Motor Skill Development: Gross, Fine, Ambidexterity

• A motor skill is a learned sequence of movements resulting in a smooth, efficient action.

Gross Motor Skills

• Gross motor skills involve large muscle movements, such as lifting the head, rolling over, sitting up, balancing, crawling, and walking.
• Development follows a pattern, with large muscles developing before smaller ones and progressing from top to bottom.
• Gross motor development is the foundation for developing skills in other areas like fine motor skills.
• By the age of two, most children can stand, walk, run, and walk up stairs.

Fine Motor Skills

• Fine motor skills involve manipulating small objects, transferring items from hand to hand, and hand-eye coordination tasks.
• They often require precise movements for delicate tasks.
• Examples include using the pincer grasp, cutting, coloring, writing, and threading beads.
• Fine motor development refers to skills using smaller muscle groups.

Ambidexterity

• Ambidexterity is a skill where there is no dominance between body symmetries, allowing fine motor skills to be performed with either extremity.
• The most common example is writing with either hand.

Voluntary Movements

• Most voluntary movements, initiated by the cerebral cortex, activate functional patterns stored in lower brain areas, including the spinal cord, brain stem, basal ganglia, and cerebellum.
• These lower centers then send control signals to the muscles.
• The cortex has a direct pathway to anterior motor neurons for fine, dexterous movements of fingers and hands.

Skeletal Muscle Contraction

• Skeletal muscle contraction is initiated by lower motor neurons in the spinal cord and brainstem.
• These neurons are located in the ventral horn of the spinal cord gray matter and in the motor nuclei of the cranial nerves.
• They send axons to skeletal muscles via ventral roots and spinal peripheral nerves or cranial nerves.
• Spatial and temporal patterns of activation are determined by local circuits within the spinal cord and brainstem.
• These circuits receive sensory input and mediate sensorimotor reflexes, also maintaining interconnections for coordinated behaviors.
• Descending pathways from higher centers modulate lower motor neuron activity by influencing local circuitry.
• Upper motor neurons are located in brainstem centers like the vestibular nuclei, superior colliculus, reticular formation, and cerebral cortex.
• Upper motor neurons initiate and guide involuntary and voluntary movements by synapsing on local circuit neurons, which connect with lower motor neurons.
• Lower motor neurons are the final common pathway for transmitting information from various sources to skeletal muscles.
• The motor system consists of four interactive subsystems:
  • Lower motor neurons that innervate skeletal muscles and local circuit neurons.
  • Upper motor neurons in the brainstem or cerebral cortex synapse with local circuit neurons or lower motor neurons.
  • The cerebellum detects and attenuates motor errors.
  • The basal ganglia prevent unwanted movements and prepare motor circuits for initiating movements.

Motor Signals

• Motor signals are transmitted from the cerebral cortex to the spinal cord directly through the corticospinal tract and indirectly through accessory pathways involving the basal ganglia, cerebellum, and brain stem nuclei.
• Direct pathways are more involved with discrete and detailed movements.
• The non-primary motor cortex is divided into the supplementary motor cortex, and the pre-motor cortex:
  • Supplementary motor cortex is involved in the execution of movement sequences, motor skill attainment, and movement selection based on sensory input.
  • Pre-motor cortex contributes to the corticospinal tract and is active during motor planning.

The Cerebrum's Role in Motor Control

• Primary motor area (M1) in the precentral gyrus initiates voluntary movement; corticospinal tracts originate here.
• Lesions to M1 cause loss of voluntary movement in the contralateral body part corresponding to the damaged motor homunculus area and a loss of the ability to implement a specific motor plan. Premotor Area:
• Located anterior to M1; involved in motor planning or praxis.
• Lesions to the premotor area cause apraxia or motor planning difficulties.
• Apraxia involves either the inability to understand task demands or the inability to access the appropriate motor plan.

