Muscle and Skeletal Systems
Chapter 50: Motor Systems & Mechanisms
Overview of Muscle Function
The physical interaction of protein filaments is required for muscle function.
Muscle activity is a response to input from the nervous system.
Muscle cell contraction relies on the interaction between:
Thin filaments (composed mainly of actin).
Thick filaments (staggered arrays of myosin).
Vertebrate Skeletal Muscle
Vertebrate skeletal muscle is responsible for moving bones and the body.
It is characterized by a hierarchical structure of smaller units:
A skeletal muscle consists of a bundle of long muscle fibers, with each fiber being a single cell extending along the muscle's length.
Each muscle fiber itself contains bundles of smaller myofibrils arranged longitudinally.
Skeletal muscle is also referred to as striated muscle due to the regular arrangement of myofilaments creating light and dark banding patterns.
The functional unit of a muscle is called a sarcomere, bordered by Z lines where thin filaments attach.
Terms/Concepts:
Muscle Bundle: Comprised of muscle fibers.
Single Muscle Fiber (cell): Single long cell.
Myofibril: Longitudinally arranged bundles of thinner filaments.
Z lines: Borders of sarcomeres.
Thick Filaments (myosin): Located at the M line in the sarcomere.
Thin Filaments (actin): Attached to Z lines.
The Sliding-Filament Model of Muscle Contraction
According to the sliding-filament model, thin and thick filaments ratchet past each other longitudinally, powered by myosin molecules.
Diagram Explanation:
Relaxed muscle: The sarcomere length is relaxed with Z lines far apart.
Contracting muscle: The sarcomere shortens as thin filaments are pulled toward the center.
Fully contracted muscle: Z lines come closer together, signifying maximum contraction.
Each myosin has:
A long tail region.
A globular head region that binds to an actin filament forming a cross-bridge, pulling the thin filament.
Muscle contraction relies on repeated cycles of binding and release between myosin heads and actin filaments.
Energy for Muscle Contraction
Glycolysis and aerobic respiration generate ATP needed to sustain muscle contraction.
During intense activity, oxygen becomes limiting; ATP is generated by lactic acid fermentation, which produces less ATP per glucose than glycolysis and can sustain contraction for approximately 1 minute.
Diagram Explanation:
Shows myosin-head configurations during contraction and ATP binding/release process.
Role of Calcium and Regulatory Proteins
Regulatory Protein: Tropomyosin and the troponin complex bind to actin strands on thin filaments in a resting muscle fiber, preventing actin and myosin interactions.
For contraction to occur, myosin-binding sites must be exposed, achieved when calcium ions (Ca^{2+}) bind to the troponin complex.
High calcium concentration: Promotes muscle fiber contraction.
Low calcium concentration: Ends contraction as regulatory proteins return to resting positions.
Diagram Explanation: Shows tropomyosin blocking and exposing myosin-binding sites.
Excitation-Contraction Coupling
The stimulus leading to muscle fiber contraction is an action potential in a motor neuron, which synapses with the muscle fiber.
Motor neuron action leads to the release of acetylcholine (ACh) at the neuromuscular junction.
ACh depolarizes the muscle, leading to its own action potential.
This action potential travels along transverse (T) tubules, prompting the release of Ca^{2+} from the sarcoplasmic reticulum (SR).
Released Ca^{2+} binds to the troponin complex, initiating muscle fiber contraction.
Muscle Relaxation
When motor neuron input ceases, the muscle cell relaxes.
SR transport proteins pump Ca^{2+} out of the cytosol, leading to repositioning of regulatory proteins on thin filaments.
Neuromuscular Disorders
Amyotrophic Lateral Sclerosis (ALS): Disease that progressively interferes with muscle fiber excitation, typically leading to fatality within five years of symptom onset.
Myasthenia Gravis: An autoimmune disorder attacking acetylcholine receptors, manageable through drugs that inhibit acetylcholinesterase or suppress the immune system.
Nervous Control of Muscle Tension
Muscle contraction can be graded, allowing voluntary adjustments in extent and strength.
Mechanisms for graded contractions:
Varying the number of contracting fibers (recruitment).
Varying the rate of fiber stimulation.
Each motor neuron may synapse with multiple muscle fibers; however, each fiber is influenced by only one motor neuron.
Motor Unit: Comprises a single motor neuron and all the muscle fibers it controls.
