Anatomical Review of Muscle Cell anatomy
Fig. 12.1
Whole muscle = many muscle cells + CT
Fig. 12.15
Individual Muscle Cell—Anatomy Review
Fig. 12.6
Organization of actin and myosin filaments
--Alternating and overlapping
Fig. 12.7
Organization of a sarcomere
Muscular System
• Skeletal Muscles and associated connective tissue
– Skeletal muscle cells=muscle fibers
FUNCTIONS
• Produces movement
– (through contraction of cells)
– Important in verbal and non-verbal communication
• Stabilizes joints and maintains posture
– (through contraction of cells)
• Produces body heat
– (through high levels of cellular respiration)
OVERVIEW OF SKELETAL MUSCLE ACTIVITY
BrainMotor Neurons, synaptic
activity--ACh
Action Potential & propagation
Ca+ channel activity
ATP production/consumption
sliding of actin & myosin filaments
production of force and movement (sometimes)
CONTROL OF SKELETAL MUSCLES
Voluntary Motor Activity Originates in Frontal Lobe of cerebral cortex
Voluntary Muscle Contraction:
Neuron Activity Begins Frontal
Lobe:
Upper motor neuron
• Decussates in medulla (~80%)
• Travels down spinal cord
through anterior or lateral
corticospinal tracts to lower motor
neuron
• Synapse with lower motor
neuron
•Lower motor neuron travels
through nerve to effector muscle
•Forms synapse—Neuromuscular
Junction—with muscle
•NT= Ach binds to nicotinic
receptors
13-10
Neurological Control of Skeletal Muscle
•
•
•
CNS (brain and spinal cord): generate motor commands that will signal muscle cells
to contract
Voluntary Activity
– Frontal lobe: initiates voluntary muscle activity
– Basal nuclei: coordinates voluntary muscle activity
– Thalamus: involved with coordination of voluntary muscle activity
– substantia nigra: coordinates muscle activity (inhibits antagonistic muscles)
– Cerebellum: coordinates muscle activity (makes adjustments based on current
body position)
Cranial reflexes
– Generate involuntary, reflexive muscle use to specific stimuli. Integrating center is in brain
•
Spinal Cord
– Spinal reflexes
•
•
Generate involuntary, reflexive muscle use to specific stimuli. Integrating center is in spinal cord
Lower motor neurons (PNS) directly innervate muscle cells
– CNS initiated commands are relayed (through synapses) to lower motor neurons which carry
A.P.s from CNS to the individual muscle cells they innervate.
Neurological Control of Skeletal Muscle
• All Skeletal Muscle cells are directly innervated by a motor
neuron
• Neuromuscular Junction:
– The chemical synapse between a motor neuron and a
muscle fiber (cell)
– Chemical synapse, always excitatory
• Motor Units: a motor neuron and all the muscle cells it
innervates
– Multiple fibers innervated by same neuron
– They contract together as a unit
Fig. 12.4
This neuron is also a lower motor neurons
Fig. 12.3
Key NMJ concepts
• Chemical synapses (as described in neuron physiology unit)
• Neurotransmitter: ACh
• Receptor: nicotinic
• Receptor Action: ACh opens ligand gated Na+ channel, Na+ enters
cells depolarizing itend plate potential
• Short-lived due to action of ACh’ase
• NMJ are always excitatory
• A single AP almost always releases enough ACh to bring the motor
end plate/muscle cell to threshold
Structure and events that occur at a NMJ
Another representation of the events at a neuromuscular junction
EXCITATION-CONTRACTION COUPLING:
from AP formation at synapse to actin-myosin interaction
• AP propagates across PM (sarcolemma of
muscle cell)
– VG Na+ channels
– Just like an AP in axon
• AP travels down T-tubels
• VG Ca+ channels (aka DHP receptors) open
– DHP receptors are coupled/linked to Ca+
release channels
• Ca+ release channels (aka ryanodine receptors) open
• Ca+ floods into cytoplasm
• Ca+ binds actin filament allowing actinmyosin interaction
Fig. 12.16
Visual representation of excitation-contraction coupling
Fig. 12.17
Flow Chart of excitationcontraction coupling
events
• REVIEW OF MYOFILAMENT ANATOMY AND
FUNCTION
Fig. 12.13
ACTIN FILAMENTS
Ca+ binds
Covers up binding sites
for myosin heads, can
move to expose binding
sites
Has binding sites of
myosin head, will be
bound by myosin during
interaction/contraction
Fig. 12.10
MYOSIN FILAMENT STRUCTURE
Myosin Head:
• Binds Actin
• Have binding site for ATP
• Will grab, pull on, and detach from actin
