Locomotor control

What is the somatic motor system

• Composed of:

  • Joints

  • Skeletal muscles

  • Spinal motor neurons and interneurons

• Controls voluntary movement through interactions between motor neurons and muscle fibers.

Hierarchical organisation of motor control

Level	Key Structures	Functions	Examples
1. Association Cortex	Prefrontal Cortex, Parietal Cortex	Planning, intention, decision-making; integrates sensory info	Choosing to pick up a cup
2. Motor Cortex	Primary Motor Cortex (M1), Premotor Cortex	Initiation and direction of voluntary movement	Sending signal to hand muscles to reach
3. Basal Ganglia & Cerebellum	Striatum, Globus Pallidus, Substantia Nigra, Cerebellar cortex	Basal Ganglia: Initiation/suppression of movementCerebellum: Coordination, error correction, learning	Correcting movement trajectory in real time
4. Brainstem	Vestibular nuclei, Reticular formation, Red nucleus	Posture, balance, gaze control; relays motor commands	Keeping balance while walking
5. Spinal Cord	Motor neurons, interneurons, Central Pattern Generators (CPGs)	Execution of movement; reflex control; rhythmic patterns	Walking, knee-jerk reflex
6. Peripheral Effectors	Skeletal muscles, joints, sensory receptors	Actual movement and proprioceptive feedback	Muscle contraction and feedback to spinal cord/brain

Somatic motor system

Somatic motor system: the joints, skeletal muscles, and spinal motor neurons and interneurons and how they communicate with each other.

The muscle spindle: proprioception

1. The muscle spindle is a sensory receptor embedded in the muscle

2. Intrafusal muscle fibers arranged in parallel with extrafusal fibers

3. Alpha α motor neurons excite the extrafusal fibers to generate muscle contraction

4. During muscle contraction, gamma γ motor neurons are also activated and contract intrafusal fibres

5. Sensory afferent axons type I (Ia axons) respond rapidly to stretch and mediate reflex adjustments when the axon is stretched

6. Afferent axons form excitatory connections with the α motor neuron innervating the same muscle and inhibitory connections with the α motor neuron innervating the antagonistic muscle

The muscle spindle: proprioception Table

Step	Component / Event	Function / Role
1	Muscle spindle is a sensory receptor embedded in the muscle	Detects changes in muscle length and contributes to proprioception
2	Intrafusal fibers run parallel to extrafusal fibers	Allow the muscle spindle to detect stretch while maintaining alignment with the contracting muscle
3	Alpha (α) motor neurons excite extrafusal fibers	Cause muscle contraction and generate movement
4	Gamma (γ) motor neurons contract intrafusal fibers during contraction	Maintain sensitivity of the spindle during shortening of the muscle
5	Ia sensory afferents respond to stretch	Send rapid feedback to the spinal cord for reflex adjustment
6	Afferent axons synapse with motor neurons:	- Excitatory to α motor neurons of the same muscle- Inhibitory to α motor neurons of the antagonistic muscle (reciprocal inhibition)

Relationship of muscles and motor neurons

1. Muscles are organised as functional antagonists

2. Reciprocal inhibition of flexors and extensors at the same joint

3. Muscle spindle: a specialized structure involved in proprioception (body sense)

What is a motor unit

• Motor unit: An α motor neuron and all the muscle fibers it innervates.

What is Somatotopic organization

• Somatotopic organization: motor neurons innervating muscles are spatially arranged in the spinal cord.

What are the 3 types of motor units

• 3 Types of Motor Units:

  • Slow (posture): Small red fibers, fatigue-resistant.

  • Fast-fatigable: Pale, large fibers, high force, quick fatigue (e.g., jumping).

  • Fast fatigue-resistant: Intermediate properties.

What are the 3 types of motor units

• Slow motor units: smaller motor unit, consist of a smaller α motor neuron innervating small red muscle fibres. Slow contraction, small

forces generated. Resistant to fatigue. E.g.Maintaining upright posture.

• Fast, fatigable motor units: larger α motor neuron innervating larger, pale muscle fibres,generate more force, sparse mitochondria therefore easily fatigued. E.g. brief exertions requiring large forces, running or jumping.

