Nerves and Muscles Flashcards
What is the structure of a nerve?
Whole nerve = bundle of fascicles
Whole nerve –> Fascicles –> Cells (neurons) –> axon
What are neurons?
The communication wires within our body. They transmit signals between different parts of our body in the form of neurotransmitters.
What are the different types of neuronal communication?
- Neuron to neuron
- Neuron to muscle (neuromuscular junction)
- Neuron to organs
What is the structure of a neuron?
INPUT ZONE
Dendrites
Cell body (soma)
Axon
Synaptic Terminals
OUTPUT ZONE
What is bioelectricity called?
Action potentials
What is an action potential?
The propagation of a change in voltage across the cell membrane down an axon.
A wave of electricity along an axon
What is depolarisation?
Cell becomes more positive
What is repolaristion?
Cell returns to resting voltage (negative)
What are the three main subtypes of muscle?
- Cardiac
- Smooth
- Skeletal
What are the layers of skeletal muscle?
- Whole Muscle (bundle of fascicles)
- Fascicles (bundle of fibres)
- Cells (myofibres)
- Myofibrils
- Myofilaments (actin and myosin)
What is the sarcolemma?
Surrounds each myofibre (cell membrane).
Barrier, keeps it all together
The sarcolemma goes into the muscle and forms the t-tubules
What is the sarcomere?
The contractile unit of a muscle. Made up of two types of myofilaments:
Actin - thin filaments (troponin complex)
Myosin - thick filaments (myosin heads)
What appearance do sarcomeres have?
A striated appearance (stripes)
What is atrophied muscle?
Caused by not exercising
What is hypertrophied muscle?
Caused by using it a lot
What is a motor unit?
Comprised of a motor neuron and all the muscle fibres that it innervates
Where are the motor neuron cell bodies located?
In the ventral (front/anterior) part of the spinal cord
How many myofibres can an axon innervate?
One axon innervates multiple myofibres
How many axons can a myofibre be innervated by?
A myofibre can only be innervated by one axon
What is motor recruitment?
The number of motor units activated at any one time to change the amount of force produced
How do ions diffuse across the cell membrane?
They require channels and they will diffuse until they reach equilibrium
What are the 4 main phases of the action potential?
- Resting membrane potential (RMP)
- Depolarisation
- Repolarisation
- Hyperpolarisation
What is the Resting Membrane Potential?
-70mV
How is the resting membrane potential established?
Establish the electrochemical gradient using the Na+/K ATPase. It pumps ions against their electrochemical gradient which uses ATP. Causes high Na+ outside the cell and K+ inside the cell which mean less positive ions come in, so the cell is more negative
When are voltage gated ion channels closed?
When the RPM of the cell is -70mV
When do voltage gated ion channels open?
-60mV for the voltage gated Na+ channel
+30mV for the voltage gated K+ channel
What happens in the early depolarisation phase?
- Upon input to the cell, the membrane voltage changes, which activates the cell
- If the voltage change reaches -60mV (threshold), the voltage-gated Na+ channels will open
- Na+ enters the cell down its electrochemical gradient
- At -60mV, the K+ channels remain closed
What happens in the depolarisation phase?
- When Na+ enters the cell, the inside becomes more positive
- Na+ continues to enter the cell until the membrane potential reaches +30mV
What happens in the repolarisation phase?
- At +30mV, the voltage gated Na+ channels close and the voltage gated K+ channels open.
- The cell is still depolarised when K+ channels open
- The opening of these channels allows K+ to leave the cell, which makes the inside more negative
What happens in the hyperpolarisation phase?
- Voltage gated K+ channels remain open until threshold of -40mV is reached and then they slowly begin to close
- K+ continues to leave the cell until the membrane potential reaches -80mV.
- Both the voltage gated Na+ and K+ channels are closed and the Na+/K+-ATPase pump re-establishes the RMP of -70mV
What is the refractory period?
The period where you aren’t able to generate another action potential.
The voltage gated Na+ channels are either already open (causing depolarisation) or inactive (during hyperpolarisation)
What does presynaptic mean?
Before the synapse
What does postsynaptic mean?
After the synapse
What is a synapse?
The junction between the presynaptic and the postsynaptic neuron
What does a neurotransmitter cross?
The synaptic cleft
What is an excitatory neurotransmitter?
Triggers the movement of Na+ into the neuron. Brings the membrane potential closer to threshold (depolarising - more positive)
What is an inhibitory neurotransmitter?
Triggers the movement of Cl- into the neuron. Moves the membrane potential away from threshold (hyperpolarising - more negative)
Where do local voltage changes summate?
At the axon hillock
When will an action potential fire?
When the net threshold at the axon hillock reaches -60mV. All the local potentials need to occur at the same time and place to summate and fire the action potential
Whats the difference between local potentials and action potentials?
Local potentials are small changes (graded response) in membrane potential at the cell body/dendrites. Chemically gated ion channels.
Action potentials occur when threshold at hillock is reached, propagate down the axon. Voltage gated ion channels.
What is myelin sheath?
Myelin is made from specialised schwann cells and is wrapped around the axon. The myelin sheath increases the speed of action potentials down the axon.
What are the 5 steps of the synaptic junction?
- Depolarisation of axon terminal causes voltage gated Ca2+ channels to open causing Ca2+ to enter the axon terminal
- Ca2+ triggers neurotransmitter to be released from vesicles into the synaptic cleft
- Neurotransmitter diffuses across the synaptic cleft
- Neurotransmitter binds to its receptor (chemically gated ion channel) on the post-synaptic membrane
- Na+ enters the post-synaptic cell which depolarises post-synaptic cell
Where are the chemically gated ion channels located?
