Lecture 15: Muscle Physiology L3 Flashcards
Explain Cardiac Muscle Structure vs skeletal muscle?
- both are striated,
arrangement of the CONTRACTILE APPARATUS is fundamentally similar to that of skeletal muscle.
Cardiac muscle:
* Structure = 8
1 * Are considerably shorter.
2 * Have branched processes
3 * Have gap junctions, allowing electrical communication between cells
4 * Have a different mechanism of excitation-contraction coupling*
5 * Have a less developed SR
- INTERCALATED DISCS = Cardiac muscle fibers branch and are interconnected by intercalated discs
- plasma membranes of adjacent cardiac muscle fibers.
8.Desmosome
Cardiac Muscle Struture: DESMOSOME
Desmosomes mechanically connect the sarcomeres in adjacent cells.
Cardiac Muscle Structure: GAP JUNCTIONS function?
allow the action potential to travel from cell to cell without the requirement for a motor nerve
CARDIAC MUSCLE STRUCTURE:
Intercalated discs contain 2 types of membrane junctions:
- Mechanically important desmosomes
- Electrically important Gap junctions
Excitation-contraction coupling: Cardiac vs skeletal muscle
Effect of removing external (extracellular) Ca2+ from the bathing
medium in isolated cardiac and skeletal muscle preparations
SLIDE 7
Excitation-contraction coupling in skeletal muscle
- Action potential in Muscle Membrane
- Release Channelss (RC)
AND Voltage sensors (VS)
to SR - SR Ca+2 pump
- Ca2+ into SR
- Ca+2 RELAXATION (Contractile apparatus)
- T-tubule
Excitation-contraction coupling: Cardiac vs skeletal muscle
Effect of removing external (extracellular) Ca2+ from the bathing
medium in isolated cardiac and skeletal muscle preparations SLIDE 9
Excitation-contraction coupling in Cardiac muscle…
- Action potential
- muscle membrane
- T-tubule Ca+2 channel
- SR Ca+2 pump in SR, CA+2
- Ca+2 out through SR CA+2 release channels.
- Na+ in, Ca+2 out
- Ca+2 in contractile apparatus
- Cardiac ECC: Primarily Ca2+-induced Ca2+ release from the SR
Excitation-contraction coupling in cardiac muscle. = 3
- Ca2+ INFLUX through the SARCOLEMMAL Ca2+ CHANNELS INDS TO AND ACTIVATES SR Ca+2 RELEASE CHANNELS.
- Stored SR Ca2+ is RELEASED INTO THE CYTOPLASM AND ACTIVATES CONTRACTION.
3.* This system of ECC is known as CALCIUM-INDUCED CALCIUM RELEASE (CICR).
Ionic basis of the cardiac action potential = 2
1 * The LONG REFRACTORY PEROID PREVENTS another AP FIRING DURING TENSION DEVELOPMENT.
2 * The Ca2+ INFLUX PRODUCING THE PLATEAU IS THE SAME
Ca2+ that is ACTIVATING Ca2+ RELEASE BY THE SR via CICR
SLIDE 12
Electrical conduction in the heart = 4
- AUTONOMIC N.S. MODULATES —>
- PACEMAKER =
- Sinoatrial (SA) Node3. - ‘SMALL FIBRES’ Delay APs
- Atrioventricular (AV) Node - BUNDLE OF His = TRANSMITS APs to VENTRICLE
Cardiac length-tension relationship compared to skeletal muscle
- The length tension curve increases the heart’s ability to contract at higher levels of stretch (ventricular filling).
- This is a fundamental mechanism responsible for the Frank Starling law of the heart, where increased diastolic volume leads to increased systolic contraction.
SLIDE 14
Motor Units and Fibre Types…
WHAT IS MOTOR UNITS?
Motor unit – Single motor neuron and the multiple muscle fibres that it INNERVATES
Motor units in various human muscles…
No of Motor Units?, Muscle fibres/unit?
- GASTROCNEMIUS
- TIBIALIS ANTERIOR
- RECTUS LATERALIS (extraocular muscle)
- GASTROCNEMIUS = ‘579 motor units, 1,934 muscle fibres/units.’
- TIBIALIS ANTERIOR = ‘445 motor units, 562 muscle fibres/units.’
- RECTUS LATERALIS (extraocular muscle) = ‘4,150 motor units, 5 muscle fibres/units.’
Motor units have varied biochemical & physiological properties
3 main types:
- S - Slow contracting fatigue resistant
‘smallest’ - FR - Fast contracting, fatigue resistant
‘medium’ - FF - Fast contracting, Fast fatigue
‘largest’
Slow motor units are generally smallest followed by FR and FF which are the largest
The different motor unit types have different properties, due to
the presence of different types of skeletal muscle fibres present in the different types of motor units.
