neurotransmission + muscles

• Glutamate is the primary excitatory neurotransmitter in the brain. GABA is the principal inhibitory neurotransmitter. The balance of glutamergic and GABAmergic tone is crucial to normal neurologic function.
• Neurotransmitters are synthesised and stored in presynaptic terminals, and released upon stimulation with specific receptors in postsynaptic cells.
• Chemical and electrical synapses are specialised biological structures found in the nervous system that allow the transmission of information between neurones.
• A neuron has two ends, the axon and the dendrites. It can have multiple branches.

  
• Neurotransmitters are stored in the axon (presynaptic neuron) in synaptic vesicles. Their release is triggered by an action potential in the form of a nerve impulse. Synaptic transmission can occur between neuronal cells, and with other non-neuronal cells.
• Electric synapses are gap junctions, which allows different molecules and ions to pass freely between cells. This connects their cytoplasms. It is composed of connexons (each of 6 connexon proteins). These make multiple cells act as one, firing simultaneously.
• Synapses between axons and dendtrites are axodentritic. Powerful synapses occur between axons and the cell body of another postsynaptic cell. These are called axosomatic synapses. Between axon terminals and postsynaptic axons they are axo-axonal.
• There are three groups of neurotransmitters; amino acids (fast synaptic transmission, can be excitatory or inhibitory, GABA, glutamic, aspartic, glycine), amines (modified amino acids, catecholamines, slow synaptic transmission, can be excitatory or inhibitory, noradrenaline, adrenaline, dopamine, serotonin, histamine), others (acetyl choline and nitric oxide)
• the minimum unit of contraction caused by a single action potential is a twitch
• the summation of frequent muscle twitches to give maximum contraction is the state of tetanus
• individual twitches in the same fibre can summate to show a range of responses to different levels of stimulation
• when maximum muscle tension is achieved, ATP is required to maintain that contraction level
• an action potential in the muscle cell activates contraction by releasing Ca2+ within the muscle
• a single action potential at the neuromuscular junction is sufficient to cause a muscle to twitc
• the tension generated by a muscle can be varied by controlling how many of its motor units are active
• a motor unit is a single motor neuron and all the muscle fibres it innervates
• an isometric contraction is when your joints do not move and your muscle fibres are kept at a constant length
• an eccentric contraction occurs when the muscle elongates.

  
• Each individual muscle fibre is surrounded by a delicate layer of connective tissue called endomysium. Muscle fibers are grouped into bundles called fascicles. Each fascicle is surrounded by perimysium. The entir ething is surrounded by epimysium.
• Each skeletal muscle fiber is long (30cm) and striated (streaky). The striations in skeletal muscle are a result of many repeating units called sarcomeres, which are the functional units of the muscle. Sarcomeres are found in myofibrils.
• myofibrils are rodlike units within a muscle fibre that contain proteins involved in contraction of the muscle. each muscle cell contains many chains of myofibrils.
• myofibrils are rodlike units within a muscle fibre that contain proteins involved in contraction of the muscle. Myocyte=muscle fibre=muscle cell
• Muscles are:

  Contractile; it has the ability to shorten in response to adequate electrical stimulation.

  Elastic; it has the ability to recoil and regain the resting length after being stretched.

  Excitable; it has the ability to receive and respond to a stimulus.

  Extensible; it has the ability to stretch when not contracted.
• Functions of muscle:

  locomotive, maintenance of posture, joint support and stabilisation, heat production for homeostasis
• a single muscle fiber cannot be innervated by more than one motor neuron, however, motor units can be of varying sizes
• smaller motor units = finer control. hands = small, posture = large + more power
• The neuromuscular junction (NMJ) consists of the axon terminal, synaptic cleft, and motor endplate of the muscle fiber.
• NMJ is the chemical synapse between a motor neuron and a muscle fiber
• Acetylcholinesterase (AChE) hydrolyses ACh into acetate and choline, and some ACh diffuses away from the synaptic cleft. Together, this terminates motor endplate depolarisation. Choline is taken up at the synaptic terminal to be recycled in the synthesis of more ACh. This is to clear the cleft and prevent constant muscle stimulation.
• Muscle fibers are surrounded by a sarcolemma (muscle membrane). Ar regular points along them, the sarcolemma extends deep into the fiber’s centre to form structures called t-tubules. This is the space along which an action potential travels.
•  When the muscle contracts, myosin repeatedly forms and breaks crossbridges with actin so that actin slides past myosin, and the sarcomere shortens. This cycle of crossbridge formation and breaking is what generates the force of contraction. It requires the presence of Ca2+ and energy in the form of adenosine triphosphate (ATP).
• The release of Ca2+ provides the link between electrical excitation of muscle, and the mechanical event of cross-bridge formation. This is referred to as "excitation-contraction coupling".
• excitation-contraction coupling is the conversion of an electrical stimulus into a mechanical response. The release of Ca2+ provides the link between electrical excitation of muscle and contraction.
• Crossbridges form independently from one another so that at any one time, there are some that are bound, and others that are not, meaning the muscle can maintain tension during the contraction, and the contraction is smooth.
• The process of progressive activation of motor units is called "recruitment". The contraction of a whole muscle may require that hundreds of motor axons fire.
• regarding muscle recruitment, a maximum response is seen when all motor units have been activated, a muscle contraction may involve recruitment of many motor units, small motor units are involved in producing fine motor movements, twitch force increases progressively as more motor nerve fibres are excited, and a twitch involves one complete cycle of contraction and relaxation.
• Muscle functions are; locomotion, maintenance of posture, support and stabilisation of joints, production of heat to maintain internal body temperature.
• Myasthenia gravis is caused when antibodies attacks receptor cells at the neuromuscular junctio