Table of Contents
Overview: What Does “Coordination of Movements” Mean?
When animals (including humans) move, dozens or thousands of muscles, joints, and sensory signals must be controlled in a highly organized way. “Coordination of movements” means:
- Selecting which muscles should be active or inactive.
- Adjusting the strength and timing of muscle contractions.
- Integrating sensory information (e.g., balance, touch, vision) with motor commands.
- Linking simple movements into complex, purposeful patterns (walking, flying, grasping).
This chapter focuses on how nervous systems produce and coordinate movement patterns, not on muscle structure itself (covered elsewhere) or on high-level decision-making.
From Simple Reflexes to Complex Movement
Reflexes as Basic Building Blocks
Many movements are partly built from reflexes:
- Reflex: a fast, automatic response to a specific stimulus with a relatively fixed pathway.
- Example: stretch reflex in vertebrates – when a muscle is stretched, receptors signal the spinal cord, which sends a signal back to contract the same muscle.
Even simple reflexes already show basic coordination:
- They link sensory input (stretch) and motor output (muscle contraction).
- They can stabilize posture and joint position without conscious control.
Reflexes do not act in isolation. The nervous system can:
- Enhance them (e.g., when you expect a heavy load).
- Suppress them (e.g., voluntary relaxation at the doctor’s reflex test).
- Combine them into larger movement patterns.
Fixed Action Patterns and Motor Programs
Some movements follow a stereotyped, species-typical sequence once triggered:
- Fixed action pattern (FAP) (often used in behavioral biology): a relatively rigid sequence of behavior given a specific “releasing” stimulus.
- At the neural level, these often correspond to motor programs: internally stored patterns of neural activity that generate coordinated movement without needing continuous moment-to-moment sensory guidance.
Examples:
- Pecking and feeding behavior in some chick species.
- Cleaning or egg-rolling behavior in certain birds.
- Invertebrate feeding programs (e.g., rhythmic chewing in snails).
Motor programs:
- Can operate even in simplified nervous system preparations.
- Are often generated by specialized neural circuits (see “central pattern generators” below).
- Can be modified in detail by sensory feedback and learning, but the overall sequence is recognizable.
Central Pattern Generators (CPGs)
What Is a Central Pattern Generator?
A central pattern generator is a neural network that can produce rhythmic, patterned output (e.g., for walking, swimming, flying) without rhythmic sensory input.
Key features:
- Located in the central nervous system (spinal cord or brain regions, or segmental ganglia in invertebrates).
- Can generate rhythmic signals even if:
- Sensory feedback is reduced or eliminated.
- Higher brain centers are inactive.
- Output can still be adjusted by sensory input and higher control, but the rhythm is intrinsic to the network.
Typical functions:
- Swimming in fishes and amphibian larvae.
- Walking, running, scratching in vertebrates.
- Insect wing beating during flight.
- Rhythmic feeding and breathing movements.
How Do CPGs Generate Rhythms?
There are two main principles (often combined):
- Pacemaker neurons
- Individual neurons with intrinsic rhythmic activity (depolarize and fire action potentials in bursts, then repolarize).
- Their rhythmic firing drives other neurons in the network.
- Reciprocal inhibition
- Two or more groups of neurons inhibit each other.
- When group A is active, it inhibits B. Over time, A fatigues or adapts, its activity declines; B becomes active and inhibits A, and so on.
- This creates alternating patterns, such as:
- Flexor vs. extensor muscles around a joint.
- Left vs. right body sides during walking or swimming.
Example:
In a vertebrate spinal cord CPG for walking:
- One set of neurons controls flexor muscles (lifting the leg).
- Another set controls extensors (pushing the leg against the ground).
- Reciprocal inhibition ensures that flexors and extensors are not active at the same time, but alternate in a coordinated pattern.
Modulation of CPGs
CPGs are flexible rather than rigid clocks:
- Speed control: Neuromodulators and descending commands from the brain can speed up or slow down the rhythm (e.g., walk → trot → gallop).
