Download somato-motor and skeletal muscle

Control of body movement The somato-motor division
Somatic Motor Pathways
 SNS, or the somatic motor system, controls contractions of
skeletal muscles
 Conscious and Subconscious Motor Commands
 Skeletal muscle contraction results in
 Posture
 Reflexes
 Rhythmic activity (locomotion, breathing)
 Voluntary movement
Somatic Motor Pathways – from brain to effector
 Always involve at least two motor neurons – upper and lower motor
neurons
 Upper motor neuron
 Cell body lies in a CNS processing center
 activity in upper motor neuron may facilitate or inhibit lower
motor neuron
 Lower motor neuron
 Cell body lies in a nucleus of the brain stem or spinal cord
 Triggers a contraction in innervated muscle
 only the axon of lower motor neuron extends outside CNS
Parts of CNS that are involved in the SNS

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Motor areas of the cerebral cortex
Basal nuclei
Cerebellum
Medulla oblongata
Descending pathways in spinal cord
Ventral horn of spinal cord
Primary Motor Cortex
 Located in the precentral gyrus in the frontal lobe in each
hemisphere
 Pyramidal cells that have long axon project to the spinal cord and
form a voluntary motor tracts called pyramidal
tracts/corticospinal tracts
 A pyramidal cell (or pyramidal neuron, or projection neuron)
is a multipolar neuron found in the cerebral cortex.
 These cells have a triangularly shaped soma
 Pyramidal neurons compose approximately 80% of the
neurons of the cortex
 Release glutamate as their neurotransmitters, making them the
major excitatory component of the cortex
 Allows conscious control of precise, skilled, voluntary
movements
Primary Motor Cortex Homunculus
 Somatotopy mapping
 Body is represented upside
down
 Although simplified in the
figure, one should remember
that:
 A given muscle is
controlled by multiple spots
on the cortex
 Individual cortical neurons
send impulses to more than
one muscle
 Neurons that control related
movements will overlap
 Neurons that control unrelated
movements do not cooperate
Posterior
Motor
Motor map in
precentral gyrus
Anterior
Toes
Jaw
Tongue
Swallowing
Primary motor
cortex
(precentral gyrus)
Figure 12.9.1
Basal nuclei of cerebrum
 Are masses of gray matter within each hemisphere deep to
lateral ventricle floor
 Provide subconscious control of skeletal muscle tone and
help coordinate learned movement patterns
 Normally do not initiate movement, but provide
general pattern and rhythm
Cerebellar Function
• Adjust ongoing movements on the basis of comparison between
arriving sensation to one previously experienced
– Posture:
• Balance
• Equilibrium
– Fine Tune Movements
• Timing
• Rate
• Range
• Force
The Cerebellum
Somatic Motor Pathways
 Three motor pathways
 Corticospinal pathway
 Medial pathway
 Lateral pathway
Somatic motor system
Corticospinal
Medial pathway
Vestibulospinal
Corticobulbar Corticospinal
Tectospinal
Lateral corticospinal
Lateral pathways
Reticulospinal
Anterior corticospinal
Rubrospinal
Body area
Voluntary
control
Skeletal
muscles of
body
Skeletal
muscles of
head
Subconscious Reflex
control
activity
Equilibrium
Ascending
pathway
Corticospinal
Upper neuron
location
Precentral gyrus
Lower neuron
location
Anterior gray
horn
Corticobulbar
Precentral gyrus
Nuclei of cranial
nerves
Reticulospinal
Brainstem nuclei
(reticular
formation)
Nucleus of cranial
nerve VIII
Superior and
inferior colliculi
Anterior gray
horn
Vestibulospinal
Auditory and Tectospinal
visual
reflexes
Distal
Rubrospinal
muscles of
upper limbs
Red nucleus of
midbrain
Anterior gray
horn
Anterior gray
horn of cervical
area
Anterior gray
horn in cervical
area
The Corticospinal /pyramidal/direct Pathway
 Upper motor neurons begin at the primary motor cortex
 Synapses with lower motor neurons occur in two tracts
 Corticobulbar (bulbar, brain stem) tracts
 move the eye, jaw, face, and some muscles of neck and
pharynx
 Synapses in motor nuclei of cranial nerves
 Corticospinal tracts
 Provide conscious control over skeletal muscles that
move various body areas
