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Synapse: A gap between two nerve cells. An impulse (better known as an action potential) travels across a synapse through a chemical medium in the form of neurotransmitters such as acetylcholine, dopamine or serotonin. There are an estimated 125 trillion synapses in the cerebral cortex alone.
The human brain structure:
Cerebrum: The largest part of the brain, the cerebrum is divided into two hemispheres separated by a structure known as the corpus callosum. The cerebrum is made out of different lobes. The occipital lobe controls visual processing. The temporal lobe controls auditory information, whilst the parietal lobe controls movement and memory.
Cerebellum: Located underneath the cerebrum, the cerebellum is smaller and carries out the functions of muscle movements and balance.
Hypothalamus: Involved in thermoregulation and hormone production from the pituitary gland.
Medulla oblongata: Controls breathing rate and heart rate (and therefore blood pressure). It is located at the base of the brain.
Different types of neurone:
Sensory: Sensory neurones send impulses from the receptor (which has detected the stimulus) to the relay neurones (inside CNS)
Motor: Motor neurones deliver the impulse from relay neurones in the CNS to effectors (which can be glands or muscles) which contract or secrete upon stimulation.
Relay: Found in the CNS, relay neurones carry the impulse from the end of the sensory neurone to the start of the motor neurone.
Resting potential:
Nerve cells have a negative membrane potential at resting state. This means there are more positive ions outside the cell than inside. How is this achieved?
The sodium-potassium pump moves 3 sodium ions out and only 2 potassium ions into the neurone.
Due to open potassium channels in the nerve cell, the potassium ions simply diffuse back out, making the membrane potential more negative.
The nerve cell therefore has a resting potential value of -70mV, which is key to remember for exam specific questions.
Action potential and propagation:
When a nerve cell is activated and needs to send an impulse in a given direction, it must first become depolarised and lose its negative membrane potential:
The stimulus will first trigger the excitement of the neurone, which opens its Na+ channels. Since Na+ has been moved initially out of the cell, it floods in down its concentration gradient.
The influx of positive Na+ ions will make the membrane more positive. Upon reaching the threshold potential of -55mV, more Na+ channels open causing more Na+ to flood int, raising the membrane potential to +30mV.
The nerve cell repolarises itself by closing the Na+ channels, opening K+ channels and allowing the Na-K pump to reestablish the -70mV resting potential.
The action potential will move as a 'wave of depolarisation' meaning that neighbouring channels will stimulate others to open.
Myelin sheath diagram:
Saltatory conduction
Myelin sheath
Schwann cell
Axon
Node of Ranvier
Nerve cell structure + function
Axon: Extensions of the cell body which conduct impulses away from the cell body.
Cell body: The larger part of the nerve cell which contains the nucleus and mitochondria.
Dendrites: Extensions of the cell body which conduct impulses towards the cell body.
Mitochondria: Make ATP through aerobic respiration.
Neurotransmitter: Chemicals released by nerve cells to help transmit an action potential across a synapse.
Synapse diagram:
Presynaptic neurone
Postsynapticneurone
Synaptic cleft
Receptor
The Synapse Dynamics (exam answer):
-AP reaches synapse and causes influx of Ca2+ into presynaptic membrane.
-Influx of Ca2+ stimulates exocytosis of vesicles containing neurotransmitter.
-Neurotransmitter exocytosed into synapse, where it diffuses across and binds to specific, complementary receptors on the postsynaptic membrane. Upon binding to the receptors (which are channels), there is an influx of Na+ into the post synaptic membrane, depolarising it by decreasing its negative charge.
-A new AP starts in the postsynaptic membrane.
-Excess neurotransmitter in the synapse is removed using enzymes in the presynaptic membrane. For example acetylcholinesterase breaks down acetylcholine and therefore stops over stimulation of the postsynaptic neurone.
Factors affecting speed of transmission:
Diameter: The larger the diameter of the axon, the faster the rate at which the impulse travels.
Temperature: The higher the temperature, the faster the rate of impulse transmission. However, above a certain temperature proteins such as ion channels could denature and therefore reduce impulse speed.
Myelination: Schwann cells deposit an insulating material called myelin over nerve cells. Since myelin cannot conduct electricity, the impulse will jump from node to node in a process known as saltatory conduction.
Summation: Temporal vs Spatial
Temporal: Temporal summation is a dynamic whereby one single neurone stimulates its neighbour neurone by increasing the frequency of impulses sent across the synapse.
In doing so, more neurotransmitter is released into the synaptic cleft and therefore it makes it more likely for the threshold potential to be reached in the next neurone along.
Spatial: Spatial summation is a dynamic whereby multiple neurones converge onto one neurone. All of the converging neurones will fire impulses onto the single neurone in a bid to again try and raise its membrane potential above the threshold value required for the initiation of an action potential.
Why have a threshold potential?
A threshold potential means it is possible to filter out any unimportant signals from nerves (non dangerous stimuli, for example)
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