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
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
Figure 12.9.1
• 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
Toes
Swallowing
Tongue
Jaw
Primary motorcortex(precentral gyrus)
MotorMotor map inprecentral gyrus
Posterior
Anterior
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
• 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
Cerebellar Function
The Cerebellum
Somatic Motor Pathways
• Three motor pathways• Corticospinal pathway
• Medial pathway
• Lateral pathway
Somatic motor system
Corticospinal
Corticobulbar Corticospinal
Lateral corticospinal
Anterior corticospinal
Medial pathway Lateral pathways
Vestibulospinal
Tectospinal
Reticulospinal
Rubrospinal
Body area Ascending pathway
Upper neuron location
Lower neuron location
Voluntary control
Skeletal muscles of body
Corticospinal Precentral gyrus Anterior gray horn
Skeletal muscles of head
Corticobulbar Precentral gyrus Nuclei of cranial nerves
Subconscious control
Reflex activity
Reticulospinal Brainstem nuclei (reticular formation)
Anterior gray horn
Equilibrium Vestibulospinal Nucleus of cranial nerve VIII
Anterior gray horn
Auditory and visual reflexes
Tectospinal Superior and inferior colliculi
Anterior gray horn of cervical area
Distal muscles of upper limbs
Rubrospinal Red nucleus of midbrain
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
Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Somatic Motor Pathways
Via basal ganglia
Initiation of Skilled MovementFrontal 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 cerebellumAllows 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.
Figure 13.17 3
A cross section of the spinal cord showing thelocations of the medial and lateral pathways
Anteriorcorticospinal
tract
Tectospinal tract
Vestibulospinal tract
Reticulospinal tract
Medial Pathway
Involved primarily with the control ofmuscle tone and gross movements ofthe neck, trunk, and proximal limbmuscles
Lateralcorticospinal
tract
Lateral Pathway
Involved primarily with the control of muscle tone and the more precise movementsof the distal parts of thelimbs
Rubrospinal tract
Somatic Motor Pathways – medial and lateral pathwaysInvoluntary pathways
Somatic motor system
Medial pathway
Vestibulospinal
Tectospinal
Reticulospinal
Figure 13.17 2
The locations of centers in the cerebrum,diencephalon, and brain stem that may issuesomatic motor commands as a result of processingperformed at a subconscious level
Nuclei of the Medial Pathway
Superior and inferior colliculi
Reticular formation
Vestibular nucleus
Motorcortex
Basalnuclei
Red nucleus
Medulla oblongata
Thalamus
Cerebellarnuclei
Body area Ascending pathway
Upper neuron location
Lower neuron location
Voluntary control
Skeletal muscles of body
Corticospinal Precentral gyrus Anterior gray horn
Skeletal muscles of head
Corticobulbar Precentral gyrus Nuclei of cranial nerves
Subconscious control
Reflex activity
Reticulospinal(medial pathways)
Brainstem nuclei (reticular formation)
Anterior gray horn
Equilibrium Vestibulospinal(medial pathways)
Nucleus of cranial nerve VIII
Anterior gray horn
Auditory and visual reflexes
Tectospinal(medial pathways)
Superior and inferior colliculi
Anterior gray horn of cervical area
Distal muscles of upper limbs
Rubrospinal(lateral pathway)
Red nucleus of midbrain
Anterior gray horn in cervical area
Somatic motor system
Lateral pathways
Rubrospinal
Somatic motor pathways - subconscious motor commandsLateral 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
Body area Ascending pathway
Upper neuron location
Lower neuron location
Voluntary control
Skeletal muscles of body
Corticospinal Precentral gyrus Anterior gray horn
Skeletal muscles of head
Corticobulbar Precentral gyrus Nuclei of cranial nerves
Subconscious control
Reflex activity
Reticulospinal(medial pathways)
Brainstem nuclei (reticular formation)
Anterior gray horn
Equilibrium Vestibulospinal(medial pathways)
Nucleus of cranial nerve VIII
Anterior gray horn
Auditory and visual reflexes
Tectospinal(medial pathways)
Superior and inferior colliculi
Anterior gray horn of cervical area
Distal muscles of upper limbs
Rubrospinal(lateral pathway)
Red nucleus of midbrain
Anterior gray horn in cervical area
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 connective tissue
surrounding each muscle fiber
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)
• In order of decreasing size…
– Myofiber - entire cell.
– Myofibrils - bundles of myofilaments inside myofiber.
– Myofilaments - actin and myosin proteins.
Skeletal Muscle organization
NucleusLight I bandDark A band
Sarcolemma
Mitochondrion
(b) Diagram of part of a muscle fiber showing the myofibrils. Onemyofibril is extended afrom the cut end of the fiber.
Myofibril
• Myofibrils are densely packed contractile elements • They make up most of the muscle volume
Myofibrils
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
http://www.brooklyn.cuny.edu/bc/ahp/LAD/C4b/C4b_muscle.html
When muscle contracts the actin filaments slide into the A/H band overlapping the myosin
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
Figure 9.6 3
Ten
sio
n
Maximum tension development
Restingphase Stimulus
Contractionphase
Relaxationphase
Time (msec)
The latent periodbegins at stimulationand typically lasts about2 msec. During thisperiod, an actionpotential sweeps acrossthe sarcolemma, and thesarcoplasmic reticulumreleases calcium ions.The muscle fiber doesnot produce tensionduring the latent period,because the contractioncycle has yet to begin.
In the contractionphase, tension rises toa peak. As the tensionrises, calcium ions arebinding to troponin, active sites on thinfilaments are beingexposed, andcross-bridgeinteractions areoccurring.
The relaxation phaselasts about 25 msec.During this period, calciumlevels are falling, activesites are being covered bytropomyosin, and thenumber of activecross-bridges is decliningas they detach. As a result,tension returns to restinglevels.
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
ContractionRelaxation
Stimulus
Single stimulus single twitch
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
Motorunit 1Recruited(smallfibers)
Motorunit 2recruited(mediumfibers)
Motorunit 3recruited(largefibers)
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 SportCreatine phosphate, almost entirely (anaerobic)
100 meter sprints, Weight lifting, diving
CP and glycogen-lactic acid (anaerobic)
200 meter sprints, basketball
glycogen-lactic acid, mainly (anaerobic)
400 meters sprints, 100 meter swim, tennis
glycogen-lactic acid and aerobic
800 meter sprints, boxing, 1 mile run
Aerobic system 10,000 meter skating, marathon