 Synapse in the anterior gray horn in the spinal cord
The corticobulbar tracts
Cranial nerve
Muscles/area
The occulomotor III
Extrinsic muscles of the eye
The trochlear IV
Extrinsic muscles of the eye
The abducens VI
Extrinsic muscles of the eye
The facial VII
muscles of facial expression
The glossopharyngeal IX
muscles involved in swallowing
The accessory XI
muscles of neck and upper back
The hypoglossal XII
tongue movements
Somatic Motor Pathways – the corticospinal pathways –
voluntary pathways
 Begins at pyramidal cells in primary motor cortex
 Primary motor cortex corresponds point by point with
specific regions of the body
 Upper axons descend into brain stem and spinal cord
 Synapse with lower motor neurons that control muscles
directly
Somatic motor system
Corticospinal
Corticobulbar Corticospinal
Lateral corticospinal
Anterior corticospinal
Somatic Motor Pathways – the corticospinal pathway
 The lateral corticospinal tracts (85 %) cross over at the level of the
medulla (pyramids)
 Exits at all levels of the spinal cord
 responsible for the control of the distal musculature
 fine control of the digits of the hand
 The anterior/ventral corticospinal (15%)
tract crosses over in
anterior gray horns before synapsing
 Exits at C1-L3
 responsible for the control of the proximal musculature
Somatic Motor Pathways
Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Initiation of Skilled Movement
Frontal Association and Primary Motor Cortex
Frontal cortex can plan and initiate movement but cannot calculate the
complex, timed sequence of muscle contraction
Send information about intended movements to the cerebellum
Basal Ganglia
Somatosensory System
Information on current position
Lateral Zone of Cerebellum
When cerebellum receives information about initiated movement it
computes the contribution that various muscles will have to make
Sends results
Dentate Nucleus
In cerebellum
Via basal ganglia
Allows the cerebellum to
modify the ongoing
movement that was initiated
by the frontal cortex
Thalamus
The cerebellum can also control a skilled movement by timing the movement and by turning on the
antagonist muscle. This happens when the movement is rapid and cannot relay on feedback.
Somatic Motor Pathways – medial and lateral pathways
Involuntary pathways
A cross section of the spinal cord showing the
locations of the medial and lateral pathways
Lateral
corticospinal
tract
Medial Pathway
Involved primarily with the control of
muscle tone and gross movements of
the neck, trunk, and proximal limb
muscles
Lateral Pathway
Involved primarily with the
control of muscle tone and
the more precise movements
of the distal parts of the
limbs
Rubrospinal tract
Reticulospinal tract
Vestibulospinal tract
Tectospinal tract
Anterior
corticospinal
tract
Figure 13.17 3
Somatic motor system
Medial pathway
Vestibulospinal
Tectospinal
Reticulospinal
The locations of centers in the cerebrum,
diencephalon, and brain stem that may issue
somatic motor commands as a result of processing
performed at a subconscious level
Motor
cortex
Thalamus
Basal
nuclei
Red nucleus
Cerebellar
nuclei
Nuclei of the Medial Pathway
Superior and inferior colliculi
Reticular formation
Vestibular nucleus
Medulla oblongata
Figure 13.17 2
Body area
Voluntary
control
Skeletal
muscles of
body
Skeletal
muscles of
head
Subconscious Reflex
control
activity
Ascending
pathway
Corticospinal
Upper neuron
location
Precentral gyrus
Lower neuron
location
Anterior gray
horn
Corticobulbar
Precentral gyrus
Nuclei of cranial
nerves
Brainstem nuclei
(reticular
formation)
Nucleus of cranial
nerve VIII
Anterior gray
horn
Superior and
inferior colliculi
Anterior gray
horn of cervical
area
Anterior gray
horn in cervical
Reticulospinal
(medial
pathways)
Equilibrium Vestibulospinal
(medial
pathways)
Auditory and Tectospinal
visual
(medial
reflexes
pathways)
Distal
Rubrospinal
muscles of
(lateral pathway)
Red nucleus of
midbrain
Anterior gray
horn
Somatic motor system
Lateral pathways
Rubrospinal
Somatic motor pathways - subconscious motor commands
Lateral pathway

