-myelinated neuron travels as a wave along the whole length of the axon membrane. This is slower than saltatory conduction (although it’ still pretty quick).

Question 28

What is a synapse?

Answer 28

The junction between a neuron and another neuron, or a neuron and an effector cell.

Question 29

What is the tiny gap between the cells at the synapse called?

Answer 29

A synaptic cleft.

Question 30

Why does the presynaptic neuron have a swelling and what is it called?

Answer 30

It is called a presynaptic knob and contains synaptic vesicles filled with chemicals called neurotransmitters.

Question 31

What happens when an action potential reaches a synapse?

Answer 31

1) It opens the voltage-gated calcium ion channels that cause the vesicles containing the neurotransmitters to fuse with the presynaptic membrane.
2) The vesicles release the neurotransmitters by exocytosis into the synaptic cleft and diffuse across to the postsynaptic membrane and bind to specific receptors.
3) When neurotransmitters bind to receptors they might trigger an action potential, cause muscle contraction or cause a hormone to be secreted.
4) Neurotransmitters are removed from the cleft so the response doesn’t keep happening so they’re taken back to the presynaptic neuron or broken down by enzymes so the products are taken back to the neuron.

Question 32

What is a cholinergic synapse?

Answer 32

Synapses that use acetylcholine.

Question 33

What type of receptors does acteylcholine bind to and what enzyme breaks it down?

Answer 33

Receptors - Cholinergic receptors

Enzyme - Acetylcholinesterase

Question 34

What is a synaptic divergence?

Answer 34

When one neuron connects to many neurons information can be dispersed to different parts of the body.

Question 35

What is a synaptic convergence?

Answer 35

When many neurons connect to one neuron information can be amplified (made stronger).

Question 36

What is summation?

Answer 36

Where the effect of neurotransmitters can be combined in order to excite the postsynaptic membrane to the threshold level to stimulate an action potential.

Question 37

What is spatial summation?

Answer 37

When neurons converge, the small amount of neurotransmitter released from each neuron can be enough altogether to reach the threshold in the postsynaptic neuron and trigger an action potential.
>Stimuli might arrive from different sources and spatial summation allows signals from multiple stimuli to be coordinated into a single response.

Question 38

What is temporal summation?

Answer 38

Temporal summaton is where two or more nerve impulses arrive in quick succession from the same presynaptic neuron. This makes an action potential more likely because ore neurotransmitter is released into the synaptic cleft.

Question 39

Why do nervous impulses only travel in one direction?

Answer 39

Receptors for neurotransmitters are only on the postsynaptic membranes so synapses many sure impulses only travel in one direction.