Motor Association Area

• Also called the prefrontal area, located in the anterior frontal lobe.
• Involved in the cognitive planning of movement.
• Lesions cause the loss of stored motor plans, resulting in the inability to cognitively understand how to carry out a previously known motor task.
• The premotor area may compensate for damage to the motor association area.

Praxis

• Praxis involves three processes:
  • Ideational praxis: the ability to cognitively understand the motor demands of a task, largely a function of the motor association area.
  • Ideomotor planning I: the ability to access the appropriate motor plan, commonly stored in the premotor area.
  • Ideomotor planning II: the ability to implement the appropriate motor plan, commonly involving M1.
• The supplementary motor area is part of the premotor area.
  • Located inside the medial longitudinal fissure, it controls bilateral posture.
  • Lesions may result in loss of bilateral control of posture; other areas like the cerebellum or vestibular system may compensate.
• The frontal eye field is located in the middle frontal gyrus, anterior to the premotor area.
  • It is responsible for visual saccades.
  • Lesions result in deviation of the eyes to the same side of the lesion.

Incoming Motor Cortex Fiber Pathways

• The motor cortex is controlled by nerve signals from the somatosensory system and, to some degree, from other sensory systems like hearing and vision.
• Once sensory information is received, the motor cortex operates in association with the basal ganglia and cerebellum to excite appropriate motor actions.
• Key incoming fiber pathways include:
  • Subcortical fibers from adjacent regions of the cerebral cortex, especially from somatosensory areas of the parietal cortex, adjacent areas of the frontal cortex anterior to the motor cortex, and the visual and auditory cortices.
  • Subcortical fibers arriving through the corpus callosum from the opposite cerebral hemisphere, connecting corresponding areas of the cortices in the two sides of the brain.
  • Somatosensory fibers arriving directly from the ventrobasal complex of the thalamus, sending cutaneous tactile signals and joint and muscle signals from the peripheral body.
  • Tracts from the ventrolateral and ventroanterior nuclei of the thalamus, which receive signals from the cerebellum and basal ganglia, coordinating motor control functions.
  • Fibers from the intralaminar nuclei of the thalamus, controlling the general level of excitability of the motor cortex.

Role of the Brain Stem

• The brain stem consists of the medulla, pons, and mesencephalon.
• It contains motor and sensory nuclei for the face and head regions, similar to the spinal cord for the neck down.
• It performs special control functions, including control of respiration, the cardiovascular system, partial control of gastrointestinal function, control of stereotyped movements, control of equilibrium, and control of eye movements.
• It serves as a way station for command signals from higher neural centers.

Cerebellum and Basal Ganglia Contributions

• The cerebellum and basal ganglia are essential for normal motor function.
• They function in association with other motor control systems, not independently.
• The cerebellum plays a key role in the timing of motor activities and smooth progression from one movement, controlling muscle contraction intensity and interplay between agonist and antagonist muscle groups.
• The basal ganglia help plan and control complex movement patterns, controlling movement intensities, directions, and sequencing for complex motor goals.

Cerebellum and Its Motor Functions

• Electrical excitation of the cerebellum doesn't cause conscious sensation or motor movement.
• Removal of the cerebellum causes abnormal body movements.
• The cerebellum is vital during rapid muscular activities like running, typing, and talking.
• Loss of the cerebellum can cause incoordination, without paralysis.
• The cerebellum helps sequence motor activities, monitoring and adjusting body movements to conform to motor signals from the cerebral motor cortex.
• The cerebellum receives updated information about muscle contraction sequences and sensory information from the body's periphery.
• It compares actual movements with intended movements and transmits corrective signals to adjust muscle activation.
• The cerebellum aids the cerebral cortex in planning the next sequential movement in advance.
• The cerebellar circuit learns from mistakes, adjusting the excitability of neurons to improve subsequent muscle contractions.