control of muscle tone and
the more precise
movements of the distal parts of the upper limbs

Upper motor neuron in the Red nucleus of the
midbrain

Found only in the cervical area
Voluntary
control
Skeletal
muscles of
body
Skeletal
muscles of
head
Subconscious Reflex
control
activity
pathway
Corticospinal
location
Precentral gyrus
location
Anterior gray
horn
Corticobulbar
Precentral gyrus
Nuclei of cranial
nerves
Brainstem nuclei
(reticular
formation)
Nucleus of cranial
nerve VIII
Anterior gray
horn
Superior and
inferior colliculi
Anterior gray
horn of cervical
area
Anterior gray
horn in cervical
area
Reticulospinal
(medial
pathways)
Equilibrium Vestibulospinal
(medial
pathways)
Auditory and Tectospinal
visual
(medial
reflexes
pathways)
Distal
Rubrospinal
muscles of
(lateral pathway)
upper limbs
Red nucleus of
midbrain
Anterior gray
horn
Three Muscle Types

All muscle tissue exhibit:

Responsiveness - The ability to receive and respond
to a stimulus

Conductivity – the ability of the impulse to travel
along the plasma membrane of the muscle cell.

Contractility - The ability to shorten

Elasticity - The ability to recoil and resume original
length
Skeletal Muscle Functions
 Movement of bones or fluids (e.g., blood)
 Maintaining posture and body position
 Stabilizing joints
 Heat generation
 Each muscle is served by one artery, one nerve, and one
or more veins
Muscle terminology
 Muscle fiber – muscle cell
 Sarcolema – cell membrane
 Sarcoplasm – cytoplasm
 Sarcoplasic reticulum – endoplasmic reticulum
Skeletal Muscle
 Connective tissue sheaths of skeletal muscle:
 Epimysium: dense regular connective tissue
surrounding entire muscle
 Perimysium: fibrous connective tissue surrounding
fascicles (groups of muscle fibers)
 Endomysium: fine
areolar
surrounding each muscle fiber
connective tissue
Microscopic Anatomy of a Skeletal Muscle Fiber
 Each fiber is a long, cylindrical cell with multiple nuclei
just beneath the sarcolemma
 Fibers are 10 to 100 m in diameter, and up to hundreds
of centimeters long
 Sarcoplasm has numerous glycosomes (granules that
store glycogen) and a unique oxygen-binding protein
called myoglobin (similar to hemoglobin)
Skeletal Muscle organization
 In order of decreasing size…
 Myofiber - entire cell.
 Myofibrils - bundles of myofilaments inside myofiber.
 Myofilaments - actin and myosin proteins.
Myofibrils
 Myofibrils are densely packed contractile elements
 They make up most of the muscle volume
Sarcolemma
Mitochondrion
Myofibril
Dark A band Light I band Nucleus
(b) Diagram of part of a muscle fiber showing the myofibrils. One
myofibril is extended afrom the cut end of the fiber.
Sarcomeres (within myofibril)
 The smallest contractile unit of a muscle
 The region of a myofibril between two successive Z
discs
 Composed of myofilaments made up of contractile
proteins – actin and myosin
Sliding Filament Model of Contraction
 Thin filaments slide past the thick ones so that the
actin and myosin filaments overlap to a greater
degree
 In the relaxed state, thin and thick filaments overlap
only slightly
 Upon stimulation, myosin heads bind to actin and
sliding begins
When muscle contracts
the actin filaments slide
into the A/H band
overlapping the myosin

