Module 5 - Section 1 - Communication (Nervous System)

Question 1

How does responding to their environment help organism survive

Answer 1

1) animals increase their chances of survival by responding to changes in their external environment eg avoiding harmful environments that are too hot or cold.

2) they respond to changes in their internal environment to make sure conditions are always optimal for their metabolism

3) plants also increase their chances of survival by responding to changes in their environment eg light intensity.

Question 2

What is a stimulus

Answer 2

Any change in the internal or external environment

Question 3

What are receptors?

Answer 3

Receptors detect stimuli. They are specific so they only detect one kind of stimulus eg pressure. Some receptors are cells eg photoreceptors are receptor cells that connect to the nervous system. Some receptors are proteins on cell surface membranes eg glucose receptors are proteins found in the cell membranes of some pancreatic cells.

Question 4

What are effectors?

Answer 4

Effectors are cells that bring about a response to a stimulus, to produce an effect. Effectors include muscle cells and cells found in glands eg the pancreas.

Question 5

How do cells communicate with each other?

Answer 5

This happens via cell signalling. Cell signalling can occur between adjacent cells or between distant cells. For example, the nervous system communicate by secreting neurotransmitters which send signals to adjacent cells. The hormonal system releases hormones which travel in blood and act as signals to distant cells. Cell surface receptors allow cells to recognise the chemicals involved in cell signalling.

Question 6

What are the three main types of neurones

Answer 6

1) sensory neurone - transmit nerve impulses from receptors to the central nervous system
2) motor neurones - transmit nerve impulses from the CNS to effectors
3) relay neurones - transmit nerve impulses between sensory and motor neurones.

Question 7

Structure of sensory neurones

Answer 7

They have short dendrites and one long dendron to carry nerve impulses from receptor cells go the cell body. One short axon that carries impulses from the cell body to the CNS. Cell body in the middle of axon.

Question 8

Structure of a motor neuron

Answer 8

They have many short dendrites that carry nerve impulses from the CNS to the cell body, and one Long axon That carries nerve impulses from the cell body to effector cells.

Question 9

Structure of a relay neurone

Answer 9

Relay neurones have many short dendrites that carry nerve impulses from the sensory neurones to the cell body, and one a on That carries nerve impulses from the cell body to motor neurones.

Question 10

What is the pathway of nervous communication?

Answer 10

1) stimulus
2) receptors
3) sensory neurone
4) CNS
5) motor neurone
6) effectors
7) response

Question 11

How do sensory receptors convert stimulus energy?

Answer 11

Different stimuli have different forms of Energy eg light or chemical energy. Sensory receptors convert the Energy of a stimulus into electrical energy. They act as transducers (something that converts one from of energy into another).

Question 12

What is the generator potential?

Answer 12

When a stimulus is detected, the cell membrane is excited and becomes more permeable, allowing more ions to move in and out of the cell, altering the potential difference. The change in potential difference due to a stimulus is the generator potential.

Question 13

What is the action potential?

Answer 13

If the generator potential is big enough, it’ll trigger an action potential (nerve impulse) along a neurone. An action potential is only triggered if the generator potential reaches the threshold level. The bigger the stimulus, the bigger the generator potential.

Question 14

Example of how receptor cells are stimulated?

Answer 14

Pacinian corpuscles are mechanorecptors found in the skin that detect mechanical stimuli eg pressure. They contain the end of a sensory neurone, called a sensory nerve ending and it is wrapped in lamellae.
When they are stimulated, the lamellae are deformed and shape of receptor shape changes and press on the sensory nerve ending. This causes deformation of stretch-mediated sodium channels in the sensory neurone’s cell membrane.
This causes the sodium ion channels to open and sodium ions to diffuse into the cell, depolarising the membrane, creating a generator potential.
If the generator potential reaches the threshold, it triggers an action potential.

Question 15

What is the resting membrane potential?

Answer 15

In a neurone’s resting state, the inside of the neurone is more negatively charged than the outside. The potential difference across the membrane is -70 mV.

Question 16

How is resting membrane potential maintained?

Answer 16

It is created and maintained by sodium-potassium pumps and potassium ion channels.
1) sodium-potassium pumps use active transport to move three sodium ions out of the neurone for every 2 potassium ions moved in. ATP is needed.
2) potassium ion channels allow facilitated diffusion of potassium ions out of the neurone down the concentration gradient.
3) as the sodium membrane is not permeable to sodium ions, when the sodium potassium ions move sodium ions out the cell, it creates a sodium ion electrochemical gradient because there are more ions outside than inside.
4) the sodium -potassium ion pumps also move k+ ions in to the neurone.
5) when the cell is at rest, Most potassium ion channels are open so the membrane is more permeable to k+ ions, so some diffuse back out potassium ion channels.

Question 17

What happens during an action potential ?

Answer 17

1) a stimulus excites the neurone cell membrane, so sodium ion channels open. The membrane becomes more permeable to Na+, so they diffuse into the neurone down the sodium ion electrochemical gradient (inside becomes more positive).

2) deploraisation - if potential difference reaches the threshold (-55 mV), voltage gated sodium ion channels open and more sodium ions diffuse into neurone (positive feedback).

3) repolarisation - at potential difference +30 mV, the sodium ion p channels close and the voltage gated potassium ion channels open. Membrane is now more permeable to potassium so potassium ions diffuse out of neurone, down the concentration gradient. Membrane starts to return to resting potential. (Negative feedback).

4) hyperpolarisation - potassium ion channels are slow to close so there is a slight “overshoot” where too many k+ ions diffuse out if the neurone so the potential difference becomes more negative than resting potential.

5) Resting potential - the ion channels are reset. The sodium-potassium pump restores the membrane to resting potential and maintain resting potential until excited by another stimulus.