Muscle Contraction Dynamics
Recruitment: Involvement of additional motor neurons increases muscle force.
Twitch: A single action potential in a motor neuron results in a single muscle twitch.
Rapidly delivered action potentials yield graded contractions through the phenomenon of summation.
Tetanus: A smooth, sustained contraction occurs when stimulation frequency is so high that muscle fibers cannot relax between stimuli.
Types of Skeletal Muscle Fibers
Skeletal muscles can be classified by ATP source or contraction speed:
Oxidative Fibers: Utilize aerobic respiration, possess numerous mitochondria, rich blood supply, and high myoglobin content.
Glycolytic Fibers: Primarily rely on glycolysis, have fewer mitochondria, larger diameters, and tire easily.
Light meat in poultry and fish consists of glycolytic fibers, while oxidative fibers comprise dark meat.
Fast-Twitch Fibers: Enable short, rapid, powerful contractions; can be either glycolytic or oxidative.
Slow-Twitch Fibers: Contract slowly but sustain longer; all are oxidative with slower Ca^{2+} pumping ability.
Muscle Fiber Composition
Most skeletal muscles contain both slow-twitch and fast-twitch fibers in varying ratios.
Example of muscle types with superior twitching capabilities:
Male toadfish's muscles can contract and relax over 200 times per second during mating calls.
Other Types of Muscle
Cardiac Muscle:
Found exclusively in the heart.
Composed of striated cells interconnected by intercalated disks.
Capable of generating action potentials independently of neural input.
Smooth Muscle:
Connective structure primarily in the walls of hollow organs (circulatory, digestive, reproductive).
Exhibits relatively slow contractions, initiated by the muscle itself or via autonomic neuron stimulation.
Lacks striations as actin and myosin are not regularly arranged; contraction regulated by Ca^{2+}.
Skeletal Systems and Locomotion
The skeletal system transfers muscle contractions into locomotion.
Skeletal muscles are attached in pairs, working antagonistically, and are coordinated by the nervous system.
Functions of skeletal systems include:
Support - Protection - Movement
Types of Skeletal Systems
Major types:
Exoskeletons: External hard parts; primarily seen in mollusks and arthropods.
Endoskeletons: Internal hard structures; present in diverse organisms from sponges to mammals.
Hydrostatic Skeletons: Fluid-filled compartments; common in cnidarians, flatworms, and annelids.
Hydrostatic Skeletons
Composed of fluid under pressure within a closed body cavity.
Primarily utilized in peristaltic movement in annelids via rhythmic muscle contractions.
Exoskeletons
Hard covering found on body surfaces.
Arthropods have a flexible, jointed exoskeleton called the cuticle, often made of chitin.
Endoskeletons
Consist of a hard skeleton encased in soft tissue.
Vertebrate endoskeletons are mainly composed of cartilage and bone.
A typical mammalian skeleton has over 200 bones, some of which are fused, others connected by ligaments at joints.
Key structures include: Skull, clavicle, scapula, ribcage, etc.
Categories of Bone Structure
Bones may be categorized based on density and structure:
Compact Bone: Dense outer layer.
Medullary Bone: Lines internal cavities; contains bone marrow in vertebrates.
Spongy Bone: Constitutes the epiphyses inside a compact bone shell.
Bone Remodeling
Bone is dynamic, capable of remodeling in response to mechanical stress during and after embryonic development.
Remodeling may lead to thickening or changes in size and shape of bone surface features.
Activities such as weight-lifting can stimulate bone remodeling, serving as a treatment for osteoporosis.
Types of Joints
Joints (Articulations): Locations where bones meet; classified into categories based on movement patterns:
Ball-and-socket Joints: Allow movement in all directions.
Hinge Joints: Permit movement in one plane.
Gliding Joints: Allow sliding between surfaces.
Combination Joints: Exhibit traits of multiple types.
Types of Locomotion
Animals actively travel using various locomotion forms, expending energy to counteract friction and gravity.
Land Locomotion:
Types include walking, running, hopping, crawling—requiring adaptations for weight-bearing.
Swimming:
Principal challenge is friction; animals adapt body shapes to minimize this, employing various swimming techniques.
Flying:
Active flight relies on wings providing enough lift to counteract gravity.
Many flying creatures possess adaptations that reduce overall body mass for effective flight.