9.2 The Mammalian Nervous System

Question 1

Name the 2 main divisions of the nervous system.

Answer 1

Central nervous system (CNS)
Peripheral nervous system (PNS - all neurons that are not part of the CNS)

Question 2

Describe the central nervous system.

Answer 2

Comprised of brain and spinal cord
Specialised system of nerve cells processes stimuli and propagates impulses

Question 3

Name the 2 main divisions of the PNS.

Answer 3

Voluntary (under conscious control)
Autonomic (not under conscious control)

Question 4

Name the 2 main divisions of the autonomic nervous system.

Answer 4

Sympathetic
Parasympathetic
They act antagonistically to regulate response of effectors e.g., heart rate

Question 5

Describe the sympathetic nervous system.

Answer 5

Usually stimulates effectors (coordinates fight-or-flight response)
Neurotransmitter noradrenaline
Ganglia are located near CNS

Question 6

Describe the parasympathetic nervous system.

Answer 6

Usually inhibits effectors (coordinates rest/digest response)
Neurotransmitter acetylcholine
Ganglia located far from CNS

Question 7

Describe the structure of the spinal cord.

Answer 7

Cylindrical bundle of nerves fibres runs from brain stem to lower back - surrounded by spinal vertebrae
Consists of nerve tissue
Grey matter - H-shaped region contains neurons
White matter - myelinated axons

Question 8

What is the function of the cerebellum?

Answer 8

Controls execution of movement
Possible role in cognition

Question 9

What is the function of the medulla oblongata?

Answer 9

Controls a range of autonomous functions including breathing and heart rate

Question 10

What is the function of the cerebrum?

Answer 10

Uppermost part of the brain that is organised into lobes which control voluntary functions

Question 11

What is the function of the hypothalamus?

Answer 11

Includes anterior pituitary gland
Involved in thermo and osmoregulation

Question 12

What is resting potential?

Answer 12

Potential difference across neuron membrane when not stimulates - about -70mV in humans

Question 13

How is resting potential established?

Answer 13

1. Membrane is more permeable to K+ than Na+
2. Sodium-potassium pump actively transports 3Na+ out of cell and 2K+ into cell
   Establishes electrochemical gradient

Question 14

Name the stages in generating an action potential.

Answer 14

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

Question 15

What happens during depolarisation?

Answer 15

1. Stimulus —> facilitated diffusion of Na+ into cell down electrochemical gradient
2. P.D across membrane becomes more positive
3. If membrane reaches threshold potential (-50mV) voltage gated Na+ channels open
   Significant influx of Na+ ions reverses P.D to +40mV

Question 16

What happens during repolarisation?

Answer 16

1. Voltage gated Na+ channels close and voltage gated K+ channels open
2. Facilitated diffusion of K+ ions out of cell down their electrochemical gradient
3. P.D across membrane becomes more negative

Question 17

What happens during hyperpolarisation?

Answer 17

1. Overshoot when K+ ions diffuse out of= P.D becomes more negative than resting potential
2. Refractory period: no stimulus is large enough to raise membrane potential to threshold
3. Voltage gated K+ channels close and sodium-potassium pump re-establishes resting potential

Question 18

Explain the importance of the refractory period.

Answer 18

No action potential can be generated in hyperpolarised sections of membrane:

1. Ensures unidirectional impulse
2. Ensures discrete impulses
3. Limits frequency of impulse transmission

Question 19

How is an action potential propagated along an unmyelinated neuron?

Answer 19

1. Stimulus leads to influx of Na+ ions - first section of membrane depolarised
2. Local electrical currents cause sodium voltage-gated channels further along membrane to open - meanwhile the section behind begins to depolarise
3. Sequential wave of depolarisation

Question 20

Describe the structure of a motor neuron.

Answer 20

Cell body: contains organelles & high proportion of RER
Dendrons: branch into dendrites which carry impulses towards cell body
Axon: long, unbranched fibre carries nerve impulse away from cell body

Question 21

Describe the additional features of a myelinated motor neuron.

Answer 21

Schwann cells: wrap around axon many times
Myelin sheath: made from myelin-rich membranes of Schwann cells
Nodes of Ranvier: very short gaps between neighbouring Schwann cells where there is no myelin sheath

Question 22

Explain why myelinated axons conduct impulses faster than unmyelinated axons.

Answer 22

Saltatory conduction: impulse jumps from one node of ranvier to another - depolarisation cannot occur where myelin sheath acts as electrical insulator
So impulse does not travel along whole axon length

Question 23

Describe the structure of a synapse.

