Nerve regeneration. Somatic nervous system

Transcription

1 Somatic nervous system Signals from CNS are sent to skeletal muscles. Final result is a muscle contraction. Motor neuron starts in CNS and its axon ends at a muscle cell. Alpha motor neuron Alpha motor neurons branch into several terminals (can be over 1000), each contacting a separate muscle cell. Nerve regeneration Axon degenerates downstream from injury Repair can happen in PNS, not often in CNS due to astrocyte scarring 1-4 mm/day 1

2 Nerve meets muscle Axon of Action potential motor neuron of motor neuron Voltage-gated calcium channels Act. pot l propagation in muscle cell Voltage-gated Na + channel acetylcholine Voltage-gated Na + channel Acetycholinesterase Motor end plate Organization of cells Sarcomere Myofibril Muscle cell 2

3 Sarcomere the unit of contraction, made of thin (actin) and thick (myosin) filaments Myofibril sarcomere sarcomere Z band Z band Z band A band Contraction of filaments Before Contraction After Contraction Z Z Z Z Length of sarcomere shortens with contraction but filament length is unchanged Myosin Actin Myosin 3

4 Actin Tropomyosin normally covers the myosin binding site on actin When calcium binds with troponin, it pulls tropomyosin away from the binding sites troponin tropomyosin myosin actin myosin binding site blocked Calcium myosin actin cross-sectional view Myosin Actin 4

5 Muscle Contraction Signal from motor neuron causes action potential in muscle cell Calcium ions released (from sarcoplasmic reticulum) Actin and myosin filaments slide relative to each other Myosin cross bridge BINDING Myosin cross bridge binds to actin POWER STROKE Cross bridge bends, pulling thin filament. DETACHMENT Cross bridge detaches and returns to original shape- *ATP required* BINDING to next actin molecule; repeat Energized Resting Detachment Myosin needs ATP to change shape Binding Bending (power stroke) Myosin has a binding site for ATPase Rigor complex 5

6 Signal coming to muscle motor neuron Sarcoplasmic reticulum (Ca +2 storage) T tubule From action potential to contraction Calcium is the link Acetylcholine released at the neuromuscular junction - action potential on muscle fiber Action potential down T tubule to sarcoplasmic reticulum at muscle fibers Calcium released from the SR to muscle fibers Terminal button Pathway review Acetylcholine Acetylcholinegated cation channel (Na+ in) Action potential T tubule A Ca+ pump in SR takes up Ca+, allows muscle to relax Myosin detachment using ATP Myosin powerstroke Cross-bridge binding 6

7 Muscles contain groups of motor units Units are recruited during motor activity Muscle force depends on # muscle fibers contracting Motor unit = motor neuron + muscle fibers it innervates The number of muscle fibers varies among different motor units. muscles can have many small units or a few large units Asynchronous recruitment of motor units delays or prevents muscle fatigue. Tension and frequency of stimulation twitch - brief contraction resulting from 1 action pot l tetanus - twitch summation from sustained Ca +2 7

8 Muscle length and force Fast and slow twitch muscle cells Differences in time when maximum tension is reached Slow twitch (Type I) - have myoglobin, many mitochondria, oxidative Fast twitch (Type IIa) - myoglobin, mitochondria, oxidative & glycol. Very Fast twitch (Type IIb) - use glycolysis, split ATP quickly Fast and slow twitch muscle cells Oxidative - resistant to fatigue, high rate of O 2 transfer from blood, recruited 1st Glycolytic - more prone to fatigue b/c less ATP produced, harder to recruit Endurance vs. Bursts of power 8

9 People are born with certain ratio of slow vs. fast twitch fibers usually an even mix in most skeletal muscles Sensation at muscle Spindle muscle fibers (deep within muscle) sense stretch, and Golgi tendon organs (in tendons) sense tension. Golgi tendon organ Intrafusal (spindle) muscle fibers 9

10 Knee spinal reflex Primary types of contraction Isometric contraction muscle tension is not enough to move load. Muscle doesn t shorten. Isotonic contraction Concentric muscle shortens to lift a load. Eccentric - shortened muscle has controlled lengthening. slowly lowering the weight Exercising your muscles Endurance training type IIx type IIa more mitochondria, glycogen, vascularization 10

11 Exercising your muscles Strength training hypertrophy of type II fibers Hypertrophy: how muscles get bigger Muscle cells have satellite cells nearby that respond to muscle injury and wear Why are muscles sore after lifting? Hypertrophy: how muscles get bigger Satellite cells: activated at microtears add nucleus to muscle cell more myofibrils made cell wider 11

12 Muscle hypertrophy vs. hyperplasia Hypertrophy Hyperplasia Are there changes in muscle recruitment w/different exercise? Overall, your CNS alters recruitment to increase efficiency of movements you train Better inhibition of antagonistic muscles Improved pattern of recruitment of muscles over a movement to gain power Are there changes in muscle recruitment w/different exercise? Movements generally recruit small motor units first (slow twitch) Thus to weight train, sufficient wt. needed to recruit more units, and thus engage type II units Endurance runners/bikers should have periods of sprinting, to engage type II Tend to be fast twitch units Tend to be slow twitch units 12

13 ATP sources at muscles 3a 1 2 3b What does creatine do? 13

14 What are muscle cramps? When muscle fibers contract without our control it is a muscle spasm or cramp Due to motor neurons being hyperexcited, often b/c of a shift in body fluids or ion levels (dehydration, low Ca, Mg, K) or vigorous activity Smooth muscle contraction Multiunit - similar to skeletal motor units Smooth muscle Single unit - gap jxns b/w muscle cells. Many cells contract as a unit. (uterus, intestine, bladder) 14

15 Cardiac muscle Pacemaker muscle cells - action potential gradually depolarizes, then repolarizes Contraction spreads from pacemaker through gap jxns Spontaneous action potential Pacemaker cell Action potential spread to other cells Gap junctions 15