Motor System I.pptx

1 Motor System: Lower Motor Neuron 2 Motor Axis of the Nervous System  Motor system:  Controlling motor body activities (muscles):  Skeletal (striated) muscles  Smooth muscles (internal organs, heart, stomach…) 3 Motor of The Axis Corticospinal the Nervous System  Tract Motor (pyramidal) system...

1 Motor System: Lower Motor Neuron 2 Motor Axis of the Nervous System  Motor system:  Controlling motor body activities (muscles):  Skeletal (striated) muscles  Smooth muscles (internal organs, heart, stomach…) 3 Motor of The Axis Corticospinal the Nervous System  Tract Motor (pyramidal) system is composed by 2 order neurons:  1st order neuron: upper motor neuron:     Starts at the motor cortex and ends at the anterior (ventral) horn of the spinal cord. Longest fibers of the CNS. Stimulates and modulates the activity of the lower motor neuron. 2nd order neuron: lower motor neuron    Starts at the anterior horn of the spinal cord and ends at the neuromuscular junction. Exits muscles. Integrated in the circuit of motor reflexes. 4 Organization of the Central Nervous System: Spinal Cord Level  Spinal cord level:   Intersection of signals conduction from periphery of body to brain and vice versa. Spinal cord is the center of processing basic sensory & motor information: o Circuits for movement and motor reflexes:    o Walking circuits Withdrawal reflex circuits Circuits that support against gravity Circuits for reflexes that control internal organ functions (e.g. blood vessels, gastrointestinal, urination…). 5 Neuronal Organization of the Spinal Cord  Sensory neurons (dorsal root ganglion) Transmit signals to higher centers (Dorsal Column-Medial Lemniscal pathway).  Synapse locally in the spinal cord to be integrated in motor reflexes.   Motor neurons (ventral horn of spinal cord) Directly innervate skeletal muscle fibers (lower motor neuron).  Alpha Motor neurons (A-alpha axons) excite extrafusal muscle fibers (motor unit) for muscle contraction.  Gamma Motor neurons (A-gamma axons) excite intrafusal muscle fibers (muscle sensory receptors) to adjust muscle sensory inputs.  In contact with the upper motor neuron.   Interneurons (dorsal horn of spinal cord) Local neurons that have interconnections with sensory & motor neurons.  Synapse with motor neurons at the anterior horn of spinal cord.  Inhibitory or excitatory interneurons.  Integrated in motor reflexes. to Brain  6 Sensory Receptors of the Muscle  Proprioceptive sensation: senses the state of muscles (starched, contracted, relaxed) and the tension practiced on the muscle.  Sensory receptors of the muscle are mechanoreceptors (i.e. stimulated by mechanical displacement of the muscle).  Slowly adapting receptors.  Muscle Spindles: Are distributed throughout the muscle belly (intrafusal fibers). Sense muscle length (stretched, relaxed, contracted) and rate of change of length.  Golgi Are Tendon Organs: located in the muscle tendon. tendon tension and rate of change of tension. Sense  Signals from muscle spindles and Golgi tendon organs: Transmitted to higher centers (e.g. cerebral cortex, cerebellum) through the dorsal column-medial lemniscal pathway. Integrated in reflexes (spinal cord). 7 Muscle Spindle  Muscle Spindle is a stretch receptor located at the intrafusal muscle fibers (do not have myosin and actin).  It signals the length of muscles and changes in length of muscles.  Stretching the muscle excites the receptor and increases the AP firing in the fiber. Contraction of the muscle inhibits the receptor and decease the AP firing in the fiber.   Signals are transmitted through two fiber types:  Primary sensory ending (Ia) fibers: transmit information about the rate of changing of the muscle length.  Secondary sensory ending (II) fibers: transmit information about the degree of stretch of the muscle. 8 Golgi Tendon Organ  Encapsulated sensory receptor through which a bundle of muscle tendon fibers passes.  Stimulated when this bundle of muscle fibers is “tensed” by the contraction or the stretching of the muscle (sensitive to muscle tension).  Signals are transmitted through Ib fibers.  Ib fibers inhibit indirectly (through interneurons) alpha motor neuron at the spinal cord to allow the muscle to relax and thus to relieve the tension on the tendon (Golgi Tendon Reflex). 9 Lower Motor Neurons  Two types of lower motor neurons (MN) are found in the ventral horn of spinal cord:  Large a MNs innervate extrafusal fibers of skeletal muscle that generate the forces needed for posture and movement (i.e. contraction).  