6 Organisms respond to changes: 15 Nervous Coordination and Muscles

Question 1

Describe the structure of a myelinated motor neurone.

Answer 1

• cell body
• dendrons to dendrites (carry nerve impulses towards cell body)
• axon (carries nerve impulses away from cell body)
• Schwann cells (surround the axon, protecting it and providing electrical insulation)
• myelin sheath (forms a covering to the axon, made up of Schwann cell membranes)
• nodes of Ranvier (contrictions between Schwann cells where there is no myelin sheath)

Question 2

Describe the establishment of a resting potential, in terms of differential membrane permeability, electrochemical gradients, and the movement of Na+ and K+.

Answer 2

In a neurone’s resting state, the outside of the membrane is more positively charged (there are more positive ions outside the cell).
Membrane is polarised - there’s a potential difference across it, the resting potential.

The resting potential is created and maintained by Na+/K+ pumps and K+ channels.
- Na+/K+ pumps move Na+ out of the neurone
- the membrane isn’t Na+ permeable so the Na+ can’t diffuse back in
- this creates a Na+ electrochemical gradient where there are more Na+ outside the cell
- Na+/K+ pumps move K+ into the neurone
- the membrane is K+ permeable so K+ diffuse back out K+ channels
- this leaves the outside of the cell more positively charged.

Question 3

What is the process of generating an action potential and going back to the resting potential?

Answer 3

1. Stimulus
   
   • a stimulus excites the neurone cell membrane, causing the Na+ channels to open.
   • membrane becomes more Na+ permeable so Na+ diffuse into the neurone down the Na+ electrochemical gradient, making the inside of the neurone less negative
2. Depolarisation
   
   • if the potential difference reaches the threshold, an action potential is generated
   • more Na+ channels open so more Na+ diffuse rapidly into the neurone
3. Repolarisation
   
   • after the potential difference lowers, the Na+ channels close and K+ channels open.
   • the membrane is more K+ permeable so K+ diffuse out of the neurone down the K+ concentration gradient
   • the membrane starts to get back to its resting potential
4. Hyperpolarisation
   
   • K+ channels are slow to close so too many K+ diffuse out of the neurone
   • the potential difference becomes more negative than the resting potential
5. Resting potential
   
   • ion channels are reset
   • Na+/K+ pump returns the membrane to its resting potential

Question 4

What is the all-or-nothing principle?

Answer 4

Once the threshold is reached, an action potential will always fire with the same change in voltage, no matter the size of the stimulus.

A bigger stimulus only causes action potentials to fire more frequently.

Question 5

What is the passage of an action potential?

Answer 5

1. When an action potential happens, some of the Na+ that enter the neurone diffuse sideways.
2. This causes Na+ channels in the next region to open and Na+ diffuse into that part.
3. This causes a wave of depolarisation to travel along the neurone.
4. The Na+ moves away from parts of the membrane in the refractory period because these parts can’t fire an action potential.

Question 6

What is the nature and importance of the refractory period?

Answer 6

During the refractory period, ion channels are recovering and can’t be opened.
It acts as a time delay between one action potential and the next.

This means that:
- action potentials don’t overlap, but pass along as discrete (separate) impulses
- there’s a limit to the frequency at which nerve impulses can be transmitted
- action potentials are unidirectional

Question 7

List the factors affecting the speed of conductance.

Answer 7

Myelination and saltatory conduction,
axon diameter,
temperature

Question 8

How does myelination affect the speed of conductance?

Answer 8

• in a myelinated neurone, depolarisation only happens at the nodes of Ranvier
• the next node is depolarised
• the impulse ‘jumps’ from node to node
• saltatory conduction
• faster than in non-myelinated neurones where the impulse travels as a wave

Question 9

How does axon diameter affect speed of conductance?

Answer 9

• action potentials are conducted quicker along axons with bigger diameters
• there’s less resistance to the flow of ions
• depolarisation reaches other parts of the cell membrane quicker

Question 10

How does temperature affect speed of conductance?

