Physiology of Behavior - Movement
Movement in Physiology
Overview of Topics
Skeletal Muscle: Covers the role and structure of skeletal muscles in movement.
Spinal Cord Reflexes: Discusses reflex actions in response to stimuli.
Brain Mechanisms: Explores brain areas involved in movement.
Complex Motor Behavior: Looks into advanced motor control mechanisms.
Deficits in Movement: Analyzes conditions that affect movement capacity.
Skeletal Muscle
Function of Skeletal Muscle:
Responsible for voluntary movement and physical actions in the body.
Attached to bones via tendons.
Muscles typically operate in opposing pairs:
Flexors: Muscles that bring limbs towards the body.
Extensors: Muscles that move limbs away from the body.
Noted that muscles contract, not flex.
Types of Muscle Fibers:
Extrafusal Muscle Fibers:
Main working fibers of muscles responsible for strength.
Contracted by alpha motor neurons.
Intrafusal Muscle Fibers:
Sensitive to the length of muscle (sensory function).
Contracted by gamma motor neurons.
Sensory Structures:
Golgi Tendon Organs:
Located in tendons, detecting tension on muscles.
Sensory Endings:
Free nerve endings and Pacinian corpuscles involved in proprioception.
Structure of Skeletal Muscle:
Composed of overlapping actin and myosin filaments.
This arrangement provides a striated appearance.
Organizational structure:
Bundle of extrafusal muscle fibers → Myofibril → Actin filament + Myosin filament.
Contraction Mechanism:
During muscle contraction, actin and myosin filaments:
Slide past each other through the action of cross bridges.
Neuromuscular Junction:
Motor Neuron Function:
Axons of motor neurons synapse on the motor endplates of muscles, releasing acetylcholine (ACh).
A single motor neuron may activate multiple muscle fibers (10s to 100s), forming a motor unit.
The strength of muscular contraction is determined by the firing rate of motor units.
Sensory Feedback:
Intrafusal fibers provide sensory feedback regarding muscle stretch, facilitating reflex actions.
Golgi tendon organs monitor tension in muscles contributing to reflexive actions (e.g., maintaining balance).
Spinal Cord Reflexes
Monosynaptic Stretch Reflex:
Controls and corrects limb movements when a muscle is quickly stretched.
Involves sensory afferents from intrafusal muscle that activate alpha motor neurons in the spinal cord.
Results in the contraction of the same muscle (e.g., maintaining posture).
Anatomical pathway involves:
One synapse—direct connection from sensory to motor neuron in the spinal cord.
Examples of Monosynaptic Stretch Reflex:
Gastrocnemius Muscle:
When stretched, muscle spindles fire, activating alpha motor neurons that contract the muscle.
Essential for maintaining upright posture and adjusting body position, such as leaning forward.
Polysynaptic Reflexes:
Involves Golgi Tendon Organs (GTO) sending signals to the spinal cord to inhibit motor neurons.
These reflexes respond to sudden tension by facilitating opposing muscle contraction.
Noted that GTOs may also inhibit motor neurons when excessive tension occurs.
Brain Mechanisms
Cortical Structures:
Primary Motor Cortex: Located in the precentral gyrus.
Responsible for the movement of specific parts of the body (contralateral control).
Multiple muscles contracted for a single movement.
Exhibits somatotopic organization—different areas control different body parts.
Motor Homunculus:
Illustrates the representation of body parts in the motor cortex, mapping out areas responsible for various muscle movements.
Motor Association Cortex:
Located anterior to the motor cortex, involved in planning movements.
Integrates inputs from visual and other sensory systems.
Involved in executing sequential movements and can be activated by observing motor actions.
Supplementary Motor Area (SMA):
Critical for linking learned behavioral sequences.
Example: Monkeys trained to complete tasks would stop mid-sequence if SMA was inhibited, indicating its role in sequential task execution.
Also applies to humans in performing series of movements (e.g., piano keys).
Premotor Cortex:
Engages in learning and executing sensory-cued movements.
Involved particularly with arbitrary stimuli (e.g., color cues).
Descending Pathways
Motor Pathway Groups:
Ventromedial Group:
Controls muscles in the trunk and proximal limbs.
Involves balance and coordinated movements; receives inputs from various systems (reticular formation, vestibular systems).
Lateral Group:
Controls independent movements of limbs (fingers, hands, face, tongue).
Inputs from the motor cortex enable precise limb control.
Detailed Pathway Functions:
Ventromedial Group:
Contains tracts for posture/balance (Vestibulospinal, Tectospinal, Reticulospinal).
Lateral Group:
Contains pathways for fine motor control and face/tongue movements (Corticospinal, Corticobulbar).
Cerebellum
Comprises 80% of brain neurons; vital for balance and timing of movements.
