Nervous System

Question 1

State the components of a nervous response.

Answer 1

• Receptors
• Effectors
  Nervous system or hormones transfer information from receptors to effectors

Question 2

Name the two main divisions of the nervous system.

Answer 2

• Central nervous system (CNS)
• Peripheral nervous system (PNS)

Question 3

What is the central nervous system (CNS)?

Answer 3

Brain and spinal cord.

Question 4

What is the peripheral nervous system (PNS)?

Answer 4

Pairs of nerves that originate from the CNS and carry nerve impulses into and out of the CNS.

Question 5

What is the dorsal root?

Answer 5

• One of two roots that emerges from the spinal cord
• Travels to the dorsal root ganglion
  Sensory neurones enter the spinal cord via the dorsal root

Question 6

What is the ventral root?

Answer 6

• One of two roots that emerges from the spinal cord
  Motor neurones leave the spinal cord via the ventral root

Question 7

What is a reflex?

Answer 7

A rapid, automatic response to a sensory stimulus by the body. It serves as a protective mechanism.

Question 8

How does a reaction differ from a reflex?

Answer 8

• Reaction is voluntary and coordinated by the brain
• Reflex is non-voluntary and does not involve the brain

Question 9

What is a nerve net?

Answer 9

• Simplest form of nervous system found in Cnidarians
  Consists of interconnected nerve cells with short
  extensions allowing a response to a limited number of stimuli

Question 10

State the three types of functional
neurones.

Answer 10

Sensory neurone
* Relay neurone
* Motor neurone

Question 11

State the function of a sensory neurone.

Answer 11

Carries nerve impulses from the
receptors to the CNS via the dorsal root. .

Question 12

State the function of a motor neurone

Answer 12

Carries nerve impulses from the CNS to
the effectors via the ventral root.

Question 13

Describe the structure of a motor
neurone.

Answer 13

Short dendrites carry impulses from
CNS to cell body
Cell body found at one end of neurone dendrites
cell body at the end of axon
Long axon carries impulses from cell body to effectors

Question 14

State the function of a relay neurone.

Answer 14

Located in the spinal cord .
Links the sensory neurone to the
motor neurone

Question 15

What is the function of dendrites?

Answer 15

Short, branched extensions of the cell body
* Receive nerve impulses from other
neurones

Question 16

Describe the cell body.

Answer 16

The region of the neurone that contains the organelles, notably the nucleus and the rough endoplasmic reticulum.

Question 17

What is the function of the axon?

Answer 17

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

Question 18

What are axon terminals?

Answer 18

Branched endings of an axon that approach the muscle fibre.

Question 19

What is a synaptic end bulb?

Answer 19

The end of an axon that is bulbous
shaped and contains synaptic vesicles filled with neurotransmitters.

Question 20

Describe the additional features of a myelinated neurone.

Answer 20

Schwann cells: wrap around axon; involved in electrical insulation, phagocytosis, nerve regeneration

Myelin sheath: made from myelin-rich membranes of Schwann cells

Nodes of Ranvier: small gaps between neighbouring Schwann cells where there is no myelin sheath

Question 21

Define action potential.

Answer 21

The temporary change in electrical potential across the membrane of an axon in response to the transmission of a nerve impulse.

Question 22

What is resting potential?

Answer 22

Potential difference (voltage) across a
neurone membrane when not stimulated (-70 mV).

Question 23

How is resting potential established?

Answer 23

Membrane more permeable to K* than Na*
Sodium-potassium pump actively transports 3Na* out of cell and 2K* into cell
* Organic phosphates and large protein anions remain in cytoplasm
Establishes electrochemical gradient: cell contents more negative than extracellular environment

Question 24

Name the stages of an action potential.

Answer 24

1. Depolarisation
2. Repolarisation
3. Hyperpolarisation
4. Return to resting potential

Question 25

What happens during depolarisation?

Answer 25

1. Stimulus causes a change in the voltage across an axon membrane, opening voltage-gated Na+ channels 2. Na* diffuse into the axon 3. Potential difference across membrane becomes more positive, membrane depolarises

Question 26

What happens during repolarisation?

