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Saturday, February 28, 2009 at 2:26am
Describe the physiology of a muscle contraction in complete detail:
When you touch a hot stove, your arm instantly reacts and jerks away from the heat. What actually takes place - to cause your arm to move away - is neuromuscular transmission. A sensory nerve detects heat, and sends a message to the central nervous system. The brain instantly sends a message by way of a motor neuron or efferent neuron to the skeletal muscle, in this case, the biceps brachii. At the neuromuscular junction, the neuron relaeases a chemical neurotransmitter called acetylcholine. The chemical diffuses across the synaptic gap, and affects the resting membrane potential.
Normally, a resting cell has a high concentration of potassium ion inside the cell, and a high concentration of sodium ion outside of the cell. In order to maintain these differing concentrations, the cell must expend energy to operate the sodium-potassium pump. The outside of the cell is lined with positive charges, while the inside of the cell is lined with negative charges. This is called the resting membrane potential. When the membrane is stimulated by the neurotransmitter, it becomes permeable to sodium, causing a switch in charge. This is known as depolarization. Repolarization occurs when the membrane becomes permeable to potassium, and then the sodium-potassium pump works to re-establish the resting membrane potential.
This excitation travels forward to the transverse tubules that run perpendicular to the myofibril. The message then crosses the cisterna and enters the sarcoplasmic reticulum which binds troponin of the actin filament. When not bound, this protein serves to inhibit myosin and actin from interacting. Now, the strand of tropomyosin moves aside, exposing the actin filament binding sites.
Actin binding sites join with the myosin hooks to form cross-bridges. An enzyme located in the myosin called ATPase releases the energy necessary. The myosin literally pulls the actin filament closer, causing the muscle to contract and shorten. When this happens, the H and I bands shorten, while the A band remains the same in length.
In order for the muscle to relax, the chemical cholinesterase must erase the effect of acetylcholine. When this happens, calcium returns to the sarcoplasmic reticulum and the muscle relaxes.
NOTE: In order to simplify the essay, a monosynaptic reflex was used, involving only a sensory and motor neuron. However, most actions have more than one synapse and involve many more neurons.
When you touch a hot stove, your arm instantly reacts and jerks away from the heat. What actually takes place - to cause your arm to move away - is neuromuscular transmission. A sensory nerve detects heat, and sends a message to the central nervous system. The brain instantly sends a message by way of a motor neuron or efferent neuron to the skeletal muscle, in this case, the biceps brachii. At the neuromuscular junction, the neuron relaeases a chemical neurotransmitter called acetylcholine. The chemical diffuses across the synaptic gap, and affects the resting membrane potential.
Normally, a resting cell has a high concentration of potassium ion inside the cell, and a high concentration of sodium ion outside of the cell. In order to maintain these differing concentrations, the cell must expend energy to operate the sodium-potassium pump. The outside of the cell is lined with positive charges, while the inside of the cell is lined with negative charges. This is called the resting membrane potential. When the membrane is stimulated by the neurotransmitter, it becomes permeable to sodium, causing a switch in charge. This is known as depolarization. Repolarization occurs when the membrane becomes permeable to potassium, and then the sodium-potassium pump works to re-establish the resting membrane potential.
This excitation travels forward to the transverse tubules that run perpendicular to the myofibril. The message then crosses the cisterna and enters the sarcoplasmic reticulum which binds troponin of the actin filament. When not bound, this protein serves to inhibit myosin and actin from interacting. Now, the strand of tropomyosin moves aside, exposing the actin filament binding sites.
Actin binding sites join with the myosin hooks to form cross-bridges. An enzyme located in the myosin called ATPase releases the energy necessary. The myosin literally pulls the actin filament closer, causing the muscle to contract and shorten. When this happens, the H and I bands shorten, while the A band remains the same in length.
In order for the muscle to relax, the chemical cholinesterase must erase the effect of acetylcholine. When this happens, calcium returns to the sarcoplasmic reticulum and the muscle relaxes.
NOTE: In order to simplify the essay, a monosynaptic reflex was used, involving only a sensory and motor neuron. However, most actions have more than one synapse and involve many more neurons.
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