Week 39 Muscles (ch 19-21)

Reflex

a simple, graded response to a specific stimulus.

Spinal reflexes

reflexes mediated by the neural circuits of the vertebrate spinal cord

Caveolae

invaginations of the cell membrane of smooth muscle that are thought to contribute to the rise of Ca2+ in the cytoplasm when the cell is activated

• Due to the lack of t-tubules and reduced SR

Muscle spindles

Proprioceptor that monitors the length of a skeletal muscle

• Consists of 8-10 intrafusal fibers (specialized muscle fibers)

  • Opposite of extrafusal fibers (: normal, “working” contractile muscle fibers)

Proprioceptor

a mechanosensory receptor that is associated with the musculoskeletal system.

• Important for the control of movement because they provide an animal with information about where the parts of its body are positioned in space

Reciprocity

the fact that muscles (or groups of muscles) tend to be arranged in antagonist pairs.

• The principle states that any signal that activates movements, whether it is the sensory input to a reflex or a command of the CNS, is coordinated to contract a set of muscles that work together (the agonists) while relaxing the opposite (antagonist) set.

Divergence

each presynaptic neuron usually contacts many postsynaptic neurons

Convergence

many presynapic neurons contact one postsynaptic neuron

Gamma motor neuron

Innervates the stretch receptor organ

Alpha motor neuron

Innervates extrafusal muscle fibers

Rhythmic behaviour

stereotyped, repetitive sequences of movement such as walking, breathing, swimming

Hypothesis of peripheral control

each movement activates receptors that trigger the next movement in the sequence

Hypothesis of central control

movement is controlled by a CPG

CPG (Central pattern generator)

a neural circuit in the CNS that can generate the sequential, patterned activation of motor neurons to antagonistic muscles that underlies a behavior pattern, without requiring sensory feedback to trigger the next movement.

Oscillators

CPGs underlying oscillary behaviour patterns

Cellular oscillator

a neuron that generates temporally patterned activity by itself, without depending on synaptic interaction with other cells.

• May generate endogenous bursts of action potentials, or they may show oscillations of membrane potential without generating any action potentials

• Ex. controlling molluscan feeding, crustacean heartbeat and crustacean scaphognathites (gill bailers)

Network oscillator

a network of neurons that interact in such a way that the output of the network is temporally patterned, although no neuron in the network functions as a cellular oscillator

• The oscillatory or pattern-generating property is said to be an emergent property of the network, resulting from cellular interactions in the network rather than from intrinsic cellular properties

• Ex. neurogenic leech heartbeat, as well as swimming in leeches, molluscs , lampreys, and clawed toad tadpoles.

Hybrid oscillator

In the nervous systems of animals, CPGs can combine the properties of both cellular oscillators and network oscillators.

Cerebral cortex

• Voluntary movements

• Primary motor cortex is a part of the _____________

  • Body regions are represented on the surface of the primary motor cortex by a somatotopic map

    • Rough map that controls movement patterns, organized to promote coordination among muscles and joints rather than to control single muscles.

Pyramidal cells

the neurons of the primary motor cortex that mediate motor responses to stimulation.

• The axons of which synapse on brainstem motor nuclei and also continue down the spinal cord as major components of the corticospinal tract

• Pyramid shaped cell bodies

Mirror neurons

neurons that are activated when an animal does a task and when it sees another animal do that task

• Code for the abstract concept of movement rather than its execution

Premotor cortex

Responsible for planning and coordinating movements, particularly in response to external stimuli.

Cerebellum

• Regulates movement indirectly, adjusting the descending motor output of other brain areas.

• Voluntary movements are still possible following _________ lesions, but they are clumsy and disordered, lacking the smooth and effortless precision of normal movements.

• Cerebellar cortex

• Deep cerebellar nuclei

Basal ganglia

set of nuclei (clusters of brain neurons) located in the forebrain and midbrain, under the cerebral hemispheres.

• The major output of the ____________ is inhibitory

  • For the initiation of a movement, this tonic inhibition is lifted, by disinhibition.

• Loop circuit

• Important in selecting movements, suppressing competing or unwanted movements, and initiating the selected movement.

Loop circuit

the output of the circuit loops back to the site of the circuit’s input

Common in the vertebrate brain, and they appear to be important for many aspects of motor control, emotions, and other brain activities

Satellite cells

muscle stem cells located between the muscle fibers and the surrounding basal lamina, responsible for muscle growth, repair and regeneration by proliferating and differentiating into new muscle cells after injury or during muscle adaptation.

Contractile proteins

actin & myosin

Striated muscle cells

• Transverse bands, giving them a striped appearance

• Reflects the organization of myosin and actin into regularly repeating units, called sarcomeres.

• Skeletal and cardiac muscle

• Each cell (fiber) contains myofibrils arranged in parallel, and each myofibril has cross-striations, which delineate sarcomeres.

Smooth (unstriated) muscle cells

• Also use actin and myosin to contract, but they are not organized into sarcomeres.

• Found in hollow, tubular organs like the intestines, uterus and blood vessels.

• Cell is spindle-shaped

• Greater proportion of actin relative to myosin than striated muscle cells.