Answer 10

• as the temperature increases, the speed of conduction increases as well
• ions diffuse faster
• after ~40C, proteins start to denature and the speed decreases

Question 11

Describe the structure of a synapse.

Answer 11

• the junction between a neurone and another neurone/ effector cell
• synaptic cleft (gap between cells)
• presynaptic neurone
  
  • synaptic knob containing synaptic vesicles filled with neurotransmitters
• postsynaptic neurone has specific receptors for specific neurotransmitters

Question 12

Describe the structure of a neuromuscular junction.

Answer 12

• synapse between a motor neurone and a muscle cell
• presynaptic neurone containing vesicles with acetylcholine within
• postsynaptic neurone with nicotinic cholinergic receptors
  
  • has lots of folds, forming clefts which store an enzyme which breaks down ACh (acetylcholinesterase)
  • has more receptors than other synapses

Question 13

What are the sequence of events involved in transmission across a cholinergic synapse?

Answer 13

1. An action potential arrives at the synaptic knob of the presynaptic neurone.
2. The action potential stimulates voltage-gated Ca2+ channels in the presynaptic neurone to open.
3. Ca2+ diffuse into the synaptic knob.
4. The influx of Ca2+ into the synaptic knob causes the synaptic vesicles to fuse with the presynaptic membrane.
5. The vesicles release acetylcholine into the synaptic cleft (exocytosis).
6. Acetylcholine diffuses across the synaptic cleft and binds to specific cholinergic receptors on the postsynaptic membrane.
7. This causes Na+ channels in the postsynaptic neurone to open.
8. The influx of Na+ into the postsynaptic membrane causes depolarisation. An action potential on the postsynaptic membrane is generated if the threshold is reached.
9. Acetylcholine is removed from the synaptic cleft so the response doesn’t continue. It’s broken down by acetylcholinesterase and the products are re-absorbed by the presynaptic neurone to make more acetylcholine.

Question 14

Why are impulses unidirectional when being transmitted across a synapse?

Answer 14

Because receptors are only on the postsynaptic membranes, impulses are unidirectional.

Question 15

What do excitatory neurotransmitters do?

Answer 15

Depolarise the postsynaptic membrane, making it fire an action potential if the threshold is reached.

Question 16

What do inhibitory neurotransmitters do?

Answer 16

Hyperpolarise the postsynaptic membrane, preventing it from firing an action potential.

Question 17

Describe spatial summation.

Answer 17

When many neurones are connected to one neurone, the small amount of neurotransmitter from each can altogether reach the threshold in the postsynaptic neurone and trigger an action potential.

If some neurones release an inhibitory neurotransmitter, then the total effect may be no action potential.

Question 18

Describe temporal summation.

Answer 18

Where two or more nerve impulses arrive in quick succession from the presynaptic neurone, an action potential is more likely because more neurotransmitter is released into the synaptic cleft.

Question 19

Compare the transmission across a cholinergic synapse and across a neuromuscular junction.

Answer 19

Similarities:
- have neurotransmitters that are transmitted by diffusion
- have receptors that cause an influx of Na+ when neurotransmitters bind
- use a Na+/K+ pump to repolarise the axon
- use enzymes to break down the neurotransmitter

Differences:
- neuromuscular junction is only excitatory, cholinergic synapse may be both excitatory or inhibitory
- neuromuscular junction only links neurones to muscles, cholinergic synapse links neurones to neurones or effectors
- neuromuscular junction only involves motor neurones, cholinergic synapse involves motor, sensory, or intermediate neurones
- action potential ends at a neuromuscular junction, a new action potential may be produced at a cholinergic synapse
- acetylcholine binds to receptors on muscle fibre membrane in neuromuscular junctions, aceytlcholine binds to postsynaptic membrane receptors in cholinergic synapses.

Question 20

What are the different effects of drugs on a synapse?

Answer 20

• some drugs are the same shape as neurotransmitters so mimic their action (more receptors are activated)
• some drugs block receptors from being activated by neurotransmitters (fewer receptors can be activated which can lead to muscle paralysis)
• some drugs inhibit the enzyme that breaks down neurotransmitters (there are more neurotransmitters in the synaptic cleft to bind to receptors and they’re there for longer which can lead to loss of muscle control)
• some drugs stimulate the release of neurotransmitter (more receptors are activated)
• some drugs inhibit the release of neurotransmitters (fewer receptors are activated)

Question 21

What do muscles act as?

