Neuromuscular A&P
Special lecture powerpoint assembled by T.A. Trendler
for her wonderful PHYSO 2A class in Fall 2008
From Saladin’s A&P Chapters 5, 11 & 12*
Given November 10 & 12
Overview
• Given only 2 days to cover 2 really important chapters
• I decided to integrate the main topics powerpoint
• We’ll cover the anatomy on Monday
• Then the physiology on Wednesday
• Okay?
MyOLOGy
Myology: study of muscular tissues (Chapter 5 in Saladin)
Myo: latin for muscle (mus = mouse, -cle = little)
Sarco: greek for flesh/body (think sarcophagus, body box)
Muscle tissue characteristics:
excitable, conductive, contractile & extensible*
*elasticity due to connective tissues
Functions:
Motion of body parts (or substances in body)
Stability of body parts (or substances in body)
Heat production (basal metabolic rate & shivering)
Types of Muscle Tissue
Skeletal* muscle tissue
striated & voluntary
studied as the muscular system
Cardiac muscle tissue
striated & autorhythmic
Smooth muscle tissue
nonstriated & involuntary
Skeletal Muscle
• Long, cylindrical, unbranched cells with
striations and multiple peripheral nuclei
• movement, facial expression, posture, breathing,
speech, swallowing and excretion
5-5
Muscular Dystrophy
• Hereditary diseases - skeletal muscles degenerate and are
replaced with adipose
• Disease of males
• appears as child begins to walk
• rarely live past 20 years of age
• Dystrophin links actin filaments to cell membrane
• leads to torn cell membranes and necrosis
• Fascioscapulohumeral MD -- facial and shoulder muscle only
11-6
Myasthenia Gravis
• Autoimmune disease - antibodies attack NMJ and bind ACh
receptors in clusters
• receptors removed
• less and less sensitive to ACh
• drooping eyelids and double vision, difficulty swallowing, weakness of
the limbs, respiratory failure
• Disease of women between 20 and 40
• Treated with cholinesterase inhibitors, thymus removal or
immunosuppressive agents
11-7
Myasthenia Gravis
Drooping eyelids and weakness of muscles of eye movement
11-8
Cardiac Muscle
• Short branched cells with striations and
intercalated discs
• one central nuclei per cell
• Pumping of blood by cardiac (heart) muscle
5-9
Cardiac Muscle 1
• Thick cells shaped like a log with uneven, notched
ends
• Linked to each other at intercalated discs
• electrical gap junctions allow cells to stimulate their neighbors
• mechanical junctions keep the cells from pulling apart
• Sarcoplasmic reticulum less developed but large T
tubules admit Ca+2 from extracellular fluid
• Damaged cells repaired by fibrosis, not mitosis
11-10
Cardiac Muscle 2
• Autorhythmic due to pacemaker cells
• Uses aerobic respiration almost exclusively
• large mitochondria make it resistant to fatigue
• very vulnerable to interruptions in oxygen supply
11-11
Smooth Muscle
• Short fusiform cells; nonstriated with only one central
nucleus
• sheets of muscle in viscera; iris; hair follicles and
sphincters
• swallowing, GI tract functions, labor contractions,
control of airflow, erection of hairs and control of
pupil
5-12
Smooth Muscle
• Fusiform cells with one nucleus
• 30 to 200 microns long and 5 to 10 microns wide
• no striations, sarcomeres or Z discs
• thin filaments attach to dense bodies scattered throughout
sarcoplasm and on sarcolemma
• SR is scanty and has no T tubules
• calcium for contraction comes from extracellular fluid
• If present, nerve supply is autonomic
• releases either ACh or norepinephrine
11-13
Types of Smooth
Muscle
• Multiunit smooth muscle
• largest arteries, iris, pulmonary air passages, arrector pili
muscles
• terminal nerve branches synapse on myocytes
• independent contraction
11-14
Types of Smooth
Muscle
• Single-unit smooth muscle
• most blood vessels and viscera as circular and
longitudinal muscle layers
• electrically coupled by gap junctions
• large number of cells contract as a unit
11-15
Stimulation of
Smooth Muscle
11-16
Stimulation of
Smooth Muscle
• Involuntary and contracts without nerve stimulation
• hormones, CO2, low pH, stretch, O2 deficiency
• pacemaker cells in GI tract are autorhythmic
• Autonomic nerve fibers have beadlike swellings called
varicosities containing synaptic vesicles
• stimulates multiple myocytes at diffuse junctions
11-17
Features of Contraction and
Relaxation
• Calcium triggering contraction is extracellular
• calcium channels triggered to open by voltage, hormones,
neurotransmitters or cell stretching
• calcium ions bind to calmodulin
• activates light-chain myokinase which activates myosin ATPase
• power stroke occurs when ATP hydrolyzed
• Thin filaments pull on intermediate filaments
attached to dense bodies on the plasma membrane
• shortens the entire cell in a twisting fashion
11-18
Features of Contraction and
Relaxation
• Contraction and relaxation very slow in comparison
• slow myosin ATPase enzyme and slow pumps that
remove Ca+2
• Uses 10-300 times less ATP to maintain the same
tension
• latch-bridge mechanism maintains tetanus (muscle tone)
• keeps arteries in state of partial contraction (vasomotor
tone)
11-19
Contraction of Smooth
Muscle
11-20
Responses to Stretch
• Stretch opens mechanically-gated calcium channels causing muscle