Question 18

What is the refractory period

Answer 18

After an action potential, the neurone cannot be excited straight away because the ion channels are recovering and they can’t be made to open. This is the refractory period. This is the time between one action potential and the next. It also makes sure action potentials are unidirectional.

Question 19

How do waves of depolarisation happen?

Answer 19

When an action potential happens, some of the Na+ ions That enter the neurone diffuse sideways. This causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that region. This causes a wave of depolarisation to travel along the neurone.

The wave moves away from regions of the membrane in the refractory period because these parts can’t Fire an action potential.

Question 20

How does the size of the stimulus affect the frequency of impulses?

Answer 20

Once the threshold is reached, the action potential will always Fire with the Same change in voltage, no matter how big the stimulus is. If the threshold isn’t reached, the action potential won’t fire. This is an All or nothing nature of action potentials.

A bigger stimulus will not create a bigger action potential but will cause them to Fire more frequently.

Question 21

What three factors affect speed of conduction

Answer 21

1) myelination
2) axon diameter
3) temperature

Question 22

How does myelination affect speed of conduction

Answer 22

Some neurones have a myelin sheath which is an electrical insulator. In the PNS, the myelin sheath is made up of Schwann cells and is wrapped around the axon /dendron. There are nodes of ranvier between Schwann cells. Sodium ion channels are concentrated here. Myelination speeds up conduction.

Question 23

What is saltatory conduction?

Answer 23

In a myelinated neurone, depolarisation only happens at the nodes of ranvier (where sodium ions can get through the membrane) .The neurone’s cytoplasm conducts enough electrical charge to depolarise their next node, so the impulse jumps from node to node. This is saltatory conduction.

In a non-myelinated neurone, the impulse travels as a wave along the length of the axon membrane. This is slower than saltatory conduction.

Question 24

How does axon diameter affect speed of conduction?

Answer 24

Action potentials are conducted quicker along axons with bigger diameters because there is less resistance to the flow of ions than in the cytoplasm of a smaller axon. With less resistance, depolarisation reaches other parts of the neurone cell membrane quicker.

Question 25

How does temperature affect speed of conduction?

Answer 25

The speed of conduction increases as the temperature increases too, because ions diffuse faster. The speed only increases up to around 40 degrees Celsius, after This the proteins begin to denature and speed decreases.

Question 26

What is a synapse?

Answer 26

It is the junction between a neurone and another neurone, or between a neurone and an effector cell.

Question 27

What is a synaptic cleft?

Answer 27

The tiny gaps between cells at a synapse

Question 28

How is a nerve impulse transmitted across a cholinergic synapse?

Answer 28

1) an action potential arrives at the synaptic knob of the presynaptic neurone. The action potential stimulates voltage-gated calcium ion channels in the presynaptic neurone to open. Calcium ions diffuse into the synaptic knob (pumped out After so by active transport)

2) the influx of calcium ions into the synaptic knob causes the synaptic vesicles to move to the presynaptic membrane. They then fuse with the presynaptic membrane. The vesicles release ACh into the synaptic cleft by exocytosis.

3) ACh diffuses across the synaptic cleft and binds to specific cholongeric receptors on the postsynaptic membrane. This causes sodium ion channels in the post synaptic neurone to open. This causes deplorisation in the post synaptic neurone. An action potential is generated if the threshold is reached in the post synaptic neurone.

4) ACh is removed from the synaptic cleft so the response doesn’t keep happening. It is broken down by the enzyme called acetylcholinesterase (AChE) and the products are reabsorbed into the presynaptic neurone and used to make more ACh.

Question 29

How can synaptic transmission be disrupted?

Answer 29

They can be affected by chemicals like drugs, toxins and poisons..

1) some chemicals are the same shape as neurotransmitters so they mimic their action at receptors. These drugs are called agonists. This means more receptors are activated. Eg nicotine mimics ACh and binds to specific cholinergic receptors in the brain.

2) some chemicals block receptors si they can’t be activated by neurotransmitters. This means fewer receptors can be activated. Eg curare blocks the effect of ACh by blocking certain cholingeric receptors at neuromuscular junctions so muscle cells can’t be stimulated leading to paralysis.

3) some chemicals inhibit the enzyme that breaks down neurotransmitters. This means more neurotransmitters in the synaptic cleft can bind to receptors. Eg nerve gases stop ACh being broken down in the synaptic cleft so it leads to loss of muscle control.

4) some chemicals inhibit the release of neurotransmitters from the presynaptic neurone so fewer receptors are activated. Eg opioids block calcium ion channels in the presynaptic neurone. This means fewer vesicles fuse with the presynaptic membrane so less neurotransmitter is released.

Question 30

What is synaptic divergence?

Answer 30

When one neurone connects to many neurones information can be dispersed to different parts of the body.

Question 31

What is synaptic convergence?

Answer 31

When many neurones connect to one neurone, information can be amplified.

Question 32

What is summation?

Answer 32

Summation is where the effect of neurotransmitters released from many neurones is added together. There are two types, spatial and temporal.

Question 33

What is spatial summation?

Answer 33

This is where two or more presynaptic neurones converge and release their neurotransmitters at the same time onto the same postsynaptic neurone. The small amount of neurotransmitter released from each of these neurones can be big enough together to reach threshold and trigger an action potential.

It allows signals from multiple stimuli to be coordinated into a single response.

Question 34

What is temporal summation?

Answer 34

This is where two or more nerve impulses arrive in quick succession from the same presynaptic neurone. This makes an action potential more likely to happen because more neurotransmitters are released into the synaptic cleft.

Question 35

What are synapses unidirectional?

Answer 35

This is because neurotransmitters are only released from presynaptic neurones and receptors for neurotransmitters Are only on the post synaptic membranes.