Structural Composition:
Two hemispheres, cortex, and deep nuclei (e.g. fastigial, dentate).
Medial zone controls trunk movements, lateral zone controls skilled movements.
Functionality:
The cerebellum processes information from movements, fine-tuning them before they reach the motor cortex for execution.
Basal Ganglia
Regulates voluntary movements, influencing motor control through cortical inputs and activity modulation.
Pathways:
Includes direct, indirect pathways for movement facilitation/inhibition. Structures like the substantia nigra influence these pathways via dopamine.
Diseases such as Parkinson's (lack of movement initiation) and Huntington's (uncontrolled movement) are linked to basal ganglia dysfunction.
Complex Motor Behavior
Mirror Neuron System:
Located in specific brain areas and involved in imitating movements by firing during observed actions.
Functional Impact:
Enhancing the capability to learn through observation, linking vision, and motor response.
Deficits in Movement
Apraxia (Dyspraxia):
Challenges in organizing familiar motor tasks; can be centered on drawing or tool use due to lesions.
Construction Apraxia: A specific type where individuals struggle with geometric relations, affecting drawing or constructing models, though proper use of objects is retained.
Movement in Physiology
Overview of Topics
Skeletal Muscle: Covers the role, structure, and cellular mechanisms of skeletal muscles in movement.
Spinal Cord Reflexes: Discusses involuntary reflex actions and their neural pathways.
Brain Mechanisms: Explores various brain areas and their hierarchical roles in motor control.
Complex Motor Behavior: Looks into advanced motor control mechanisms, including learning and imitation.
Deficits in Movement: Analyzes conditions that affect movement capacity due to neurological impairment.
Skeletal Muscle
Function of Skeletal Muscle:
Responsible for voluntary movement, locomotion, and maintaining posture.
Attached to bones via tough, fibrous connective tissues called tendons.
Muscles typically operate in opposing pairs or groups, known as antagonistic pairs:
Flexors: Muscles that decrease the angle of a joint, bringing limbs towards the body's midline (e.g., biceps contracting to bend the arm).
Extensors: Muscles that increase the angle of a joint, moving limbs away from the body (e.g., triceps contracting to straighten the arm).
It is important to note that muscles contract (shorten), rather than 'flex', to produce movement.
Types of Muscle Fibers:
Extrafusal Muscle Fibers:
These are the main contractile fibers of muscles, providing the force for movement.
Innervated and contracted by large myelinated axons of alpha motor neurons.
Intrafusal Muscle Fibers:
These are specialized sensory fibers, delicate and embedded within muscle spindles.
They are sensitive to the length and rate of change of length of the muscle, providing crucial proprioceptive feedback.
Contracted by smaller gamma motor neurons, which adjust the sensitivity of the muscle spindle.
Sensory Structures:
Muscle Spindles: Located within the muscle belly, these detect changes in muscle length and stretch velocity. They are critical for the stretch reflex.
Golgi Tendon Organs (GTOs): Encapsulated receptors located in the tendons, near the muscle-tendon junction. They detect and respond to changes in muscle tension, protecting muscles from excessive force.
Free Nerve Endings: Detect pain and temperature.
Pacinian Corpuscles: Detect rapidly adapting pressure and vibration, contributing to proprioception.
Structure of Skeletal Muscle:
Skeletal muscle fibers are composed of smaller units called myofibrils, which in turn contain contractile proteins actin (thin filaments) and myosin (thick filaments).
The organized, overlapping arrangement of actin and myosin filaments within sarcomeres gives skeletal muscle its characteristic striated appearance.
Organizational hierarchy: Whole Muscle → Muscle Fascicle (bundle of extrafusal fibers) → Muscle Fiber (cell) → Myofibril → Sarcomere (functional contractile unit) → Actin filament + Myosin filament.
Contraction Mechanism (Sliding Filament Theory):
During muscle contraction, the actin and myosin filaments do not shorten themselves but slide past each other.
This sliding action is mediated by myosin cross bridges (heads) binding to actin, pivoting, and detaching in a cycle. This process requires ATP for energy.
The release of calcium ions (Ca^{2+}) from the sarcoplasmic reticulum, triggered by an action potential, binds to troponin, which then causes tropomyosin to move, exposing myosin-binding sites on the actin filaments.
Neuromuscular Junction (NMJ):
Motor Neuron Function: The specialized synapse between an alpha motor neuron and a muscle fiber.
Axons of motor neurons release the neurotransmitter acetylcholine (ACh) into the synaptic cleft, which binds to nicotinic receptors on the muscle fiber's motor endplate.
This binding causes depolarization of the muscle fiber membrane (end-plate potential), leading to an action potential that propagates throughout the muscle fiber, initiating contraction.
