The Nervous System

Question 1

What are the 2 parts that make up the human nervous system and what do they consist of?

Answer 1

Central nervous system (CNS) brain and spinal cord
Peripheral nervous system (PNS) nerves and ganglia on the outside of the CNS

Question 2

Define a stimulus

Answer 2

A detectable change in the external or internal environment of an organism eg heat

Question 3

Define a receptor

Answer 3

Detects a stimulus and converts this to electrical energy to send the information as an electrical impulse to the CNS via neurones

Question 4

Define an effector

Answer 4

Receives electrical impulses from the CNS and brings about a response eg muscles and glands

Question 5

What does a sensory neurone do?

Answer 5

Carries impulses from sense receptors into CNS

Question 6

What does a motor neurone do?

Answer 6

Carries impulses from CNS to effector organ (muscle or glands)

Question 7

What do relay neurones do?

Answer 7

They carry impulses from sensory neurones (or other relay neurones) to a motor neurone found in the spinal cord

Question 8

Compare the structure of the 3 main types of neurone

Answer 8

Sensory - cell body in the middle of the neurone whereas motor and relay cell body at the start of the neurone.
Motor and sensory neurones have a longer axon

Question 9

What is the function of the axon?

Answer 9

Extension of the cytoplasm that transmit impulses away from the cell body to the axon endings

Question 10

What is the function of dendrites?

Answer 10

Many thin extensions of cytoplasm that receive impulses from other nerve cells and transmit impulses towards the cell body

Question 11

What is the function of the cell body/centron?

Answer 11

Contains the nucleus and groups of ribosomes in the cytoplasm (Nissl granules) that synthesise neurotransmitters

Question 12

What is the function of the synaptic end bulb?

Answer 12

Swelling at the end of an axon in which neurotransmitter is stored

Question 13

What is the function of axon endings/terminals?

Answer 13

Secretes neurotransmitter by exocytosis into synaptic cleft

Question 14

What is the function of Schwann cells?

Answer 14

They surround peripheral neurones in vertebrates and grow around the axon to form a multi-layered myelin sheath

Question 15

What is the function of the myelin sheath?

Answer 15

Formed as Schwann cells grow around the axon in peripheral neurones. Provide electrical insulation to speed up nerve impulse transmission

Question 16

What is the function of the nodes of Ranvier?

Answer 16

Intervals in the myelin sheath between adjacent Schwann cells

Question 17

What is the function of the spinal cord?

Answer 17

The transmission of neural signals between the brain and the rest of the body. It also contains neural circuits that can independently control numerous reflexes

Question 18

What is the spinal cord made up of?

Answer 18

A central area of grey matter which mainly contains nerve cell bodies. It is surrounded by white matter which consists of nerve fibres surrounded by myelin sheath.

Question 19

The spinal cord is surrounded by membranes called __________

Answer 19

Meninges

Question 20

Describe the placement of different neurones in the spinal cord

Answer 20

Sensory neurone from the peripheral nervous system enter the spinal cord on the dorsal root and the cell bodies of the sensory neurone is found in the dorsal root ganglion. Motor neurones leave via ventral roots

Question 21

The brain may override a response using __________ ________ ________

Answer 21

Inhibitory nerve fibres

Question 22

What is the hole in the middle of the spinal cord called?

Answer 22

Central canal

Question 23

Describe what is happening at the resting potential of a neurone (salty banana)

Answer 23

• 3 Na+ ions are pumped out of the axon for every 2 K+ ions that are pumped in by the sodium potassium pump
• Voltage-gated potassium ion channels, some of which are open, allow K+ to diffuse back out of the axon
• As the voltage gated sodium ion channels are closed the membrane is much more permeable to K+ than Na+
• The neurone has a resting potential of -70mV
• Some K+ channels allow K+ ions to leave the axon through leakage

Question 24

Describe what happens during depolarisation of a neurone

Answer 24

• The energy of a stimulus causes some voltage gated Na+ channels to open. The voltage gated potassium ion channels close.
• If a threshold is reached, the permeability of the membrane to sodium ions increases, and they rush into the axon, depolarising them membrane
• The negative charge of -70mV inside the axon rapidly becomes a positive charge of +40mV
• The membrane is said to be depolarised

Question 25

Describe what happens during the repolarisation and hyperpolarisation of the membrane

Answer 25

- The voltage gated sodium ion channels close. The voltage gated potassium ion channels open. - This causes K+ to rapidly diffuse out of the axon, reducing the potential difference across the membrane - An overshoot causes the membrane to become hyperpolarised

Question 26

Describe what happens during the refractory period

Answer 26

- During this time, the concentrations of K+ and Na+ are restored to that of the resting potential - During this time, this portion of the axon cannot transmit another action potential, ensuring that transmission is in one direction only

Question 27

What does the all or nothing law state?

