Chapter 2 Neuromuscular Fundamentals PPT Series 2C EXSC 314 Chapter 2 Neuromuscular Fundamentals PPT Series 2C

Neural Control of Voluntary Movement Overview: Muscle contraction results from stimulation by the central nervous system (CNS). Every muscle fiber is innervated by a somatic motor neuron, and when an stimulus is provided, results in muscle contraction.

Neural Control of Voluntary Movement The genesis/stimulus for skeletal muscle action is processed in varying degrees at different levels of the CNS The CNS may be divided into five levels of control Cerebral cortex Basal ganglia Cerebellum Brain stem Spinal cord

Neural Control of Voluntary Movement - Cerebral Cortex The cerebral cortex (word origin = bark) is the outer layer of the cerebrum and maintains the greatest level of control. Provides for the creation of voluntary movement as aggregate muscle action, but not as specific muscle activity Interprets sensory stimuli from the body, to a degree, to determine desired response

Neural Control of Voluntary Movement - Basal Ganglia Basal ganglia are a group of structures at the top of the midbrain Basal ganglia communicate with the cerebral cortex, thalamus, and brainstem, and other several areas. Associated with the control of voluntary movements Controls maintenance of postures and equilibrium, integrates balance, rhythmic movements. Controls procedural learning movements such as driving a car.

Neural Control of Voluntary Movement - Cerebellum Located in the posterior inferior brain. Function is to coordinate and regulate muscular activity. The Cerebellum does not initiate movement. The Cerebellum does play a major role in coordination, precision, and accurate timing of movement The Cerebellum integrates sensory impulses as it receives input from the spinal cord and from other parts of the brain. This integration allows the cerebellum to fine-tune motor activity by controlling timing and intensity of muscle activity. FYI – The Cerebral and cerebellar peduncles (stalk like connecting structures) communicate with other regions of the brain and CNS to integrate and coordinate movement

Neural Control of Voluntary Movement - Brain Stem Lower region of the CNS that “connects” the brain to the spinal cord. Includes midbrain, pons, and medulla. Transmits sensory neural impulses from the periphery and motor impulses to the periphery Reticular functions, integrating cardiorespiratory control, arousal and alertness, consciousness. Brain Stem

Neural Control of Voluntary Movement - Spinal Cord Is a cylindrical bundle of neural fibers that is the common pathway between CNS and peripheral nervous system (PNS) It extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column Integrates various simple and complex spinal reflexes There are different tracts within the cord. Ascending and tracts transmit different forms of sensory information, whereas descending tracts carry motor signals to the musculature.

Neural Control of Voluntary Movement - Peripheral Nervous System (PNS) Functionally, the PNS is divided into sensory and motor divisions Sensory or afferent nerves bring impulses from receptors in skin, joints, muscles, and other peripheral aspects of the body to the CNS Motor or efferent nerves carry impulses to outlying regions of the body from the CNS

Neural Control of Voluntary Movement - Spinal Nerves - Provide both motor and sensory function for their respective portions of the body - Named for the location from which they exit the vertebral column - From each of side of spinal column there is/are: 8 cervical nerves 12 thoracic nerves 5 lumbar nerves 5 sacral 1 coccygeal nerve

Neural Control of Voluntary Movement – Sensory Function Dermatome Map Sensory function ( one of the ascending tracts of the spinal cord relays this information) of spinal nerves is to provide information to the CNS regarding skin sensation Dermatome - Defined area of skin supplied by a specific spinal nerve

Neural Control of Voluntary Movement Myotome - a group of muscles innervated by a single spinal nerve root Myotomes are are clinically useful as they can determine if damage has occurred to the spinal cord, and at which level the damage has occurred Certain spinal nerves are also responsible for reflexes

Neural Control of Voluntary Movement - Neurons Basic functional units of the nervous system responsible for generating and transmitting impulses Consist of: A neuron cell body One or more branching projections known as dendrites, which transmit impulses to the neuron and cell body An axon, which is an elongated projection that transmits impulses away from neuron cell bodies

Neural Control of Voluntary Movement – Neuron Types Neurons are classified into three types according to the direction in which they transmit impulses Sensory neurons transmit impulses to the spinal cord and brain from all parts of the body Motor neurons transmit impulses away from the brain and spinal cord to muscle and glandular tissue Sensory neurons Motor neurons Interneurons Interneurons are central or connecting neurons that conduct impulses from sensory neurons to motor neurons

Proprioception and Kinesthesis Proprioception is an awareness of the position of the body and body segments. Activity performance is significantly dependent on proprioception which also incorporates the “senses” Example: Seeing when to lift our hand to catch a flying ball Proprioceptors Internal receptors located in skin, joints, muscles, and tendons that provide feedback relative to tension, length, and contraction state of muscle, position of body and limbs, and movements of joints

Proprioception and Kinesthesis Kinesthesis - Conscious awareness of the position and movement of the body in space (ie. is sensation or perception of motion) Proprioceptors work in combination with other sense organs to accomplish kinesthesis.