• Fast, fatigue-resistant motor units

Regulation of muscle force: the size principle

• The number of motor units active at any one time determines the amount of force produced by a muscle

• Gradual increase in tension results from the recruitment of motor units in a fixed order according to their sizE

What is the size principle

• Size Principle: Motor units are recruited from smallest to largest to control force output smoothly.

Reflexes (Muscle spindle,reflex arc and knee jerk reflex)

• Muscle spindle: Detects stretch, consists of intrafusal fibers parallel to extrafusal fibers.

• Reflex arc:

  • Ia afferents excite α motor neurons (same muscle) and inhibit antagonists.

  • Gamma motor neurons maintain spindle sensitivity during contraction.

• Knee-jerk reflex is an example of this stretch reflex system.

Locomotion is characterised by 2 highly stereotyped pattern of movements

• a swing phase, during which the leg (having been extended to the rear) is flexed, raised off the ground, swung forward, and extended again to contact the ground

• a stance phase, during which the leg is in contact with the ground, moving backward in relation to the direction taken by the body

Spinal Control of Movement

• Flexion-crossed extension reflex: When one leg withdraws (flexes), the other extends.

• Central Pattern Generators (CPGs):

  • Neural circuits that generate rhythmic patterns (e.g., walking).

  • Can function without sensory input or brain signals, though inputs refine the pattern.

  • Built with pacemaker neurons.

  • Example in Lampreys: Rhythm generated by NMDA receptor properties.

What are CPGs

neural circuits that generate patterns of neural activity that underlie rhythmic motor behaviours

Examples: walking, swimming, scratching, chewing and feeding

What’s a limit of CPGs

However: CPG merely generates a suitable pattern of output; sensory feedback and inputs from the brain adjust the output to suit particular environmental demands

CPGs in Lampreys

Neurotransmitter	Type	Released by	Main Functions in CPG	Role in Swimming
Glutamate	Excitatory	Excitatory interneurons	- Activates motor neurons and interneurons   - Generates rhythmic bursting activity	Drives contractions on each side; initiates and maintains rhythmic movement
GABA	Inhibitory	Inhibitory interneurons	- Modulates excitatory signals  - Regulates burst timing and frequency	Fine-tunes the rhythm; helps stabilize and shape the swimming pattern
Glycine	Inhibitory	Commissural inhibitory interneurons	- Inhibits neurons on the opposite side  - Enables reciprocal (left-right) inhibition	Coordinates alternating muscle activity for smooth, undulating swimming motion

What’s the Molecular Basis of CPGs

• NMDA Receptors:

  • Allow Ca²⁺ and Na⁺ influx.

  • Voltage-dependent block by Mg²⁺ creates oscillatory potential in pacemaker neurons

How does the Generation of rhythmic activity in CPG neurons (7)

Steps in Rhythmic Activity Generation:

1. Initiation by Brainstem Input

   • Descending signals from the brainstem (e.g., reticulospinal neurons) activate the spinal CPG.

2. Excitatory Neuron Activation (Glutamate)

   • Excitatory interneurons release glutamate to stimulate motor neurons and other interneurons.

   • This initiates bursting activity in one side of the spinal cord.

3. Intrinsic Bursting of Neurons

   • Some neurons have ion channels (Na⁺, Ca²⁺, K⁺) that generate rhythmic bursts even without input.

   • These properties help maintain the rhythm after it starts.

4. Reciprocal Inhibition (Glycine)

   • Commissural inhibitory interneurons release glycine to inhibit the opposite side of the spinal cord.

   • This prevents both sides from firing at the same time, creating left-right alternation.

5. Feedback Inhibition (GABA/Glycine)

   • Other inhibitory interneurons provide feedback to excitatory neurons, limiting burst duration and intensity.

   • This shapes the timing and stability of the rhythm.

6. Intersegmental Coordination

   • Rhythmic activity spreads down the spinal cord from one segment to the next.

   • This creates a traveling wave of alternating contractions along the body (swimming motion).

7. Sustained Rhythmic Pattern

   • The balance of excitation and inhibition keeps the rhythm going as long as needed.

   • Sensory feedback and brain input can modulate the speed or strength of the rhythm.

Descending pathways (2)

1. Groups of myelinated nerve fibers that carry motor information from the brain or brainstem to effector's muscles, via the spinal cord.

