Lecture 2 Notes: Neuromotor and Sensory Basis for Motor Control
Neuromotor system
Focus of Lecture 2: Neuromotor and sensory basis for motor control, covering Neuromotor system and Sensory inputs (Vision, Touch, Proprioception).
Part A: Neuromotor system – Nervous system, neurons, brain and spinal cord, motor units.
Part B: Sensory inputs – Vision, Touch, Proprioception.
Neuromotor system overview
CNS (Central Nervous System) and PNS (Peripheral Nervous System) are involved in the control of coordinated movement.
CNS comprises the brain and spinal cord; it is the centre of coordination, integrating sensory information, generating responses, and sending responses to muscles to initiate movement.
PNS consists of nerves that connect the spinal cord with other parts of the body and is subdivided into:
Afferent (sensory) divisions
Efferent (motor) divisions
Sensory input and motor output are organized to support voluntary and involuntary control through somatic and autonomic pathways.
Sensory inputs and autonomic control
Sensory input comes from receptors in skin, muscles, joints, and internal organs.
Central nervous system processes sensory information and coordinates responses.
Peripheral nervous system subdivisions include:
Somatic nervous system: Directs voluntary movements.
Autonomic nervous system: Regulates involuntary bodily activities. Subdivisions:
Parasympathetic nervous system: Governs bodily activities during restful conditions.
Sympathetic nervous system: Prepares body for stressful or emergency situations.
Input from sensory receptors responding to external stimuli contributes to perception and action planning.
The Neuron (nerve cell)
Basic component of the nervous system; humans have about 200\,{,}000{,}000{,}000 neurons (commonly described as ~200 billion).
Neuron function: transmits neural impulses from one part of the body to another (neural transmission).
Structure (typical neuron):
Cell body: contains nucleus; metabolic centre of the cell.
Dendrites: branches from the cell body; receive information from other cells; ranges from 1 to thousands per neuron.
Axon (nerve fibre): single extension from the cell body with branches; sends information from the neuron to other neurons or muscles.
Axon terminal: releases neurotransmitters to pass information to the next cell.
Myelin sheath: insulates some axons and speeds neural impulses.
Transmission pathway: electrical impulse (action potential) travels along the axon to transmit information.
Types and functions of neurons
Three main types: 1) Sensory neurons (afferent):
Also known as afferent neurons; carry information from sensory receptors to the CNS.
Different structure compared to other neurons.
2) Motor neurons (efferent):Also known as efferent neurons; carry information from CNS to skeletal muscles to execute movements.
Alpha motor neurons: predominantly in the spinal cord; innervate skeletal muscles; transmit information to contract muscles and perform movements.
Gamma motor neurons: in fibers of skeletal muscles; linked to muscle spindles which detect changes in muscle length and velocity.
3) Interneurons: specialized neurons that originate and terminate in the CNS; connect sensory and motor neurons; can be sources of synapses from brain to motor neurons or from sensory/spinal nerves to brain.
Neural transmission and reflex arc (conceptual)
Afferent (sensory) information travels to the CNS.
Efferent (motor) information travels from CNS to muscles/glands.
Interneurons integrate information within CNS to coordinate responses.
A simple sensory-to-motor loop can be illustrated as sensory endings in skin → sensory neuron → interneuron in spinal cord (reflex involvement) → upper motor neuron → spinal cord (lower motor neuron) → muscle contraction.
Central nervous system (CNS) and motor control: four main brain structures
The brain contains four structural components most directly involved in voluntary movement:
1) Cerebrum (Cerebral cortex)
2) Diencephalon
3) Cerebellum
4) Brainstem
The Cerebrum (Cerebral cortex)
Largest section of the brain; consists of right and left cerebral hemispheres; covered by the cerebral cortex (outer surface).
Hemispheric specialization:
Left hemisphere (verbal/logical): Speaking, Reading, Writing, Science, Maths, right-hand (RH) touch.
Right hemisphere (non-verbal): Spatial tasks, Creativity, Art, Music, left-hand (LH) touch.
Connected by the corpus callosum (band of nerve fibres).
Cerebral cortex is the highest centre of processing for interpretation and integration of sensory information, planning, and organization of movement.
Four lobes of the cortex: Frontal, Parietal, Occipital, Temporal.
Four lobes and their primary roles
Frontal lobe: Central for voluntary movement control.
Parietal lobe: Central for perception of sensory information.
Occipital lobe: Critical for perception of visual information.
Temporal lobe: Important for memory.