Proprioception and Kinesthesis Proprioceptors specific to joints and skin Meissner’s corpuscles Ruffini’s corpuscles Pacinian corpuscles Krause’s end-bulbs Proprioceptors specific to muscles Muscles spindles Golgi tendon organs (GTO)

Proprioception and Kinesthesis - Muscle Spindles Concentrated primarily in muscle belly between the fibers Sensitive to stretch and rate of stretch Insert into connective tissue within muscle and run parallel with muscle fibers Number of spindles varies depending upon level of control needed Greater concentration of muscle spindles in the hands than in the thighs Muscle Spindle

Proprioception and Kinesthesis - Muscle Spindles Muscle spindles and myotatic or stretch reflex Rapid muscle stretch occurs Impulse is sent to the CNS CNS activates motor neurons of the muscle and cause the spindle to contract

Proprioception and Kinesthesis - Muscle Spindles Example - Knee jerk or patella tendon reflex Reflex hammer strikes the patella tendon Causes a quick stretch of the musculotendinous unit of the quadriceps In response, the quadriceps fires and the knee extends The more forceful the tap, the greater the reflexive contraction

Proprioception and Kinesthesis - Muscle Spindles Stretch reflex may be utilized to facilitate a greater response Example - Quick, short squat before attempting a jump Quick stretch placed on muscles in the squat enables the same muscles to generate more force in subsequently jumping off the floor

Proprioception and Kinesthesis - Golgi Tendon Organ (GTO) Found serially in the tendon close to the muscle–tendon junction Sensitive to both muscle tension and active contraction Much less sensitive to stretch than muscle spindles are Requires a greater stretch to be activated

Proprioception and Kinesthesis - Golgi Tendon Organ (GTO) Tension in tendons and GTO increases as the muscle contracts, which activates GTO GTO stretch threshold is reached Impulse is sent to the CNS CNS causes the muscle to relax and facilitates activation of the antagonists as a protective mechanism GTO protects us from an excessive contraction by causing its muscle to relax

Proprioception and Kinesthesis – Pacinian, Ruffini's, and Meissner’s Corpuscles

Proprioception and Kinesthesis - Pacinian Corpuscles Concentrated around joint capsules, ligaments, tendon sheaths, and beneath the skin. External pressure causes the Pacinian Corpuscle to deform. Activated by rapid changes in the joint angle and by pressure changes affecting the capsule Activation lasts only briefly and is not effective in detecting constant pressure Helpful in providing feedback regarding the location of a body part in space following quick movements such as running or jumping

Proprioception and Kinesthesis - Ruffini’s Corpuscles Located in deep layers of the skin and the joint capsule Activated by strong and sudden joint movements and pressure changes Reaction to pressure changes are slower to develop. Activation is continued as long as pressure is maintained Essential in detecting even minute joint position changes and providing information as to the exact joint angle.

Proprioception and Kinesthesis - Meissner's Corpuscles Located in the skin and other subcutaneous tissues Important for receiving stimuli from touch Not as relevant to the discussion of kinesthesis

Muscle Tension Development - All or None Principle When a muscle contracts, contraction occurs at all fibers within a particular motor unit Motor unit - Consists of a single motor neuron and all muscle fibers it innervates and functions as a single unit

Muscle Tension Development - All or None Principle Typical muscle contraction The number of motor units responding, thus, the number of muscle fibers contracting within the muscle may vary significantly from relatively few to virtually all, depending on the number of muscle fibers within each activated motor unit and the number of motor units activated Regardless of number, individual muscle fibers within a given motor unit will either fire and contract maximally or not at all.

Factors Affecting Muscle Tension Development Difference between a muscle contracting to lift a minimal resistance versus lifting a maximal resistance is the number of muscle fibers recruited The number of muscle fibers recruited may be increased by: Activating those motor units containing a greater number of muscle fibers Activating more motor units Increasing the frequency of motor unit activation

Factors Affecting Muscle Tension Development Number of muscle fibers per motor unit varies significantly (Fine Control) From less than ten in muscles requiring a precise and detailed response such as the muscles of the eye (Strength Control) To as many as a few thousand in large muscles that perform less complex activities such as the quadriceps