A single motor neuron and all the muscle fibers it innervates constitute a motor unit. The number of muscle fibers per motor unit varies, from a few (for fine control like eye muscles) to hundreds (for powerful movements like leg muscles).
The strength of muscular contraction is regulated by two main mechanisms: recruitment (activating more motor units) and the firing rate of individual motor units.
Sensory Feedback:
Intrafusal fibers within muscle spindles provide continuous sensory feedback regarding muscle stretch and length, crucial for adjusting muscle activity and facilitating reflex actions.
Golgi tendon organs monitor the tension exerted by muscles on tendons. When excessive tension occurs, GTOs initiate a reflex that inhibits the motor neurons of the contracting muscle, preventing damage (autogenic inhibition).
Spinal Cord Reflexes
Monosynaptic Stretch Reflex (Myotatic Reflex):
This is the simplest reflex, involving only one synapse between a sensory neuron and a motor neuron in the spinal cord.
It controls and corrects limb movements in response to an unexpected or quick stretch of a muscle.
Mechanism: Stretch of the muscle (e.g., by tapping the patellar tendon) activates muscle spindles. Sensory afferent neurons (Type Ia fibers) from the spindles directly excite alpha motor neurons in the spinal cord.
This leads to the rapid contraction of the same stretched muscle (e.g., knee jerk reflex). Concurrently, interneurons are activated to inhibit antagonistic muscles (reciprocal inhibition), allowing for smooth movement.
Essential for maintaining upright posture, adjusting body position, and preventing falls.
Examples of Monosynaptic Stretch Reflex:
Patellar Reflex (Knee-Jerk): Tapping the patellar tendon stretches the quadriceps femoris muscle, activating its muscle spindles, leading to quadriceps contraction and leg extension.
Gastrocnemius Muscle Reflex: When the triceps surae (gastrocnemius and soleus) is stretched (e.g., by leaning forward), muscle spindles fire, activating alpha motor neurons that contract the muscle, helping to pull the body back and maintain balance.
Polysynaptic Reflexes:
These reflexes involve multiple synapses and typically one or more interneurons between the sensory and motor neurons.
Golgi Tendon Organ Reflex (Inverse Myotatic Reflex): Mediated by GTOs, this reflex responds to excessive muscle tension. When GTOs detect high tension, they send signals via Ib afferent fibers to the spinal cord. These fibers synapse with inhibitory interneurons, which in turn inhibit the alpha motor neurons of the contracting muscle, causing it to relax. This prevents muscle and tendon injury.
Flexor Withdrawal Reflex: A protective reflex in response to a painful stimulus (e.g., touching a hot object). Nociceptive afferents activate interneurons in the spinal cord, which then excite flexor motor neurons to withdraw the limb and inhibit extensor motor neurons. This is often accompanied by the Crossed Extensor Reflex in the contralateral limb, where extensors are activated to support the body's weight.
Brain Mechanisms
Cortical Structures:
Primary Motor Cortex (M1): Located in the precentral gyrus of the frontal lobe.
Directly involved in the execution of voluntary movements, controlling specific parts of the body, primarily influencing contralateral muscles.
It dictates the force, speed, and direction of movements.
A single movement often involves the coordinated contraction of multiple muscles, orchestrated by M1.
Exhibits somatotopic organization, meaning different areas of the M1 control different body parts, forming a distorted map known as the motor homunculus.
Motor Homunculus: A graphic representation illustrating the disproportionate allocation of cortical space to body parts requiring fine motor control (e.g., hands, face, tongue) compared to those requiring less precise control (e.g., trunk).
Motor Association Cortex: Located anterior to the primary motor cortex (in the frontal lobe, including premotor and supplementary motor areas) and includes parts of the parietal cortex.
Involved in the planning and initiation of movements, integrating sensory information to prepare for action.
Plays a crucial role in the selection of movements appropriate to the context and in the execution of sequential movements.
Can be activated by observing motor actions, suggesting its role in motor learning and understanding intentions.
Supplementary Motor Area (SMA): Located on the medial aspect of the frontal lobe, anterior to M1.
Critical for internally generated movements and the planning and execution of sequences of movements, especially learned behavioral sequences that are performed without external cues.
Example: Monkeys trained to complete complex tasks would stop mid-sequence if SMA was inhibited, indicating its role in the internal orchestration of ongoing movement chains (e.g., playing a piano piece from memory).
Premotor Cortex (PMC): Located on the lateral surface of the frontal lobe, anterior to M1.
Primarily involved in externally guided movements and learning and executing sensory-cued movements.
Engages when movements are made in response to arbitrary stimuli (e.g., moving a hand based on a color cue).
Also contains mirror neurons and plays a role in observational learning and preparing for action based on external visual or auditory signals.
Descending Pathways
These pathways originate in the cerebral cortex and brainstem, carrying motor commands to the spinal cord.