Answer 23

1. Presynaptic neuron ends in synaptic knob - contains lots of mitochondria, endoplasmic reticulum & vesicles of neurotransmitter
2. Synaptic cleft
   Postsynaptic neuron: has complementary receptors to neurotransmitter

Question 24

Explain the role of acetylcholine.

Answer 24

Causes muscle contraction at motor end plate
Causes excitation at preganglionic neurons
Causes inhibition at postganglionic neurons

Question 25

What happens to acetylcholine from the synaptic cleft?

Answer 25

1. Hydrolysis into acetyl and choline by acetylcholinesterase 2. Acetyl & choline diffuse back into presynaptic membrane 3. ATP is used to reform acetylcholine from storage in vesicles

Question 26

What happens in the presynaptic neuron when an action potential is transmitted between neurons?

Answer 26

1. Wave of depolarisation travels down presynaptic neuron, causing voltage-gated Ca2+ channels to open 2. Vesicles move towards & fuse with presynaptic membrane 3. Exocytosis of neurotransmitter into synaptic cleft

Question 27

How do neurotransmitters cross the synaptic cleft?

Answer 27

Via simple diffusion

Question 28

What happens in the postsynaptic neuron when an action potential is transmitted between neurons?

Answer 28

1. Neurotransmitter binds to specific receptor 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 29

What happens in an inhibitory synapse?

Answer 29

1. Neurotransmitter binds to an opens Cl- channels on postsynaptic membrane & triggers K+ channels to open 2. Cl- moves in & K+ moves out via facilitated diffusion 3. P.D becomes more negative: hyperpolarisation so no action potential is generated

Question 30

How does nicotine work?

Answer 30

Absorbed in lungs - travels to brain via bloodstream Similar shape to acetylcholine = bids to cholinergic receptors Stimulant effect causes symptoms of sharpness & relaxation

Question 31

How does lidocaine work?

Answer 31

Absorbed through skin or injected Blocks voltage gated Na+ ion channel = prevents postsynaptic neuron from depolarising Prevents transmission of pain signals so acts as anaesthetic

Question 32

How does cobra venom work?

Answer 32

Blocks acetylcholine receptors on diaphragm = muscles cannot contract during inhalation Disrupts normal breathing

Question 33

Name the 2 types of photoreceptor cells located in the retina.

Answer 33

Cone cells Rod cells

Question 34

Where are rod and cone cells located in the retina?

Answer 34

Rod: distributed evenly around periphery but NOT in central fovea Cone: mainly central fovea No photoreceptors at blind spot where ganglion axon fibres form optic nerve

Question 35

Explain why rod cells do not generate action potentials in the dark.

Answer 35

1. Na+ enters outer segment of red cell via on-specific cation channels - active transport of Na+ out of inner segment = rod cell is slightly depolarised 2. Action potential = voltage gated Ca2+ channels open - triggers exocytosis of glutamate 3. Glutamate acts as inhibitory neurotransmitter to hyperpolarisation bipolar neuron

Question 36

Explain how rod cells generate an action potential in the light.

Answer 36

1. Rhodopsin pigment bleaches when it absorbs light & breaks down into opsin + retinal 2. Opsin closes cation channels via hydrolysis - active transport of Na+ out of inner segment continues 3. Rod cell becomes hyperpolarised - no glutamate is release, so no inhibitor signal 4. Bipolar neuron depolarises

Question 37

Describe the pigments in rod and cone cells.

Answer 37

Rod: rhodopsin absorbs all wavelengths of light Cone: 3 types of iodopsin which absorbs red, blue or green wavelengths of light

Question 38

Describe the visual acuity of rod and cone cells.

Answer 38

Rod: many told cells synapse with 1 bipolar neuron - low resolution Cone: 1 cone cell synapse 1 bipolar neuron so there is no retinal convergence = high resolution

Question 39

Describe the light sensitivity of rod and cone cells.

Answer 39

Rod: very sensitive due to spatial summation of subthreshold impulses = vision in low-light conditions Cone: less sensitive = vision in bright light

Question 40

Describe how light causes a change in the release of glutamate from rod cells.

Answer 40

Rhodopsin is involved Retinal and opsin is formed from rhodopsin Na+ channels are closed Na+ is moving out of the cell Stops glutamate/neurotransmitter release

Question 41

Explain the role of mitochondria in the functioning of rod cells.

Answer 41

Produces ATP Needed for active transport of ions