2 Nervous Coordination And Muscles

Question 1

What are the the principles of coordination for the nervous system?

Answer 1

• communication by nerve impulses
• transmission by neurones and is rapid
• nerve impulses travel to specific parts of body
• response is localised, rapid and short-lived
• effect usually temporary and reversible

Question 2

What are the the principles of coordination for the hormonal system?

Answer 2

• communication by hormones
• transmission by blood stream and is relatively slow
• hormones travel to all parts of the body, but only target cells respond
• response is widespread, slow and often long-lasting
• effect may be permanent and irreversible

Question 3

Describe the structure of a myelinated motor neurone.

Answer 3

• a cell body
• dendrons
• an axon
• Schwann cells
• a myelin sheath
• nodes of ranvier

Question 4

Describe the cell body of a myelinated motor neurone.

Answer 4

Contains all the usual cell organelles, including a nucleus and large amounts of RER. This is associated with production of proteins and neurotransmitters

Question 5

Describe the dendrons of a myelinated motor neurone.

Answer 5

Extensions of cell body which subdivide into smaller branched fibres, called dendrites, that carry nerve impulses towards the cell body.

Question 6

Describe the axon of a myelinated motor neurone.

Answer 6

A single long fibre that carries nerve impulses away from the cell body.

Question 7

Describe the schwann cells of a myelinated motor neurone.

Answer 7

Surround axon, protecting it and providing electrical insulation. They also carry out phagocytosis and play a part in nerve regeneration. Schwann cells wrap themselves around axon many times, so that layers of their membranes build up around it

Question 8

Describe the myelin sheath of a myelinated motor neurone.

Answer 8

Forms a covering to the axon and is made up of the membranes of the Schwann cells. These membranes are rich in a lipid known as myelin. Neurones with myelin sheath called myelinated neurones

Question 9

Describe the nodes of Ranvier of a myelinated motor neurone.

Answer 9

Constrictions between adjacent Schwann cells where there’s no myelin sheath. Constrictions are 2-3 μm long and occur every 1-3mm in humans.

Question 10

What is the function of sensory neurones?

Answer 10

Transmit nerve impulses from a receptor to an intermediate or motor neurone. Have 1 dendron that’s often very long. It carries the impulse towards the cell body and 1 axon that carries it away from the cell body.

Question 11

What is the function of motor neurones?

Answer 11

• Transmit nerve impulses from an intermediate or relay neurone to an effector, such as a gland or muscle.
• have long axon and many short dendrites.

Question 12

What is the function of intermediate/relay neurones ?

Answer 12

Transmit impulses between neurones, for example, from sensory to motor neurones. They have numerous short processes.

Question 13

How does the phospholipid bilayer of the axon plasma membrane control the movement of sodium and potassium ions across axon membrane?

Answer 13

Phospholipid bilayer of axon plasma membrane prevents sodium and potassium ions diffusing across it.

Question 14

How does the proteins known as channel proteins control the movement of sodium and potassium ions across axon membrane?

Answer 14

Proteins have channels, called ion channels, which pass through them. Some of channels have ‘gates’, which can be opened or closed so that sodium or potassium ions can move through by facilitated diffusion at any one time, but not on other occasions. There are different gated channels for sodium and potassium ions. Some channels remain open all time so sodium and potassium ions move unhindered through them by facilitated diffusion.

Question 15

How does the carrier proteins control the movement of sodium and potassium ions across axon membrane?

Answer 15

Carrier proteins actively transport potassium ions into the axon and sodium ions out of the axon. This mechanism can be called a sodium potassium pump.

Question 16

What happens as a result of the various controls to control the movement of sodium and potassium ions across axon membrane?

Answer 16

The inside of an axon is negatively charged relative to the outside. This is known as resting potential and ranges from 50-90 millivolts, but is usually 65mV in humans. In this condition the axon is said to be polarised.

Question 17

How is a resting potential established in a neurone?

Answer 17

• sodium ions actively transported out of axon by sodium-potassium pumps
• potassium ions actively transported into axon by sodium potassium pump
• active transport of sodium ions greater than that of potassium ions, so 3 sodium ions move out for every 2 potassium ions that move in.
• as a result, more sodium ions in tissue fluid surrounding axon than in cytoplasm, and more potassium ions in cytoplasm than in tissue fluid, this creating an electrochemical gradient.
• sodium ions begin to diffuse back naturally into axon while potassium ions begin to diffuse back out of axon
• however, most of gates in channels that allow potassium ions to move through are open, while most of gates in channels that allow sodium ions to move through are closed.

Question 18

What is an action potential?

Answer 18

A change that occurs in the electrical charge across the membrane of an axon when its stimulated and a nerve impulse passes.

Question 19

What’s the first stage of a nerve impulse?

Answer 19

• at resting potential some K+ voltage gated channels are open but sodium voltage gated channels are closed.

Question 20

What’s the second stage of a nerve impulse?

Answer 20

Energy of stimulus causes some Na+ voltage-gated channels in the axon membrane to open and therefore Na+ ions diffuse into axon through these channels along their electrochemical gradient. Being positively charged, they trigger a reversal in the potential difference across the membrane.

Question 21

What’s the third stage of a nerve impulse?

Answer 21

As sodium ions diffuse into axon, so more sodium channels open, causing an even greater influx of sodium ions by diffusion.

Question 22

What’s the fourth stage of a nerve impulse?

Answer 22

Once action potential of around +40mV has been established, voltage gates on sodium ion channels close + voltage gates on potassium ion channels start to open.

Question 23

What’s the fifth stage of a nerve impulse?

Answer 23

With some potassium voltage-gated channels now open, the electrochemical gradient that was preventing further outward movement of K+ ions is now reversed, causing more K+ ion channels to open. This means more K+ ions diffuse out, starting repolarisation of axon.

Question 24

What is the final stage of a nerve impulse?

Answer 24

Outward diffusion of these potassium ions causes a temporary overshoot of the electrical gradient, with the inside of the axon being more negative than usual (hyperpolarisation) the crossbow gates on the K+ ion channels now close and activities of the sodium-potassium pumps once again cause sodium ions to be pumped out and potassium ions in. Resting potential of -65mV is re-established and axon said to be repolarised.