• Lack t-tubules, troponin and nebulin. Reduced SR but typically have caveolae (: invaginations of the cell membrane that are thought to contribute to the rise of Ca2+ in the cytoplasm when the cell is activated)

• Innervated by the autonomic nervous system (ANS)

Tendon

attatches the skeletal muscles to bones

Sarcolemma

cell membrane of a muscle cell

Myofibrils

• Each muscle fiber contains hundreds of parallel _________

• Has repeating transverse bands

  • A bands, I bands, Z disc (Z line) etc

• Made up of repeating sarcomeres

Sarcomere

portion of a myofibril between two Z discs

• Many repreating ___________ make up the myofibril

Myofilaments

• Myofibrils contain 2 kinds of ____________

• Thick filaments composed of mainly myosin

• Thin filaments composed of mainly actin

• Titin and nebulin are giant proteins that help align actin and myosin.

Titin and nebulin

giant proteins that help align actin and myosin.

Thick filaments

Composed of mainly myosin

• Has the myosin heads (cross-bridges) that “climb” the actin at the cost of ATP

• Titin holds it straight at the center of the sarcomere

  • Elastic

Thin filaments

Composed of mainly actin

• Anchored to proteins in the Z disc

• Troponin (TN) and tropomyosin (TM) regulate the process of contraction by controlling whether or not the myosin cross-bridges can interact with the _____ filaments

• Nebulin is inelastic; it runs the length of a _____ filament and stabilizes it

H zone

Segment of myofibril

Thick filaments only

I band

Segment of myofibril

Thin filaments only

Acetylcholine (ACh)

neurotransmitter released from the motor neuron during an action potential

Excitation

depolarization of the muscle fiber

Excitation–contraction coupling

the relationship between depolarization and contraction

T-tubules (transverse tubules)

system of tubules continuous with the sarcolemma

• Invaginations occur at regular intervals along the length of the sarcolemma.

• Continuation of the outer sarcolemma so the tubule lumen is continuous with extracellular space

• When the sarcolemma is depolarized, they conduct this excitation deep into the interior of the muscle fiber

• Close contact with SR

SR (sarcoplasmic reticulum)

a branching lacework of tubules contained entirely within the muscle fiber

• Adapted from the endoplasmic reticulum (ER) of other cells

• Each myofibril is enveloped in ____

• The membrane maintains a low Ca2+ concentration within the cytoplasm and a high concentration of Ca2+ within the ___ by active transport

• ___ compartment: ___ between two t-tubules

  • Form a sleeve of branching tubules around each myofibril

• Terminal cisternae

• Once an action potential conducted along the sarcolemma depolarizes the t-tubule, Ca2+ ions are released from the ___ into the cytoplasm.

Terminal cisternae

enlarged sacs next to the t-tubules

In resting muscle, Ca2+ is largely confined to the ____________ of the SR.

Antagonistic pairs

muscles that are opposites, one shortens and the other lengthens

• Arranged around joints

Isometric

muscle contraction where muscle stays the same length

• Sarcomeres shorten slightly

• Ex. holding a position or resisting an external force

Concentric

muscle contraction where muscle shortens

Lengthening/eccentric

muscle contraction where muscle is longer than usual and is resisting stretch imposed by an external force

• Ex. hiking down an incline, the quadriceps muscles on the top of your thighs are actively contracting, but the muscles are actually longer than they are when the knee is not bent.

• Thought to produce minor damage to muscle fibers that lead to delayed soreness following exercise.

Isotonic

“same tension”, tension matches the load which leads to muscle shortening

• Ex. concentric and eccentric contractions

Summation

when a muscle is stimulated more than once within a brief period, the successive twitches produced add to each other, so the overall response is greater than the twitch response to a single stimulus

Tetanus

the maximum contractile response the muscle can achieve

Tonic (skeletal) muscle fibers

• Found in postural muscles of lower vertebrates

• Relatively rare

• Do not generate action potentials but do undergo changes in membrane potential.

• Contract more slowly than any other types of vertebrate muscle fibers, and their slow cross-bridge cycling permits long-lasting contractions with low energetic costs.

Twitch fibers

• Generate action potentials

  • Each AP gives rise to a muscle twitch

• 3 types based on differences in isoforms of the myosin ATPase and metabolic features of the cells: SO, FOG and FG

Slow oxidative (SO)

• Split ATP half as fast as the others → contracts slower

• Mitochondria-rich

  • Make ATP principally by aerobic catabolism → slow to fatigue

• Small diameters of the fibers

• Rich in myoglobin

  • Fibers are red in colour

• Well supplied capillaries

• Adapted for isometric postural functions and for small, slow movements

Fast oxydative glycolytic (FOG)

• Intermediate between SO and FG

• Capable of rapid tension development

• Relatively rich in mitochondria

• Make ATP aerobically → resistant to fatigue

• Adapted for repeated movements such as locomotion

Fast glycolytic (FG)

• Large diameters of the fibers

• Fewer capillaries

• Little myoglobin

  • Fibers are white in colour

• Few mitochondria

• Make ATP mainly by anaerobic glycolysis

  • Rich in glycogen (which fuels anaerobic glycolysis)

  • Quickly accumulate lactic acid and fatigue rapidly

• Used for occasional, forceful, fast movements such as leaps or bursts of speed in escape or prey capture.