Factors Affecting Muscle Tension Development Motor unit must first receive a stimulus via an electrical signal known as an action potential for the muscle fibers in the unit to contract Subthreshold stimulus Not strong enough to cause an action potential Does not result in a contraction Threshold stimulus Stimulus becomes strong enough to produce an action potential in a single motor unit axon All of the muscle fibers in the motor unit contract Submaximal stimulus Stimuli that are strong enough to produce action potentials in additional motor units Maximal stimulus Stimuli that are strong enough to produce action potentials in all motor units of a particular muscle

Factors Affecting Muscle Tension Development As stimulus strength increases from threshold up to maximal, more motor units are recruited, and the overall muscle contraction force increases in a graded fashion

Factors Affecting Muscle Tension Development Greater contraction forces may also be achieved by increasing the frequency or motor unit activation Phases of a single muscle fiber contraction or twitch Stimulus Latent period Contraction phase Relaxation phase

Factors Affecting Muscle Tension Development Latent period Brief period of a few milliseconds following a stimulus Contraction phase Muscle fiber begins shortening Lasts 25-40 milliseconds Relaxation phase Follows the contraction phase Last about 50 milliseconds

Factors Affecting Muscle Tension Development Summation When successive stimuli are provided before the relaxation phase of the first twitch is complete, the subsequent twitches combine with the first to produce a sustained contraction Generates a greater amount of tension than a single contraction would produce individually As the frequency of stimuli increase, the resultant (wave) summation increases accordingly producing increasingly greater total muscle tension Tetanus Occurs when the stimuli are provided at a frequency high enough that no relaxation can occur between contractions.

Factors Affecting Muscle Tension Development Treppe ( German for “stairs”) Occurs when a series of maximal stimuli delivered to the muscle at a frequency just below tetanizing frequency immediately following the previous stimulus. The tension with each twitch during each twitch increases until maximal height is reached and a plateau has formed. This gradual rise is believed to result form the gradual increase in Ca++ ion concentration in the sarcoplasm

Muscle Length–Tension Relationship Maximal ability of a muscle to develop tension and exert force varies depending upon the length of the muscle during contraction. Greatest amount of tension can be developed when a muscle is stretched between 100% and 130% of its resting length When a muscle is stretched beyond 100% to 130% of its resting length, amount of active tension generated significantly decreases

Muscle Length–Tension Relationship Generally, depending on muscle involved: A proportional decrease in ability to develop tension occurs as a muscle is shortened The ability to develop contractile tension is essentially reduced to zero when shortened to around 50% to 60% of resting length Example - Increasing the ability to exert force Squatting slightly to stretch the calf, hamstrings, and quadriceps before contracting the same muscles concentrically to jump

Muscle Length–Tension Relationship Example- Reducing the ability to exert force Isolating the gluteus maximus by maximally shortening the hamstrings with knee flexion

Muscle Force–Velocity Relationship When a muscle is contracting (concentrically or eccentrically) the rate (velocity) of muscle length change is related to the amount of force that can be produced by the muscle and the amount of resistance applied When contracting concentrically against a light resistance, a muscle is able to contract at a high velocity

Muscle Force–Velocity Relationship As resistance increases, the maximal velocity at which a muscle is able to contract decreases Eventually, as the load increases, the velocity decreases to zero resulting in an isometric contraction As the load increases beyond the muscle’s ability to maintain an isometric contraction, the muscle begins to lengthen resulting in an eccentric contraction

Muscle Force–Velocity Relationship Increases in the load results in the velocity of lengthening Eventually, the load may increase to a point where the muscle can no longer resist, resulting in an uncontrollable lengthening or dropping of the load Inverse relationship between concentric velocity and the amount of force production (ie. Lighter resistance = Higher velocity)

Muscle Force–Velocity Relationship As the force needed to cause the movement of an object increases, the velocity of concentric contraction decreases Somewhat proportional relationship between eccentric velocity and force production As force needed to control an object’s movement increases, the velocity of eccentric lengthening increases, at least until when control is lost

Stretch-Shortening Cycle Sequencing and timing of contractions can enhance the total amount of force produced When a muscle is suddenly stretched, resulting in an eccentric contraction that is followed by a concentric contraction of the same muscle, the total force generated in the concentric contraction is greater than that of an isolated concentric contraction Functions by integration of the GTO and the muscle spindle (see previous slides)

Stretch-Shortening Cycle Elastic energy is stored during the eccentric stretch phase, transitioned, and utilized in the concentric contraction phase A stretch reflex is elicited in the eccentric phase of the motion, which subsequently increases activation of the muscle that was stretched, resulting in a more forceful concentric contraction To be effective, the transition phase must be immediate. The shorter the transition phase, the more effective the force production

Reciprocal Inhibition or Innervation Antagonist muscle groups must relax and lengthen when the agonist muscle group contracts This reciprocal innervation effect occurs through reciprocal inhibition of the antagonists Activation of the motor units of the agonists causes a reciprocal neural inhibition of the motor units of the antagonists This reduction in neural activity of the antagonists allows them to subsequently lengthen under less tension

