Applicable for A-Level, IB, DSE, AP-Level Exams
Neurones are specialised cells to carry nerve impulses in terms of action potentials. The 3 main types are sensory, motor and relay neurones.
The nerve impulse is the process of depolarisation, repolarisation and hyperpolarisation of action potentials. As sodium ion channels open, it causes depolarisation, causing the inner membrane potential to be less negative compared to the outside. As we reach the maximum potential, the voltage gated Na+ channels close and the voltage gated K+ channels open. Hence K+ ions diffuse back out of the neurone, causing repolarisation. Hyperpolarisation is caused the temporary overshoot of the diffusion of K+ ions. Na+-K+ pump pumps Na+ out and K+ in for repolarisation.
The influx of Na+ ions on region of the neurone causes a positive feedback: As Na+ ion channels are opened in one area, this causes neighbouring Na+ ion channels to open as well, causing depolarisation down the axon. This is the premise of how an action potential is propagated. The presence of myelin sheth allows for saltatory conduction: where the action potential jumps from nodes of Ranvier to another.
Synapses are the connection points between adjacent neurones. The presence of neurotransmitter vesicles only in the pre-synaptic neurone as well as the presence of neuroreceptors only on the post-synaptic neurone ensures the unidirectionality of action potentials. In addition, temporal and spatial summation allows for the effect of neurotransmitters to be added together.
Synaptic transmission occurs as follows: As action potential causes the opening of Ca2+ ions, Ca2+ moves neurotransmitter vesicles to the pre-synaptic membrane and releases the neurotransmitter. As the neurotransmitter diffuses across the synaptic cleft to bind to the post-synpatic receptor, this causes depolarisation on the post-synaptic neurone and if the threshold is reached, an action potential is generated.
Muscles are composed of myofibres containing myofibrils. Within a myofibril, sacromeres contain actin and myosin filaments. The A-band consists of overlapping myosin and actin filaments and the I-band conly consist of actin filaments. Z-lines mark the start and end of each sacromere and are situated in the middle of actin filaments.
Skeletal muscle contraction are described by the sliding filament theory:
As Ca2+ ions bind the protein attached to tropomysoin, tropomyosin moves and exposes the myosin-actin binding site. Using the hydrolysis fo ATP to ADP + Pi, the myosin head binds to the actin. It then bends and performs a power stroke causing the contraction of the sacromere - myosin then detaches from the actin binding site and restores to its original position for the second contraction using energy from ATP to ADP + Pi as well.