Table of Contents
Overview
Muscles transform chemical energy into mechanical work and, together with the skeleton and nervous system, generate movement. In this chapter, the focus is on how muscles are organized at the tissue and organ level and how their activity results in movement of body parts or whole organisms. Cell-level aspects of muscle contraction (such as ion channels and action potentials) are handled in the parent sections on excitation and the function of muscle cells; here we concentrate on the integration into movement.
Types of Muscle and Their Functional Roles
In animals, especially vertebrates, muscles are grouped into three main types. For movement in the everyday sense (walking, flying, swimming, grasping), skeletal muscle is the key player, but the other types influence movement indirectly.
Skeletal Muscle
- Structure: Attached to bones (or exoskeleton elements) by tendons.
- Control: Voluntary (somatic nervous system).
- Function: Produces rapid, often powerful contractions; responsible for posture and all conscious body movements.
Functional subtypes (not going into cellular details, but into movement roles):
- Postural (tonic) muscles
- Maintain body position against gravity (e.g., back muscles).
- Often active for long periods with relatively low force.
- Important for static movement: standing, holding a pose.
- Phasic (movement) muscles
- Generate fast, strong contractions.
- Used for dynamic movement: jumping, sprinting, rapid eye movements.
- Fast vs. slow muscle groups (at organ level)
- Some muscles are compositionally enriched in “fast” or “slow” fibers.
- Fast muscles: e.g., some eye muscles, wing muscles of flying animals — rapid changes, fine control.
- Slow muscles: e.g., antigravity muscles in the legs — endurance, posture.
Cardiac Muscle
- Forms the heart, responsible for the circulation of blood.
- Involuntary and rhythmically active.
- Indirectly crucial for movement by supplying oxygen and nutrients to working skeletal muscles and removing waste products.
Smooth Muscle
- Found in walls of hollow organs (intestine, bladder, blood vessels, airways).
- Involuntary.
- Influences movement by:
- Controlling blood flow to muscles (vasodilation, vasoconstriction).
- Regulating airway diameter, affecting breathing mechanics.
- Producing peristaltic movements in the gut that move food (internal “movement”).
From Muscle to Movement: The Motor System
Movement depends not only on the muscle tissue itself but also on its arrangement and attachment to supporting structures.
Muscles, Tendons, and Skeleton
Most macroscopic movements in animals rely on muscle–skeleton systems:
- Muscles generate force by shortening.
- Tendons transmit this force to bones or exoskeletal elements.
- Joints provide pivot points, converting linear muscle shortening into rotational or translational movement.
Two or more muscles often work in coordinated groups:
- Agonist (prime mover): main muscle responsible for a particular movement.
- Antagonist: muscle that produces the opposite movement.
- Synergists: assist the agonist or help stabilize joints.
Example: At the human elbow
- Biceps brachii: agonist for forearm flexion.
- Triceps brachii: antagonist, extends the forearm.
- Synergists: help stabilize the shoulder and elbow during the movement.
This arrangement allows:
- Precise control of movement amplitude and speed (by balancing agonist and antagonist).
- Joint stabilization (co-contraction of both sides).
Levers and Mechanical Advantage
Bones often act as levers, and joints as fulcrums. The way muscles attach determines:
- Force advantage: ability to generate large forces over a short distance.
- Speed advantage: ability to produce large, fast displacements but with reduced force.
Three conceptual lever types exist; in the body, many joints function as third-class levers:
- Fulcrum at one end, load at the other, effort (muscle attachment) between them.
- Example: Human forearm when lifting a weight.
- Consequence: High speed and range of motion of the hand, but muscles must generate forces much greater than the load.
Other lever arrangements:
- Second-class levers (rare in humans, common in some animal limbs) favor force at the expense of speed.
- Lever design reflects evolutionary trade-offs between power, speed, endurance, and control.
Muscles and Body Support
Movement occurs relative to a support structure:
- Hydrostatic skeleton: In many invertebrates (e.g., earthworms, some cnidarians).
- Fluid-filled cavity enclosed by muscles.
- Muscles change the shape of the fluid-filled compartment (volume remains constant), resulting in movement.
- Exoskeleton: In arthropods (insects, crustaceans, spiders).
- Rigid outer skeleton; muscles attach to its inner surface.
- Joints in the exoskeleton permit movement.