Reciprocal Inhibition or Innervation Example - Compare the ease of: Stretching hamstrings when simultaneously contracting the quadriceps Stretching hamstrings without contracting the quadriceps

Angle of Pull Angle between the line of pull of the muscle and the bone on which it inserts (angle toward the joint) With every degree of joint motion, the angle of pull changes Joint movements and insertion angles involve mostly small angles of pull

Angle of Pull Angle of pull decreases as the bone moves away from its anatomical position through the local muscle group’s contraction Range of movement depends on the type of joint and bony structure Most muscles work at angles of pull less than 50 degrees Amount of muscular force needed to cause joint movement is affected by the angle of pull

Angle of Pull Rotary component (vertical component) - Component of muscular force that acts perpendicular to the long axis of the bone (lever) When the line of muscular force is at 90 degrees to the bone on which it attaches, all of the muscular force is rotary force (100% of the force is contributing to movement) All of the force is being used to rotate the lever about its axis The closer the angle of pull to 90 degrees, the greater the rotary component

Angle of Pull At all other degrees of the angle of pull, one of the other two components (non-rotary) of force are operating in addition to the rotary component Rotary component continues with less force to rotate the lever about its axis Second force component is the horizontal or non- rotary component and is either a stabilizing component or a dislocating component, depending on whether the angle of pull is less than or greater than 90 degrees

Angle of Pull If the angle is less than 90 degrees, the force is a stabilizing force because its pull directs the bone toward the joint axis

Angle of Pull If angle is greater than 90 degrees, the force is dislocating because its pull directs the bone away from the joint axis

Angle of Pull Sometimes, it is desirable to begin with the angle of pull at 90 degrees Chin-up (pull-up) Angle makes the chin-up easier because of the more advantageous angle of pull Compensate for the lack of sufficient strength

Uniarticular, Biarticular, and Multiarticular Muscles Uniarticular muscles Cross and act directly only on the joint that they cross Example - Brachialis, which can only pull the humerus and ulna closer together Biarticular muscles - Cross and act on two different joints Depending on certain factors, biarticular muscles may contract and cause motion at either one or both of its joints Two advantages over uniarticular muscles Can cause and/or control motion at more than one joint Are able to maintain a relatively constant length due to shortening at one joint and lengthening at another joint

Uniarticular, Biarticular, and Multiarticular Muscles Muscle does not actually shorten at one joint and lengthen at the other Concentric shortening of the muscle to move one joint is offset by the motion of the other joint, which moves its attachment of the muscle farther away This maintenance of a relatively constant length results in allows the muscle a continuous exertion of force (examples follow)

Uniarticular, Biarticular, and Multiarticular Muscles Example 1 - Hip and knee biarticular muscles Concurrent movement pattern occurs when both the knee and hip extend at the same time If only the knee were to extend, the rectus femoris would shorten, and its ability to exert force similar to the other quadriceps muscles would decrease, However, relative length of the rectus femoris and subsequent force production capability are maintained due to its relative lengthening at the hip joint during extension of the hip with the knee

Uniarticular, Biarticular, and Multiarticular Muscles Example 2 - Hip and knee biarticular muscles Countercurrent movement pattern occurs in kicking During the lower extremity forward movement phase, the rectus femoris concentrically contracts to flex the hip and extend the knee These two movements, when combined, increase the tension or stretch on the hamstring muscles both at the knee and hip

Uniarticular, Biarticular, and Multiarticular Muscles Multiarticular muscles act on three or more joints due to the line of pull between their origin and insertion crossing multiple joints Principles relative to biarticular muscles apply to multiarticular muscles; however, multiarticular muscles can reduce work load of muscles with less articulations

Active and Passive Insufficiency As a muscle shortens, its ability to exert force diminishes Active insufficiency is reached when the muscle becomes shortened to the point that it cannot generate or maintain active tension The muscle cannot shorten any further Passive insufficiency is reached when the opposing muscle becomes stretched to the point where it can no longer lengthen and allow movement

Active and Passive Insufficiency Easily observed in either biarticular or multiarticular muscles when the full range of motion is attempted in all the joints crossed by the muscle Example - Rectus femoris contracts concentrically to both flex the hip and extend the knee Can completely perform either action one at a time but actively insufficient to obtain the full range at both joints simultaneously (countercurrent movement pattern)

Active and Passive Insufficiency Similarly, hamstrings cannot usually stretch enough to allow both maximal hip flexion and maximal knee extension because of passive insufficiency As a result, it is virtually impossible to actively extend the knee fully when beginning with the hip fully flexed or vice versa END OF PPT SERIES 2C