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BIOL 1117 (81)
Lecture

Muscles: Excitable Cells, Nerve-Muscle Relationship, Fiber Behaviors

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Department
Biology
Course
BIOL 1117
Professor
Christopher Richardson
Semester
Fall

Description
L18-Muscle Cells 10/21/13 • Electrically Excitable Cells o muscle fibers and neurons are electrically excitable cells o Because in response to stimulation, muscle and nerve cell plasma membrane experiences voltage change o electrophysiology: the study of the electrical activity of cells o voltage (electrical potential): the voltage between two points is equal to the electrical potential difference between those points. o Voltage or electrical potential is energy per unit charge o Electrical potential is the energy required to move a unit of electrical charge to a particular place in a static or nonmoving electric field o resting membrane potential of unstimulated skeletal muscle cell is about -90 mV; the negative sign indicates that the negative charge is relatively greater on the inside than the outside of the cell o in an unstimulated (resting) cell: +  excess sodium ions (Na ) in the extracellular fluid (ECF)  excess potassium ions (K ) in the intracellular fluid (ICF)  more anions (negative ions) on the inside of the plasma membrane than on the outside  in the ICF: anions such as proteins, nucleic acids, and phosphates that cannot penetrate the plasma membrane  These anions make the inside of the plasma membrane negatively charged by comparison to the outer surface  In resting muscle, also more K+ leaking out than NA+ leaking in at rest  The membrane is therefore polarized or charged due to difference in amount of charge across the membrane o stimulation to the muscle fiber or nerve cell  gates of ion channels open in the plasma membrane +  Na immediately diffuses down its concentration gradient into the cell  These cations override the negative charges in the ICF  depolarization: inside of the plasma membrane becomes briefly positive + +  immediately, Na gates close and K gates open; K+ gates are slow to open  K rushes out of cell; repelled by the Na+ charge and partly because of the concentration gradient +  repolarization: the loss of positive K turns the membrane negative again.  RMP is the stable voltage in a resting muscle or nerve cell  action potential: quick up-and-down voltage shift from the negative RMP to a positive value, and back negative value again.  This voltage shift of depolarization followed by repolarization is Action Potential  an action potential at one point on a plasma membrane causes another one to happen immediately in front of it, which triggers a wave of action potentials along a nerve called a nerve impulse.  Wave of Action Potential also travels along cell membranes of muscle fibers which leads to muscle contractions • The Nerve-Muscle Relationship o skeletal muscle only contracts when stimulated by a nerve o if nerve connections are severed or poisoned, a muscle is paralyzed o somatic motor neurons: nerve cells whose cell bodies are in the brainstem and spinal cord that serve skeletal muscles o somatic motor fibers: axons that lead to the skeletal muscle stimulate muscle fibers  each nerve fiber branches out to a number of muscle fibers  a motor unit is a single motor fiber or nerve fiber and all the muscle fibers innervated by branches of the motor fiber. • Motor Units o muscle fibers of one motor unit  spread throughout the muscle  All contract together  when stimulated, produce weak contraction over wide area  effective contraction usually requires the contraction of several motor units at once o On average, each motor neuron innervates about 200 muscle fibers, but a wide variation of n
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