10.1 The Physiological Actions Implementing Movement – Contraction of Muscles
Effector skeletal muscles move limbs and body parts in desired directions by pulling in a coordinated manner, typically in conjunction with antagonistic partners pulling in opposite directions. To accomplish the pulling, motor neurons synapsing at neuromuscular junctions initiate contractions. Depolarization derived from ACh opening nicotinic ionotropic receptors filters through the muscle fiber from the sarcolemma into the inner core where myofibrils reside, releasing calcium from storage in the sarcoplasmic reticulum. This calcium initiates and maintains a contractile machinery where myosin heads extending from thick filaments pull actin chains inward by forming cross-links and cocking inward, then releasing and re-binding with power strokes until the calcium dissipates. This activity represents a multiple-molecule-coordinated event occurring at different speeds and efficiencies depending on the arrangement of muscle fiber connections to the tendons, and on the efficiency of myosin head cross-bridge cycling. The pulling can either move a limb or slow externally forced movement by eliciting force attempting to counteract loads (e.g., shortening or lengthening contractions). This gets more complex when different antagonist muscles play tug-of-war across joints to produce desired movement speeds, as the same systems are in play. Force increase can be elicited by higher frequency activation of select populations of muscle fibers, or increasingly recruiting larger populations of muscle fibers to pull in synergistic (same) or close to the same directions. Muscles simultaneously pulling in distinct directions tend to produce intermediate movement trajectories between the pull directions. Blood-rich fibers are commonly recruited first. They are more resistant to fatigue. Those utilizing stored glycogen alone for their energy (ATP) source are recruited for final thrusts.
10.2 Eliciting Contractions from Lower Levels – Lower Motoneurons and Reflex Arcs
Distinguishing between upper and lower motoneurons (UMNs, LMNs) is important to divide the overall action of motor control into the conscious selection and patterning activity (upper) versus the implementation and minor adjustments to maintain bodily status quo (lower). We described how LMNs extend into the periphery to synapse with muscle fibers, and how larger LMNs synapse with more fibers than smaller, thus forming the larger and smaller motor units. Motor units activate connected muscles through neuromuscular junctions relatively simultaneously. Fibers synapsed upon typically pull in the same direction.
LMNs are activated from their central nervous system residence either at the brainstem or spinal cord, either via descending UMNs in the context of conscious intended actions or via reflex arcs creating quick local lower corrective actions while monitoring circumstances with sensory feedback. Within the spinal cord, LMNs are arranged at the ventral spinal grey in a pattern where medial LMNs connect to medial body parts. The more lateral the LMN residence, the more lateral the activated body part (e.g., finger movement most lateral in the cervical cord). Also within the spinal cord: reflex arc circuitry and circuitry supporting central pattern generation coordinate simple levels of locomotion.
Spindle fibers and Golgi tendon organs are the two main sensory feedback systems for muscles. They cover stretch and muscle tension, respectively. Muscle spindles detect stretch via the longitudinal pull on intrafusal muscle fibers, which can be tightened by the gamma motoneuron system to maintain sensitivity at critical moments. Golgi tendon organs detect tension when muscles pull against a load, producing inward pressure by the collagen fibers constituting tendons. They can help ensure a soft touch when needed. Proprioceptive sensory systems also extend into skin vibration and stretch sensations contributing to more complex actions at the lower level.
10.3 Our Brain Gets Involved – Responsibilities of Upper Motor Systems
Upper system control, via upper motoneuron sources, combines the efforts of the prefrontal cortices, the basal ganglia, the cerebellum, the premotor cortices, and the primary motor cortices. Initial maneuvers based on our needs are formulated within the prefrontal cortices. Inappropriate or inconsiderate behaviors may stem from damage within prefrontal cortex in general, or more specifics may be distorted as plans get further formulated in premotor cortices. After a big picture “what do we want” is formulated, movements to accomplish this grow in increasingly specific ways, like deciding which limbs, directions, and sequences of movement will accomplish our objectives. We build repertoires within the basal ganglia. The premotor cortices dip into the underlying striatum to engage circuits compiling and connecting movement repertoires in conjunction with the correct groups of muscles (ensembles) for the task.
As the details of movement plans are “fleshed out,” the basal ganglia loop feeds modulatory control back into the premotor cortices, and the cerebellum becomes involved by increasingly anticipating/appreciating how the intensity of movements needs to be tailored for smoothness. The cerebellum orders and manages this effort, paying attention to limb location and possible adjustment. The compiled patterning, including smoothing from the cerebellum, is fed forward into the primary motor cortex, encouraging proper sets of UMNs to activate and promote preferred movements with the proper emphasis (likely continuously reinforced by the cerebellum).
M1 primary motor cortex neurons descend into the brainstem and spinal cord to elicit movements of either the head/viscera or the body/limbs. Thus, the planned movements are fed forward toward areas closer to their physical enactment. If these signals are blocked, weakness and spasticity result because this releases lower motoneurons from control of activation and reflex circuitry through which many descending upper motoneuron signals actuate control.