Muscles & Muscle Tissue
A. Functions
1. provide energy for movement
2. maintain posture
3. thermogenesis
4. maintain hollow organ volume
B. characteristics
1. excitability
2. contractility
3. extensibility
4. elasticity
C. Three types
1. Skeletal
A) long, cylindrical cells, multi-nucleated, non-branching, and voluntary
B) striated – caused by special arrangement of myofilaments
C) stimulus from nervous system
2. Cardiac
A) only in heart
B) striated, mononucleated, branching, and involuntary
C) intercalated discs – gap junctions between cells; distinguishing characteristic
D) stimulus is intrinsic
3. Smooth
A) found lining the digestive, respiratory & reproductive tracts; also surrounding
blood vessels
B) mononucleated, no striations (distinguishing characteristic), and involuntary
C) stimulus from nervous system, some hormones, or on its own (stretch)
D. Structure of Skeletal Muscle
1. Whole muscle – bundle of fascicles
A) epimysium – CT layer surrounding whole muscle
2. Fascicle – bundle of muscle fibers (cells)
A) perimysium – CT layer surrounding and tying together fascicles
3. Muscle fiber – bundle of myofibrils
A) endomysium – CT layer surrounding and tying together muscle fibers
B) sarcolemma
C) sarcoplasm
D) transverse-tubules (T-tubules) – inward projections of the sarcolemma; unite with
the sarcoplasmic reticulum
4. Myofibrils – composed of 2 types of myofilaments
A) thin – composed of 3 protein fibers
1) actin – contractile protein; contains myosin binding sites
2) troponin – regulatory protein
3) tropomyosin – regulatory protein; combines with troponin to form the troponin-
tropomyosin complex

a) when the muscle is relaxed , the troponin-tropomyosin complex
blocks the myosin binding sites on the actin
B) thick – composed of 1 protein fiber
1) myosin – contractile protein; golf-club shaped a) head – will bind to the myosin binding sites on the actin during contraction b) tail – intertwined to hold the myosin fibers together
5. Sarcomere – specialized arrangement of myofilaments
A) functional unit of muscle
6. Sarcoplasmic reticulum (SR) – fluid filled tubes surrounding each myofibril
A) storage site of calcium (Ca ++ )
B) adjacent terminal cisternae unite with the T-tubules to form the triad
1) the terminal cisternae are enlarged portions of the SR surrounding each T-tubule
2) allow an impulse to be transmitted from the T-tubules to the SR
E. Skeletal Muscle Contraction
1. Involves a motor unit
A) a single muscle may have many motor units
2. Three steps
A) Nerve-Muscle Communication
1) Occurs at the neuromuscular junction a) motor-end plate b) synaptic cleft
2) Process: a) an impulse travels down the motor neuron b) ACh is released from the neuron into the synaptic cleft c) ACh binds to receptors on the motor-end plate causing chemical-gated Na +
channels to open d) Na + moves into the muscle fiber causing depolarization of the motor-end
plate e) depolarization of the motor-end plate causes an opening of voltage-gated Na +
channels in the sarcolemma f) this causes an action potential to be transmitted along the length of the
sarcolemma
B) Excitation-Contraction Coupling
1) involves T-tubules and sarcoplasmic reticulum (SR)
2) process: a) AP travels down the sarcolemma, also travels down the T-tubules b) AP is then transferred to the SR at its intersection with the T-tubule c) the AP causes an opening of Ca ++ release channels in the SR d) Ca ++ floods the sarcoplasm surrounding the thin & thick filaments e) Ca ++ binds to troponin causing a shifting of the troponin-tropomyosin
complex exposing the myosin binding sites on the actin
C) Sliding Filament Mechanism
1) involves the thin and thick filaments