Basal Ganglia and Their Motor Functions

• Basal ganglia work with the cerebral cortex and corticospinal motor control system.
• Basal ganglia receive input signals from, and return output signals to, the cerebral cortex.
• Principal role: works with the corticospinal system to control complex motor activity patterns.
• Damage to the basal ganglia impairs the cortical system's ability to provide patterns; writing becomes crude.
• Patterns that require basal ganglia intervention include cutting paper, hammering nails, shooting a basketball, throwing a baseball, shoveling dirt, vocalization, controlled eye movements, and other skilled subconscious movements.
• Basal ganglia coordinate using excitatory (glutamate) and inhibitory (GABA) neurons.
• The basal ganglia work in two distinct pathways:
  • Direct pathway: initiates movement which turns on the motor cortex by disinhibiting thalamic control of motor planning.
  • Indirect pathway: terminates movement by turning off the motor cortex by activating inhibition. which results in the inhibition of activity in the indirect pathway, which leads to the facilitation of movement.
• Glutamate excitatory neurons from the thalamus stimulate movement by projecting to motor regions of the cerebral cortex.
• Neurons from the GPi and SNpr steadily release GABA, inhibiting thalamic neurons and suppressing unwanted movement.
• When a movement is wanted, information is sent from the cortex to the striatum (CSP), where glutamate neurons excite striatum neurons, releasing GABA in the GPi and SNpr, stopping the inhibition of thalamic neurons involved in the desired movement, thus opening the gate for the movement to occur.
• The SNpc modulates the direct pathway by sending neurons to the striatum through the nigrostriatal pathway, where they release dopamine, facilitating activity in the direct pathway.
• The indirect pathway is involved with inhibiting movement: cortex projections activate GABA neurons in the striatum causing the activation of GABA neurons which exert an inhibitory effect on glutamate neurons in the subthalamic nucleus which activate GABA neurons and inhibit thalamic neurons from traveling to the motor regions of the cerebral cortex to stimulate movement
• Activity from these two pathways inhibits these thalamic neurons and thus inhibits movement; the activity in the indirect pathway antagonizes the activity of the direct pathway and acts to keep unwanted movements from occurring
• SNpc modulate the activity of the indirect pathway through dopamine release.
• Dopamine depletion causes difficulties in initiating movement.

The Spinal Level

• The spinal cord has local movement patterns for all muscle areas, such as withdrawal reflexes from pain.
• Complex rhythmical motions like walking and reciprocal motions are programmed in the cord.
• High levels of motor control can command these patterns into action or inhibit them.

The Hindbrain Level

• The hindbrain provides two major functions for motor control:
  • Maintenance of axial tone for standing.
  • Continuous modification of muscle tone based on vestibular apparatus information for maintaining equilibrium.

The Motor Cortex Level

• The motor cortex provides activating motor signals to the spinal cord.
• It issues sequential and parallel commands to activate cord patterns, change intensities, or modify timing.
• Corticospinal system can bypass cord patterns with higher-level patterns from the brain stem or cerebral cortex.
• Cortical patterns are complex and can be learned; cord patterns are mainly determined by heredity.

Associated Functions of the Cerebellum

• The cerebellum enhances the stretch reflex by transmitting it through the cerebellum and back to the cord.
• The cerebellum makes postural movements (especially from the equilibrium system) smooth and continuous at the brain stem level.
• At the cerebral cortex level, the cerebellum provides extra motor force at the start of a movement, turns on antagonist muscles to stop the movement, and programs muscle contractions for smooth transitions between rapid movements.
• Cerebellum mainly functions when muscle movements must be rapid; slow movements can occur without it.

Associated Functions of the Basal Ganglia

• The basal ganglia help the cortex execute subconscious patterns of movement and plan multiple parallel and sequential movement patterns to accomplish a task.
• Motor patterns that require the basal ganglia include writing, throwing a ball, and typing.
• The basal ganglia also allow action in response to stimuli.

Equilibrium

• The receptors are the hair cells in the semicircular canals, utricle, and saccule located in the inner ear.
• The semicircular canals respond to angular acceleration and deceleration (rotation) and linear acceleration and deceleration.
• The utricle and saccule respond to gravity and changes in head position.