6- Nerve impulses, Synaptic transmission, Skeletal muscles

Describe the structure of a myelinated motor neurone

Describe resting potential

Inside of axon has a negative charge relative to outside (as more positive ions outside compared to inside)

Explain how a resting potential is established across the axon membrane in a neurone

• Na+/ K+ pump actively transports:

  • (3) Na+ out of axon AND (2) K+ into axon

• Creating an electrochemical gradient:

  • higher K+ conc inside AND higher Na+ conc outside

• Differential membrane permeability:

  • more permeable to K+= move out by facilitated diffusion

  • less permeable to Na+ (closed channels)

What are the stages of depolarisation and generation of an action potential?

1. Stimulus

2. Depolarisation

3. Repolarisation

4. Hyperpolarisation

5. Resting potential

Explain how changes in membrane permeability lead to depolarisation and the generation of an action potential

1. Stimulus

   • Na+ channels open; membrane permeability to Na+ increases

   • Na+ diffuse into axon down electrochemical gradient (causing depolarisation)

2. Depolarisation

   • If threshold potential reached, an action potential is generated

   • As more voltage-gated Na+ channels open (positive feedback effect)

   • So more Na+ diffuse in rapidly

3. Repolarisation

   • Voltage-gated Na+ channels close

   • Voltage-gated K+ channels open; K+ diffuse out of axon

4. Hyperpolarisation

   • K+ channels slow to close so there’s a slight overshoot- too many K+ diffuse out

5. Resting potential

   • Restored by Na+/ K+ pump

Describe the all-or- nothing principle

• For an action potential to be produced, depolarisation must exceed threshold potential

• Action potentials produced are always same magnitude/ size/ peak at same potential

  • bigger stimuli instead increase frequency of action potential

Explain how the passage of an action potential along non- myelinated axons results in nerve impulses

• Action potential passes as a wave of depolarisation

• Influx of Na+ in one region increases permeability of adjoining region to Na+ by causing voltage-gated Na+ channels to open so adjoining region depolarises

Explain how the passage of an action potential along myelinated axons results in nerve impulses

• Myelination provides electrical insulation

• Depolarisation of axon at nodes of Ranvier only

• Resulting in saltatory conduction (local currents circuits)

• So there is no need for depolarisation along whole length of axon

Suggest how damage to the myelin sheath can lead to slow responses and/ or jerky movement

• Less/ no saltatory conduction; depolarisation occurs along whole length of axon

  • so nerve impulses take longer to reach neuromuscular junction; delay in muscle contraction

• Ions/ depolarisation may pass/ leak to other neurones

  • causing wrong muscle fibres to contract

Describe the nature of the refractory period

• Time taken to restore axon to resting potential when no further action potential can be generated

• As Na+ channels are closed/ inactive/ will not open

Explain the importance of the refractory period

• Ensures discrete impulses are produced (action potentials don’t overlap)

• Limits frequency of impulse transmission at a certain intensity (prevents over reaction to stimulus)

  • higher intensity stimulus causes higher frequency of action potentials

  • but only up to certain intensity

• Also ensures action potentials travel in one direction- can’t be propagated in a refractory region

In the second half of the refractory period an action potential can be produced but requires greater stimulation to reach threshold

Describe the factors that affect speed of conductance

Myelination:

• depolarisation at NOdes of Ranvier only= saltatory conduction

• impulse doesn’t travel/ depolarise whole length of axon

Axon diameter:

• bigger diameter means less resistance to flow of ions in cytoplasm

Temperature:

• increases rate of diffusion of Na+ and K+ as more kinetic energy

• but proteins/ enzymes could denature at a certain temperature

Describe the structure of the synapse

What are cholinergic synapses?

synapses that use the neurotransmitter acetylcholine (ACh)

Describe transmission across a cholinergic synapse

AT PRE-SYNAPTIC NEURONE:

1. Depolarisation of pre-synaptic membrane causes opening of voltage-gated Ca2+ channels

   • Ca2+ diffuse into pre-synaptic neurone/ knob

2. Causing vesicles containing ACh to move and fuse with pre-synaptic membrane

   • releasing ACh into the synaptic cleft (by exocytosis)

AT POST-SYNAPTIC NEURONE:

3. ACh diffuses across synaptic cleft to bind to specific receptors on post- synaptic membrane

4. Causing Na+ channels to open

   • Na+ diffuse into post- synaptic knob causing depolarisation

   • if threshold is met, an action potential is initiated

Explain what happens to acetylcholine after synaptic transmission

• It is hydrolysed by acetylcholinesterase

• Products are reabsorbedby the presynaptic neurone

• To stop overstimulation- if not removed it would keep binding to receptors, causing depolarisation

Explain how synapses result in unidirectional nerve impulses

• neurotransmitter only made in/ released from pre-synaptic neurone

• receptors only on post-synaptic membrane

Explain summation by synapses

• Addition of a number of impulses converging on a single post-synaptic neurone

• causing rapid buildup of neurotransmitter (NT)

• so threshold more likely to be reached to generate an action potential

Importance- low frequency action potentials release insufficient neurotransmitter to exceed threshold

Describe spatial summation

• Many pre-synaptic neurones share one synaptic cleft/ post- synaptic neurone

• Collectively release sufficient neurotransmitter to reach threshold to trigger an action potential

Describe temporal summation

• one pre-synaptic neurone releases neurotransmitter many times over a short time