Answer 26

1. Membrane potential reaches +40 mV 2. Voltage-gated Na+ channels close and voltage-gated K+ channels open 3. Facilitated diffusion of K+ ions out of cell down their electrochemical gradient 4. Potential difference across membrane becomes more negative, membrane repolarises

Question 27

What happens during hyperpolarisation?

Answer 27

1. 'Overshoot' when K* ions diffuse out 2. Potential difference becomes more negative than resting potential 3. Membrane hyperpolarises, preventing another impulse occuring

Question 28

What is the refractory period?

Answer 28

Time period after an action potential during which further action potentials are prevented.

Question 29

Why is the refractory period important?

Answer 29

Ensures that action potentials can only be propagated in one direction.

Question 30

Describe the 'all or nothing' law

Answer 30

Principle that states that all stimuli above a certain threshold value will generate the same size of action potential, regardless of the strength of the stimulus.

Question 31

State the factors affecting the speed of conduction of a nervous impulse

Answer 31

* Temperature * Axon diameter * Myelin sheath

Question 32

Explain why myelinated axons conduct impulses faster than unmyelinated axons.

Answer 32

Saltatory propagation: Impulse 'jumps' from one node of Ranvier to another (depolarisation cannot occur where myelin sheath acts as electrical insulator). Therefore, impulse does not travel along whole axon length.

Question 33

How does temperature affect the speed of conduction of nervous impulses?

Answer 33

The higher the temperature, the faster the speed of conduction

Question 34

How does axon diameter affect the speed of conduction of nervous impulses?

Answer 34

The larger the diameter, the faster the speed of conduction.

Question 35

What is a synapse?

Answer 35

The junction between two nerve cells or a nerve cell and an effector.

Question 36

What is the function of synapses?

Answer 36

Electrical impulse cannot cross junction Neurotransmitters send impulses between neurones or from neurones to effectors Summation of sub-threshold impulses New impulses can be initiated in several different neurones for multiple simultaneous responses

Question 37

Describe the structure of a synapse.

Answer 37

Presynaptic neurone ends in synaptic knob * Synaptic knob contains a high concentration of mitochondria, endoplasmic reticulum and vesicles of neurotransmitter Synaptic cleft, 20-30 nm gap Postsynaptic neurone has complementary receptors to neurotransmitter (ligand-gated Na* channels)

Question 38

What is the synaptic cleft?

Answer 38

A small gap between neurones across which a nerve impulse is transmitted via neurotransmitters.

Question 39

Describe synaptic transmission in the presynaptic neurone.

Answer 39

1. Wave of depolarisation travels down presynaptic neurone, causing voltage-gated Ca2+ channels to open 2. Ca2+ cause vesicles of acetylcholine to move towards and fuse with presynaptic membrane 3. Exocytosis of neurotransmitter (e.g. acetylcholine) into synaptic cleft

Question 40

How do neurotransmitters cross the synaptic cleft?

Answer 40

Via simple diffusion

Question 41

Describe synaptic transmission in the postsynaptic neurone.

Answer 41

1. Acetylcholine diffuses across synaptic cleft and binds to specific receptors on postsynaptic membrane 2. Ligand-gated Na* channels open 3. If influx of Na+ ions raises membrane to threshold potential, action potential is generated

Question 42

How is the merging of impulses prevented during synaptic transmission?

Answer 42

* Active transport of Ca2+ out of synaptic knob Role of cholinesterase * Reabsorption of neurotransmitters

Question 43

Describe the role of cholinesterase in synaptic transmission.

Answer 43

* Hydrolyses acetylcholine in the postsynaptic neurone * Products diffuse back across the cleft

Question 44

What is the effect of organophosphates on the transmission of impulses?

Answer 44

* Act as cholinesterase inhibitors, preventing the hydrolysis of acetylcholine in the postsynaptic neurone * Results in continuous stimulation of the neurone

Question 45

Where are organophosphates commonly found?

Answer 45

Component of insecticides.

Question 46

How do psychoactive drugs (e.g. amphetamine) affect the transmission of impulses?

Answer 46

* Excitatory drugs Stimulate the release of neurotransmitters such as noradrenaline