6.5 Neurons and Synapses Flashcards
What do neurons do?
Neurons transmit electrical impulses
What systems are used for internal communication of the body?
- endocrine system - nervous system
What does endocrine system consist of?
The endocrine system consists of glands that release hormones
What does the nervous system consist of?
consists of nerve cells called neurons. They help with internal communication by transmitting nerve impulses
Explain how the nerve impulse passes along a neuron. (8 marks)
○ in resting potential ○ sodium is pumped out by the active transport and potassium in ○ a concentration gradient builds up electrical potential / voltage ○ negative inside compared to outside ○ in action potential ○ must pass threshold level ○ sodium channels open and ions diffuse into neuron ○ membrane depolarized ○ potassium diffuse out across membrane through ion channels ○ active transport of ions once more ○ slower in un-myelinated neuron than in myelinated ○ an action potential in one part of the neuron causes the action potential to develop in the next section
What is a nerve impulse?
Is an electrical signal, an action potential starts at one end of neuron and propagates along the axon to the other end of the neuron
Where do nerve impulses travel along in a neuron?
Neurons have narrow out growths called nerve fibres where impulses travel along.
Explain how a nerve impulse is transmitted from a neuron to a muscle
• impulse reaches the motor end plates / synaptice knobs / boutons / synaptic terminals • synaptic vesicles contain neurotransmitter / acetylcholine • calcium enters through the presynaptic membrane • calcium causes the vesicle to move to and fuse with the membrane / causes exocytosis • neurotransmitter / acetycholine released into the synaptic cleft • crosses / diffuses across the synoptic cleft to the muscle fibre membrane / postsynaptic membrane • binds to receptor sites • causes depolarisation of the muscle fibre membrane / postsynaptic membrane • by opening sodium gates • threshold of stimulation must be reached / all or nothing effect • enzyme / acetylcholinesterase breaks down the neurotransmitter / acetylcholine • depolarisation causes sarcoplasmic reticulum to release calcium ions • calcium ions cause / enable muscle contraction
What is a synapses?
Synapses are junctions between neurons and between neurons (meeting point between two neurons)
What are the cells called that send the signals to another cell?
Pre-synaptic neuron
What is a neurotransmitter?
A neurotransmitter is a chemical that sends signals across synapses.
What is an example of a neurotransmitter?
Acetylcholine
When nerve impulse (action potential) reaches the synaptic knob before it reaches the axon terminal, what happens?
The electrical impulse (depolarization) causes calcium channels to open.
What happens when an electrical impulse in a pre-synaptic cell causes calcium channels to open?
Calcium ions diffuse into the axon terminal of the pre-synaptic neuron
How does the neurotransmitter move across the synaptic cleft from the pre-synaptic neuron to the post synaptic neuron?
Through simple diffusion it moves across the synaptic cleft
What are dendrites?
short branched nerve fibres used to transmit impulses between neurons like in the brain
What are axons?
very long elongated nerve fibres for example those that transmit impulses from toes to spinal cord
What is myelin?
Material that COAT along most the nerve fibre’s length which consists PHOSPHOLIPID BILAYER deposited by SCHWANN CELLS
What do Schwann cells do?
deposit a layer of myelin by growing around the nerve fibre several times which creates Schwann cells and thus a myelin sheath
What are the gaps called between the myelin?
Node of Ranvier
How does the nerve impulse travel along a myelinated nerve fibre?
Jumps from one node of ranvier to the next. This is called Saltatory conduction
Does a nerve fibre travel quicker on a myelinated or unmyelinated nerve fibre?
Saltatory conduction (myelinated) is faster than continuous transmission as it can jump.
What is resting potential?
A neuron that is not transmitting a signal that has potential difference
How is potential created?
An imbalance of positive and negative charges across the membrane
How is RESTING Potential (imbalance of charge across membrane) created?
- Using sodium potassium pumps.
- The ions pumped in and out are unequal
- Concentration gradient created (2 K+ into the cell, while 3 Na+ go out) as different in concentration and charges
- Membrane is more permeable to K+ ions, so Na+ concentration gradient is steeper (Na+ does not diffuse easily)
- Negatively charged protein inside nerve fibre increase the charge imbalance
- resting membrane potential is -70 mV
How does action potential occur?
Rapid change in membrane potential from depolarization (negative to positive) and re-polarization (positive to negative) ONLY when THRESHOLD POTENTIAL is reached
How does depolarization occur?
When local currents (natural polarization) occurs, and threshold of around -55 mV is reached:
- SODIUM CHANNELS OPEN, so Na+ can diffuse down concentration gradient (into the cell)
- this makes the charge inside as positive as the outside - membrane potential becomes +30 mV
How does re polarization occur?
- rapidly happens after depolarization
- Na+ channels close, K+ channels open
- K+ ions diffuse down concentration gradient (move out of cell)
- inside of cell becomes negative again
- K+ channel remains open until -70mV potential difference
- diffusion of K+ repolarizes neuron but does not restore RESTING potential (K+ and Na+ gradients are not the same)
Why does the propagation of action potential occur?
ion movements that depolarize one part of a neuron, causing depolarization of a neighbouring neuron that is still polarized . (Na ions moving along axon due to concentration gradient )
Can a nerve impulse travel in two directions?
No, can only be initiated at one terminal be passed to on by the other terminal. The period after depolarization also prevents action potential backwards
What are local currents?
- Inside the cell, there is more Na+ in the depolarized part
- Inside cell, Na+ diffuses from depolarized part to not yet polarized part to reduce potential difference from -70 mV to -50mV - Na+ channels open at threshold potential
What is a synaptic cleft?
the gap between pre-synaptic neuron and post-synaptic neuron is called synaptic cleft and is 20 nm wide
What does the influx of calcium in the pre-synaptic cell cause?
Vesicles containing neurotransmitters move to membrane and fuse
How are neurotransmitters released into the synaptic cleft?
exocytosis
What happens after neurotransmitters diffuse across the synaptic cleft?
Bind to to receptors on post-synaptic membrane which cause adjacent Na+ channels to open where Na+ ions can diffuse down concentration gradient into new neuron
What is caused when Na+ ions diffuse from synaptic cleft into post-synaptic cell
Threshold potential is reached that triggers action potential which depolarizes along the neuron
What happens to the neurotransmitters after they activate receptors?
-broken down and removed from synaptic cleft
How are acetylcholin produced?
Produced by pre-synaptic cell by combining choline and acetyle group (held in vesicles of neuron)
If a spider touches you compared to an elephant, how is the action potential different?
The strength is ALWAYS THE SAME, only the frequency increases to highlight urgency
Why does acetylcholine only bind to the receptor for a short time?
enzymes in the synaptic cleft break acetylcholin down quickly.
What happens to broken down choline?
Reabsorbed by presynaptic cell and converted back into active neurotransmitter
When neurotransmitters are released into synapse, does this always cause threshold potential?
If not ENOUGH neurotransmitters are released, and ENOUGH Channels are opened, threshold potential cannot be reached
What are the different areas of science that come together to research memory and learning?
medicine & physiology, engineering and optical microscopy, chemistry and biochemistry, genetics, molecular biology and neurobiology
