14.3 Motor Responses

The motor system is a complex network that controls movement. It involves various brain regions, the spinal cord, and muscles working together. This system processes sensory input, plans actions, and executes movements through a series of coordinated steps.

Descending pathways, like the corticospinal tract, carry signals from the brain to the spinal cord. These pathways control different aspects of movement, from fine motor skills to posture and balance. The system relies on feedback loops and reflexes to adjust and fine-tune movements in real-time.

Motor System Processing Stream

Components of motor system processing

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• Premotor cortex receives input from sensory areas and plans movements
• Primary motor cortex (M1) executes voluntary movements by sending signals to lower motor neurons
• Basal ganglia modulates motor activity and helps initiate movements, receiving input from cerebral cortex and sending output to thalamus and brainstem (caudate nucleus, putamen, globus pallidus)
• Cerebellum fine-tunes and coordinates movements, receiving input from sensory systems and sending output to brainstem and thalamus
• Brainstem contains motor nuclei that control cranial nerves and relay signals to spinal cord (midbrain, pons, medulla oblongata)
• Spinal cord contains lower motor neurons that directly innervate skeletal muscles

Pathway of motor commands

1. Premotor cortex and primary motor cortex (M1) in the frontal lobe initiate motor commands
2. Corticospinal tract (pyramidal tract) carries motor signals from cortex to spinal cord
   • Axons from M1 descend through the internal capsule
   • Most axons decussate (cross) at the pyramids in the medulla oblongata
   • Lateral corticospinal tract crosses at the medulla and descends contralaterally in the lateral white matter of the spinal cord
   • Anterior corticospinal tract consists of uncrossed axons that descend ipsilaterally in the anterior white matter of the spinal cord
3. Lower motor neurons (alpha motor neurons) in the anterior horn of the spinal cord receive signals from the corticospinal tract
   • Axons of lower motor neurons exit the spinal cord via ventral roots and form peripheral nerves
4. Neuromuscular junctions are where axon terminals of lower motor neurons synapse with skeletal muscle fibers to cause contraction
   • The neurotransmitter acetylcholine is released at the neuromuscular junction to initiate muscle contraction

Descending Motor Pathways

Descending motor pathways: structure vs function

• Pyramidal (corticospinal) tract originates from the primary motor cortex (M1) and controls fine, precise, and voluntary movements, especially of distal limbs (hands, fingers)
• Extrapyramidal tracts originate from brainstem motor nuclei, receive input from various brain regions, and control posture, balance, and stereotyped movements
  • Rubrospinal tract originates from the red nucleus and controls flexor muscle tone
  • Tectospinal tract originates from the superior colliculus and controls head and neck movements in response to visual stimuli
  • Reticulospinal tracts originate from the reticular formation and control axial and proximal limb muscles for posture and balance (sitting, standing)
  • Vestibulospinal tracts originate from the vestibular nuclei and control balance and posture in response to vestibular input (inner ear)

Neurological Connections and Movement Initiation

Neurological initiation of movement

• Cortical motor areas (premotor cortex and M1) receive input from:
  • Sensory cortices provide information about the body and environment (somatosensory, visual, auditory)
  • Basal ganglia help initiate and select appropriate movements
  • Cerebellum fine-tunes and coordinates planned movements
• Cortical motor areas integrate sensory information and motor planning to generate motor commands
• Motor commands are sent via the corticospinal tract to lower motor neurons in the spinal cord
• Lower motor neurons directly innervate skeletal muscles, causing them to contract and generate movement

Motor Control and Coordination

Hierarchical organization of motor control

• Upper motor neurons in the cerebral cortex and brainstem initiate and control voluntary movements
• Lower motor neurons in the spinal cord directly innervate skeletal muscles
• Motor units consist of a single lower motor neuron and all the muscle fibers it innervates

Sensory feedback and motor control

• Proprioception provides information about body position and movement, essential for coordinating motor responses
• Central pattern generators in the spinal cord produce rhythmic motor patterns for activities like walking and breathing

Reflex Arcs

Reflex arcs in motor responses

• Stretch reflex (myotatic reflex) is a monosynaptic reflex that maintains muscle tone and resists sudden changes in muscle length
  • Receptors are muscle spindles that detect muscle stretch
  • Afferent neuron is the Ia sensory fiber that carries the signal to the spinal cord
  • Efferent neuron is the alpha motor neuron that innervates the same muscle
  • Example is the patellar tendon reflex (knee-jerk reflex)
• Golgi tendon reflex (inverse myotatic reflex) is a polysynaptic reflex that protects muscles and tendons from excessive tension
  • Receptor is the Golgi tendon organ that detects muscle tension
  • Afferent neuron is the Ib sensory fiber that carries the signal to the spinal cord
  • Interneuron is an inhibitory interneuron that inhibits the alpha motor neuron of the same muscle
  • Efferent neuron is the alpha motor neuron, and its innervation to the same muscle is reduced
• Withdrawal reflex (flexor reflex) is a polysynaptic reflex that protects the body from harmful stimuli
  • Receptors are nociceptors and mechanoreceptors in the skin
  • Afferent neuron is a sensory fiber that carries the signal to the spinal cord
  • Interneurons include excitatory interneurons that stimulate flexor muscles and inhibitory interneurons that inhibit extensor muscles
  • Efferent neurons are alpha motor neurons that innervate flexor muscles to withdraw the limb and extensor muscles to relax
  • Example is withdrawing your hand from a hot stove