Spinal Motor Control and Muscles

Spinal Motor Control

• Spinal motor control manages much of our motor output, using simple circuits between receptors, the spinal cord, and muscles.
• The brain isn't always needed for simple movements: these are fast and reliable.

Muscles

• Motor Unit: A single motor neuron and all the individual skeletal muscle fibers it innervates.
• Motor Pool: All motor neurons innervating a specific muscle.

Neuromuscular Junctions

• Acetylcholine is the neurotransmitter used at the neuromuscular junction.
• Acetylcholinesterase rapidly breaks down acetylcholine, closing channels.
• Motor end-plate: the receptive area on the muscle fibre at the neuromuscular junction.

Muscles (cont.)

• Muscles are attached to bones by tendons.
• Fast muscle fibers are forceful but fatigue quickly; they are poorly vascularized.
• Slow muscle fibers are weaker but highly enduring; they are well-vascularized.

Flexors & Extensors

• Synergistic muscles work together to produce the same movement.
• Antagonistic muscles produce opposite movements.
• Isometric contractions increase muscle tension without movement.

Receptors in Tendons & Muscles

• Golgi Tendon Organs: Embedded in tendons, they respond to muscle tension and help prevent over-contraction.
• Muscle Spindles: Embedded in muscles, they respond to changes in muscle length and provide proprioception.

Extrafusal & Intrafusal Muscle

• Intrafusal motor neurons adjust the length of intrafusal muscles, maintaining appropriate tension on muscle spindles. Without these neurons, spindles would be unresponsive to stretch during muscle contraction.

The Stretch Reflex

• The stretch reflex adjusts muscle tension in response to external forces.
• Intrafusal motor neurons adjust the length of intrafusal fibers.
• The reflex is monosynaptic (fast).

Muscle-Spindle Feedback System

• Unexpected forces trigger increased firing in spindle afferent neurons.
• This in turn increases firing in biceps motor neurons, maintaining the arm's original position.

Withdrawal Reflex

• Sensory neurons carry pain signals to interneurons on flexors initiating withdrawal in 1.6 ms.
• The withdrawal reflex happens before the brain receives the signal.
• Reciprocal innervation: when one muscle contracts, its antagonist relaxes.
• Inhibitory interneurons mediate this relaxation.
• Co-contraction helps with smooth movement.

Recurrent Collateral Inhibition

• Function: to distribute workload across motor pools.
• Process: An axon collateral excites Renshaw cells (inhibitory interneurons) in the ventral horn.
• Inhibiting the motor neuron that activated it.
• Reinforcement of rest after motor firing..

Walking: A Complex Sensorimotor Reflex

• Walking relies on visual, somatosensory, and vestibular information.
• It involves integrated movements of the trunk, legs, feet, and arms.
• The walking program is plastic, adapting to changing terrain and forces.

Central Sensorimotor Program Theory

• Lower levels of the sensorimotor system contain programmed activities.
• Complex movements activate a combination of these commands.
• Execution does not require higher-level oversight..
• Sensorimotor programs are not concerned with details only the goal.

Characteristics of Central Sensorimotor Programs

• Motor equivalence: one task can be achieved with different muscles or same muscles in different ways.
• Central sensorimotor programs use different muscles or body parts to achieve the same task.
• Control is high in the system.

Characteristics of Central Sensorimotor Programs (cont.)

Sensory information for controlling sensorimotor programs may not need to be conscious.

• Finger position when picking up objects is consistent with their actual sizes.
• not with the sizes we perceive them.

Characteristics of Central Sensorimotor Programs (cont.)

• Species-typical behavior may develop without practice.
• Fixed action patterns are a complete, instinctive behavioral sequence.

Characteristics of Central Sensorimotor Programs (cont.)

• Practice can create or refine central sensorimotor programs.
• Transferring control to lower levels frees up higher areas.
• Increased speed in tasks is achieved when lower levels process information simultaneously.

Characteristics of Central Sensorimotor Programs (cont.)

• Response chunking combines programs into larger units of programs.
• Smaller units such as key presses become chunks in typing and larger units like words and sentences arise out of key press chunks.

Characteristics of Central Sensorimotor Programs (Final)

• Acquired motor skills can be disrupted by conscious thought.
• Higher-level areas can interfere with well-established lower-level control mechanisms.

Functional Neuroimaging of Sensorimotor Learning

• Dorsolateral Prefrontal Cortex: active primarily during new motor sequences and conscious motor commands
• Premotor Cortex: plays a role in sensory-guided movements and new sequences
• Supplementary Motor Cortex: controls automatic movements and well-practiced motor sequences.
• Primary Somatosensory & Motor Cortices: active during both new and well-practiced motor sequences.
• Posterior Parietal Cortex & Cerebellum: active primarily during new motor sequences

Neuroplasticity and Sensorimotor Learning

• Sensorimotor learning changes in both cortical and subcortical levels
• Thalamic inputs and oligodendrocytes (supporting cells) in subcortical white matter increase during learning.

Bridging Spinal Injuries

• Spinal injuries can cause complete paralysis.
• Bridging severed spinal tracts is a focus of many new technologies.