Nervous Coordination Flashcards
What are the conditions of a neurone’s resting state?
It is positively charged on the outside and negatively charged on the inside, as there are more positive ions on the outside
How is the resting potential maintained?
- The sodium-potassium pump moves sodium ions out the neurone, but as the membrane is not permeable to sodium, it doesn’t diffuse back in
- This creates a sodium ion electrochemical gradient, as it is more positive on the outside then the inside
- The sodium-potassium pumps also move potassium ions into the neurone, the membrane is permeable to potassium ions so potassium ions diffuse back out through potassium ion channels
- Resulting in the outside of the cell being positively charged compared to the inside
What happens during the stimulation of cell membranes?
- Na⁺ ion channels in the cell membrane open
- Na⁺ ions flood into the neurone
- PD changes to be more positive on the inside
What happens during depolarisation?
- If PD is above threshold value, then the membrane becomes depolarised
- More Na⁺ ion channels open and a sharp increase in PD occurs
What happens during repolarisation?
- Na⁺ ion channels close and K⁺ ion channels open
- Na⁺ ions are transported back out of the neurone so PD is more negative
What happens during hyperpolarisation?
- PD becomes slightly more negative than the resting potential
- It prevents the neurone from being re-stimulated instantly
Why can’t neurone cell membranes get excited after an action potential?
- Due to the fact that the ion channels are recovering and can’t be opened. The period when both sodium and potassium ion channels are closed, it is called the refractory period
How do action potentials have an all or nothing nature?
- Once the threshold is reached, an action potential will fire with the same change in voltage
- If the threshold is not reached, an action potential will not fire
How do action potentials move along a neurone?
- Some sodium ions that enter diffuse sideways
- This causes sodium ion channels in the next region to open and sodium ions to diffuse into that region
- This causes a wave of depolarisation to travel across the neurone
How does myelination affect the speed of conduction of action potentials?
- Depolarisation only happens at the nodes of Ranvier
- The neurone’s cytoplasm conducts enough electrical charge to depolarise the next node, in a process called sultatory conduction
- If unmyelinated, the action potential travels across the whole membrane and is slower than sultatory conduction
How does axon diameter affect the speed of conduction of action potentials?
- The bigger the diameter, the quicker the speed of conduction because their is less resistance to the flow of ions
- Depolarisation is able to reach other parts of the neurone quicker
How does temperature affect the speed of conduction of action potentials?
- As temperature increases, the speed of conduction increases because ions diffuse faster
- After 40℃, the proteins start to denature and the speed decreases
What is a synapse?
- A junction between a pre-synaptic and post-synaptic neurone / effector cell
What happens when an action potential reaches the end of a neurone?
- When an action potential reaches the end of a neurone, it causes neurotransmitters to be released into the synaptic cleft and diffuse to the post-synaptic membrane and bind to specific receptors
- Once the bind, they can trigger an action potential causing either muscle contraction of the secretion of a hormone
- Neurotransmitters are removed from the cleft so the response is not continuous
Why are impulses unidirectional?
- The travel in one direction as the receptors are only found on the post-synaptic membrane
How are nerve impulses transmitted across a cholinergic synapse?
- An action potential arrives at the synaptic knob of the presynaptic neurone, which stimulates voltage-gated calcium ion channels to open
- Calcium ions then diffuse into the synaptic knob, causing the synaptic vesicles to move to and fuse with the presynaptic membrane
- ## The vesicles release acetylcholine (ACh) into the synaptic cleft called exocytosis and diffuse across to bind to specific cholinergic receptors on the postsynaptic
How are nerve impulses transmitted across a cholinergic synapse?
- An action potential arrives at the synaptic knob of the presynaptic neurone, which stimulates voltage-gated calcium ion channels to open
- Calcium ions then diffuse into the synaptic knob, causing the synaptic vesicles to move to and fuse with the presynaptic membrane
- The vesicles release acetylcholine (ACh) into the synaptic cleft called exocytosis and diffuse across to bind to specific cholinergic receptors on the postsynaptic membrane
- This causes sodium ion channels to open and release sodium ions into the postsynaptic knob causing depolarisation
- An action potential is generated if the threshold is reached
- ACh is removed from the synaptic cleft so the response does not keep happening. It is broken down by the enzyme acetylcholinesterase (AChE) and the products are reabsorbed by the pres-synaptic neurone
What is a neurotransmitter?
- A chemical that allows an action potential to be transmitted across a synapse
What do excitatory neurotransmitters do?
- They depolarise the postsynaptic membrane, making it fire an action potential if the threshold is reached
What do inhibitory neurotransmitters do?
- They hyperpolarise the postsynaptic membrane, preventing it from firing an action potential
What is spatial summation?
- When many neurones connect to one neurone
- The small amount of neurotransmitters released could be enough to trigger an action potential
- But if some of neurones release an inhibitory neurotransmitter then the total effect might result in no action potential
What is temporal summation?
- When 2 or more nerve impulses arrive in quick succession from the same presynaptic neurone
- Therefore an action potential is more likely as more neurotransmitters are released into the synaptic cleft
What is a neuromuscular junction?
- A synapse between a motor neurone and a muscle cell
- They use ACh which bind to nicotinic cholinergic receptors
What are the differences between cholinergic synapses and neuromuscular synapses?
- The postsynaptic membrane has loads of folds that form clefts, which stores AChE
- The postsynaptic membrane has more receptors
- ACh is always excitatory, an action potential will normally trigger a response
How do excitatory drugs affect synaptic transmission?
- Some drugs have the shape of the neurotransmitter so mimic their action to receptors so more receptors are activated eg, Nicotine
- Some drugs inhibit the enzyme that breaks down neurotransmitters, so more neurotransmitters are in the synaptic cleft binding to receptors eg, Nerve gases
- Some drugs stimulate the release of neurotransmitters from the presynaptic neurone so more receptors are activated eg, Amphetamines
How do inhibitory drugs affect synaptic transmission?
- Some drugs block receptors so they can’t be activated, so fewer receptors are activated resulting in muscle failure/being paralysed eg, Curare
- Some drugs inhibit the release of neurotransmitters from the presynaptic neurone so fewer receptors are activated eg, Alcohol
What are sensory neurones?
Neurones that carry nervous impulses from receptors to the CNS
What are motor neurones?
Neurones that carry impulses from the CNS to the effector
What are relay neurones?
- Intermediate neurones
- Receive impulses from the sensory neurone to the motor neurone
What are dendrites?
Carry nervous impulse to the cell body
What are axons?
Carry nervous impulses away from the cell body
What are the nodes of Ranvier?
The gaps between the Schwann cells
How does temperature affect the rate of transmission?
- It increases kinetic energy
- So ions move more quickly across the membrane
What happens at a neuromuscular junction?
- An action potential arrives at the synaptic knob at the end of the motor neurone
- It depolarises the membrane of the synaptic knob causing Ca²⁺ ion channel to open + Ca²⁺ ions to diffuse in
- The conc starts to increase causing the vesicles to fuse with the presynaptic membrane and ACh is released (exocytosis)
- ACh binds to receptors on the postsynaptic membrane opening Na⁺ ion channels + they diffuse in and depolarise the membrane
- If threshold is reached and action potential is generated
How is ACh removed from the synaptic cleft?
- AChE breaks down ACh
- The products are reabsorbed by the presynaptic neurone and reused to synthesise more ACh
What is ACh?
Acetylcholine
What is AChE?
Acetylcholinesterase
Why is it important that ACh is removed?
- To stop any further action potential from being continuously generated in the postsynaptic cell
What is a cholinergic synapse?
- A synapse between 2 neurones