Fig. 12.9
Filament Interaction:
• Myosin grabs and pulls on actinfilaments slide across one another
• Zone of filament overlap increases
• Sarcomeres get shorter cell shortens=contraction
FILAMENT INTERACTION: SLIDING FILAMENT THEORY OF MUSCLE CONTRACTION
• Myosin and actin filaments
interact
• Myosin pulls on actin
• Filaments slide past one
another increasing zone of
overlap
• Sarcomeres get smaller
• Results in contraction of
muscle and production of
tension (i.e., pulling force)
Fig. 12.14
When Ca+ binds
troponin,
tropomyosin moves
to expose myosin
binding sites as
shown in diagram
NOTE:
This is show as if
you were viewing
the filaments along
their short axis—
different perspective
then other diagram
Fig. 12.10
Fig. 12.11
Fig. 12.12
Table 9.02
Table 12.2
Figure 9.12
Production and Control of Tension
• Contraction produces
pulling force known as
tension
Twitch: a single contraction. The result of a single
AP/excitation contraction coupling event
Latent:
•AP propagation, Ca release, Ca
build up in sarcoplasm
Contraction:
• Active cross bridging/contraction,
Ca+ available
Relaxation:
• Ca+ decreasing in sarcoplasm,
diminished and eventual lack of
crossbridging/contraction
Figure 9.41
Relationship between time of stimulus, AP, and tension
Factors that influence tension (i.e., strength of contraction)
• Action Potential (stimulus) frequency
• Number of active fibers/number of motor units
activated
• Fiber length (amount of actin-myosin overlap)
Figure 9.19
Green: single twitches, complete relaxation between
Orange: partial relaxation between two stimuli stimuli
wave summation—second contraction stronger than first
Purple: two stimuli with no relaxation between
contraction stronger than that with a single stimulus
Summation (temporal/frequency)
• Increased stimulation rate  increased tension/strength
– Build up/availability of Ca+ in cytoplasm
• Incomplete/unfused tetanus—stimulation frequency allows partial contractions
• Complete/fused tetanus – stimulation frequency does not allow any relaxation phase.
Figure 9.20
• Recruitment = strength of contraction proportional to number of motor units
activated
– E.g., ↑ motor unit = ↑ tension/strength
• Rotating through motor units allows prolonged contraction with reduced fatigue
•
•
•
Optimal resting length = optimal overlap of filaments  ↑ cross bridging  ↑ tension
Too long = too little overlap  not enough crossbridging  ↓ tension
Too short = no room left to contract & fiber mis-alignment  ↓ tension
• Energetics
Energetics
ATP needed for:
• Energizing head
• Detaching head from myosin
• Power Ca+ pumps that transport Ca+
into SR (from cytoplasm)
ATP production through:
• Aerobic respiration
• Anaerobic respiration
• Creatine Phosphate
Stored energy sources and how much muscle
contraction they can sustain.
Resting Muscle
•
•
•
Primary substrate plasm fatty acids
ATP production ˃ ATP consumption/demand
Surplus ATP used to:
– Creatine  CP
– Glucose  glycogen
Fig. 12.24
Moderate activity
•
•
•
Substrates = plasm fatty acids & glucose/glycogen
ATP production can meet ATP consumption/demand
Aerobic respiration dominates
Heavy/intense activity
• Primary substrate glucose (from glycogen)
• Aerobic respiration is insufficient to meet needs
• Anaerobic respiration occurs
– Produces lactic acid
Cori Cycle
• Lactic acid from anearobic activity of muscle  blood  liver
• Liver uses ATP (produced aerobically) to synthesize glucose from lactic acid
• Liver glucose  blood  skeletal muscle*
* liver cells = only type that can release glucose
5-15
Figure 9.22
Fig. 12.22
Intensity-substrate use patterns (short duration)
• As intensity increases:
– Fatty acids  glycogen
– Plasma borne substrate  intracellular substrates
Fig. 12.22
Intensity-substrate use patterns (long duration)
• As intensity increases:
– Fatty acids  glycogen
– Plasma borne substrate  intracellular substrates
– Why no graph for Heavy exercise of 90-120 min duration?
SUBSTRATE USE AT ACTIVITY ONSET
• Stored ATP
• CP
• Anaerobic Respiration
“instant” energy that is
available immediately
Ongoing activity + cardiopulmonary response
-- increased O2 delivery
• Aerobic Respiration (if mild to moderate intensity)
There are 3 types of muscle fibers
Table 12.3
Fig. 12.26
Table 12.4
Figure 9.13
Figure 9.24