Basic Classification & Characteristics of Human Skeletal Muscle Fibres
SLOW (slow twitch) = TYPE I Slow, Oxidative (RED)
CHARATERISTICS = 7
- SPEED OF CONTRACTION: slow
- STRENGTH OF CONTRACTION: low
- FATIGUABILITY: low
- AEROBIC CAPACITY: high
- ANAEROBIC CAPACITY: low
- SIZE: small
- MYOSIN ATPase Activity: slow
FAST TWITCH = FAST
TYPE IIa Fast OXIDATIVE GLYCOLYTIC
CHARACTERISTICS = 7
- SPEED OF CONTRACTION: fast
- STRENGTH OF CONTRACTION: high
- FATIGUABILITY: high
- AEROBIC CAPACITY: medium
- ANAEROBIC CAPACITY: medium
- SIZE: medium
- MYOSIN ATPase Activity: high
Basic Classification & Characteristics of Human Skeletal Muscle Fibres
TYPE IIb FAST, GLYCOLYTIC (WHITE)
CHARATERISTICS = 7
- SPEED OF CONTRACTION: fastEST
- STRENGTH OF CONTRACTION: highEST
- FATIGUABILITY: highEST
- AEROBIC CAPACITY: medium
- ANAEROBIC CAPACITY: HIGH
- SIZE: LARGEST
- MYOSIN ATPase Activity: highEST
MOTOR TYPE – MOTOR UNIT CHARACTERISTICS – FIBRES INNERVATED
- 3
S - Slow contracting fatigue resistant - Type I
FR - Fast contracting, Fatigue resistant - Type IIa
FF - Fast contracting, Fast Fatigue - Type IIb
Twitch and tetanus responses in fast and slow muscles
slide 21
Fibre type proportions
in sprint versus endurance elite athletes.
100m swimmer = more FAST (TYPE II) FIBRES (LIGHTLY STAINED)
Long Distance Cyclist = more SLOW (Type ii) fibre (STAINS BLACK)
EFFECTS OF TRAINING ON FIBRE TYPE…
- Muscle fibre type be established early in life and remains stable
- In human muscle, slow twitch (type I) and fast twitch (type II)DO NOT CHANGE THIER RELATIVE PROPORTIONS GREATLY AS THE RESULT OF TRAINING.
- HOWEVER, THEIR SIZE AND OXIDATIVE CAPACITIES IMPROVE.
Emerging evidence suggests:
Proportions of type IIa and type IIb fibres can change in response to training.
DISTANCE RUNNING VS WEIGHT TRAINING….
- Distance running: Promotes a shift from fast twitch, glycolytic (type IIb) to fast twitch, oxidative, glycolytic (type IIa) fibres
- Weight training: Promotes shift from type IIa to type IIb
Graded contractions:
Motor unit recruitment in skeletal muscle
- Recruitment of:
Motor unit 1 = 3/8 (38% max. force) - Motor units 1 & 2 = 6/8 (75% of max. force)
- All motor units = 8/8 (100% of max. force)
- A proportion of available motor units are recruited to provide the level of force required to suit a given task.
Slow motor units are generally smallest followed by FR and FF.
(GRADED CONTRACTION: Motor unit recruitment in skeletal muscle)
-Power required, Recruitment Order, Low force required
Low force required: Only slow units recruited
Power required: Fast units recruited
Recruitment Order:
-> MOTOR UNITS:
S —> FR —> Fint —> FF
-> MUSCLE FIBRES:
Type I –> Type IIa –> Type II(int) –> TYPE IIb
F(int) – Fast intermediate type
Type II(int) – intermediate type
Graded contractions:
Can also involve changes in stimulation frequency
– In some muscles, motor neuron firing frequency may also be used to increase force (i.e. by summation) (e.g. smaller distal muscles).
– This can be used in combination with motor recruitment.
Trophic Effects of Nerves On Muscles: FAST VS SLOW TWITCH, 4
- Research suggests that biochemical and physiological differences between fast and slow muscle fibres are DETERMINED TO SOME DEGREE BY THE NERVE FIBRES THAT INNERVATE THEM.
- FAST TWITCH: Receive intermittent nerve impulses of high frequency
- SLOW TWITCH: Receive continuous nerve impulses of low frequency.
- The PATTERN OF ELECTRICAL IMPULSES COMING DOWN THE NERVE AND INVADING THE MUSCLE, ‘
— INFLUENCE THE PATTERNS OF GENE EXPRESSION IN THE MUSCLE AND PROMOTES A PARTICULAR FIBRE TYPE.
Trophic Effects Of Nerves On Muscles: Cross-re innervation experiments.
- Slow muscle becomes faster
- Fast muscle becomes slower
SLIDE 32
Trophic Effects Of Nerves On Muscles
Mechanism ?
The mechanism responsible for this may be that: 3
- Different trophic substances are released from the different axon terminals influencing the TRANSCRIPTION OF GENETIC INFORMATION IN THE MUSCLE FIBRES.
- Differences in steady impulse activity of the motor nerves and the corresponding amount of contractile activity the muscle experiences ALTERS GENE TRANSCRIPTION.
- Both 1 & 2
