Transcript Biol 155 Human Physiology - University of British Columbia

Smooth Muscle
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Characteristics
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Not striated
Dense bodies instead of Z
disks as in skeletal muscle
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Have noncontractile
intermediate filaments
Ca2+ required to initiate
contractions
Types
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Visceral or unitary
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Function as a unit
Multiunit
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Cells or groups of cells act
as independent units
Smooth Muscle Contraction
Electrical Properties of Smooth
Muscle
Functional Properties of
Smooth Muscle
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Some visceral muscle exhibits autorhythmic
contractions
Tends to contract in response to sudden stretch
but no to slow increase in length
Exhibits relatively constant tension: Smooth
muscle tone
Amplitude of contraction remains constant
although muscle length varies
Smooth Muscle Regulation
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Innervated by autonomic nervous system
Neurotransmitter are acetylcholine and
norepinephrine
Hormones important as epinephrine and
oxytocin
Receptors present on plasma membrane
which neurotransmitters or hormones bind
determines response
Cardiac Muscle
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Found only in heart
Striated
Each cell usually has one nucleus
Has intercalated disks and gap junctions
Autorhythmic cells
Action potentials of longer duration and longer
refractory period
Ca2+ regulates contraction
Cardiac Muscle
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Elongated, branching cells containing 1-2 centrally located nuclei
Contains actin and myosin myofilaments
Intercalated disks: Specialized cell-cell contacts
Desmosomes hold cells together and gap junctions allow action potentials
Electrically, cardiac muscle behaves as single unit
Cardiac myocyte action potential:
Refractory Period
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Absolute: Cardiac muscle cell completely
insensitive to further stimulation
Relative: Cell exhibits reduced sensitivity to
additional stimulation
Long refractory period prevents tetanic
contractions
AP-contraction relationship:
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AP in skeletal muscle is very
short-lived
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AP is basically over before an
increase in muscle tension can
be measured.
AP in cardiac muscle is very
long-lived
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AP has an extra component,
which extends the duration.
The contraction is almost over
before the action potential has
finished.
Functions of the Heart
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Generating blood pressure
Routing blood
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Ensuring one-way blood flow
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Heart separates pulmonary and systemic
circulations
Heart valves ensure one-way flow
Regulating blood supply
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Changes in contraction rate and force match
blood delivery to changing metabolic needs
Orientation of cardiac muscle fibres:
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Unlike skeletal muscles,
cardiac muscles have to
contract in more than
one direction.
Cardiac muscle cells are
striated, meaning they
will only contract along
their long axis.
In order to get
contraction in two axis,
the fibres wrap around.
Circulation circuits:
Heart Wall
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Three layers of tissue
Epicardium: This serous membrane of smooth
outer surface of heart
 Myocardium: Middle layer composed of cardiac
muscle cell and responsibility for heart
contracting
 Endocardium: Smooth inner surface of heart
chambers
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Valve function:
Coronary circulation:
Pacemaker potential:
Cardiac conducting system:
EKG:
Heart Sounds
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First heart sound or “lubb”
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Second heart sound or “dupp”
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Atrioventricular valves and surrounding fluid vibrations
as valves close at beginning of ventricular systole
Results from closure of aortic and pulmonary semilunar
valves at beginning of ventricular diastole, lasts longer
Third heart sound (occasional)
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Caused by turbulent blood flow into ventricles and
detected near end of first one-third of diastole
Heart sounds:
Cardiac Arrhythmias
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Tachycardia: Heart rate in excess of 100bpm
Bradycardia: Heart rate less than 60 bpm
Sinus arrhythmia: Heart rate varies 5% during
respiratory cycle and up to 30% during deep
respiration
Premature atrial contractions: Occasional
shortened intervals between one contraction
and succeeding, frequently occurs in healthy
people
Mean Arterial Pressure (MAP)
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Average blood pressure in aorta
MAP=CO x PR
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CO is amount of blood pumped by heart per minute
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CO=SV x HR
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SV: Stroke volume of blood pumped during each heart beat
HR: Heart rate or number of times heart beats per minute
Cardiac reserve: Difference between CO at rest and
maximum CO
PR is total resistance against which blood must be
pumped
Pressure relationships:
Cardiac Cycle
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Heart is two pumps that work together, right
and left half
Repetitive contraction (systole) and relaxation
(diastole) of heart chambers
Blood moves through circulatory system
from areas of higher to lower pressure.
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Contraction of heart produces the pressure
Factors Affecting MAP
Regulation of the Heart
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Intrinsic regulation: Results from normal functional
characteristics, not on neural or hormonal regulation
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Starling’s law of the heart
Extrinsic regulation: Involves neural and hormonal
control
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Parasympathetic stimulation
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Supplied by vagus nerve, decreases heart rate, acetylcholine
secreted
Sympathetic stimulation
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Supplied by cardiac nerves, increases heart rate and force of
contraction, epinephrine and norepinephrine released
Heart Homeostasis
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Effect of blood pressure
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Effect of pH, carbon dioxide, oxygen
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Chemoreceptors monitor
Effect of extracellular ion concentration
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Baroreceptors monitor blood pressure
Increase or decrease in extracellular K+ decreases heart
rate
Effect of body temperature
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Heart rate increases when body temperature increases,
heart rate decreases when body temperature decreases
Baroreceptor and Chemoreceptor
Reflexes