- Endoskeleton: In vertebrates and some invertebrates.
- Internal skeleton with muscles attached externally.
- Usually allows larger body sizes and more complex motion patterns.
The interplay between type of skeleton and muscle arrangement strongly shapes an animal’s movement capabilities (burrowing vs. flying, swimming vs. running).
Motor Units and Coordination of Force
A motor unit consists of a motor neuron and all the muscle fibers it innervates (within a single muscle). The organization of motor units explains how muscles generate graded force and precise movements at the organ level.
Fine vs. Gross Movements
- Small motor units (few fibers per neuron):
- Found in muscles requiring precise control (eye muscles, fingers).
- Many units can be independently recruited for delicate adjustments.
- Large motor units (hundreds to thousands of fibers per neuron):
- Found in powerful, postural, or locomotor muscles (thigh, back).
- Less fine control, but stronger contraction when recruited.
Graded Force Production
Muscle force can be regulated by:
- Recruitment of motor units
- At low effort, only a few, often fatigue-resistant units are active.
- As demand increases, additional and usually larger, more powerful units are recruited.
- Frequency of activation
- Repeated action potentials from the motor neuron increase the tension produced by its fibers.
- Smooth, sustained contractions (useful for movement) occur when activation frequencies and unit recruitment are well coordinated.
At the behavioral level, this organization allows:
- Smooth initiation and cessation of movement.
- Modulation of strength and speed.
- Economical use of energy: only the necessary units are activated.
Reflexes and Automatic Movements
Higher chapters discuss sensory input and neural circuits in detail; here we focus on their motor implications.
Reflex Movements
Reflexes are rapid, automatic responses to specific stimuli, involving:
- A reflex arc (receptor → sensory neuron → CNS → motor neuron → effector muscle).
For movement:
- Stretch reflexes (e.g., knee-jerk reflex) help maintain muscle length and posture.
- Withdrawal reflexes protect from harmful stimuli (e.g., pulling the hand away from heat).
- These reflexes operate without conscious control, though they can be modulated by higher brain centers.
Central Pattern Generators (CPGs)
Many rhythmic movements are produced by central pattern generators in the spinal cord or brainstem:
- Neural networks that can generate rhythmic motor patterns without requiring sensory feedback for each cycle.
- Examples:
- Walking and running in vertebrates.
- Swimming in fish.
- Wing beating in many insects.
- Breathing movements.
CPGs provide:
- Basic rhythm and pattern of movement (e.g., left–right alternation of limbs).
- Flexibility: sensory input and higher brain commands can modify speed, timing, or switch from walking to running.
Thus, complex movement often arises from the combination of:
- Inherent rhythmicity (CPGs).
- Reflex adjustments (based on sensory feedback).
- Voluntary control (descending signals from the brain).
Whole-Body Movement Strategies
Different animals achieve locomotion in distinct physical environments (land, water, air). While the cellular mechanism of muscle contraction is similar, the arrangement of muscles and skeleton and the movement pattern differ.
Locomotion on Land
Main constraints: gravity, friction, and the need for support.
Walking and Running
- Involves cyclical movements of limbs.
- Typical pattern:
- Stance phase: limb on ground, supporting body weight and propelling forward.
- Swing phase: limb off ground, moving to next foothold.
- Gait patterns differ (walk, trot, gallop), changing:
- Number of limbs in contact with the ground.
- Stability and speed.
- Leg muscles act as:
- Motors (generating propulsive force).
- Brakes (controlling deceleration).
- Springs (storing and releasing elastic energy in tendons).
Jumping
- Concentrated, powerful contraction of limb extensor muscles.
- Often aided by elastic elements (tendons, specialized structures) that store energy and then release it rapidly.
- Seen in frogs, kangaroos, many insects.
Climbing
- Requires strong and coordinated control of limb and trunk muscles.
- Grasping and pulling movements.
- Often involves fine control of small muscles in hands, feet, claws, or adhesive pads.
Swimming
Movement in water must overcome drag; buoyancy partly counteracts gravity.
Undulatory Swimming
- Waves of muscle contraction pass along the body or tail.
- Seen in fish, eels, many aquatic invertebrates.
- Lateral or vertical bending of the body pushes against water, creating forward thrust.
- Muscle blocks (myomeres) are arranged along the trunk and contract in sequence.
