Module 5: Neuronal Communiaction

Question 1

Role of sensory receptors

Answer 1

Sensory receptors respond to stimuli in the internal or external environment of an organism and can create action potentials.

Question 2

Receptors

Answer 2

Receptors are specialised cells that can detect stimuli.

The stimulus is actually a change in energy levels in the
environment.

Receptors are transducers as they convert one type of energy into another.

The convert one form of energy into electrical energy as an action potential (nerve impulse).

Question 3

Sensory neurone

Answer 3

Carries action potential from sensory receptor to CNS.

Long dendron, short axon.

Question 4

Relay neurone

Answer 4

Connects sensory and motor neurone in CNS.

Short dendrites, no dendron, short axon.

Question 5

Motor neurone

Answer 5

Carries action potential from CNS to effector muscles.

Short dendron, long axon

Question 6

What is a stimulus?

Answer 6

Change in the environment causes sodium voltage gated channels to open and sodium to diffuse into the receptor.

Question 7

Differences between types of neurone

Answer 7

• Motor neurone have their cell body in the CNS and have a long axon that carries the action potential out to the effector.
• Sensory neurones have a long dendron carrying the action potential to the cell body, which is outside the CNS. They have a short axon.

Question 8

Myelinated neurones

Answer 8

Schwann cells wrap tightly around the neurone so the sheath consists of several layers of membrane and thin cytoplasm from the schwann cell.

At intervals of 1-3 mm along the neurone there are gaps in the myelin sheath. These are called the nodes of ranvier.

The action potential jumps from one node to the next. This makes the conduction more rapid. This is called saltatory conduction.

Question 9

Non-myelinated neurones

Answer 9

The action potential moves along the neurone in a wave.

Non-myelinated neurones only carry a impulse over a shorter distance. They are often used in coordinating body functions such as breathing, digestive system. Therefore, the increased speed of transmission is not important.

Question 10

Advantages of myelination

Answer 10

Myelinated neurones can transmit an action potential much more quickly than non-myelinated neurones.
This enables a more rapid response to a stimulus.

Question 11

Pacinian corpuscles

Answer 11

A pacinian corpuscle is a pressure sensor- (detects changes in pressure on the skin)

It is an oval-shaped structure with rings of connective tissue wrapped around the end of a nerve cell.
When the pressure on the skin changes this deforms the ring of connective tissue, which push against the nerve ending.

It is only sensitive to changes in pressure that deform the connective tissue. When the pressure is constant they stop responding.

Question 12

Changing membrane permeability

Answer 12

Cells associated with the nervous system have sodium and potassium channels. These channels have a gate that can open/close the channel.

When the membrane is deformed by the changing pressure the Na+ channel open. This allows Na+ to diffuse into the cell creating a generator potential.

The membranes contain sodium/potassium pumps that actively pump ions in/out the cell. For every 3 Na+ ions pumped out, 2 K+ ions are pumped in. The conc of Na+ out of the cell increases while the conc of K+ in the cell increases. The membrane is more permeable to K+ ions, so some leak out of the cell. (Less permeable to Na+).

This creates an ionic conc gradient across the membrane. The cell is negatively charged inside compared with the outside.

When the cell is inactive it is said to be polarised, negatively charged inside compared with outside.

Question 13

Creating nerve impulses

Answer 13

A nerve impulse is achieved by changing the permeability of Na+. The Na+ gated channels open, Na+ diffuses down a conc gradient into the cell. This makes the cell less negative (depolarisation) creating a potential difference across the membrane.

The larger the stimulus the more Na+ gated channels open. If enough Na+ enters the cell, the potential difference across the cell membrane changes significantly and will initiate an action potential.

Question 14

Structure of neurones

Answer 14

Neurones are specialised cells

• Many are very long- transmit the action potential over a long distance .
• The membrane has gated ion channels
• Sodium/ Potassium pumps
• Cell body containing nucleus, mitochondria and ribosomes
• Dendrites carry impulses towards the cell body
• Axon carries impulses away from the cell body
• Myelinated sheath and nodes of ranvier

Question 15

Function of neurones

Answer 15

The impulse is transmitted along the neurone as an action potential.

The action potential is carried as rapid depolarisation of the membrane caused by influx of Na+ ions.

Question 16

Neurones at rest

Answer 16

When a neurone is not transmitting an action potential it is said to be at rest.

It is actively pumping ions, for every 3 Na+ ions out of the cell it pumps 2 K+ ions in to the cell. This keeps the charge negative.

The gated sodium channels are kept closed. This keeps a conc gradient.

The plasma membrane is more permeable to K+ ions than Na+ ions so some leak through.

There are also large organic anions in the cell cytoplasm.

The cell membrane is said to be polarised. The potential difference across the cell membrane is about -60mv. This is called the resting potential.

Question 17

Generating an action potentail

Answer 17

At rest- higher conc of Na+ outside than inside the neurone.
If Na+ channels open, Na+ will quickly diffuse down a conc gradient into the cell from surrounding tissue fluid. This causes depolarisation of the membrane.

