2.1.2 movement

Sequential Steps and Brain Regions Involved:
1. Decide to move
  - Involves prefrontal cortex
2. Assess current body position relative to target/where you want to move (subconscious)
  - Involves primary somatosensory cortex and posterior parietal cortex
3. Plan the movement (subconscious - which muscles, how much force, sequence/timing)
  - Involves premotor cortex and supplementary motor area
4. Initiate movement (movement commands from the substantia nigra and basal ganglia travel to the motor cortex via the thalamus)
  - Involves substantia nigra within the midbrain of the brainstem, and basal ganglia (caudate nucleus, subthalamic nucleus, putamen, globus pallidus) within the cerebrum.
5. Execute movement (signals descend from primary motor cortex to skeletal muscle)
  - An action potential arriving at the neuromuscular junction triggers a muscle fibre action potential, initiating excitation-contraction coupling and muscle contraction to produce movement.
6. Monitor and modulate movement
  - Involves pathway between cerebellum and motor cortex, and sensory information from skeletal muscle, skin and joints (sensory feedback loops provide information to adjust the force and coordination as needed during task execution)
  - Primary somatosensory cortex, posterior parietal cortex, and visual cortex are active to determine if movement was successful.

Role of Sensory Information:
- Pre-Movement Planning: Afferent inputs provide spatial context and enable predictive motor calculations.
- Proprioception: Specialised muscle spindles containing intrafusal fibers detect muscle length and stretch; sensitivity is adjusted by gamma motor neurons to maintain body position awareness.

Pathway Overview:
- The corticospinal tract is the primary descending pathway transmitting voluntary motor commands to skeletal muscles in the trunk and limbs.
- The corticospinal tract primarily originates in the primary motor cortex (can be from the premotor or supplementary motor areas) and synapses onto alpha motor neurons, either directly or indirectly via spinal interneurons.
Subdivisions of the Corticospinal Tract:
- Lateral Corticospinal Tract:
  - Proportion: The larger division of the tract.
  - Function: Controls skeletal muscles of the limbs.
  - Decussation: Axons cross the midline in the medulla oblongata.
  - Control Pattern: Contralateral control (right motor cortex controls left limb muscles; left motor cortex controls right limb muscles).
- Anterior Corticospinal Tract:
  - Proportion: The smaller division of the tract.
  - Function: Controls skeletal muscles of the trunk.
  - Decussation: Axons cross the midline locally within the spinal cord.
Signal Path and Neuromuscular Execution:
- Upper motor neuron axons descend through the tract to synapse onto lower motor neurons (alpha motor neurons) in the ventral horn of the spinal cord.
- $\alpha$ -motor neurons transmit action potentials via peripheral nerves to extrafusal muscle fibers, causing contraction.

Upper Motor Neurons:
- Location: Cell bodies are located in the cerebral cortex (primary motor cortex, premotor cortex, supplementary motor area).
- Axonal Path: Axons descend through the brainstem and spinal cord within descending motor tracts (e.g., corticospinal tract).
- Function: Formulate, initiate, and convey voluntary motor commands to spinal or brainstem circuits.
- Synaptic Target: Synapse onto lower motor neurons or interneurons in the ventral horn of the spinal cord or brainstem nuclei.
Lower Motor Neurons (Alpha Motor Neurons):
- Location: Cell bodies reside in the ventral horn of the spinal cord (for trunk/limbs) or brainstem motor nuclei (for head/neck).
- Axonal Path: Axons project out of the CNS through ventral roots and peripheral nerves (or cranial nerves).
- Function: Serve as the final direct path to excite extrafusal skeletal muscle fibres and generate physical force.
- Synaptic Target: Directly innervate extrafusal muscle fibres at the neuromuscular junction.
Summary Comparison:
- Anatomical Boundary: Upper motor neurons remain entirely within the central nervous system, whereas lower motor neuron axons extend into the peripheral nervous system to reach muscles.
- Functional Role: Upper motor neurons organise and direct motor commands from higher brain centres, while lower motor neurons directly execute muscle contraction.

Alpha motor neurons innervate extrafusal muscle fibres; they contract to produce movement (directly driving skeletal muscle movement)
Gamma motor neurons innervate intrafusal muscle fibres, adjust muscle spindle sensitivity, and keep the muscle spindle taut and sensitive to stretch.
Beta motor neurons are large motor neurons that innervate both extrafusal and intrafusal muscle fibres in skeletal muscles.