http://www.brooklyn.cuny.edu/bc/ahp/LAD/C4b/C4b_muscle.html
Events at the Neuromuscular Junction
 Skeletal muscles are stimulated by somatic motor
neurons
 Axons of motor neurons travel from the central nervous
system via nerves to skeletal muscles
 Each axon forms several branches as it enters a muscle
 Each axon ending forms a neuromuscular junction with
a single muscle fiber
Events in Generation of muscle contraction
1. Local depolarization (end plate potential):
 ACh binding opens chemically (ligand) gated ion
channels
 Simultaneous diffusion of Na+ (inward) and K+
(outward)
 More Na+ diffuses, so the interior of the
sarcolemma becomes less negative
 Local depolarization – end plate potential
Excitation-Contraction (E-C) Coupling
 Sequence of events by which transmission of an AP along
the sarcolemma leads to sliding of the myofilaments
 Latent period:
 Time when E-C coupling events occur
 Time between AP initiation and the beginning of
contraction
 Voltage-sensitive proteins stimulate Ca2+ release from SR
 Ca2+ is necessary for contraction
AP FLIX - Excitation-Contraction (E-C) Coupling
Muscle Twitch
 Response of a muscle to a single, brief threshold
stimulus
 Three phases of a twitch:
 Latent period: events of excitation-contraction
coupling
 Period of contraction: cross bridge formation;
tension increases
 Period of relaxation: Ca2+ reentry into the SR;
tension declines to zero
Tension
Maximum tension development
Resting
phase Stimulus
Contraction
phase
Relaxation
phase
Time (msec)
The latent period
begins at stimulation
and typically lasts about
2 msec. During this
period, an action
potential sweeps across
the sarcolemma, and the
sarcoplasmic reticulum
releases calcium ions.
The muscle fiber does
not produce tension
during the latent period,
because the contraction
cycle has yet to begin.
In the contraction
phase, tension rises to
a peak. As the tension
rises, calcium ions are
binding to troponin,
active sites on thin
filaments are being
exposed, and
cross-bridge
interactions are
occurring.
The relaxation phase
lasts about 25 msec.
During this period, calcium
levels are falling, active
sites are being covered by
tropomyosin, and the
number of active
cross-bridges is declining
as they detach. As a result,
tension returns to resting
levels.
Figure 9.6 3
Motor Unit: The Nerve-Muscle Functional Unit
 Motor unit = a motor neuron and all (four to several
hundred) muscle fibers it supplies
 Small motor units in muscles that control fine
movements (fingers, eyes)
 Large motor units in large weight-bearing muscles
(thighs, hips)
 Muscle fibers from a motor unit are spread
throughout the muscle so that a single motor unit
causes weak contraction of entire muscle
Force of Muscle Contraction
 The force of contraction is affected by:
 Length-tension relationship — muscles contract most
strongly when muscle fibers at relaxation are at 80–
120% of their normal resting length
 Frequency of stimulation —  frequency allows time
for more effective transfer of tension to
noncontractile components
 Number of muscle fibers stimulated (recruitment)
 Relative size of the fibers — hypertrophy of cells
increases strength
Length-tension relationship
 The force of muscle contraction depends on the length
of the sarcomeres before the contraction begins
 On the molecular level, the length reflects the
overlapping between thin and thick filaments
 The tension a muscle fiber can generate is directly
proportional to the number of crossbridges formed
between the filament
Response to Change in Stimulus Frequency
 A single stimulus results in a single contractile
response — a muscle twitch
Single stimulus
single twitch
Contraction
Relaxation
Stimulus
A single stimulus is delivered. The muscle
contracts and relaxes
Response to Change in Stimulus Frequency
 Increase frequency of stimulus (muscle does not have
time to completely relax between stimuli)
 Ca2+ release stimulates further contraction  temporal
(wave) summation
 Further increase in stimulus frequency  unfused
(incomplete) tetanus
 If stimuli are given quickly enough, fused (complete)
tetany results
 Rarely happens in the body – mostly in lab conditions
Response to Change in Stimulus Strength
 Threshold stimulus: stimulus strength at which the
first observable muscle contraction occurs
 Muscle contracts stronger as stimulus strength is
increased above threshold
 Contraction force is controlled by recruitment
(multiple motor unit summation), which brings more
and more muscle fibers into action
Response to Change in Stimulus Strength
 Size principle: motor units with larger and larger fibers
(cells) are recruited as stimulus intensity increases
Motor
unit 1
Recruited
(small
fibers)
Motor
unit 2
recruited
(medium
fibers)
Motor
unit 3
recruited
(large
fibers)
Contraction does not always shorten a muscle