2) process: a) ATP is split by ATPase on the myosin head resulting in an “energized”
myosin head i) this occurs at the end of the previous contraction ii) ADP & P stay attached to the myosin head b) the energized myosin head binds to the exposed binding site on the actin c) using the energy from ATP, the myosin head swivels inward pulling the thin
filament towards the center of the sarcomere = power stroke i) the ADP & P are released d) ATP binds to the myosin head causing it to break away from the binding site e) the ATP is split by ATPase, re-energizing the myosin head f) the process repeats and will continue as long as ATP and Ca ++ are present
F. Relaxation – 2 mechanisms
1. Acetylcholinesterase
A) breaks down ACh in the synaptic cleft
2. Ca ++ active transport pumps
A) found in the walls of the sarcoplasmic reticulum
B) pump Ca ++ back into SR
G. Muscle Metabolism
1. Muscle stores enough ATP for about 4-6 sec of work
2. Three processes provide muscle cells with more ATP
A) phosphagen system
1) creatine kinase transfers a phosphate group from creatine phosphate (CP) to an
ADP molecule creating 1 ATP for each CP
2) allows for about 10-15 seconds of energy for maximum activity
B) fermentation (anaerobic) – no oxygen needed; occurs in cytoplasm
1) incomplete oxidation of glucose
2) produces 2 pyruvic acid & 2 ATP from 1 glucose molecule
3) pyruvic acid is converted to lactic acid
4) allows for about 30-40 seconds of max activity
C) cellular (aerobic) respiration – requires oxygen; occurs in mitochondria
1) complete oxidation of glucose
2) produces 6 CO
2
, 6 H
2
O, & 32 ATP from one glucose molecule
3) can also use fatty acids & amino acids
4) amount of energy produced depends on fitness of individual
H. Muscle fatigue
1. Can be caused by a number of factors
A) inadequate O
2
B) glucose/glycogen depletion
C) ACh depletion
D) lactic acid accumulation

2. Oxygen debt – the amount of O
2
necessary to restore the muscle to normal state
I. Muscle Contractions
1. Muscle twitch – response of the motor unit to a single impulse
A) latent period
B) contractile period
C) relaxation period
2. Graded muscle responses
A) the way muscles normally function
B) creates smooth contractions of varying strength
C) 2 primary graded responses
1) wave summation – increases the strength of contraction by increasing the
frequency of the stimulus a) tetanus
2) multiple motor unit summation (recruitment) – increases the strength of
contraction by activating more motor units
D) treppe
1) stronger contractions resulting from no increase in stimulation
2) possibly due to increased Ca ++ availability and temperature
3) basis behind “warming up” before exercise
E) muscle tone
1) slight contraction seen in “resting” muscles
2) keeps muscles firm, healthy, and ready to respond
F) types of contractions
1) isometric contraction a) muscle does not change in length b) tension increases
2) isotonic contraction a) muscle length changes b) tension remains constant c) 2 types
1) concentric a) muscle shortens as it contracts
2) eccentric a) muscle lengthens as it contracts
J. Muscle Fiber Types
1. Slow (Red) Oxidative Fibers
A) rely on aerobic metabolism
1) more myoglobin
2) more capillaries
3) more mitochondria
B) long, slow contractions
C) fatigue resistant
2. Fast (White) Glycolytic Fibers

A) rely on anaerobic metabolism
1) less myoglobin
2) fewer capillaries
3) fewer mitochondria
B) rapid, powerful contractions
C) fatigue easily
3. Fast (Pink) Oxidative Fibers
A) similar to red fibers except:
1) faster contractions
2) can use anaerobic metabolism
3) fatigues more easily than red fibers but not as easily as white fibers
K. Benefits of Exercise
1. increases in fiber size and strength
2. increased muscle tone
3. increase in blood supply, therefore increased RBCs
4. increased cardiovascular & respiratory function
5. lowers BP
L. Smooth Muscle Contraction
1. Same principle as skeletal muscle but a slower and longer sustained contraction
2. Differences include:
A) no T-tubules or sarcomeres
B) Ca ++ from SR & ECF
C) no troponin-tropomyosin complex
1) calmodulin instead of troponin
2) myosin light chain kinase instead of ATPase
D) contracts in response to norepinephrine as well as ACh
E) contracts in response to certain hormones (ex. oxytocin)
F) contracts in response to stretch
M. Cardiac Muscle Contraction
1. Again, same contractile principle as skeletal except:
A) contracts continuously
B) contracts as a unit
C) stimulus is intrinsic, but also neural and hormonal control
D) Ca ++ from SR & ECF
E) cannot undergo tetanus
N. Muscular Disorders
1. Myasthenia gravis – characterized by drooping upper eyelids, difficulty swallowing &
talking, and generalized muscle weakness
A) results from loss of ACh receptors
2. Rigor mortis – muscle stiffness following death
A) results from lack of ATP to break the myosin cross-bridges
3. Atrophy – loss of muscle mass
A) results from immobilization or loss of neural stimulation

B) occurs to a small extent from non-use of healthy muscles
4. Muscular dystrophy – a group of inherited muscle destroying diseases
A) muscles enlarge due to fat and CT deposits while muscles fibers atrophy