Small g MNs innervate intrafusal muscle fibers (muscle spindles) to modulate (adjust) sensory inputs. 10 Motor Unit    Muscle fibers are innervated by nerve fibers (motor neurons) to cause muscle contraction. One nerve fiber innervates multiple muscle fibers. The motor unit is the smallest subunit of muscle fibers that can be controlled by a single motor neuron. The synapse between the motor neuron and muscle fiber is called ‘Neuromuscular Junction’. It releases Acetylcholine. o Small muscles that require precision (e.g. hand, eye, laryngeal…) have low innervation ratio (i.e. one nerve innervates 2-3 muscle fibers). o Large muscles that do not require precision (e.g. soleus, quadriceps…) have high innervation ratio (i.e. one nerve innervates 80100 muscle fibers). 11 Force Gradation - Summation  A muscle twitch (contraction) is the tension developed in response to one motor neuron stimulation.  Summation means the adding together of individual muscle twitches to increase the intensity of overall muscle contraction.  Summation occurs in two ways:   Multiple fiber summation:  Increasing the number of motor units contracting at the same time (recruitment) Frequency summation  Increasing the frequency of stimulation of one motor unit. 12 Multiple fiber summation - Size Principle  Size Principle: depending on the intensity of the stimulation, motor units are recruited in an orderly fashion according to their size:    Smallest motor units are recruited first (for weak stimulation) Largest motor units are recruited last (for strong stimulation) It allows the graduations of muscle force from small steps (weak contraction) to great steps (strong contraction). 13 Frequency Summation & Tetanization   As the frequency of the signal increases, individual twitchs summate (no recovery form previous contactions). When the frequency reachs a critical level, the summated twitchs fuse together to form one continuous contacation, Tetanization (tonic state):   Contraction reaches a Max level, no response to further stimulations. Ca2+ ions are maintained in the sarcoplasm, the contractile state of the muscle is sustained (myosin & actin stay attached). https://www.youtube.com/watch?v=_IGbNiN3I-I 14 Integration center of the Spinal Cord  Sensory neurons:  Afferent fibers from muscle spindle & Golgi tendon,  Cell body in the dorsal root ganglion,  Synapse in the spinal cord directly with motor neurons (monosynaptic) or interneurons (polysynaptic).  Motor neurons:  Cell body located in the anterior horn of the spinal cord,  Directly innervate skeletal muscle fibers (lower motor neuron):    Alpha Motor neurons (A-alpha axons) excite extrafusal muscle fibers (motor unit) for muscle contraction.  Gamma Motor neurons (A-gamma axons) excite intrafusal muscle fibers (muscle sensory receptors) to adjust muscle sensory inputs. In contact with descending fibers (i.e. upper motor neuron Interneurons:  Local neurons in the spinal cord that have interconnections with sensory & motor neurons,  Synapse with motor neurons at the ventral horn of spinal cord.  Inhibitory or excitatory interneurons,  Modulate the activity the motor reflexes. 15 Spinal Cord Reflexes  Spinal cord reflex: is a mechanism by which a sensory impulse is automatically converted into a motor response.  Involuntary (i.e. no need of upper brain levels) and rapid (seconds) mechanism that is controlled by the spinal cord integration center.  Protective mechanism to the body (e.g. prevent an over stretch/contraction of the muscle, withdrawal from a painful stimulus, keep balance…).  Spinal Cord Reflexes:  Stretch reflexes:   Patellar reflex (Knee jerk reflex). Biceps reflex.  Golgi Tendon Reflex.  Withdrawal Reflexes:   Withdrawal reflex. Crossed extensor reflex. 16 Stretch Reflexes  Mechanism of a stretch Reflex:   Stretching the muscle activates the muscle spindles. Associated sensory fibers (Ia & II) transmit impulses to the spinal cord:  1. Sensory fibers synapse directly (monosynaptic) with a motor neuron to cause the contraction of the stretched muscle (extensor).  2. Sensory fibers synapse with an interneuron that inhibit the motor neuron of antagonist muscles (flexors) to prevent it from contracting (i.e. relax). Stretch Reflex - Neuronal Circuits Example: Patellar reflex (Knee Jerk Reflex)  Stretch (patellar) reflex causes a sudden kicking movement of the lower leg in response to a sharp tap on the patellar ligament.  Function of the stretch reflex: is to oppose sudden changes in muscle length (stretching). Over-stretching = tear.  