ELEC 811 Skeletal Muscle Anatomy and Function. Skeletal muscles act on bones to produce movement of the limb and to move (lift and carry) objects.

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1 ELEC 811 Skeletal Muscle Anatomy and Function The primary function of a skeletal muscle is to generate forces, by contracting; these forces allow us to move through and interact with our environment Skeletal muscles act on bones to produce movement of the limb and to move (lift and carry) objects. Image from: anphys/skel_mus.htm

2 Skeletal muscle anatomy A muscle is a bundle of individual muscle cells or fibres Each fibre is surrounded by connective tissue, called the endomysium

3 Skeletal muscle anatomy The fibres are organized in bundles called fascicles* Each fascicle is surrounded by connective tissue, called the perimysium * These are also called compartments.

4 Skeletal muscle anatomy A number of fascicles or compartments are bundled to form a skeletal muscle The muscle is surrounded by a connective tissue sheath called the epimysium

5 Skeletal muscle anatomy At the ends of the muscle, the endomysium, perimysium and epimysium merge to form the tendon, which connects the muscle to bone

6 Skeletal muscle anatomy gross anatomy There are 5 different shapes of muscles in the human body: circular, convergent, parallel, pennate and fusiform. We are primarily interested in pennate and fusiform muscles. In pennate muscles, the muscle fibres are at an angle to the tendons In fusiform muscles, the muscle fibres are in line with the tendons tendon tendon Unipennate and bipennate arrangement Fusiform arrangement

7 Agonist and antagonist muscles Muscles generate force by contracting to pull the tendon of insertion (usually further away from the centre of the body) closer to the tendon of origin (usually closer to the centre of the body. In order to move a limb in opposing directions, muscles are often organized in pairs called agonist-antagonist pairs. The biceps brachii contracts to move the forearm closer to the upper arm or flex the elbow. The triceps brachii contracts to move the forearm away from the upper arm or extend the elbow.

8 Motor control the brain Motor programming is done in the pre-motor cortex, supplementary motor area and other associated areas. The primary motor cortex receives information from the motor programming areas, the cerebellum and basal ganglia.

9 Areas of the brain The motor cortex is at the rear portion of the frontal lobe. area 6 the pre-motor cortex area 4 the primary motor cortex The cerebellum is the structure tucked under the back part of the brain. The cerebellum precisely regulates the sequence and duration of movement. The basal ganglia are located deep in the cerebrum. They function to initiate and regulate motor commands. See: The Brain from Top to Bottom: Body Movement and the Brain

10 Motor control the motor nerves Motor commands are relayed from the motor cortex, along the corticospinal tract to the muscles. The corticospinal tract is comprised of - motoneurons, which are the nerves which make contact with the muscle cells.

11 The motor unit (MU) Each -motoneuron connects with several muscle fibres a motoneuron and all muscle fibres innervated by the motoneuron are called a motor unit. -motoneuron with terminal branches n muscle fibres

12 Motoneuron and muscle cell characteristics Motoneurons vary in their morphology (size and shape) and electrophysiological characteristics (action potential conduction velocity). Muscle cells vary in their morphology and contractile characteristics contraction time and sensitivity to fatigue. The muscle cells within a motor unit will have uniform contractile characteristics. Muscle fatigue is a physiological process resulting in the decline of muscle output force or power over the time course of a sustained muscle contraction.

13 Motor unit types Three types of muscle fibre have been identified type I, IIa and IIb. Each motor unit will include only one type of muscle fibre. Motor Unit Type Mechanical Properties Electrical Properties Other Slow twitch or Type I Slow, small force twitch; smaller fibre diamter and MU size Slower nerve conduction velocity Recruited at low force levels Fast, fatigueresistant or Type IIa Fast force twitch Intermediate nerve conduction velocity Recruited at intermediate force levels Fast, fatiguing or Type IIb Fast, large force twitch; large fibre diameter and MU size Faster nerve conduction velocity Recruited at high force levels From: Merletti and Parker, Electromyography,

14 Motor unit contraction u(t) Activation :excitationcontraction coupling a(t) Muscle contraction dynamics F m (t) A neural excitation signal, u(t), is received from the - motoneuron. A muscle activation state, a(t), is generated via the excitationcontraction coupling mechanism this activates the muscle contractile process. The contractile mechanism in the muscle cells turns on and force is generated via muscle contraction dynamics. All muscle cells in the motor unit contract to develop an output force F m (t). Adapted from Zajac, 1989.

15 The force twitch Potential (mv) Fast twitch motor unit t(ms) Slow twitch motor unit Potential (mv) t(ms) -motoneuron potentials Force (mn) Force (mn) Motor unit force twitches t(ms) t(ms) EMD EMD An action potential in an -motoneuron results in contraction of all muscle fibres in the motor unit and generation of a force twitch. EMD = electromechanical delay. Adapted from: Merletti and Parker, 2004

16 Smooth force production When a motor unit is excited by its -motoneuron, it will produce a short duration force twitch In order to get longer, smooth force generation, the motor unit must be continuously excited. Excitation pulses Force t(ms) t(ms)

17 Modulating output force in a muscle The level of force generated by a muscle is modified in two ways: altering the number of active motor units recruitment and decruitment altering the rate of excitation impulses transmitted to a motor unit rate coding

18 References R. Merletti and P.A. Parker, Electromyography, IEEE Press, Piscataway, NJ, F. Zajac, Muscle and tendon: properties, models, scaling and application to biomechanics and motor control, CRC Crit. Rev. Biomed. Eng., 17(4): , The Brain from Top to Bottom: Body Movement and the Brain, Skeletal muscles, kel_mus.htm