Central sulcus: Fold in the cortex that separates parietal lobe from frontal lobe and the primary motor cortex from the primary somatosensory cortex.
Functional areas of the cerebral cortex
Motor areas (directly involved in movement):
Primary motor cortex (initiation and execution of movement): Final output to spinal cord to cause contraction of specific muscles; starts with premotor planning and ends with primary motor output.
Premotor cortex: Anterior to primary motor cortex; organises movements before initiation; plans movement; heavily linked to external sensory information; coordinates rhythmic movements; enables transitions between sequential movements (e.g., keyboard typing, piano).
Supplementary Motor Area (SMA): Medial surface of frontal lobe near primary motor cortex; controls preparation and organization of movement; larger role in movements generated from memory.
Sensory areas:
Primary Somatosensory Cortex (somatosensory cortex): Located just posterior to the central sulcus in the parietal lobe; processes somatic sensory information (pain, temperature, pressure).
Association areas:
Association areas: Integrate information from multiple sensory cortices to form plans for action; pass plans to motor areas.
Subcortical brain areas important in motor control
Basal Ganglia (collection of four nuclei): Caudate nucleus, Putamen, Substantia nigra, Globus pallidus.
Functions: Movement initiation; scaling/modifying movement parameters (velocity, direction); force modulation; receive information from cerebral cortex and brainstem; send information to brainstem.
Diencephalon:
Thalamus: Relay station; receives and integrates sensory information from spinal cord and brainstem; sends information to cerebral cortex; important for attention, mood, and perception of pain.
Hypothalamus: Critical centre for endocrine control and homeostasis (body temperature, hunger, thirst, energy use).
Cerebellum:
Structure: Two hemispheres; white matter under cortex; includes connections to red nucleus and spinal cord; receives information from sensory systems, spinal cord, and other brain regions; regulates motor movements.
Functions:
Error detection and correction system; compares actual movement with intended movement using sensory information; makes corrections.
Timings and accuracy in coordination; hand-eye coordination, movement timing, posture/balance.
Involved in learning motor skills.
Brainstem (brainstem components involved in motor control): Pons, Medulla, Reticular formation.
Pons: Bridge between cerebral cortex and cerebellum; involved in control of various body functions (e.g., chewing) and balance.
Medulla: Sensory and motor pathways cross and merge; regulates heartbeat and respiration.
Reticular formation: Integrates sensory and motor information; can directly or indirectly influence motor and cognitive activity; influences arousal (attention and alertness).
The spinal cord and motor control
Spinal cord relays messages and has a vital role in subconscious movement (reflexes) and moment-to-moment control (timing of muscle activation, minor adjustments to movement).
Information travels through specific pathways (tracts).
Structure: Grey matter and White matter; grey matter contains neuron cell bodies in an H-shaped area; White matter contains myelinated axons.
Grey matter horns:
Dorsal (posterior) horns: Receive sensory information and transmit sensory info.
Ventral (anterior) horns: Transmit motor information from CNS to skeletal muscles.
Interneurons (e.g., Renshaw cells) reside in the ventral horn and can influence motor activity (e.g., inhibition).
A typical transverse section showing ventral horn, dorsal horn, grey/white matter.
Spinal sensory pathways (ascending tracts):
Dorsal column: Transmits pressure, touch, proprioception.
Spinothalamic system: Transmits pain and temperature (and some touch/pressure).
Spinal motor pathways (descending tracts):
Pyramidal tracts (corticospinal tracts): Main motor pathway for upper motor neuron signals from cortex/brainstem; majority (about 90\%) cross over to the contralateral side at the brainstem level.
Extrapyramidal tracts (non-pyramidal, brainstem pathways): Do not cross to the opposite side or cross variably; involved in postural control and fine motor control of hand and finger movements; about 10\% do not cross.
The motor unit and muscle control
The motor unit = an alpha motor neuron and all skeletal muscle fibers it innervates.
When a motor neuron fires, all its connected muscle fibers contract; there is no partial contraction of a motor unit.
Approximate total number of alpha motor neurons in the spinal cord is about 2\times 10^{5}.
The number of muscle fibers per motor unit varies by movement type:
Fine movements (e.g., eye movements): about 1 fiber per motor unit.
Gross movements (e.g., posture control): up to around 700 fibers per motor unit.
Types of motor units and fatigue characteristics
Skeletal muscles contain slow-twitch (Type I) and fast-twitch (Type II) fibers.
Type I (slow-twitch): Innervate slow-twitch fibers; smaller force, less fatigue.