In the generator region of a neurone, the gated channels are opened by the action of the synapse. When a few voltage gated channels open they allow a few Na+ and produce a small depolarisation- known as generator potential. It may go no further. However, when more voltage gated channels open, the generator potentials combine to produce a large depolarisation. If the depolarisation reaches a particular magnitude it passes a threshold and will cause an action potential.

Most Na+ channels in a neurone open by changes in the potential difference across the membrane- they are called voltage-gated channel. When there are sufficient generator potentials to reach the threshold potential they cause the voltage-gated channels to open. This is an example of positive feedback.

The opening of the voltage-gated channels allows a large influx of Na+. Depolarisation reaches +40mv inside the cell.

Question 18

What is positive feedback?

Answer 18

A mechanism that increases a change taking the system further away from the optimum.
- A small depolarisation of the membrane causing a change that increases the depolarisation further.

Question 19

Stages of an action potential:

Answer 19

1) The membrane is at rest- polarised inside being-60mv. There is a higher conc of K+ inside than outside.
2) Na+ channels open and some Na+ diffuse into the cell.
3) The membrane depolarises- it becomes less negative and reaches the threshold of -50mv.
4) Positive feedback causes voltage-gated channels to open and many Na+ diffuse in. The inside becomes positive compared to outside. The potential difference across the plasma membrane reaches +40mv.
5) The Na+ channels close and K+ channels open.
6) K+ ions diffuse out of the cell bringing the potential difference back to negative inside the cell- this is called repolarisation.
7) The potential difference over shoots slightly, making the cell hyper polarised.
8) The original p.d. is restored bringing the cell to it’s resting state.

Question 20

Hyperpolarisation

Answer 20

Too many K+ ions diffuse out.

p.d. under -70mv.

Question 21

The refractory period

Answer 21

After an action potential the Na+ and K+ are in the wrong places.
For a short time after each action potential it is impossible to reach another action potential.
This is known as the refractory period and allows the cell to recover after an action potential.
It also ensures that action potentials are transmitted in only one direction.

Question 22

What is a synaptic cleft?

Answer 22

The small gap in between 2 neurones.

Action potentials cant get through this gap.

Question 23

Cholinergic synapse

Answer 23

Use acetylcholine (ACh) as neurotransmitter.

Question 24

Synaptic knob

Answer 24

Swelling at the end of the presynaptic neurone.

Contains:

• many mitochondria
• SER, which packages the neurotransmitter into vesicles
• Vesicles containing acetylcholine
• Voltage-gated calcium ion channels on the cell surface membrane

Question 25

The post-synaptic membrane

Answer 25

Contains sodium ion channels that can respond to the neurotransmitter.
These channels consist of 5 polypeptide molecules. 2 of these have as receptor site that is complementary in shape to a molecule of acetylcholine. When the ACh is present in the synaptic cleft it binds to the 2 receptor sites and causes the sodium ion channel to open.

Question 26

Transmission across the synapse

Answer 26

1) An action potential arrives at the synaptic knob.
2) The voltage-gated calcium ion channels open.
3) Calcium diffuse into the synaptic bulb.
4) The calcium ions cause the synaptic vesicles to move and fuse with the pre-synaptic membrane.
5) Acetylcholine is released by exocytosis.
6) Acetylcholine molecules diffuse across the cleft.
7) ACh molecules bind to receptor sites on the sodium ion channels in the post-synaptic membrane.
8) The sodium ion channels open.
9) Sodium ions diffuse across the post-synaptic membrane into the post-synaptic neurone.
10) A generator potential is created.
11) Generator potentials combine to reach threshold potential.
12) A new action potential is created in the post-synaptic neurone. It passes down the neurone.

Question 27

The role of acetylcholinesterase

Answer 27

Enzyme
Found in the synaptic cleft
Hydrolyses acetylcholine to acetic acid and choline
This stops the transmission of signals, so that the synapse does not continue to produce an action potential in the post-synaptic neurone.

The acetic acid and choline are recycle. They re-enter the presynaptic neurone and combine to produce ACh using ATP from respiration in the mitochondria.

Question 28

The role of the synapses

Answer 28

The synapses control the communication along the nervous system.

They:

1) Transmit information between neurones.
2) Ensure one way transmission of impulses- vesicles with ACh only in pre-synaptic knob, recpetors for ACh only found in post-synaptic membrane.
3) Acclimatisation- Synapses stop responding to stimuli to avoid overstimulation of effectors which could cause damage- run out of neurotransmitter vesicles after repeated stimulation.
4) Divergence of nervous pathways- 1 presynaptic neurone diverges into several post-synaptic neurones- transmits to several parts of the nervous system.

Question 29

What is spatial stimulation?

Answer 29

Occurs when action potentials arrive from converging presynaptic neurones cause a few vesicles each to be released into the same synapse causing an action potential in the postsynaptic neurone.