Appendage-Based Swimming
- Crustaceans, some insects (e.g., water beetles), and many vertebrates (frogs, turtles) use limbs as paddles or flippers.
- Muscles move limbs to generate forward thrust and then reposition them with minimal resistance.
Flying
Flying demands high power output, rapid contraction cycles, and fine control.
- Wing muscles attach to the skeleton in ways that convert short muscle shortening into broad wing strokes.
- Two principal strategies in insects:
- Direct flight muscles: attached directly to wings; allow precise steering.
- Indirect flight muscles: deform the thorax to flap wings; can beat at extremely high frequencies.
- In birds and bats:
- Large pectoral muscles power the downstroke.
- Flight muscles are arranged to balance weight, power, and control.
Movement Coordination and Control
Muscles do not act alone; they are part of integrated sensorimotor circuits.
Feedback for Movement Accuracy
- Proprioceptors: sensory receptors in muscles, tendons, and joints that report body position and movement to the CNS.
- Their input is used to:
- Adjust ongoing movements (e.g., correcting balance if you stumble).
- Coordinate multiple joints (e.g., reaching and grasping).
The integration of:
- Feedforward control (planned, predicted motor commands).
- Feedback control (correction based on sensory input)
allows precise, smooth movements in changing environments.
Motor Learning
While details of learning and memory are covered elsewhere, muscle-related aspects include:
- Practice-dependent improvement of movement sequences (e.g., playing an instrument, sports).
- Changes in:
- Patterns of muscle activation (timing, recruitment).
- Distribution of work among muscle groups.
- Efficiency and energy use.
At the systems level, this leads to:
- More accurate, faster, more economical movement.
- The possibility of automating complex sequences, freeing attentional resources.
Energy Use and Fatigue in Movement
The biochemistry of ATP and metabolism is treated in other chapters; here we connect it briefly to movement.
Energy Demand of Muscular Work
- Active movement requires ATP for:
- Cross-bridge cycling in muscle fibers.
- Ion pumping (maintaining excitability).
- During light activity:
- Mainly aerobic energy supply (cellular respiration).
- During intense, brief activity:
- Additional anaerobic processes and stored energy reserves contribute.
Muscle Fatigue and Endurance
At the level of movements and behavior:
- Short, intense efforts can lead to rapid fatigue:
- Decline in maximum force and movement speed.
- Endurance activities rely on:
- Stable ATP supply, efficient oxygen delivery.
- Muscles and circulatory system adapted to prolonged activity.
Training and adaptation can:
- Increase endurance (e.g., enhanced blood supply to muscles).
- Improve strength and power (e.g., increased muscle mass and neural coordination).
Movement in Plants and Simple Organisms
Movement is not restricted to animals; other organisms also move, but often using different mechanisms.
Movement in Unicellular Organisms
- Many protists move using:
- Flagella: whip-like structures driven by internal protein machinery.
- Cilia: short, numerous appendages for swimming or moving particles.
- Pseudopodia: temporary extensions of the cell for crawling (amoeboid movement).
- These movements are based on cytoskeletal rearrangements, not on muscle.
Movement in Plants
Plants lack muscles but still show movement:
- Growth movements: directional growth toward light or gravity (tropisms).
- Turgor movements: rapid changes in cell water content cause leaves or leaflets to move (e.g., closing of mimosa leaves, Venus flytrap snap).
- These processes rely on:
- Ion transport and water movement.
- Changes in cell wall properties.
The common theme: force generation by structural change inside cells, but via mechanisms distinct from animal muscle contraction.
Summary
- Muscles convert chemical energy into mechanical work and, together with skeletal structures and the nervous system, enable movement.
- Skeletal muscles are primary effectors of voluntary movement; cardiac and smooth muscles support movement indirectly.
- Movement emerges from the arrangement of muscles on skeletons (or fluid-filled cavities), lever mechanics, and coordinated neural control via motor units, reflexes, and central pattern generators.
- Different locomotion strategies (walking, swimming, flying) reflect adaptations of muscle and skeletal systems to physical constraints of land, water, and air.
- Precise and efficient movement depends on feedback from sensory systems, motor learning, and appropriate energy supply.
- Even organisms without muscles, such as plants and many unicellular organisms, achieve movement through alternative cellular and structural mechanisms.