Somatic Nervous System

• have cell bodies in spinal cord
• axons or motor fibers (large and myelinated) leave the cord and travel in a spinal nerve to the muscle it innervates
• axons branch into many terminals at the muscle and each muscle fiber (cell) is innervated by one terminal
• can only stimulate muscle contractions, cannot not relax with AP

1 motor unit = 1 motor neuron and all the muscle fibers it innervates

• neurons controlled by CNS 
• presynaptic input (excitatory and inhibitory) from higher levels of CNS to cell bodies, in cord, of motor neurons
• CNS has many areas of control (4 motor cortex regions, basal nuclei, cerebellum, brainstem)

• the junction of an axon terminal and the muscle fiber
• has a cleft – the space between the nerve terminal and muscle membrane

Axon Terminal of Motor Neuron

ú synthesized in cytoplasm from choline à stored in vesicles à released by exocytosis when action potential depolarizes axon terminal membrane and there is a calcium influx

§has cholinergic (nicotinic) receptors  à which ACH binds to à ion channels open in end-plate à end plate potential (EEP) = a graded potential à reaches threshold à initiates muscle cell membrane action potential à propagated over the muscle fiber membrane

acetylcholinesterase

breaks down ACH, end-plate recovers to its RMP

Respiration and the Neuromuscular Junction

• depends on alternate contraction and relaxation of skeletal muscles (diaphragm and intercostals)
• paralysis of muscles leads to: 

1. excessive stimulation à continuous contractions (spastic paralysis)
2. blocked stimulation à no contraction (flaccid paralysis)

Clostridium Botulinum Toxin

botulism = food poisoning from improperly canned foods

blocks release of ACH à flaccid paralysis à respiratory failure

§ toxin is lethal in small quantities (<.0001 mg)

Botulinum Toxin Type A (Botox-A or BTX-A)

• least toxic type is injected into muscles for therapeutic uses:
• stop teeth clenching or grinding (one injection = 6-78 week benefit)
• treat spasticity resulting from a stroke (12 wks of benefit), brain or spinal cord injury
• cosmetic purposes – stays locally where it was injected

Cholinesterase inhibitors

inhibit cholinesterase enzyme so ACH is not destroyedàcontinuous depolarization àskeletal muscle paralysis (spastic paralysis) à death b/c of continuous contraction of respiratory muscles

Types of Muscle

1. skeletal
2. Smooth - lines walls of hollow organs and tubes, bv, digestive tract, airways, bladde
3. Cardiac - in heart

• attached to the skeleton for contraction of muscle à moves bones à produces motor activity
• plays a role in homeostasis
• obtaining, chewing and procuring food
• breathing
• generation of heat to maintain body temp
• for movements away from harm

Categories of Muscle

anatomical type

o involuntary – cardiac and smooth (innervated by ANS)

 Skeletal Muscle Components

àwhole muscle attached to bone by tendon

àmuscle organized into fascicles

àmuscle fiber made of myofibrils

àmade of sarcomeres

bundles of muscle fibers wrapped in connective tissue (perimysium)

Muscle Fibers

§ sarcoplasmic reticulum

contains cellular organelles, enzymes, glycogen, myoglobin

• extensive intracellular membrane system surrounding each myofibril to store high concentrations of calcium
• membranes are very close to membranes of the T-tubules

bands aligned in muscle fibers that fill most of the sarcoplasm

a myofibril between two successive Z-lines

• the z-line is the anchor for actin
• contain myofilaments, Actin (thin filament) and Myosin (thick filament), that are the contractile proteins of the muscle
•  creating bands
• A bands = dark = myosin and actin overlap
• I band = light = only actin filament
• z-line runs through center to separate sarcomeres so band is part of two sarcomeres
• have crossbridges that extend from the myosin filaments (are the heads)

Sarcoplasmic Reticulum

• extensive intracellular membrane system surrounding each myofibril to store high concentrations of calcium
• membranes are very close to membranes of the T-tubules

extensions of the cell membrane containing ECF

calcium ATPase Pumps in SR membrane use active transport to take calcium back into SR

àpropagation of AP down motor nerve fiber to terminal

àterminal releases ACH neurotransmitter

àACH binds to nicotinic receptors on end plate

àopen ACH-gated protein channels

àsodium diffuses in à depolarization à EPP à AP

à AP travels along muscle fiber membrane (including T-tubules)

