Striated Muscle: Length-Tension Relationship

isometric contraction

the length of the muscle remains constant during contraction, no movement at the joint

isotonic contraction

the muscle changes length while the tension remains constant

motor control hierarchy: highest level

includes areas of the brain that play a role in planning, memory, emotions, and motivation, provides info to local motor neurons to determine final outcome

motor control hierarchy: middle level

cross-talk between sensory inputs and areas that regulate modes of movement including fine motor control, provides info to local motor neurons to determine final outcome

motor control hierarchy: local level

peripheral nervous system sensory receptors, brainstem and spinal cord interneurons, and the motor neuron cell bodies

reflex arcs

stimulus, receptor or sensor, afferent signaling pathway, integrating center, efferent signaling pathway, effector, response

muscle spindles (intrafusal muscle fibers)

small specialized muscle cells that have a central non-contractile region flanked by contractile end regions, embedded in muscle groups of the body and stretch and contract as the muscle groups they inhabit change length

two types of spindles: nuclear chain fibers

respond to the magnitude of a musclet

two types of spindles: nuclear bag fibers

respond to the speed and size of a muscle stretch

muscle spindle feedback

as a muscle is stretched the sensory afferent increases its rate of action potentials

golgi tendon organs

sensory afferents embedded in connective tissue capsules where muscle tendons attach to bones, signaling pattern of sensory afferents is dependent on tension by the tendon due to muscle contraction or external loads

smooth muscles

cells are 30 – 200 µm long and 3 – 8 µm in diameter, spindle-shaped, one centrally located nucleus, thick and thin filaments but no myofibrils, cells are physically attached to one another by desmosomes

smooth muscle lines hollow structures and organs such as:

arteries, arterioles, airways, stomach, intestines, urinary bladder, uterus

role of calcium for skeletal muscle

amount of calcium released from the SR as a result of an action potential is sufficient to briefly saturate all of the troponin calcium binding sites (resulting in twitch)rol

role of calcium in smooth muscle

cytosolic calcium concentration can be increased in a graded manner, enters through voltage-gated calcium channels, ligand-gated calcium channels, and stretch-activated cation

calmodulin

calcium sensor in smooth muscle

length-tension relationship for smooth muscle

the optimum length that permits max tension generation is found over a broader range than skeletal muscle

smooth muscle can constract:

when there is an action potential, there is a subthreshold depolarization, there is not change in membrane potential

changes in calcium concentration can be due to the action of:

voltage-gated channels, calcium release channels, receptor-activated channels, IP3-gates channels, store-operated channels, mechanically gated channels

single-unit smooth muscles

common, are connected by gap junctions so that excitation of one cell results in the excitation of the entire collection of interconnected muscle cells

multi-unit smooth muscles

rare, activate individually after reception of the activating signal

pacemakers

when threshold for voltage-gated calcium channels is reached the cells depolarize, repolarization requires closing of the calcium channels and opening of voltage-gated potassium channels

slow waves

a stimulus will only reach threshold when the underlying slow wave is near the crest of a cycle, the result is a periodic contraction pattern times to match frequency of the slow waves