Stretching the muscle activates sensory fibers of muscle spindles:  Sensory fibers excite monosynaptically (directly) alpha motor neurons of the stretched muscle (i.e. Quadriceps) and cause it to contract (extend of the lower leg).  Sensory fibers excite interneurons that in turn inhibit alpha motor neurons of the antagonist muscle (i.e. Hamstrings) to prevent it to contract (i.e. relax). Polysynaptic (indirect) inhibition.  Abnormality of the reaction suggest that there may be damage to the CNS or PNS. 17 18 Stretch Reflex - Neuronal Circuits Example: Biceps reflex  Biceps reflex causes a sudden kicking movement of the lower arm in response to a sharp tap on the biceps tendon.  This reflex opposes further stretch of the muscle.  Stretching the muscle activates the muscle spindles:  Sensory fibers excite monosynaptically (directly) alpha motor neurons of the stretched muscles (Biceps & Branchialis) and cause them to contract (flexion of the lower arm).  Sensory fibers excite interneurons that in turn inhibit alpha motor neurons of the antagonist muscle (Triceps) to prevent it to contract (i.e. relax). Polysynaptic (indirect) inhibition. (Triceps) (Biceps) (Branchialis)  Abnormality of the reaction suggest that there may be damage to the CNS or PNS. Damping Function of the Stretch Reflexes in Smoothing Muscle Contraction.    An important function of the stretch reflex is its ability to prevent jerkiness of body movements, damping (smoothing) function. Motor signals from spinal cord to muscles are often not stable and vary in intensity (e.g. too strong, too weak…), cause jerky muscle contractions. Neuronal circuit of the stretch reflex acts to dampen (stabilize) the signals for smooth contraction (i.e. averaging the signal). Animal with intact muscle spindle sensory nerves Smooth muscle contraction Animal with sectioned muscle spindle sensory nerves Jerky muscle contraction 19 20 Golgi Tendon Reflex Opposite Function: of the stretch reflex (inverse stretch reflex). to relieve excess tension on the tendon (i.e. prevent it from tearing). Contracting of a muscle (e.g. Quadriceps) creates a tension on the tendon attached to the same muscle and thus activates the Golgi tendon organs. Golgi tendon sensory fibers (Ib) transmit impulses to spinal cord: Sensory neurons synapse with inhibitory interneurons that inhibit alpha motor neurons and cause the contracted muscle (e.g. Quadriceps) to relax. Sensory neurons synapse with excitatory interneurons that excite alpha motor neurons of antagonist muscles (e.g. Hamstrings) and cause it to contract. 21 Withdrawal Reflex Also known as the Flexor reflex  The withdrawal reflex is initiated by a painful stimulus that causes automatic withdrawal of the threatened body part.  Pain sensory neurons synapse with a pool of spinal cord interneurons:  Excite motor neurons of the flexor muscle (e.g. Biceps) and cause muscle to contract.  Inhibit motor neurons of the extensor muscle (e.g. Triceps) and cause the muscle to relax. 22 Withdrawal and Crossed Extensor Reflex - Neuronal Circuits  Two reflexes: Withdrawal & Crossed Extensor reflexes:  Painful stimulus elicits:   Flexor reflex (i.e. withdrawal reflex) in affected limb (limb withdrawal), Extensor reflex (i.e. crossed extensor reflex) in the opposite limb (limb extension):   Contraction of the extensor muscle and relaxation of the flexor muscle. This series of reflexes serves to push body away from the stimulus, but also to shift weight to the opposite limb (i.e. prevent falls). 23 Motor Reflexes  Stretch/Golgi Tendon Reflex: https://www.youtube.com/watch?v=z3Z1NhXCLO0  Withdrawal/Crossed Extensor Reflex: https://www.youtube.com/watch?v=4oUCcIt3TtU 24 Stretch Reflex - clinical applications  Abnormality in the stretch reflexes response suggests that there may be damage to the CNS or PNS.  Hyporeflexia: weak (or absent) reflex response:    The problem is in the lower motor neuron. Disruption of either afferent or efferent fibers (PNS) in the stretch reflex loop (e.g., cut in the fibers, neuropathies, nerve or root compression…). Localisation of a spinal segment:    Patellar reflex corresponds to L4 Biceps reflex corresponds to C5-C6 Hyperreflexia: strong reflex response:    The problem is in the upper motor neuron. The upper motor neuron has an inhibitory control on the stretch reflex. Loss of central command on the spinal cord leads to an enhanced stretch reflex (e.g., CNS lesions affecting the motor pathways). L4 C5-C6