response
• food entering the esophagus brings on peristalsis
• Stress-relaxation response necessary for hollow organs that gradually fill
(urinary bladder)
• when stretched, tissue briefly contracts then relaxes
• Must contract forcefully when greatly stretched
• thick filaments have heads along their entire length
• no orderly filament arrangement -- no Z discs
• Plasticity is ability to adjust tension to degree of stretch such as empty
bladder is not flabby
11-21
“Gross” Muscle Anatomy
Skeletal muscles are organs
Muscle cells are called “fibers” (myofibers)
Nervous tissue -> sensory & motor neurons
Blood vessels (lined by epithelia)
Connective tissue wrappers
endomysium, perimysium, epimysium, fascia,
tendons vs. aponeuroses
collagen is extensible and elastic*
A Myofiber
MYOFiber specializations
Multiple flattened nuclei just inside cell membrane
• fusion of multiple myoblasts during development
satellite cells nearby can multipy/produce some new myofibers
Sarcolemma with transverse (T) tubules that penetrate the cell
• carry electric current to cell interior
Sarcoplasm is filled with
• myofibrils (bundles of myofilaments)
• glycogen for stored energy and myoglobin for binding oxygen
Sarcoplasmic reticulum = smooth ER
• network around each myofibril
• dilated end-sacs (terminal cisternae) store calcium
• triad = T tubule and 2 terminal cisternea
11-27
Thick Filaments
• Made of 200 to 500 myosin molecules
• 2 entwined polypeptides (golf clubs)
• Arranged in a bundle with heads directed outward in a spiral
array around the bundled tails
• central area is a bare zone with no heads
11-28
Thin Filaments
• Two intertwined strands fibrous (F) actin
• globular (G) actin with an active site
• Groove holds tropomyosin molecules
• each blocking 6 or 7 active sites of G actins
• One small, calcium-binding troponin molecule on each
tropomyosin molecule
11-29
Elastic Filaments
• Springy proteins called titin
• Anchor each thick filament to Z disc
• Prevents overstretching of sarcomere
11-30
Regulatory vs. Contractile
Proteins
• Myosin and actin are contractile proteins
• Tropomyosin and troponin = regulatory proteins
• switch that starts and stops shortening of muscle cell
• contraction activated by release of calcium into sarcoplasm and its
binding to troponin,
11-31 • troponin moves tropomyosin off the actin active sites
Overlap of Thick and Thin
Filaments
11-32
Striations = Organization of
Filaments
• Dark A bands (regions) alternating with lighter I bands (regions)
• anisotrophic (A) and isotropic (I) stand for the way these regions affect polarized light
• A band is thick filament region
• lighter, central H band area contains no thin filaments
• I band is thin filament region
• bisected by Z disc protein called connectin, anchoring elastic and thin filaments
• from one Z disc (Z line) to the next is a sarcomere
11-33
Striations and
Sarcomeres
11-34
Relaxed and Contracted
Sarcomeres
• Muscle cells shorten because their individual
sarcomeres shorten
• pulling Z discs closer together
• pulls on sarcolemma
• Notice neither thick nor thin filaments change length
during shortening
• Their overlap changes as sarcomeres shorten
11-35
Nerve-Muscle
Relationships
• Skeletal muscle must be stimulated by a nerve or it will not
contract
• Cell bodies of somatic motor neurons in brainstem or spinal
cord
• Axons of somatic motor neurons = somatic motor fibers
• terminal branches supply one muscle fiber
• Each motor neuron and all the muscle fibers it innervates =
motor unit
11-36
Motor Units
• A motor neuron and the muscle fibers it
innervates
• dispersed throughout the muscle
• when contract together causes weak contraction
over wide area
• provides ability to sustain long-term contraction
as motor units take turns resting (postural
control)
• Fine control
• small motor units contain as few as
20 muscle fibers per nerve fiber
• eye muscles
• Strength control
• gastrocnemius muscle has 1000
fibers per nerve fiber
11-37
Neuromuscular Junctions
(Synapse)
• Functional connection between
nerve fiber and muscle cell
• Neurotransmitter (acetylcholine/ACh) released from
nerve fiber stimulates muscle cell
• Components of synapse (NMJ)
• synaptic knob is swollen end of nerve fiber (contains ACh)
• junctional folds region of sarcolemma
• increases surface area for ACh receptors
• contains acetylcholinesterase that breaks down ACh and causes
relaxation
• synaptic cleft = tiny gap between nerve and muscle cells
• Basal lamina = thin layer of collagen and glycoprotein over
all of muscle fiber
11-38
The Neuromuscular
Junction
11-39
Neuromuscular Toxins
• Pesticides (cholinesterase inhibitors)
• bind to acetylcholinesterase and prevent it from degrading ACh
• spastic paralysis and possible suffocation
• Tetanus or lockjaw is spastic paralysis caused by toxin of
Clostridium bacteria
• blocks glycine release in the spinal cord and causes
overstimulation of the muscles
• Flaccid paralysis (limp muscles) due to curare that competes
with ACh
• respiratory arrest
11-40
End of Lecture 11a
• During lab
• see slides of all three muscle tissues
• look at models of same
• review muscles (organs)