Nervous Communication Flashcards
Describe the resting membrane potential
Ina neurones resting state, the outside of the membrane is positively charged compared to the inside. This is because there are more positive ions outside the cell than inside, so the membrane is polarised. The potential difference across the membrane when it’s at rest is about -70mV and is called the resting potential
How is the resting potential of a membrane created
Sodium-potassium pumps use active transport to move three sodium ions out of the neurone for every two potassium ions that moved in. This requires ATP. Potassium ion channels allow facilitated diffusion of potassium ions out of the neurone down their concentration gradient. The sodium-potassium pumps move sodium ions out of the neurone but they are unable to diffuse back in, creating a sodium ion electrochemical gradient (more positive sodium outside than inside). The sodium-potassium pumps also move potassium ions in to the neurone. When the cell is at rest, most potassium ion channels are open. This means that the membrane is permeable to potassium ions so they diffuse back out through the potassium ion channels
Action potential sequence
The stimulus excites the neurone cell membrane, causing the sodium ion channels to open. The membrane becomes more permeable to sodium ions so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient. This makes the inside of the neurone less negative. If the potential difference reaches the threshold, more sodium ions channels open causing more sodium ions to diffuse rapidly into the neurone. The cell become depolarised. At a potential difference of +30mV, the sodium ions channels close and potassium ion channels open. The membrane is more permeable to potassium ions so potassium ions diffuse out of the neurone down the potassium ion concentration gradient. This starts to get the membrane back to its resting potential by repolarising it. However, potassium ion channels are slow to close meaning more potassium ions diffuse out of the neurone than they should. This is known as hyperpolarisation and makes the inside of the cell more negative than its resting potential. Once the ion channels are reset, the sodium potassium pump returns the membrane back to its resting potential.
What is the refractory period?
The period of recovery for the membrane. Period of time where the cell membrane cannot be excited as the ion channels are recovering and cannot open
How does the action potential move along a neurone
When an action potential happens, some of the sodium ions that enter the neurone diffuse sideways. This causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part. This causes a wave of depolarisation to travel along the neurone. The wave moves away from the parts of the membrane in the refractory period because these parts can’t fire an action potential
What is the role of the refractory period
Acts as a time delay to ensure:
- action potentials don’t overlap but pass along as discrete impulses
- there’s a limit to the frequency at which nerve impulses can be transmitted
- action potentials are unidirectional
What is the action potential all or nothing theory
Threshold reached, action potential will occur - bigger stimulus doesn’t mean bigger action potential but causes action potentials to fire more frequently
Threshold not reached then no action potential
Structure of myelinated motor neurone
Myelin sheath surrounding the axon made up of Schwann cells. In between Schwann cells are nodes of Ranvier. Sodium ion channels are concentrated at the nodes of Ranvier
How are impulses passed along a myelinated neurone
The neurones cytoplasm conducts enough electrical charge to depolarise the next node of Ranvier so the impulse jumps from node of Ranvier to node of Ranvier. This is called saltatory conduction and is very fast
How are impulses passed along a non myelinated neurone
The impulse travels as a wave along the whole length of the axon membrane
How does axon diameter affect action potential conductivity?
Action potentials are conducted quicker along axons with bigger diameters because there’s less resistance to the flow of ions than in the cytoplasm of a smaller axon. With less resistance, depolarisation reaches other parts of the neurone cell membrane quicker
How does temperature affect action potential conductivity?
The speed of conduction increases as the temperature increases too, because ions diffuse faster. The speed only increases up to 40 degrees Celsius- after that the proteins begin to denature and the speed decreases
What is a synapse
Junction between 2 neurones or between a neurone and an effector cell
Structure of a synapse
Presynaptic membrane contains a synaptic knob. Synaptic knob contains vesicles filled with neurotransmitters. Postsynaptic membrane has receptors on its surface. Synaptic cleft between presynaptic membrane and postsynaptic membrane
How do action potentials travel across synapses
When an action potential reaches the end of a neurone is causes the neurotransmitters inside the vesicles to be released into the synaptic cleft. The diffuse across to the postsynaptic membrane and bind to specific receptors. This causes an action potential. Neurotransmitters and e removed from the cleft so the response doesn’t keep occurring
How do synapses ensure impulses are unidirectional
Receptors are only found on the postsynaptic knob
How is a nerve impulse transmitted across a cholinergic synapse
An action potential arrives at the synaptic knob of the presynaptic neurone. The action potential stimulates voltage gated calcium ion channels in the presynaptic neurone causing them to open. Calcium ions diffuse into the synaptic knob. The influx of calcium ions into the synaptic knob causes the synaptic vesicles to move to the presynaptic membrane. They then fuse with the membrane. The vesicles release the acetylcholine (neurotransmitters) into the synaptic cleft. This is called exocytosis. Acetylcholine diffuses across the synaptic cleft and binds to cholinergic receptors on the postsynaptic membrane. This causes sodium ion channels in the postsynaptic neurone to open. The influx of sodium ions into the postsynaptic membrane causes depolarisation. An action potential on the postsynaptic membrane is generated of the threshold is reached. Acetylcholine is removed from the synaptic cleft by acetylcholinesterase to prevent the response from continuing.