Type IIa (fast-twitch, fatigue resistant): Generate more force, somewhat resistant to fatigue.
Type IIb (fast-twitch, fatigable): Larger force, short duration, fatigue quickly.
Implication: The type of motor unit recruited influences motor control and performance. For example, short-duration, discrete, gross motor skills (e.g., hitting a cricket ball, kicking a football) recruit fast-twitch units for quick power.
Motor unit recruitment and the size principle
The amount of force a muscle contraction generates depends on the number of muscle fibers activated (i.e., active motor units).
Recruitment increases the number of active motor units to achieve the desired force.
Size principle: Recruit smallest (low-threshold) motor units first, then progressively recruit larger (higher-threshold) motor units as needed.
Order example: Type I (slow-twitch, fatigue-resistant) → Type IIa (fast-twitch fatigue-resistant) → Type IIb (fast-twitch fatigable).
This ordered recruitment allows for smooth and efficient movements.
Formal statement (conceptual): The set of active motor units at a given force level is such that the smallest units are activated first, with larger units added as demand increases.
From intent to action: neural control of voluntary movement
Movement begins with intent to perform a skill and proceeds through a hierarchical organization from high to low levels of the neuromuscular system.
High-level planning in association areas and premotor areas informs motor planning; the motor cortex sends commands down through the thalamus and brainstem to the spinal cord.
The cerebellum and basal ganglia adjust commands generated by the cortex to refine movement (timing, coordination, scaling).
The brainstem relays commands from the cortex to the spinal cord for execution of movement.
This hierarchy ensures integration of perception, planning, execution, and feedback control.
Sensory inputs: Vision, Touch, Proprioception
Vision, touch, proprioception (and vestibular information) are key components of motor control.
Vision is the dominant sensory system used for perception and action.
Vision provides pre-movement information and feedback during movement, and post-movement information about achievement of action goals.
The environment provides constant sensory information; perception is the interpretation of sensory input; perceptual-motor processes combine this with stored information to produce motor performance.
Exteroceptive information includes vision and hearing; interoceptive information includes proprioception (kinaesthesis) and vestibular inputs.
Vision in motor control
Vision is the dominant sensory system; we rely on it more than other senses (visual dominance) but it can mislead when in conflict with other senses (e.g., motion illusions or stationary car at traffic lights).
Structural elements of the eye:
Cornea: Transparent surface that refracts light.
Pupil: Central opening of the iris; lets light into the eye.
Lens: Changes shape to project a sharp image onto the retina.
Neural elements:
Retina: Contains rods and cones; transduces light into neural signals.
Fovea: Centre of retina; highest acuity.
Rods: Peripheral vision; black-and-white, low acuity.
Cones: Central vision; colour vision, high acuity.
Optic nerve: Carries visual information from retina to visual cortex.
Visual pathways:
Where pathway (dorsal stream): Ambient/peripheral vision; involved in spatial awareness and action guidance.
What pathway (ventral stream): Focal vision; involved in object recognition and form representation.
Visual fields:
Central vision (focal): 2–5 degrees of visual field; conscious; high acuity; identifies objects (what is it?).
Peripheral vision (ambient): Up to ~200 degrees horizontally; subconscious; lower acuity; locates objects in space (where is it?).
Binocular vision provides depth perception (important for reaching, grasping, navigating cluttered pathways, intercepting moving objects).
Monocular vision provides object features such as shape, size, texture, contrast, and overlap.
Perception–action coupling: Vision and action influence each other; the gaze is usually directed ahead of the movement; perception offers opportunities for movement.
Techniques to study vision in motor control:
Eye movement recording tracks focal vision and gaze fixation strategies; experts show efficient search strategies with fewer fixations and longer fixations.
Occlusion techniques:
Temporal occlusion: Stop video at various times to assess when information is used.
Spatial occlusion: Occlude specific events or characteristics (e.g., opponent’s arm/hand).
Video-based temporal occlusion and in-situ temporal occlusion (field-based applications).
Spatial occlusion: Isolates specific parts of the action to see what information is needed.
Role of vision in performance examples:
Kicking a ball: Vision guides movement planning and execution; per previous studies, skilled performers use early visual information efficiently.
Cricket batting: Experts monitor ball release, perform visual saccades to predicted bounce points, and track ball trajectory for 100–200 ms post-bounce.
Blocked vision studies show that higher-skilled batters rely more on early visual information for performance advantage.
Touch and motor control
Tactile sensory information coming from the skin is important for motor control.
