Hierarchical organization of motor network of spinal cord
Lowest to highest: Spinal cord Cerebellum Basal ganglia Motor cortex
What are reflexes in the spinal cord controlled by
Midbrain
What are the midbrain and spinal activity controlled by
Forebrain
First level in regulation of sensorimotor functions
Spinal cord
Dermatome
Area innervsted by afferent fibers of. Enron in single DRG
Myotome
Muscles innervated by axons exiting cord via single ventral root
LMN
Final common pathway for muscle activation
Motor unit (4 components)
Motor neuron (cell body)
Efferent fibers
Motor end plate
Innervated muscle fibers
Damage to LMN
Disruption of entire unit
Damage to axons branches weakening of the muscle
Innervation ratio
Number of muscle fibers per axon 1-30 (fine)
1-3000 (gross)
Temporal summation
Refers to increasing the rate of firing of individual motor units
Spatial summation
Recruiting a greater number of motor units
3 major fasciuli of white matter of spinal cord
Dorsal
Lateral
Ventral
Dorsal
Ascending sensory
Lateral
Ascending sensory and descending motor
Ventral
Ascending sensory and descending motor
Fasciculi
Bundles of longitudinal axons
Descending tracts: corticospinal tracts
Originating from pyramidal cells (betz)
Precentral gyrus (primary motor, broadmann 4)
Premotor cortex (8)
Somesthetic (3,2,1)
Supplementary motor cortex (6)
Lateral corticospinal trAct
Cortical control of skeletal muscles during skilled movements
Anterior corticospinal tract
I crossed corticospinal eventually crossing before synapse for on motor neurons
Extrapyramidal tracts
Tectospinal
Rubrospinal
Vestibulospinal
Tectospinal
Regulation of neck and body movements for startle reflexes
Rubrospinal
Regulation of muscle tone for limb extension against gravity
Originates in red nucleus
Fibers cross midline after leaving red nucleus and continue down spinal cord
May assist pyramidal system in controlling voluntary movements
Inhibits extensively alpha and gamma motor neurons
Facilitates flexor muscles and inhibits extensor muscles
Vestibulospinal
Regulation of body adjustment to stabilize head
Originates in vestibular apparatus in ear
Courses through pins, medulla
Terminates at spinal cord
Helps body maintain posture and balance
Facilitates activity of extensor muscles
Inhibits activity of flexor muscles
Reticulospinal tract
Reticular formation
Fibers terminate on gamma motor neurons
Influences on spinal nerves
Maintains upright posture
Allows some voluntary and gross motor movements
Reticular formations and reticulospinal tract dontakl built in reflective motor patterns
Fasciculi gracious and cuneatus
Ascending
Discrimitive touch and vibration from lower and upper halves
Of body
Lateral sponothalamic tract
Mediation of pain and temperature
Spinocerebellar tract
Ascending
Unconscious propriception from muscles of extremities and limbs
Alph motor neurons
9-16um
1 alpha neuron for about 200 muscle fibers
Rapid impulse conduction
Innervation of extrafusal fibers of muscles
Voluntary and reflexive movements of head, trunk, and extremities
Extrafusal fibers
Contractile elements of skeletal muscles
Gamma motor neurons
Greater in number with small diameter
Slow impulse conduction
Regulation of spindle fibers length modulating excitation of annulospiral sensory primary endings
Regulation of stretch reflex muscle tone
Firing of gamma motor neuron (efferent)
- Shortening is detected by sensory receptors
- Detection is directed back to spinal cord and synapses with alpha motor neuron
- Alpha motor neuron directs impulses back to extrafusal fibers
- Extrafusal fibers contract until they are the same length as the muscle spindles
Internuerons
Numerous specialized cells
Integration of sensory and motor functions of CNS
Renshaw cells
Capable of inhibiting alpha motor neurons
Produces a negative feedback response
Turns off alpha motor neuron after it fires and prepares to fire again
30x as many interneurons and motoneurons
Reflex response
Stereotypes movement to sensory stimulation
Neuronal circuitry
Muscle spindles Afferebt fibers Alpha motor fibers Efferent fibers Reflexive contraction of muscle fibers
Gamma motor neurons
Innervate muscle spindles (intrafiaal fibers)
Slower conduction than alpha
Primary role is to regulate length of spindle fibers
Controlled by brainstorm, Rec formation, and vestibular system
Gamma efferent leave through ventral nerve root, contract end portions of intrafusal fibers, attach central part of muscle spindles
Muscle spindles
Specialized receptors to detect degree and rate of muscle length change
Consists of intrafusal fibers (3-5 specialized)
If stretched, sends impulses in the afferent fiber from the spindle
Afferent pulses travel to alpha-motor neurons via large diameter
Result is contraction of muscles
Golgi tendon organs
Sensitive to degree of muscle tension during contraction
Reflexive inhibitor for preventing muscle damage
Prevent excessive contractions
Regulate motor neurons via interneurons
Two types of afferent spindles
Type IA: fast conduction fibers
Type II: slow conducting fibers
Intrafusal fibers are divided into
Nuclear bag:
Many nuclei
Central portion innervsted by IA fibers
Mediate dynamic responses
Nuclear chain:
Fewer nuclei
Central portion innervated by IA, end portions by II fibers
Mediate static responses