Isotonic contraction: muscle shortens because
muscle tension exceeds the load

Isometric contraction: no shortening; muscle
tension increases but does not exceed the load
Type of muscle contraction - Isotonic Contractions
 Muscle changes in length and moves the load
 Isotonic contractions are either concentric or
eccentric:
 Concentric contractions — the muscle shortens
and does work
 For example, concentric contraction is used to
lift a glass from a table
 Eccentric contractions — the muscle contracts as it
lengthens
 Example – someone pulls your arm straight
while at the same time you try to keep the arm
locked in one position
Type of muscle contraction - Isometric Contractions
• The load is greater than the tension the muscle is able to
develop
• Tension increases to the muscle’s capacity, but the muscle
neither shortens nor lengthens
Muscles need energy to contract

ATP is the only source used directly for contractile activities

Muscles store few high-energy molecules

ATP - Available stores of ATP are used in 4–6 seconds

Creatine phosphate (CP)

Most energy stored as glycogen

May account for 1.5% of total muscle weight

Enables extended periods of muscle contractions
Figure 9.9 2
ATP production in muscles – 3 sources
 Glycolysis (anaerobic: does not require oxygen)

Occurs in sarcoplasm

Produces 2 ATP and 2 pyruvate molecules for each glucose
 Aerobic metabolism

Provides 95% of ATP demands of resting muscle cell

Occurs in mitochondria


Primarily through electron transport chain activity
Creatine phosphate (CP)