Answer 27

**’An action potential is either initiated or it is not and it is always the same size’** - the size of the impulse is independent of the size of the stimulus - the speed of conduction of the stimulus is not altered by the intensity of the stimulus - a large stimulus will simply produce a greater frequency of impulses (this ensures low level stimuli are filtered out)

Question 28

Describe nerve impulse transmission in a non-myelinated neurone

Answer 28

- Sodium ions rapidly diffuse into the axon during depolarisation and diffuse down their concentration gradient to the next part of the axon due to local current effects. - This begins to depolarise the adjacent section of the membrane causing voltage gated sodium ion channels to open in these regions, allowing more sodium to diffuse in, generating an action potential and depolarising this part of the membrane. This spreads a wave of depolarisation along the axon - At the site of the initial action potential, sodium ion channels are inactivated and cannot reopen again until the resting potential has been reestablished

Question 29

Describe nerve impulse transmission in myelinated neurones

Answer 29

- The axon is surrounded by Schwann cells that form a myelin sheath to act as an electrical insulator - The nerve impulse jumps from one Node of Ranvier to the next, as myelin is impermeable to ions. Therefore depolarisation can only take place at the nodes where there is an abundance of ion channels and sodium potassium pumps. - The local currents caused by the diffusion of sodium ions are elongated and the speed of transmission is increased **(saltatory propagation)**

Question 30

What are the 3 factors affecting the speed of conduction of an impulse?

Answer 30

- Temperature - Diameter of axon - Myellinated

Question 31

What effect does temperature have on the speed of conduction of an impulse?

Answer 31

Ions move faster at a higher temperature as they have more kinetic energy. Warm blooded animals transmit nervous impulses more quickly and have faster responses

Question 32

What effect does the diameter of the axon have on the speed of conduction of an impulse?

Answer 32

The larger the diameter, the greater its volume in relation to the area of the membrane. More Na+ can flow along the axon so impulses travel faster.

Question 33

What effect does myelin have on the speed of conduction of an impulse?

Answer 33

This electrical insulation of the axon speeds up the rate of transmission as the action potential jumps from node to node in a process called saltatory propagation

Question 34

Compare the nervous system in a hydra and a human

Answer 34

- Hydra have a nerve net whereas humans have a CNS - Hydra do not have a myelin sheath whereas humans do - Hydra have a slow conduction speed whereas humans have a fast conduction speed - Hydra have the ability to regenerate neurones whereas humans don’t

Question 35

What are the 2 types of synapses?

Answer 35

- **Electrical** - a tiny gap that is small enough for an electrical impulse to be transmitted directly across - **Chemical** - involves a gap. Branches of axons lie close to dendrites and the impulse is transmitted across the synaptic cleft chemically as a neurotransmitter before being converted back to an electrical impulse

Question 36

Describe the mechanism involved in a synaptic transmission

Answer 36

- An action potential arrives at the axon terminal. This causes voltage gated calcium ion channels to open. Calcium ions diffuse rapidly into the presynaptic knob. - The influx of calcium ions causes vesicles containing acetylcholine to fuse with the presynaptic membrane - The acetylcholine is released into the synaptic cleft by exocytosis, diffuses over the synaptic cleft and binds to specific receptors on the post synaptic membrane - This causes the sodium ion channels in the post synaptic membrane to open and Na+ diffuses in, depolarising the post synaptic membrane. If the depolarisation reaches the threshold value an action potential is generated in the post synaptic membrane - Acetylcholine in the receptors is broken down by acetylcholinesterase into ethanoic acid and choline which diffuse back into the axon terminal through the presynaptic membrane - ATP is then required to re-synthesise and package the neurotransmitters into vesicles - Calcium ions use a different protein to be actively transported back out of the synaptic knob to reset its electrochemical gradient