What do excitatory neurotransmitters do
Excitatory neurotransmitters depolarise the postsynaptic membrane making it fire an action potential if the threshold is reached
What do inhibitory neurotransmitters do
Inhibitory neurotransmitters hyperpolarise the postsynaptic membrane preventing it from firing an action potential
Spatial summation
Lots of neurones connect to one neurone so all release neurotransmitters that add together to reach the threshold
Temporal Summation
Two or more nerve impulses arrive in quick succession from the same presynaptic neurone
A neuromuscular junction is
A neuromuscular junction is a synapse between a motor neurone and a muscle cell
Difference in neuromuscular junction compared to cholinergic synapse
- the postsynaptic membrane has lots of foods that form clefts. These clefts store the enzyme that breaks down acetylcholinerase
- the postsynaptic membrane has more receptors than other synapses
- acetylcholine is always excitatory at a neuromuscular junction but not always the case between two neurones
Smooth muscle
Contracts without conscious control and found in the walls of internal organs
Cardiac muscles
Contract without conscious control and found in the heart
Skeletal muscle
Consciously moved and are attached to bones by tendons
Three types of muscle
Smooth muscle
Cardiac muscle
Skeletal muscle
Antagonistic pairs are
Muscles that work together to move a bone
One contracts while the other relaxes and vice Versa
Contracting muscle in antagonistic pairs called
Agonist
Relaxing muscle in antagonistic pairs called
Antagonist
Structure of skeletal muscles
Skeletal muscle is made up of muscle fibres. The cell membrane of muscles fibre cells is called the sarcolemma. Bits of the sarcolemma foldnineards across the muscle fibre and stick into the sarcoplasm. These folds are called transverse T tubules and they help to spread electrical impulses throughout the sarcoplasm so they reach all parts of the muscle fibre. The sarcoplasmic reticulum runs through the sarcoplasm. Muscle fibres contain lots of myofibrils which are made up of proteins
Structure of myofibrils
Myofibrils contain thank and thick myofilaments that move past each other to make muscles contract. The thick myofilaments are made of myosin (protein) and the thin myofilaments are made of actin (protein)
Myofibrils are made up of sarcomeres. Ends of each sarcomere is z line. Middle of each sarcomere is the m line. The m line is in the middle of the myosin filaments. The m line is in the middle of the h zone. H zone only contains myosin filaments
Explain light and dark bands in myofibrils
Dark bands contain myosin and actin overlapping. Light bands contain actin. Dark bands are A bands. Light bands are I bands.
Sliding filament theory
Myosin and actin filaments slide over one another to make the sarcomere contract. The simultaneous contractions of lots of sarcomeres means the myofibrils and muscle fibres contract. Sarcomeres return to their original length as the muscle relaxes
What happens to the bands during contraction of the sarcomeres
A band = stays the same length
I band = gets shorter
H zone = gets shorter
Muscle contraction
At rest the actin myosin binding site on the actin filament is blocked by tropomyosin. When an action potential from a motor neurone stimulates a muscle cell, it depolarises the sarcolemma. Depolarisation spreads down the T tubules to the sarcoplasmic reticulum. This causes the sarcoplasmic reticulum to release stored calcium ions into the sarcoplasm. Calcium ions binds tropomyosin causing it to change shape and unblock the actin myosin binding site on the actin filament. The myosin globular head on the myosin filament binds to the actin myosin binding site, forming an actin myosin cross bridge. Calcium ions activate the ATPase which releases energy needed for muscle contraction. The energy released caused the myosin globular head to bend, which pulls the actin filament along. Another ATP molecule breaks the actin myosin cross bridge so the myosin head is released from the actin myosin binding site. The myosin head then attached to a different binding site forming a new actin myosin cross bridge and the cycle is repeated. When the muscles is no longer being stimulated, calcium ions leave their binding sites and are actively transported back into the sarcoplasmic reticulum. This causes tropomyosin molecules to reblock the actin myosin binding site so that myosin heads can no longer bond and the muscle is no longer contracted
Slow twitch muscle fibres
Contract slowly Good for endurance Releases energy slowly Aerobic respiration Lots of mitochondria Lots of blood vessels Rich in myoglobin
Fast twitch muscle fibres
Contract quickly Sprints Short bursts of energy Tired quickly Anaerobic respiration Not much myoglobin
Slow twitch muscle fibres
Contract slowly Good for endurance Releases energy slowly Aerobic respiration Lots of mitochondria Lots of blood vessels Rich in myoglobin
Fast twitch muscle fibres
Contract quickly Sprints Short bursts of energy Tired quickly Anaerobic respiration Not much myoglobin
What is summation
Where the effect of neurotransmitters released from many neurones is added together
