Control of body movement - The somato-motor division.

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


• 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


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

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


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




Voluntary 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


Tectospinal(medial pathways)




Rubrospinal(lateral pathway)

Red nucleus of midbrain



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




Voluntary 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


Tectospinal(medial pathways)




Rubrospinal(lateral pathway)

Red nucleus of midbrain


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

Control of body movement - The somato-motor division.

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Transcript of Control of body movement - The somato-motor division.