Mechanoreceptors located in the dermis (greatest concentration in fingertips) provide CNS with information about temperature, pain, and pressure.
Research methods commonly compare task performance with finger anesthesia; tactile information influences movement accuracy, consistency, and force adjustments during contact with external objects.
Proprioception and motor control
Proprioception: sensory information from within the body about limb, trunk, head position and movement; sometimes called kinaesthesis (sense of movement).
Requires the appreciation of changes in position and force; multiple receptors provide this information.
Primary proprioceptors:
Muscle spindles: Lie in parallel with muscle fibres; detect changes in muscle length (stretch) and velocity (speed of stretch); provide feedback on limb movement position, direction, and velocity.
Golgi tendon organs (GTO): In skeletal muscle near tendon insertion; detect changes in muscle tension (force/ stretch); less sensitive to length changes.
Joint receptors: Located in joint capsules and ligaments; detect changes in force, rotation, and joint angle; especially sensitive at extreme joint angles or positions.
The vestibular system
Provides an overall sense of body movement or position; detects head position and movement; works with the visual system to coordinate eye movements.
Key components: Otolith organs and Semicircular canals (three canals).
Functions: Detect angular acceleration, head/body acceleration, and gravitational forces.
Hierarchy of movement control (conceptual model)
Motor commands flow from higher to lower levels:
Primary motor cortex and nonprimary motor areas influence downstream structures.
The cerebellum and basal ganglia adjust and refine commands.
The brainstem passes commands from cortex to spinal cord.
Muscles execute the resulting movement.
Schematic: Basal ganglia, Cerebellum, Thalamus + Brainstem, Spinal cord, Muscles.
Learning activities and practical notes
Group learning activity involves constructing a brain model (wearing a shower cap) and labeling lobes, central sulcus, and cortex areas using markers/post-its.
Draw and label 4 lobes and the cerebellum; identify the central sulcus and the primary motor and somatosensory cortices.
List the functions of: Somatosensory cortex, Sensory association areas, Primary motor cortex, Premotor cortex, SMA, and Cerebellum.
Spinal cord and pathways (recap)
Spinal cord functions:
Relays messages; subconscious control of movement (reflexes); moment-to-moment control via timing and adjustments.
Tracts carry information; there are ascending (sensory) and descending (motor) pathways.
Key ascending tracts:
Dorsal column: Pressure, touch, proprioception.
Spinothalamic: Pain and temperature (and some touch/pressure).
Key descending tracts:
Pyramidal (corticospinal) tracts: Main motor pathway for voluntary movement; originate from cortex/brainstem; most cross to contralateral side at brainstem (90%).
Extrapyramidal tracts: Brainstem pathways that often do not cross; involved in postural control and fine motor control of hands and fingers (10%).
Review and conceptual questions
Differences between the central and peripheral nervous systems are foundational for understanding motor control.
Key brain areas responsible for movement and motor control include: Cerebral cortex (motor and sensory areas), Basal ganglia, Cerebellum, Brainstem, Thalamus, and Spinal cord.
Functions of motor cortex and sensory cortex include initiation/execution of movement and processing of sensory information, respectively.
Basic neuron structure and neural transmission: cell body, dendrites, axon, axon terminal; action potentials; myelin; synapses; neurotransmitters.
Specialized sensory receptors for vision, proprioception, and vestibular sense include rods/cones, muscle spindles, Golgi tendon organs, joint receptors, otoliths, and semicircular canals.
Mathematical and numerical notes (quick references)
Total neurons: 200\,000\,000\,000\;\text{neurons}
Alpha motor neurons in spinal cord: \approx 2\times 10^{5}
Motor unit fiber counts (typical extremes):
Fine movement units: ≈ 1 fiber per motor unit
Gross movement units: up to ≈ 700 fibers per motor unit
Motor neuron crossover:Approximately 90\% cross to contralateral side; about 10\% remain ipsilateral (extrapyramidal pathways involvement varies)
Visual fields:
Focal vision: 2{-}5^{\circ}
Ambient/peripheral vision: up to ≈ 200^{\circ} horizontally
Speed of information processing and timing references (examples):
Visual information processing for anticipatory actions often relies on early visual cues; gaze typically directed ahead of movement trajectory.
In sports demonstrations, experts show particular saccade patterns and post-bounce tracking windows (e.g., ball tracking up to 100–200 ms post-event in batting).
If you want, I can restructure these notes into a printable PDF-friendly outline or convert to a condensed study guide with bolded keywords and quick-reference terms. Also, I can add a short glossary of terms used in the lecture.