Against what?

Answer 21

Antagonistic pairs against an incompressible skeleton.

Question 22

What is the gross structure of skeletal muscle?

Answer 22

• attached to bones by tendons
• ligaments attach bones to other bones
• pairs of skeletal muscle contract and relax to move bones at a joint

Question 23

What is the microscopic structure of skeletal muscle?

Answer 23

• made up of long muscle fibres
• cell membrane of muscle fibre cells is called the sarcolemma
• cytoplasm of muscle fibre cells is called sarcoplasm
• bits of the sarcolemma fold inwards and stick into the sarcoplasm (transverse tubules) which help spread electrical impulses
• sarcoplasmic reticulum stores and releases Ca2+
• lots of mitochondria to provide ATP for muscle contraction
• contain many nuclei
• have lots of myofibrils

Question 24

What is the structure of a myofibril?

Answer 24

• contain thick myosin and thin actin
• alternating pattern of dark and light bands
  
  • dark bands contain thick myosin filaments and some overlapping thin actin filaments (A-bands)
  • light bands contain thin actin filaments only (I-bands)
• made up of sarcomeres
• the ends of each sarcomere are marked with a Z-line
• in the middle of each sarcomere is the M-line (middle of the myosin filaments)
• around the M-line is the H-zone (only contains myosin filaments)

Question 25

What is the role of actin and myosin in myofibril contraction?

Include the structure of actin and myosin.

Answer 25

• myosin filaments have hinged globular heads that can move back and forth
• each myosin head has a binding site for actin and a binding site for ATP
• actin filaments have binding sites for myosin heads (actin-myosin binding sites)
• tropomyosin is found between actin filaments, helping myofilaments move past each other
• in a resting muscle, tropomyosin blocks the actin-myosin binding site (myofilaments can’t slide past each other because the myosin heads can’t bind)

Question 26

What is the role of Ca2+ and ATP in myofibril contraction (the cycle of actin-myosin cross bridge formation)?

What is the process of this cycle, including tropomyosin?

Answer 26

1. When an action potential stimulates a muscle cell, depolarisation spreads to the sarcoplasmic reticulum.
2. The sarcoplasmic reticulum releases stored Ca2+ into the sarcoplasm.
3. Ca2+ cause tropomyosin to change shape, pulling the tropomyosin out of the actin-myosin binding site.
4. This exposes the binding site, allowing the myosin head to bind.
5. An actin-myosin cross bridge is formed when a myosin head binds to an actin filament.
6. Ca2+ ions activate ATP hydrolase which hydrolyses ATP to provide the energy needed for muscle contraction.
7. This energy released causes the myosin head to bend, which pulls the actin filament along.
8. Another ATP molecule provides the energy to break the actin-myosin cross bridge, so the myosin head detaches.
9. The myosin head then reattaches to a different binding site further along the actin filament.
10. This cycle is repeated as long as Ca2+ ions are present.

Question 27

What are the roles of ATP and phosphocreatine in muscle contraction?

Answer 27

They provide the energy for muscle contraction.
ATP is mainly generated by respiration.
Phosphocreatine regenerates ATP as it is stored in muscles and acts as a reserve supply of phosphate. The phosphate can immediately combine with ADP to form ATP.

Question 28

Describe the structure, location, and general properties of slow and fast skeletal muscle fibres.

Answer 28

Slow twitch:
- contract slowly
- good for endurance activities
- can work for a longer time
- energy’s released slowly through aerobic respiration so they have lots of mitochondria, blood vessels, and myoglobin
- located in muscles like the back or calves

Fast twitch:
- contract quickly
- good for intense exercise
- can work for a shorter time
- energy’s released quickly through anaerobic respiration using glycogen so they have few mitochondria, blood vessels, or myoglobin
- have a store of phosphocreatine
- located in muscles like the biceps or eyes.