- Pattern changes: The same CPG network can produce different gaits or movement patterns under different neuromodulatory states.
- Adaptation to environment: Sensory feedback (from joints, muscles, skin) fine-tunes the rhythm to maintain balance and adapt to uneven ground.
Thus, CPGs provide the basic rhythm and pattern, while sensory input and higher brain centers adjust their output to the current situation and behavioral goals.
Hierarchy in Motor Control
Movement coordination is usually organized in layers or levels.
Lower Level: Spinal Cord and Segmental Ganglia
In vertebrates, the spinal cord (and in many invertebrates, segmental ganglia) control:
- Basic reflex arcs (e.g., withdrawal from painful stimuli).
- Local pattern generators for rhythmic movements (walking, swimming, scratching).
- Simple integration between neighboring segments (front and hind legs; adjacent body segments).
These circuits allow many movements to continue without constant “orders” from the brain. For instance:
- A decerebrate cat (with higher brain centers disconnected under laboratory conditions) can still produce stepping movements when its spinal cord is stimulated and the limbs are placed on a moving treadmill.
This shows that much of the detailed timing of leg muscle activity is handled at these lower levels.
Intermediate Level: Brainstem and Cerebellum
The brainstem and cerebellum help coordinate posture, balance, and smooth execution:
- Brainstem:
- Controls basic muscle tone and posture.
- Houses vestibular centers that use balance information from the inner ear.
- Sends descending pathways to spinal cord networks, activating or biasing certain patterns (e.g., initiating locomotion).
- Cerebellum:
- Compares intended movement (from motor commands) with actual movement (from sensory feedback).
- Corrects timing and amplitude of movements.
- Essential for fine-tuning and learning of precise, coordinated movements (e.g., playing an instrument, accurate reaching, eye–hand coordination).
Damage to the cerebellum leads to:
- Inaccurate, overshooting movements (dysmetria).
- Difficulty timing and sequencing complex movements.
- Problems with balance and gait.
Higher Level: Motor Cortex and Other Forebrain Areas
In vertebrates with complex brains (especially mammals, birds):
- Motor cortex plans and initiates voluntary movements.
- It can:
- Select which CPGs to start or stop.
- Alter strengths and timing of signals to different muscles.
- Generate entirely new movement sequences through learning.
- Basal ganglia and related structures:
- Involved in action selection: deciding which movement program to run at a given time, suppressing irrelevant or competing actions.
- Dopamine and other neuromodulators strongly influence these circuits.
Thus, the forebrain doesn’t manage every contraction but rather chooses and shapes patterns that lower centers then execute in detail.
Coordination Among Multiple Limbs and Body Parts
Left–Right Coordination
Many movements involve symmetry or alternating patterns:
- Swimming undulations in fishes and tadpoles.
- Walking or running in tetrapods.
- Wing beating in flying insects and birds.
Neural networks ensure that:
- The left and right sides of the body are appropriately phased:
- Sometimes nearly synchronous (e.g., hopping).
- Sometimes alternating (left leg forward while right leg back).
- These relationships change with gait (walk, trot, gallop) or speed, often through adjustments in the coupling between left and right CPGs.
Front–Back and Limb–Body Coordination
Multiple limbs must be coordinated with body movements and posture:
- Timing of forelimb vs. hindlimb steps.
- Bending of the spine to extend stride length in running mammals.
- Coordination between limb movement and head/eye movements to stabilize gaze and track targets.
This involves:
- Interconnections between spinal segments and brainstem centers.
- Feedback from joints, muscles, and the vestibular system.
Coordination of Fine Movements
Some animals perform highly precise movements:
- Manipulation with hands or tentacles.
- Tongue projection in some amphibians.
- Beak and tongue coordination in birds.
These movements rely on:
- Higher density of motor neurons dedicated to small muscle groups.
- Precise cortical control and strong sensory feedback (touch, proprioception).
- Specialized neural circuits in the spinal cord and brainstem for hand or face regions.