Dendrites and cell body (soma)
Where are the voltage gated ion channels located?
Axon hillock, axon and axon terminals
What happens at the neuromuscular junction
- Depolarisation of axon terminal causes voltage gated Ca2+ channels to open causing Ca2+ to enter the axon terminal
- Ca2+ triggers neurotransmitter (ACh) to be released from vesicles into the synaptic cleft
- ACH diffuses across the synaptic cleft
- ACH binds to its ACH-receptor (chemically gated ion channel) on the motor end plate of the myofibre
- Na+ enters the myofibre which increases the membrane potential from -70mV to -60mv causing depolarisation of the myofibre
- The action potential propagates along the sarcolemma of the myofibre.
Once the action potential propagates along the sarcolemma of the myofibre, what happens?
- When the action potential arrives at the t-tubule, it initiates Ca2+ release from the Sarcoplasmic reticulum (the internal Ca2+ store)
- Ca2+ in the sarcoplasm diffuses to the myofilaments
- This initiates cross bridge cycling - the sarcomere shortens causing muscle contraction
What happens in cross bridge cycling?
- Cross bridge forms - Ca2+ binds to troponin, moving tropomyosin off the myosin binding sites and myosin binds to actin
- Power stroke - ADP +Pi dissociates from myosin. Myosin head flexes (power stroke) and tension develops
- Cross bridge detaches - ATP binds to myosin head causing it to detach from actin
- Myosin head reactivates - ATP is hydrolysed into ADP + Pi. The myosin head is recocked into an active state
What happens in the relaxation phase of the Excitation-Contraction Coupling?
Ca2+ in the sarcoplasm diffuses away from the myofilaments and is taken back up into the SR (this uses ATP). This terminates cross bridge cycling because the sarcomere lengthens causing relaxation
What is the length-tension relationship?
The relationship between muscle length, and the amount of force that is produced.
What happens if the sarcomeres are too short?
There is too much overlap between actin and myosin so myosin can not effectively move the z-lines closer together
What happens if the sarcomeres are too wide?
Overstretching means the myosin can not effectively bind to actin to create cross bridges
What is a twitch contraction?
A single action potential produces a short duration of contraction
What is summation contraction?
When another fibre is stimulated before the relaxation is completed, the subsequent contraction develop a higher tension. Not all Ca2+ is removed from the sarcoplasm because the next stimulus comes along very quickly. The build up of Ca2+ causes more cross bridges and more tension
What is tetanic contraction?
Force produced by a fibre at its maximum.
Incomplete tetanus - A muscle fibre produces maximum tension during rapid cycles of contraction and relaxation
Complete tetanus - When relaxation phase is eliminated by higher frequency stimuli
What muscles will a single twitch occur in?
Only in ocular muscles
What is isometric contraction?
No change in muscle length - still able to generate force due to shortening of the sarcomeres
What is isotonic contraction?
Change in muscle length
What is concentric contraction?
As the muscle contracts, it shortens in length
What is eccentric contraction?
As the muscle contracts, it lengthens
What is the ATP storage?
Muscle doesn’t have a lot of ATP storage so it needs to be replaced when used.
What happens during the first 6 seconds of short duration, high intensity exercise?
The ATP storage is used first.
What is the direct phosphorylation (creatine phosphate) energy system for the cell?
Coupled reaction of creatine phosphate and ADP. CP shuttle, provides fast but little ATP.
Energy source: creatine phosphate
What happens during the first 10 seconds of short duration, high intensity exercise?
ATP is formed from creatine phosphate and ADP (direct phosphorylation)
What is the anaerobic pathway energy system for the cell?
No oxygen required.
Glycolysis and lactic acid formation. Glucose breakdown forms 2 ATP molecules and lactic acid.
Energy source: glucose
What happens during the first 30-40 seconds to the end of short duration, high intensity exercise?
Glycogen stored in muscles is broken down to glucose, which is oxidised to generate ATP (anaerobic pathway)
What is the aerobic pathway energy system for the cell?
Oxygen required.
Aerobic cellular respiration. Glucose and fatty acids combined with oxygen can make a lot of ATP in the mitochondria.
Energy source: glucose, pyruvic acid, free fatty acids from adipose tissue, amino acids from protein catabolism
What happens during prolonged-duration exercise?
ATP is generated by breakdown of several nutrient energy fuels by aerobic pathway.
What are white muscle fibres?
- Type II B muscles
- Fast twitch muscle fibres
- High intensity, short duration
What are pink muscle fibres?
- Type II A muscles
- immediate muscle fibres
- Mid intensity, mid duration
What are red muscle fibres?
- Type I
- Slow twitch muscle fibres
- Blood vessels
- Low intensity, long duration
What type of muscle fibres would Track Cyclists have?
- Bigger, large muscles
- Fast, white, anaerobic muscle fibres
What type of muscle fibres would Road Race Cyclists have?
- Smaller, lean muscles
- Slow, red, aerobic muscle fibres
Where is ATP being used? (4)
- Na+/K+ ATPase pumps ions against the electrochemical gradient preparing for action potential to occur
- Output of synaptic cleft - exocytosis
- Re-uptake of Ca2+ in internal stores
- Cross bridge de-attachment, to prepare for contraction to occur
What happens to neurons if you run out of ATP?
- Loss of action potentials
- Loss of neuronal communication
What happens to skeletal muscle if you run out of ATP?
- No cross bridge de-attachment
- Remain in contractile state, muscles won’t relax
- Rigor Mortis - when dead the body isn’t producing ATP so the muscles stay contracted.
