Motor systems - function • locomotor, postural • eye movements • breathing • nutritional • speech and communication • defense • reproduction • manipulation Movements • reflexive - fast, involuntary coordinated patterns of muscle contraction and relaxation elicited by peripheral stimuli • rhythmic - chewing, swallowing, scratching, walking, breathing • voluntary movements - initiated to accomplish a specific goal • conscious processes are not necessary for moment-tomoment control of movement Sensory information are important in movement control • to trigger behaviorally meaningful motor acts - withdrawal, coughing or swallowing • to control of an ongoing motor pattern – breathing, locomotion • to influence the level of activity of one muscle or a group of close synergists – reflexes • to detect and counteract any disturbance of body posture – skin, vestibular system, vision, proprioreceptors • when object is held by the fingers – skin receptors Motor commands • are derived from sensory inputs (sensorimotor transformations) Motor learning Motor unit • the elementary unit of motor control • one motor neuron innervates from a few to several thousand muscle fibers Reflexes • are automatic, stereotyped movements in response to stimulation of periphery receptors • but reflexes are flexible - under normal conditions they can be adapted to a task Proprioreception • precise information about: - the length of the muscle and force exerted - joint position and angel • e.g. muscle spindle, Golgi tendon organ The stretch and tendon reflexes • control the length of the muscle and its prevention from generating excessive tension • stabilization of posture • provide a mechanism for compensating for small changes in load and intrinsic irregularities in the muscle contraction • cutaneous reflexes produce complex movements that serve protective and postural functions • central motor commands and cognitive processes can alter synaptic transmission in spinal reflex pathway • e.g. the strength of the monosynaptic reflex declines as we progress from standing to walking to running Damage to the CNS produce characteristic alterations in reflex responses and muscle tone • areflexia or hyporeflexia: often indicate a disorder of one or more of the components of the peripheral reflex pathway (also from lesion of the CNS) • hyperreflexia: indicates the lesion of the CNS • paresis • plegia Central pattern generators • are neuronal networks capable of generating a rhythmic pattern of motor activity without phasic sensory input (walking, swimming, respiration, ..) • the basic pattern produced by a CPG is usually modified by sensory information from peripheral receptors and signals from other regions of the CNS Locomotion Phases of the step cycle: swing and stance Locomotion • important sensory information: proprioception, tactile receptors in feet, visual • central pattern generators in spinal cord • mesencephalic locomotor region - initiation and speed • postural stability during locomotion - corresponding structures • goal directed locomotion - cerebellum, basal ganglia, sensorimotor areas (motor cortex, posterior parietal cortex) Posture • to maintain a steady stance (body orientation) in the presence of gravity - tonic activation of antigravity muscles (neck, back and leg extensors) • to maintain equilibrium (balance) during different conditions, e.g. motor planning, anticipatory postural adjustments • balance control is also influenced by emotional state • automatic postural response is a synergistic activation of a group of muscles with the goal of maintaining equilibrium, it is not a simple reflex ! Automatic postural responses counteract unexpected disturbances Stance determines postural response Postural control • important afferent information: somatosensory, vestibular and visual • spinal cord, reticular formation • cerebellum (vestibulo- and spinocerebellum), basal ganglia • suplementary motor area, sensorimotor cortex Voluntary movements differ from reflexes • are initiated by an internal decision to act • involve choices between alternatives • are organized to achieve some goal in the near or distant future • context-dependent associations with sensory inputs • the effectiveness improves with experience and learning Control of motor behaviour • involves a sequence of neuronal operations that select, plan and execute a movement • parietal, premotor, prefrontal and primary motor regions of the cerebral cortex • the primary motor cortex plays an important role in the generation of motor commands • corticospinal pathway: from primary motor cortex, premotor cortices and parietal cortex Direct and indirect pathways • fine and precise finger movements – direct corticospinal tract to the lateral alpha motoneurons • postural adjustment – indirect pathways to the medial alpha motoneurons (through interneurons bilaterally) Grasping • intention – motivational subcortical areas • identification and localization of the object in space – posterior parietal cortex • planning – premotor cortex and SMA • choosing the proper motor program from cerebellum and basal ganglia • movement execution – primary motor cortex and premotor cortex (area 6) Cerebellar functions • control of balance and muscle tone • movement error correction, movements coordination • motor learning • motor planning and movement execution • cognitive functions: - timing of serial events - judging the elapsed time in cognitive tasks - comparing speed of moving objects - word-association task Cerebellum - motor learning Cerebellar dysfunction 1) hypotonia (pendular reflexes) 2) ataxia of stance (astasia) and gait (abasia) 3) ataxia = abnormal execution of multi-jointed voluntary movements dysmetria, dysdiadochokinesis 4) intention tremor Basal ganglia Major functional roles of the basal ganglia • selection of appropriate voluntary movement, and the simultaneous suppression of unwanted movement • action selection, execution of automatic movements • control of motivated behaviour, mood → adaptive shaping of behaviour and action selection • motor (skills) and non-motor (habits) learning Bazal ganglia disorders • hypokinetic syndrom - akinesis, bradykinesis, rigidity, tremor (resting) • hyperkinetic syndrom - hypotonia, dyskinesis (e.g. chorea, ballism) The control of gaze • conjugate X disconjugate • six extraocular muscles form three complementary pairs, that are controlled by three cranial nerves Saccadic eye movements To redirect the fovea on visual target in the environment • shift the fovea rapidly to a visual target • driven e.g. by the existence of object of interest in the visual field Smooth pursuit movements To keep the fovea on visual target in the environment. • keep the image of a moving target on the fovea • driven by slow moving object Vestibulo-ocular reflex To stabilize the eye during head movement. • i.e. to hold images still on the retina during brief head movements • driven by signals from the vestibular system Optokinetic reflex To stabilize the gaze • i.e. to hold images during sustained head rotation or translation • driven by visual stimuli Vergence eye movements To keep the fovea on visual target in the environment. • move the eyes in opposite directions while redirecting gaze from near to far point → the image is positioned on the same place on both foveae • driven by retinal disparity Fixation system • hold the eyes still during intent gaze • this requires suppression of eye movement Eye movements control • motivational systems - choose significant objects in the environment as target for eye movements • cortex: posterior parietal (area 7) - attention frontal eye field (area 8) – motor commands → superior colliculus → brain stem - reticular formation - motor programming: eye position and velocity