• sufficient neurotransmitter to reach threshold to trigger an action potential

Describe inhibition by inhibitory synapses

• Inhibitory neurotransmitters hyperpolarise postsynaptic membrane as:

  • Cl- channels open= Cl- diffuse in

  • K+ channels open= K+ diffuse out

• This means inside of axon has a more negative charge relative to outside/ below resting potential

• So more Na+ required to enter for depolarisation

• Reduces likelihood of threshold being met/ action potential formation at post- synaptic membranes

Importance- both excitatory and inhibitory neurones forming synapses with the same post- synaptic membrane gives control of whether it ‘fires’ an action potential

Describe the structure of a neuromuscular junction

Very similar to a synapse except:

• receptors are on muscle fibre sarcolemma instead of postsynaptic membrane and there are more

• muscle fibre forms clefts to store enzyme e.g. acetylcholinesterase to break down neurotransmitter

Compare transmission across cholinergic synapses and neuromuscular junctions

In BOTH: transmission is unidirectional

CHOLINERGIC SYNAPSE:

• neurone to neurone (or effectors, glands

• neurotransmitters can be excitatory or inhibitory

• action potential may be initiated in postsynaptic neurone

NEUROMUSCULAR JUNCTION:

• (motor) neurone to muscle

• always excitatory

• action potential propagates along sarcolemma dow T tubules

Use examples to explain the effect of drugs on a synapse

• Some drugs stimulate the nervous system, leading to more action potentials, e.g.;

  • similar shape to neurotransmitter

  • stimulate release of more neurotransmitter

  • inhibit enzyme that breaks down neurotransmitter= Na+ continues to enter

• Some drugs inhibit the nervous system, leading to fewer action potentials, e.g.;

  • inhibit release of neurotransmitter e.g. prevent opening of calcium ion channels

  • block receptors by mimicking shape of neurotransmitter

Describe how muscles work

• work in antagonistic pairs= pull in opposite directions e.g. biceps/ triceps

  • one muscle contracts (agonist), pulling on bone/ producing force

  • one muscle relaxes (antagonist)

• skeleton is incompressible so muscle can transmit force to bone

ADVANTAGE- the second muscle required to reverse movement caused by the first (muscles can only pull) and contraction of both muscles helps maintain posture

Describe the gross and microscopic structure of skeletal muscle

• Made of many bundles of muscle fibres (cells) packaged together

• Attached to bones by tendons

• Muscle fibres contain:

  • sarcolemma (cell membrane) which folds inwards (invagination) to form transverse (T) tubules

  • sarcoplasm (cytoplasm)

  • multiple nuclei

  • many myofibrils

  • sarcoplasmic reticulum (endoplasmic reticulum)

  • many mitochondria

Describe the ultrastructure of a myofibril

• Made of 2 types of long protein filaments, arranged in parallel

  • myosin- thick filament

  • actin- thin filament

• Arranged in functional units called sacromeres

  • ends- Z-line/ disc

  • middle- M-line

  • H zone- contains only myosin

Explain the banding pattern to be seen in myofibrils

• I-bands= light bands containing only thin actin filaments

• A-bands= dark bands containing thick myosin filaments (and some actin filaments)

  • H zone contains only myosin

  • darkest region contains overlapping actin and myosin

Give an overview of muscle contraction

• Myosin heads slide actin along myosin causing the sacromere to contract

• Simulanteous contraction of many sacromeres causes myofibrils and muscle fibres to contract

• When sacromeres contract (shorten)…

  • H zones get shorter

  • I band get shorter

  • A band stays the same

  • Z lines get closer

Describe the roles of actin, myosin, calcium ions, tropomyosin and ATP in myofibril contraction

Describe the role of phosphocreatine in muscle contraction

• A source of inorganic phosphate (Pi) = rapidly phosphorylates ADP to regenerate ATP

  • ADP+ phosphocreatine = ATP + creatine

• Runs out after a few seconds= used in short bursts of vigorous exercise

• Anaerobic and alactic

Compare the structure, location and general properties of slow and fast skeletal muscle fibres (slow twitch)

GENERAL PROPERTIES:

• specialised for slow, sustained contractions (e.g. posture, long distance running)

• produce more ATP slowly (mostly) from aerobic respiration

• fatigues slowly

LOCATION:

• high proportion in muscles used for posture e.g. back, calves

• legs of long distance runners

STRUCTURE:

• high conc. of myoglobin= stores oxygen for aerobic respiration

• many mitochondria= high rate of aerobic respiration

• many capillaries= supply high conc. of oxygen/ glucose for aerobic respiration and to prevent build-up of lactic acid causing muscle fatigue

Compare the structure, location and general properties of slow and fast skeletal muscle fibres (fast twitch)

GENERAL PROPERTIES:

• specialised for brief, intensive contractions (e.g. sprinting)

• produce less ATP rapidly (mostly) from anaerobic respiration

• fatigues quickly due to high lactate concentration

LOCATION:

• high proportion in muscles used for fast movement e.g. biceps, eyelids

• legs of sprinters

STRUCTURE:

• low levels of myoglobin

• lots of glycogen= hydrolysed to provide glucose for glycolysis/ anaerobic respiration which is inefficient so large quantities of glucose required

• high conc. of enzymes involved in anaerobic respiration (in cytoplasm)

• store phosphocreatine