2. They can be functionally divided into two groups: Pyramidal (voluntary) and extrapyramidal (involuntary) tracts.

Diagram of A sensorimotor system

What is The Brain Control of Movement

• Descending Motor Pathways:

  • Pyramidal tracts (voluntary): Corticospinal and corticobulbar tracts.

  • Extrapyramidal tracts (involuntary): Rubrospinal, vestibulospinal, etc.

• Motor cortex (M1):

  • Initiates voluntary movement.

  • Some axons synapse directly with spinal motor neurons (especially in primates).

  • Somatotopically organized (motor homunculus).

• Premotor cortex: Plans movements; activates before movement occurs.

  • Contains mirror neurons (respond during action observation).

How does Planning and directing voluntary movements in relation to premotor cortex,primary motor cortex and the somatosensory cortex

1. Premotor Cortex – Planning & Preparation

• Function: Plans and organizes movements before they happen.

• Role:

  • Selects the correct movement based on external cues (like seeing an object).

  • Coordinates with the prefrontal cortex for decision-making and intention.

  • Sends information to the primary motor cortex to execute the plan.

2. Primary Motor Cortex (M1) – Execution

• Function: Executes voluntary movements by sending motor commands to muscles.

• Role:

  • Receives the movement plan from the premotor cortex.

  • Activates specific muscles through descending motor pathways (e.g., corticospinal tract).

  • Controls the force, direction, and speed of movements.

3. Somatosensory Cortex – Feedback and Adjustment

• Function: Processes sensory input from the body (touch, pressure, proprioception).

• Role:

  • Provides real-time feedback about body position and movement.

  • Communicates with the motor cortices to adjust movements during execution.

  • Ensures movements are precise and accurate by detecting errors (e.g., if you’re off-target).

What is The Basal Ganglia Function

• Regulates initiation and termination of movement by influencing upper motor neurons.

• Works via disinhibition:

  • Direct Pathway: Facilitates movement via D1 dopamine receptors.

  • Indirect Pathway: Inhibits competing movements via D2 dopamine receptors.

How does dopamine work

Dopamine activates intended motor programs through the direct pathway and suppresses competing motor programs through the indirect pathway

Basal Ganglia Disorders (2)

• Parkinson’s Disease:

  • Loss of dopaminergic neurons.

  • Overactive indirect pathway → movement suppression.

  • Symptoms: bradykinesia, tremors, shuffling gait.

• Huntington’s Disease:

  • Loss of medium spiny neurons in the indirect pathway.

  • Overactive direct pathway → excess involuntary movement.

Why is C. elegans used as a Model

• Simple nervous system: 302 neurons, 118 types, full connectome mapped.

• Advantages:

  • Transparent, genetically tractable, small.

  • Suited for optogenetics and calcium imaging.

• Dopaminergic system modulates locomotion and learning.

• Major neurotransmitter pathways present

How does Whole-Brain Imaging in C. elegans Work

• Fluorescent calcium indicators allow real-time tracking of single-cell activity.

• NeuroPAL: multicolor neuron atlas for brain-wide mapping.

• Useful for studying sensorimotor integration:

  • Stimulus → neural activity → behavior.

Knee Jerk Reflex steps (6)

1. Tap on the Patellar Tendon = A light tap just below the kneecap (patella) stretches the quadriceps muscle in the thigh.

2. Stretch of the Muscle Spindle = This stretch activates muscle spindles, which are proprioceptive sensory receptors located within the muscle. They detect the rapid lengthening of the quadriceps.

3. Activation of Ia Sensory Afferents = The Ia afferent fibers (sensory neurons) from the muscle spindle rapidly carry the signal to the spinal cord (lumbar region).

4. Monosynaptic Excitation of Alpha Motor Neurons = In the spinal cord, the Ia afferent fibers make a direct excitatory synapse onto the alpha (α) motor neurons that innervate the same quadriceps muscle.

5. Muscle Contraction = The activated α motor neurons stimulate the quadriceps to contract, producing the knee extension (the leg kicks forward).

6. Reciprocal Inhibition of Antagonist Muscle = Simultaneously, the Ia afferents also activate interneurons that inhibit the α motor neurons of the antagonistic muscle (the hamstrings). This ensures the hamstrings relax, allowing smooth extension of the knee.