àcauses SR to open calcium channels à release calcium into myofibrils

àcalcium pumped back in after fraction of a second

Myosin

• has a tail with two heads (crossbridges) directed toward the z-lines for:
• actin to bind
• ATPase to bind
• when the actin binding site head binds to actin à sarcomere contracts toward the center
• 1 myosin filament contains 200+ myosin molecules

· backbone of the actin filament

· when resting they are covering the sites for myosin cross bridges to prevent myosin form interacting with actin à preventing continuous contraction

• hold Tropomyosin in position
• has 3 subunits
• one binds to actin
• one has affinity for Tropomyosin
• one has affinity for calcium ions

when calcium binds à protein changes shape à Tropomyosin slides off cross bridge binding sites à myosin cross bridges bind to actin à muscle contraction

interactions between actin and myosin

• the filaments do not shorten, they slide past each other (cross bridge activity) to shorten the length of the sarcomere à to contract muscle fiber
• length shortens by filaments pulling the z line closer together

Cross Bridge Activity  

                                         i.     as long as calcium is still available

Relaxation of contracted muscle

due to calcium being returned to SR

§ tension in contractile proteins transmitted via tendons (with series of elastic components) to the bone to move bone

Contraction of a whole muscle

§ the heavier the load the lower the velocity (time of muscle fiber shortening)

Strength of a Contraction

§ used by the tetanus virus for rapid stimulation à maximum contraction force

o ex – when trying to lift heavy weight… there is tension in muscles but you cant lift it

o ex – lifting the heavy load

ATP’s Role in Muscle Contraction

· ATP needed for active transport of calcium back into SR

 Sources of ATP

· oxidation phosphorylation

creatine phosphate

too much creatine à increased amino acid level in blood à increases levels in cells à cells cant use all of it because they have a max storage amount à gets filtered into kidneys à too much in kidneys takes water out of the body à dehydration à acute kidney failure

Glycolysis

§ produces ATP more rapidly but requires large supply of glucose

§ brain is tired

type of exercise à changes neuronal pattern of activityà adaptation

· anaerobic high intensity, short duration exercises à hypertrophy à increase synthesis of actin and myosin filaments

o color of fiber = red

in back and leg muscles for maintaining posture

o color of fiber = red

in leg muscles

o color of fiber = white

muscle enlargement (increased diameter in fast-glycolytic muscles)

testosterone in males promotes synthesis of myosin and actin (anabolic) à thicker fibers

hereditary disorder of progressive degeneration of the contractile elements leading to decreased muscle mass and loss of strength

disuse of a muscle

Anabolic Androgenic steroids

o behavioral changes (hostility, addictive, “roid rage”)

o has thick filaments but many more thin filaments that are anchored to dense bodies (internal surface of membrane)

from increases cytosolic calcium à bind to calmodulin (similar to troponin) à activates myosin à forms cross bridges with actin

o most of calcium from ECF when AP opens calcium channels in membrane

· receptor proteins for neurotransmitters are located all over the membrane

o specific drugs

• can be stretched greatly and still develop considerable tension  (ex=bladder)
• slow contraction to conserve energy but still generate considerable tension
• cross bridges remain attached longer and maintain tension with less ATP used

By Brain:

• depends on converging input of excitatory and inhibitory to motor neurons
• if sum of input = threshold EPSP à AP à contraction of fibers in motor unit

§ brain stem nuclei receives info from many basal nuclei, cerebellum, motor cortex

· motor regions produce a motor program for a specific voluntary task and sequence of movements

· deficiency of dopamine NT in nuclei à Parkinson’s Disease à increases muscle tone or rigidity, involuntary movements, slow, resting tremors

o input to cortical motor areas for roles in planning voluntary activity

o nystagmus = oscillating eye movements

response that occurs automatically without conscious effort

o conditional (acquired) reflex = learned, practiced response

AP goes from sensory receptor à afferent pathway à integrating center of CNS à efferent pathway à effector

ordinary muscle fibers innervated by alpha motor neurons distributed throughout the muscle

specialized muscle fibers of the spindle

• the central part of a muscle fiber is non-contractile
• the sensory endings (primary annulospiral endings) wrap around the central part to detect changes in fiber length during stretching
• the end is the contractile part innervated by gamma motor neurons

o only go from muscle to spinal cord back to muscle, don’t go to areas of brain

o excessive tension à afferent AP via interneurons inhibit alpha motor neurons to the same muscle à relaxation of muscle = a protective reflex to prevent damaging muscles or tendons with excessive contractions