Motor unit

one motor neuron and all the muscle fibers it controls

Recruitment (of motor units)

increasing the number of active motor units to make a stronger muscle contraction

Multiterminal innervation

each muscle fiber of a tonic muscle receives many synaptic contacts (from the same neuron) distributed over its length.

• Not like how each fiber of a twitch muscle has a single end-plate contact near the middle of the fiber

Arthropod plan

• Has both multiterminal and polyneural innervation (: arthropod muscle fibers are innervated by more than one motor neuron)

• As in vertebrate tonic muscle, each neuron in arthropod skeletal muscle branches to provide multiterminal innervation to several muscle fibers

• Arthropod muscle fibers typically do not generate all or-nothing action potentials

  • Except insect flight muscles.

• Some arthropod muscles are innervated by both excitatory and inhibitory motor neurons, allows peripheral inhibition

• Have muscle fibers that vary in time to contract

  • Short-sarcomere fibers contract quickly, and long-sarcomere fibers contract slowly

Dense bodies

what the actin filaments are attached to in smooth muscle

Single-unit

smooth muscle cells where the muscle cells are electrically coupled by gap junctions

• Causes groups of muscle cells to be depolarized and contract together

• Ex. gastrointestinal tract and small-diameter blood vessels

• Often spontaneously active, with electrical activity propagating from cell to cell via the gap junctions

• Can be activated by stretch

Multiunit

smooth muscle cells with few if any gap junctions, so the muscle cells function as independent units

• Innervated by autonomic nerves and individual cells are under more direct neural control than in the alternative

• May or may not generate action potentials, and they may be activated hormonally or by local chemical stimuli as well as neurally.

• Not stretch sensitive

• Ex. hair/feather erectors, large arteries, eye and respiratory airways

Tonic smooth muscles

smooth muscles that maintain contractile force for long periods

• Ex. those in the airways and certain sphincter muscles

• Do not generate spontaneous contractions or action potentials

Phasic smooth muscles

smooth muscles that show rhythmic or intermittent activity

• Ex. muscles in stomach and small intestine

• Contract rapidly, produce spontaneous contractions and generate action potentials that propagate through gap junctions from cell to cell.

MLCK (myosin light chain kinase)

• Activated by Ca2+ binding to calmodulin

• Adds a phosphate group to myosin

• Enables myosin to use ATP by increasing myosin ATPase activity

MLCP (myosin light chain phosphase)

• Activated when Ca2+ is released from calmodulin due to ion channels expelling it from the cell

• Removes the phosphate group from myosin

• Makes myosin inactive, unable to use ATP due to ATPase inactivity

Postmiotic

cells no longer divide by mitosis, so the number of cells cannot increase

Hypertrophy

muscle increasing in bulk by adding structural proteins to individual cells, not by adding new cells by mitosis

Atrophy

losing muscle mass due to lack of use

• Muscle becomes smaller because individual muscle fibers lose actin and myosin components of myofibrils.

• Can also be caused by losing cells, a phenomenon seen in some disease states and in aging

Endurance exercise

• Ex. long-distance running, cycling or swimming

• Involves repetitive actions that generate relatively low forces

• Slow-twitch fibers (SO) called type I

  • Have the slowest myosin isoform and thus the slowest rates of cross-bridge cycling. SO fibers also have a slow Ca2+-ATPase isoform in the SR. They contain abundant mitochondria and tend to be fatigue resistant.

• Elicits changes in fiber type, increased capillary density and increased mitochondrial density

Resistance exercise

• Ex. stair-running or weight lifting

• Fewer repetitions of movements that generate large forces

• Fast-twitch fibers (FOG and FG) that depend more on anaerobic metabolism are important in resistance exercises.

  • FOG (also called Type IIa) fibers have a myosin isoform that hydrolyzes ATP faster than does the myosin isoform of SO muscle fibers, and a fast Ca2+-ATPase isoform in the SR; they are relatively resistant to fatigue.

  • FG (in humans, also called Type IIx) fibers have the fastest myosin isoform and a fast SR Ca2+-ATPase isoform. FG fibers have the fastest speed of contraction. They contain relatively few mitochondria, and fatigue easily.

• Causes hypertrophy and changes in fiber type of Type IIx fibers into Type IIa fibers

Power

force * shortening velocity

Needle biopsy

the procedure used to obtain muscle samples

Cytokine vascular endothelial growth factor (VEGF)

Signal protein triggering angiogenesis

Taper

a period of reduced intensity training

Myostatin

• A growth factor found to be expressed specifically in developing and adult skeletal muscles.

• Function: regulating muscle mass

• Without _________’s regulatory effects, hypertrophy (increase in fiber size) occurs as a result of increased protein synthesis and satellite cell activation.

• _________ binds to a receptor on the muscle cell membrane to induce intracellular signaling sequences that control growth of the cell. The growth factor negatively regulates additional features of the whole muscle, including the amount of fat deposited between muscle fibers.