Motor Systems PDF

Motor system Receptors and afferent pathways are responsible for sensation Motor units are responsible for muscle contraction representing the final efferent pathway and effectors The sensory and motor commands are integrated Sensory component (afferent system) Sends in...

Motor system Receptors and afferent pathways are responsible for sensation Motor units are responsible for muscle contraction representing the final efferent pathway and effectors The sensory and motor commands are integrated Sensory component (afferent system) Sends information of Integrative component external/internal (Central) environment Integrates peripheric information and produces a response Motor-autonomic component (efferent system) Sends information to effectors Sensory information is required to execute a motor command  Motor systems Motor system allow motor actions, such as reflexes and voluntary movements, at high speed and with great accuracy. They control movement and posture 1) they are intimately related to sensory information: their function also depends on the sensory system; 2) they have a hierarchic organization.  The motor system is organized on three hierarchically different levels Cerebral cortex 1st motor areas cortical level Thalamus 2nd level Basal ganglia encephalic trunk (brainstem) Cerebellum Trunkg Afferent system Spinal cord Sensitive Interneurons Motor neurons receptors Other sensitive afferents 3rd level Muscles contraction and movement spinal cord Integration between the motor and sensitive systems Movement and posture depend on: 1) Reflex actions, coordinated in the spinal cord (spinal reflexes) 2) Voluntary actions, controlled by higher centers  Encephalic trunk (brainstem) Contains two main systems: 1) the medial system, for posture control; 2) the lateral system, which mainly controls the distal muscles of the limbs. Nuclei controlling the movements of the head and eyes. The cortical motor areas The primary motor cortex, premotor cortex, and the supplementary motor area constitute the highest hierarchical level. They project to the spinal cord both directly and through the brainstem. These areas are essential for the planning of movement.  Motor activity based on spinal reflexes Involuntary motor responses triggered by specific stimuli, which can be modulated by higher centers of the CNS. The reflex arch is the basic neuronal circuit involved in reflexes. sensory receptors 1 2 3 muscles  Reflexes Receptors 1 2 Effectors 3 1. Receptors 2. Sensitive afferent fibers (primary sensory neurons) transmit information from receptors to the CNS 3. Integration center in the spinal cord (for limbs and trunk) and trunk (for the head) 4. Efferent motor fibers ( -motorneurons) command muscles 5. Effectors, the muscle system  Reflexes can either be: Mono-synaptic it is the simplest circuit, composed only by a primary sensory neuron and a motor neuron. The afferent fiber directly contacts the a motor-neuron (only one synapse. Multi-synaptic One or more interneurons (many synapses) are interposed between afferent fiber and a motor-neurons. A proprioceptive reflex can be activated by the neuro-muscular spindle (muscle proprioceptor) The muscle spindle is a stretch receptor that measures the length of the muscle. It consists of intrafusal muscle fibers enclosed in a sheath (spindle). Spindles are in parallel to extrafusal fibers, they are receptors that provide information on muscle length. 1. When the muscle lengthens (is stretched) the spindles are stretched 2. The stretch causes the sensory neuron in the spindle to transmit APs to the spinal cord, where it synapses with a motor-neurons 3. Activation of motor-neurons that innervate the muscle  Myotatic reflex is a phasic stretch reflex Example: the patellar reflex Contraction of the quadriceps muscle (leg extension) caused by the percussion of the patellar tendon. Beginning The extensor muscles contract and the flexors are released allowing the extension of the leg The afferent fibers activate the a motor neurons of the quadriceps muscle (extensor), whose contraction extends the leg Beginning: Receptor: The hammer stroke on the The neuromuscular tendon elongates the spindle is stretched and The afferent fibers activate inhibitory quadriceps muscle becomes active interneurons that inhibit the a motor neurons of the flexor muscle, which is released Reflexes mediated by spindles can either be: phasic (tendon reflexes, i.e. patellar reflex): the muscle contracts only after a quick stretch and it is immediately released as the stimulus ends (elongation). tonic (tonic myotatic reflex) is at the basis of muscular tone: the muscle stays in a the contraction phase as long as the neuromuscular spindle remains in its active elongated status, even if the muscle is in contraction. The tonic myotatic reflex is dependent on g motorneurons projecting to the spindle. The shortening of the muscle causes shortening of the spindle, with a consequent decrease of information to the spinal cord. The muscle could not maintain a tonic contraction. g motorneurons projections contract the polar parts of the spindle keeping the central (sensory) part elongated during the shortening of the muscle. The information sent to the spinal cord does not decrease and the muscle remains tonically contracted even in the absence of external stimulus. Gamma projections increase the sensitivity of the neuromuscular spindles, allowing operation at all lengths of the muscle during movements and postural adjustments. Golgi tendon organs are involved in inverse myotatic reflex Located in the muscle-tendon junction, they are stimulated by the increase of the muscle tension during a contraction. Afferent fibers activate interneurons in spinal cord and inhibit the a motor-neurons of the same muscle (relaxation) excite interneurons that activate a motor- neurons of the antagonist muscle (contraction).  The inverse myotatic (multi-synaptic) reflex: Increased force in the muscle à Activation of Golgi tendon organs à Release of the ipsilateral muscle and contraction of the antagonist.  Flexor reflex (multi-synaptic) Activated by nociceptive/touch stimuli. Characterized by the flexion of the stimulated limb to avoid the painful stimulus. The flexor reflexes of the lower limbs are often accompanied by the cruciate extensor reflex aimed at maintaining balance and posture. Vie ascendenti per dolore e aggiustamento posturale Midollo spinale Recettori Stimolo doloroso Estensori inibiti Contrazione estensori permette spostamento Contrazione flessori per allontanare il piede dallo peso sulla gamba sinistra stimolo doloroso Flessori inibiti Reflex movements can be modulated by information from 1) higher nervous centers (brainstem and motor cortex); 2) sensory information that triggers reflexes. Modulation from encephalon Reflex Modulation from movements sensory information Voluntary movements require integration in the cerebral cortex (motor) and can be initiated even in the absence of external stimuli. Voluntary movements include three phases: 1) Programming (premotor cortex and supplementary motor cortex) 2) Initiation (primary motor cortex) 3) Execution The execution of the movement is continuously monitored by signals from the muscular and articular proprioceptors. The harmony of execution depends on the function of the basal ganglia and of the cerebellum.