Creatine assembled from amino acids

Facilitates regeneration of ATP

ADP + CP  ATP + C
Muscle Metabolism: Energy for Contraction
ATP demand and production at different activity levels
 At rest
 Demand for ATP is low
 Surplus ATP produced by mitochondria (aerobic)
 Used to build up CP and glycogen reserves
 At moderate activity levels
 Demand for ATP increases
 ATP production by mitochondria (aerobic metabolism)
meets demand
ATP demand and production at different activity levels
 At peak activity levels
 Mitochondria can provide only ~1/3 ATP demand
 Glycolysis provides most ATP (anaerobic pathways
due to low oxygen)
 Excess pyruvate converts to lactic acid
 Diffuses into the bloodstream
 Used as fuel by the liver, kidneys, and heart
 Converted back into pyruvic acid by the liver
 Decreases intracellular pH
 Can affect enzymatic activities and cause fatigue
Muscle Fatigue
 Physiological inability to contract or sustain the expected
power output
 It is a reversible condition
 Fatigue can potentially occur at any of the points involve
in muscle contraction – from the brain to the muscle
fibers or in any of the systems that are responsible to
supply oxygen to the muscles
 Total lack of ATP occurs rarely during states of
continuous contraction
Muscle Fatigue
 Central fatigue mechanisms – arise from the CNS
 Includes subjective feeling of tiredness and desire to cease
activity
 Suggested reasons include low pH, failure to produce
enough ACh
 Peripheral fatigue mechanisms – anywhere between the
neuromuscular junction and the muscle
 Lack of glycogen
 Ionic imbalances (K+, Ca2+, Pi) interfere with E-C coupling
Muscle Fatigue
 An endurance exercise training can delay onset of
fatigue by increasing oxidative capacity:
 Increased number of mitochondria
 Increased level of oxidative enzymes
 Increased number of capillary beds to muscle
Muscle Performance
 Power
 The maximum amount of tension produced
 Endurance
 The amount of time an activity can be sustained
 Power and endurance depend on
 The types of muscle fibers
 Physical conditioning
Muscle Fiber Types
 Muscle fiber type is defined by 2 criteria
 Speed of contraction – determined by speed in which
ATPases split ATP
 The two types of fibers are slow and fast
 ATP-forming pathways
 Oxidative fibers – use aerobic pathways
 Glycolytic fibers – use anaerobic glycolysis
Muscle Fiber Type: Functional Characteristics
 These two criteria define three categories
 Slow oxidative fibers contract slowly, have slow acting
ATPases, and are fatigue resistant (contain myoglobin)
 Fast oxidative fibers contract quickly, have fast ATPases, and
have moderate resistance to fatigue
 Fast glycolytic fibers contract quickly, have fast ATPases, and
are easily fatigued (large glycogen reserve, few mitochondria)
Terms of muscle diameter changes
 Hypertrophy – increased total mass of muscle
 Result of increased number of the filaments in
each fiber
 The enzyme systems also increase (mainly
enzymes for glycolysis)
 If the muscles are not used for many weeks, the rate
of decay of contractile units is more rapid results in
atrophy (decrease in mass)
Adjustment of muscle length
 It is a type of hypertrophy in which muscles are
stretched to greater than normal length
 That causes the addition of new sarcomeres at the end
of the fibers
Anaerobic endurance
 Refers to the length of time muscle contraction can continued to be
supported by glycolysis and existing energy reserve of ATP and CP
 It is limited by:
 Amount of ATP and CP
 Amount of glycogen
 Ability of the muscle to tolerate lactic acid
 The use of resistance to muscular contraction to build the strength
and size of skeletal muscles.
 Muscle fatigue occurs within 2 minutes of start of maximal activity
Effects of Resistance Exercise (anaerobic)
• Resistance exercise results in:
• Muscle hypertrophy (due to increase in fiber size)
• Increased mitochondria, myofilaments, glycogen
stores, and connective tissue
• Examples – weight lifting, Machines that offer
resistance
Aerobic exercise
 Aerobic exercise is physical exercise that intends to improve the
oxygen system
 Leads to increased:
 Muscle capillaries
 Number of mitochondria
 Myoglobin synthesis
 Results in greater endurance, strength, and resistance to fatigue
 May convert fast glycolytic fibers into fast oxidative fibers
 Examples: running, swimming, cycling
Effects of Exercise - Aerobic exercise
 The length of time a muscle can continue to contract while
being supported by mitochondrial activities
 Among the recognized benefits of doing regular aerobic
exercise are:
 Strengthening the muscles involved in respiration, to
facilitate the flow of air in and out of the lungs
 Strengthening and enlarging the heart muscle, to
improve its pumping efficiency and reduce the resting
heart rate, known as aerobic conditioning
 Strengthening muscles throughout the body
 Improving circulation efficiency and reducing blood
pressure
Energy systems used in various sports
Energy system
Sport
Creatine phosphate, almost 100 meter sprints, Weight lifting, diving
entirely (anaerobic)
CP and glycogen-lactic acid 200 meter sprints, basketball
(anaerobic)
glycogen-lactic acid, mainly 400 meters sprints, 100 meter swim,
(anaerobic)
tennis
glycogen-lactic acid and
aerobic
800 meter sprints, boxing, 1 mile run
Aerobic system
10,000 meter skating, marathon