Motor Pathway Groups:
Ventromedial Group (Medial Pathways):
Controls muscles in the trunk and proximal (upper) limbs.
Primarily involved in maintaining balance, coordinating body movements, and posture.
Receives inputs from various systems, including the reticular formation and vestibular systems.
Detailed Pathway Functions:
Vestibulospinal Tracts: Originate in the vestibular nuclei (receiving input from the inner ear's balance organs); responsible for head and neck movements, balance, and posture in response to head position.
Tectospinal Tract: Originates in the superior colliculus (midbrain, involved in visual reflexes); responsible for reflexive movements of the head and neck in response to visual or auditory stimuli.
Reticulospinal Tracts: Originate in the reticular formation (brainstem); involved in coarse limb movements, posture, muscle tone, and locomotion. Also involved in modulating gamma motor neuron activity.
Lateral Group (Lateral Pathways):
Controls independent, precise movements of the distal limbs (fingers, hands, feet) and fine movements of the face and tongue.
Inputs from the motor cortex enable highly skilled and precise limb control.
Detailed Pathway Functions:
Corticospinal Tract (Pyramidal Tract): Originates predominantly in the primary motor cortex. The largest descending motor pathway.
Lateral Corticospinal Tract: Crosses in the medulla (pyramidal decussation) and descends contralaterally, controlling fine, voluntary movements of the distal limbs.
Anterior Corticospinal Tract: Descends ipsilaterally (with some fibers crossing in the spinal cord), controlling movements of the trunk and proximal limbs.
Corticobulbar Tract: Controls movements of the face, head, and neck. It projects from the motor cortex to motor nuclei of cranial nerves in the brainstem, which then innervate muscles for facial expressions, chewing, swallowing, and tongue movements.
Cerebellum
Comprises approximately 80% of the brain's neurons and is crucial for balance, coordination, motor learning, and accurate timing of movements.
Structural Composition:
Consists of two hemispheres, a highly folded cerebellar cortex, and deep cerebellar nuclei (e.g., fastigial, interposed, dentate).
Functionally divided into:
Medial zone (vermis and flocculonodular lobe): Controls trunk and proximal limb movements, balance, and eye movements (vestibulocerebellum/spinocerebellum).
Lateral zone (cerebrocerebellum): Controls skilled movements, especially of the distal limbs, and is involved in planning and motor learning.
Functionality:
The cerebellum acts as a 'movement comparator'. It receives vast amounts of sensory information about body position and ongoing movements (proprioception) as well as motor commands from the cerebral cortex.
It compares the intended movement with the actual movement feedback and then sends corrective signals back to the motor cortex (via the thalamus) and brainstem to fine-tune ongoing movements and ensure precision, smoothness, and coordination.
Plays a critical role in motor learning, adapting and improving movements over time through trial and error (error correction mechanism).
Basal Ganglia
A group of subcortical nuclei (e.g., striatum, globus pallidus, substantia nigra, subthalamic nucleus) that plays a crucial role in initiating and regulating voluntary movements, modulating motor control through a system of excitatory and inhibitory pathways.
Pathways:
Direct Pathway: Facilitates movement. Cortical input excites the striatum, which then inhibits the internal segment of the globus pallidus (GPi). GPi normally inhibits the thalamus, so its inhibition disinhibits the thalamus, allowing it to excite the motor cortex and promote movement.
Indirect Pathway: Inhibits movement. Cortical input excites the striatum, which inhibits the external segment of the globus pallidus (GPe). Inhibition of GPe disinhibits the subthalamic nucleus (STN), which then excites GPi, leading to increased inhibition of the thalamus and suppression of movement.
The substantia nigra influences these pathways via dopamine. Dopamine excites the direct pathway (promoting movement) and inhibits the indirect pathway (reducing inhibition of movement), thus overall facilitating movement.
Diseases:
Parkinson's Disease: Caused by the degeneration of dopaminergic neurons in the substantia nigra. This leads to a reduction in dopamine, impairing the ability to initiate movements (bradykinesia/akinesia), leading to rigidity, tremor at rest, and postural instability.
Huntington's Disease: An inherited neurodegenerative disorder characterized by the degeneration of inhibitory neurons in the striatum, particularly affecting the indirect pathway. This results in excessive, uncontrolled, jerky movements (chorea) and cognitive decline.
Complex Motor Behavior
Mirror Neuron System:
A network of neurons located in specific brain areas, including the premotor cortex, supplementary motor area, and parietal lobe (inferior parietal lobule).
These neurons fire not only when an individual performs an action but also when they observe another individual performing the same or similar action.
Functional Impact:
Proposed to play a vital role in imitation learning, understanding the actions and intentions of others, empathy, and developing language.
It links vision with a motor response, providing a neural basis for