Sensory Feedback and Proprioception
Proprioception: Sensing Body Position and Movement
Proprioceptors are sensory receptors that inform the nervous system about:
- Muscle length and rate of change (muscle spindles in vertebrates).
- Tendon tension (Golgi tendon organs).
- Joint angles and movement.
- In invertebrates: stretch receptors in muscles or body wall; specialized organs in limbs.
They provide:
- Continuous information for adjusting ongoing movements.
- Rapid reflex pathways to prevent overstretching or excessive force.
- Data for fine-tuning complex, learned actions.
Without proper proprioceptive input:
- Movements become clumsy and poorly targeted.
- The animal may struggle to maintain posture and balance, even if muscle strength is intact.
Role of Other Senses in Movement Coordination
Other senses also contribute:
- Vision:
- Guides reaching, pointing, grasping.
- Allows prediction of where moving objects will be.
- Helps maintain direction and orientation in space.
- Vestibular system (inner ear in vertebrates):
- Senses head position and acceleration.
- Triggers reflexes that stabilize eyes and posture (e.g., vestibulo-ocular reflex to keep gaze fixed when the head moves).
- Touch and pressure:
- Detect contact with surfaces or obstacles.
- Adjust grip force and foot placement.
Coordination of movements is therefore a continuous cycle:
- Motor commands cause movement.
- Movement changes sensory signals.
- New sensory feedback adjusts ongoing motor output.
This is often referred to as sensorimotor integration.
Coordination in Invertebrates vs. Vertebrates
Invertebrate Examples
Invertebrates, despite often having smaller nervous systems, show remarkably rich coordination:
- Insects:
- Walking uses a characteristic “tripod gait” in many species (three legs on the ground, three swinging).
- Local rules in each leg’s neural circuitry (segmental ganglia) determine when a leg should lift or touch down, based on load and position.
- Signals between neighboring ganglia coordinate legs to produce stable gaits and adapt to obstacles.
- Crustaceans:
- Coordinated beating of swimmerets or appendages for swimming, often controlled by segmental rhythm-generating circuits.
- Cephalopods (squid, octopus):
- Very complex motor coordination in tentacles and body shape.
- Large brain regions devoted to integrating sensory input and controlling flexible limbs.
These systems demonstrate that high-level, cortex-like structures are not strictly necessary for sophisticated movement coordination; distributed networks can achieve a great deal.
Vertebrate Examples
In vertebrates, the principles are similar but often embedded in larger, more layered structures:
- Fish and amphibian larvae:
- Spinal CPGs control swimming; rhythmic bending alternates left and right along the trunk.
- Reptiles, birds, mammals:
- Spinal CPGs plus brainstem and cerebellum coordinate walking, flying, and complex limb use.
- Mammalian cortex allows fine voluntary control and elaborate sequences (tool use, art, language-related gestures).
Learning and Adaptation of Movement Patterns
Coordination is not fixed from birth; many movements are refined through practice:
- Motor learning:
- Repetition strengthens and optimizes neural pathways.
- The cerebellum and motor cortex undergo structural and functional changes.
- Early, rough movements (e.g., in juvenile animals) become smoother, faster, and more efficient.
- Complex skills (song in birds, tool use in primates, human sports and musical performance) require prolonged learning and continuous adjustment.
Thus, coordination of movements combines:
- Innate components (reflexes, basic CPG structures, species-typical patterns).
- Experience-dependent refinement (learning, practice, adaptation to body growth and environmental demands).
Summary
- Coordination of movements is the organized control of muscles and body parts to achieve purposeful behavior.
- It relies on layered neural control:
- Local reflexes and CPGs at spinal or segmental levels.
- Brainstem and cerebellum for posture, balance, and fine-tuning.
- Higher brain centers for planning, selection, and voluntary control.
- Sensory feedback, especially proprioception, vision, and vestibular input, is essential for accurate, adaptable movement.
- In both invertebrates and vertebrates, movement patterns emerge from interacting neural networks rather than single control centers.
- Many motor patterns are partly innate but are shaped and improved through learning and experience.