Neurobiology 303 -- Chapter 15 Outline

Muscle and its control

types of muscle:    visceral (smooth muscle in gut, blood vessels, etc)
                                  cardiac (in the heart)
                                  skeletal (types: fast twitch, slow twitch, tonic)

visceral and cardiac muscle is controlled involuntarily by the autonomic
    nervous system (sympathetic and parasympathetic)
    skeletal muscle is controlled voluntarily by the somatic nervous system

muscle proteins: actin and myosin
    actin -- thin filaments, associated with tropomyosi
    myosin -- thick filaments, have crossbridges
    sarcomere -- interdigitating array of actin and myosin
    myofibril -- chain of sarcomeres attached end-to-end

muscle fiber contracts when actin and myosin slide past one another
    sequence of events:
        resting state: crossbridges cocked but unbound to actin,
            ADP bound to myosin and tropomyosin in place
        muscle cell AP -- free calcium (Ca++) becomes available
        tropomyosin displaced, myosin crossbridges bind to actin
        power stroke -- ADP released, crossbridges spring back,
            actin and myosin slide past one another -- contraction!
                each sarcomere shortens by only 12 nm!
        ATP binds to myosin, causing crossbridges to release actin
        ATP split to ADP, cocking myosin crossbridges
        calcium is removed and tropomyosin is replaced

regulation of calcium in skeletal muscle:
    resting state -- calcium sequestered in sarcoplasmic reticulum
    sarcoplasmic reticulum -- intracellular membrane meshwork
    muscle cell AP -- calcium released from sarcoplasmic reticulum
    calcium reabsorbed by sarcoplasmic reticulum upon repolarization

cardiac muscle:
    capable of contracting on its own without input from motoneurons
    cardiac AP is very long and drawn out compared with skeletal AP
    individual cardiac muscle cells interconnected with gap junctions
        so cardiac muscles contract synchronously
    innervated by autonomic nervous system
        sympathetic: thoracic ganglia
            post-ganglionic transmitter -- norepinephrine, acts at
            beta-adrenergic receptor via G protien cascade to
            increase rate of depolarization of cardiac muscle cells
        parasympathetic: vagus nerve (cranial nerve X)
            post-ganglionic transmitter: acetylcholine, acts at two
            different muscarinic receptors; one works via a second
            messenger cascade to reduce the magnitude of a
            voltage sensitive calcium current; the other works via a
            G protein cascade to open potassium channels; both
            work to slow the rate of depolarization and prolong
            hyperpolarization in cardiac muscle cells

smooth muscle:
    of two types: multiunit and unitary
        multiunit -- muscle cells are not coupled by gap junctions,
                           and each is innervated by several autonomic
                           motoneurons
        unitary -- muscle cells are coupled by gap junctions, and the
                            activity of groups of cells is indirectly modulated by autonomic
                            motoneurons

skeletal muscle:
    also called striated muscle because of its striated appearance
    each muscle cell is formed from fusion of many embryonic cells
    skeletal muscle cells are not coupled by gap junctions
    innervated by somatic motoneurons
    skeletal muscle fibers are of several distinct types:
        tonic -- contract weakly and slowly (hundreds of ms)
                    rich in mitochondria, can contract for long periods
                    found in lower verts and inverts, control posture
        twitch -- contract strongly and quickly (tens of ms)
            constitute bulk of skeletal muscle in all animals
            can be further subdivided on basis of ATP production
                slow-twitch oxidative (SO)
                fast-twitch oxidative glycolytic (FOG)
                fast-twitch glycolytic (FG)
                oxidative are weak but fatigue resistant while glycolytic are strong but fatigue easily
                skeletal muscle fibers can be further distinguished biochemically
                all skeletal muscles contain a mixture of muscle fiber types

innervation of skeletal muscle
    motor unit -- the muscle fibers controlled by one motoneuron
    motor units can contain many muscle fibers or just a few
    the number of motor units in a muscle is simply the number of
        motoneurons innervating it
    innervation differs for vertebrate and invertebrate skeletal muscle
    vertebrates: muscle fibers are often innervated by one motoneuron only
       because vertebrate skeletal muscle fibers can produce APs
    invertebrates: a fiber may be innervated by several motoneurons
       each muscle fiber may receive several neuromuscular synpases,
       because the muscle fibers cannot produce APs

types of skeletal muscle fiber innervation patterns:
    unineuronal
    polyneuronal
    uniterminal
    polyterminal

muscles can only pull, so they usually work in antagonistic pairs as in:
    circular and longitudinal muscles in soft-bodied animals
    flexor and extensor muscles in animals with hard skeletons
    muscles that work together are called synergists

control of skeletal muscle tension by motoneurons
    recruitment -- regulates the number of active motor units
    frequency -- regulates the frequency of firing of motoneurons
    synaptic integration -- various motoneuron inputs (inverts only)

matching and size principal in skeletal muscle innervation
    small motoneurons (in axon diameter and soma size): tonic
        innervate slower twitch muscle
        fire first (more excitable)
    large motoneurons (in axon diameter and soma size): phasic
        innervate faster twitch muscle
        fire later (less excitable)
        motor units are recruited in order of size (smallest to largest)
        regardless of task
        smaller neurons are activated first because they have a higher
            current density and higher input resistance
        neural innervation can influence the fiber characteristics of muscle
        for example, a slower twitch muscle fiber re-innervated by a
        large, phasic motoneuron will develop more fast twitch characteristics

motoneuron morphology
    vertebrates: multipolar, branching dendritic tree and single axon
        located in gray matter of cord or in brainstem nuclei
    invertebrates: monopolar, single neurite branches in neuropil
        located in ganglion of body segment

control of motoneurons
    organized around specific movements, and hierarchical

biofeedback
    a human subject given electromyographic feedback can learn to
        control even single motor units