Somatic Motor System Overview
Somatic Motor System Overview
Motor Neurons
* Spinal Nerves - Anterior horn of spinal cord
* Cranial Nerves - Motor Nuclei in Brainstem
Motor Control Centers
* Motor aera of cerebral cortex, basal nuclei, cerebellum, brainstem centers, and spinal cord
Pathwasy connecting control centers to one another and to motor neurons
* Corticospinal and Corticobulbar tracts (Main Voulantary)
* Medial Pathways and Lateral Pathways (helping and Muscle Tone/Reflexes)
Somatic Motor Neurons Innervate Skeletal Muscles
Somatic Motor Neurons Innervate Skeletal Muscles
Spinal Cord
* MN cell bodies - anterior gray horn of spinal cord
* MN axons - ventral (anterior) root of every spinal nerve
* Type A fibers - large diameter, mylenated 140 m/S
* inervate skeletal muscles of arms, legs, body trunk
Somatic Motor Neuorns
Somatic Motor Neuorns
Brainstem:
* MN cell bodies - cranial nerve motor nuclei in medulle, pons, and midbrain
* MN Axons - in some cranial nerves
* Eye Muscles _ CN III, IV, VI
* Muscles of Mastication CN V
* Muscles of Facial Expression CN VII
* Muscles of Neck and Pharynx CN IX, XI
* Muscles of Tounge CN XII
Control of Skeletal Muscle Contraction
Control of Skeletal Muscle Contraction
- Reflex - sensory information to motor neurons
- Voulntary - cerebral cortex plans and initiates voluntary muscle movements
- Subconscious - Various brainstem centers help control basic movements
- Cerebellum and basal nuscli - moderate ongoing muscle activity via input to subconsious and voulntary centers
Reflexes
Reflexes
- Relfflexes are rapid, Autonomic predictiable responses to stimuli
- Neural reflex pathways invloves sensory input to CNS and motor neuron output to effectors
- Can be modified by input from higher brain centers
Reflex Pathways
Reflex Pathways
- Activation of sensory nerves by a stimulus
- AP in sensory neuron propagates into CNS
- NT released onto all neurons on which sensory neuron synapses (divergence)
- Information processing ocurs at many levels of nervous system
- Motor Neuron receives multiple inputs that determine AP frequency (Convergince)
- Strength and Duration of skeletal muscle contraction determined by MN AP frequence
Reflexes of Spinal and Cranial Nerves
Reflexes of Spinal and Cranial Nerves
Monosynaptic Reflex (no Interneurons)
* Sensory neuron synapses directly on a motor neuron
Polysynaptic Reflex
* At least one interneuron between sensory afferent and motoer efferent
* Longer delay between stimulus nd repsonse
* Multiple SC segments or BS nuclei interact to form more complex responses
Stretch Reflex
Stretch Reflex
- Monosynaptic reflex
- Postural Reflex - helps maintain upright position
- Reflexively adjusts skeletal muscle length and tone
- Patellar (Knee jerk) reflex
- Sensory receptors are muscle spindles stretch receptors
- Sensory neuron axon has excitatory synapse into motor neuron
Polysnaptic reflexes
Polysnaptic reflexes
- Produce more complicated responses
- Invlove many interneurons
- Are intersegmental in distrobution
- Invlove reciprocal inhibition (EPSPs and IPSPs)
- Invlove muscle groups ipsiliateral and contralateral to stimulus
- Several reflexes may cooperate to produce a coordinated response
Flexor and Crossed Extensor Reflexes
Flexor and Crossed Extensor Reflexes
- Polysynaptic Reflex
- Withdrawl reflex and balancing reflex
- Sensory receptors are pain receptors
- Excitatory input to flexor motor neurons and inhibitory input to extensory neurons of ipsilateral leg
- Inhibitory input to flexor motor neurons and excitatory input to extensors motor neurons of contralateral leg
Reflexes can be Modified by input from higher brain centers
Reflexes can be Modified by input from higher brain centers
- Reinforcement = facilitation that enhances spinal nerve and cranial nerve reflexes
- Inhibition = supression that reduces spinal and cranial reflexes
- CNS damage can result in abnormal reflexes
- Hyperreflexia of stretch reflex ex. patellar
- Nortmal plantar reflex replaced by abnormal reflex called Babinski sign
During Development
During Development
- Spinal and cranial reflexes apperar early
- Withdrawl from pain
- Input from higher brain centers that modifiy reflexes develop as CNS matures
- interneuron synapse formation
- Mylenation of axons
- Descending inhibitory pathways
Control of Skeletal Muscle Contraction
Control of Skeletal Muscle Contraction
- Reflex - mono- or poly-synaptic input pathways to motor neurons
- Voulntary - cerebral coretx plans and initiates voluntary muscle movements
- Subconscious - various brainstem centers help control basic movements
- Cerebellum and Basal Nuclei modulate ongoing muscle activity to subconcious and voulntary centers
Motor Aeras of the Cerebral cortex Voulntary control
Motor Aeras of the Cerebral cortex Voulntary control
- Primary motor cortex - precentral gyrus
- Corresponds point by point with spesific regions of opposite side of the body
- Motor Homunculus “little Person”
- Aera of cortex devoted to spesific region of body is proportional to number of motor units, not to body aera size
Motor Aeras of the cerebral cortex voulntary control
Motor Aeras of the cerebral cortex voulntary control
- Premotor cortex = somatic motor associtation aera
- Interpret incoming data and coordinate learned motor responces, plan next movement
- Speech center
- Eye field
Corticospinal and Corticobulbar Pathways
Corticospinal and Corticobulbar Pathways
Provides voluntary skeletal muscle control via rapid direct pathway
* Cell body in primary motor cortex
* Axon decussates (crosses over) before synapsing onto motor neuron
Corticobulbar Tracts
* Synapse onto motor neurons in cranial nerve nuclei
Corticospinal Tracts ( anterior and Spinal)
* Synapse on motor neurons in the anterior gray horns of the spinal cord
The medial and Lateral Pathways
The medial and Lateral Pathways
Medial Pathway - primarily controll of muscle tone of the neck, trunk, and proximal limb muscles
* Can assist with gross movements of these regions
* Upper motor neurons in the vestibular neclei, superior and inferior colliculi, and the reticular formation
Lateral Pathway - muscle tone of the distal parts of the limbs
* Extend only to the cervical region of the spinal cord
* Role is generally insignificant compared to the motor pathways along the lateral corticospinal tracts
Brainstem motor control centers subconscious control
Brainstem motor control centers subconscious control
Vestibular Nuclei
* Sensory input from receptors from equilibrium and balance
Tectum of Mesencephlan
* Superior Colliculi - visual input - visual startle input
* Inferior Colliculi - auditory input - auditory startle reflex
Reticular Formation
* Input from many sensory and motor centers
Basal Nuclei (no direct pathways to motor Neurons)
Basal Nuclei (no direct pathways to motor Neurons)
- Basal Nuclei adjsut motor comands issued in processing centers
- Provide background patterns of movement involved in voulntary motor movements
- Background positions of the trunk or limbs
- Rythmic cycles of movement in walking or running
Cerebellum
Cerebellum
Cerebellum monitors proprioceptive information, visula information and vestibular sensations
* Minimizes number of motor comands used to perform a movement
* Makes a movvement efficent, smooth, and percisley controlled
* Compares ongoing activity with previousley learned activity patterns
