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24 Cards in this Set

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  • Back
What kind of junctions, and to what, do the lower motor neurons of the spinal cord's anterior horn form?
Neuromuscular junctions with skeletal muscle.
Where to anterior horn motor neurons receive sensory feedback from? (3)
From the muscles they control as well as from synergist and antagonist muscles.
The linkage of peripheral sensory input and anterior horn neruons form the substrate for a number of Spinal Reflexes.
The activity of lower motor neurons in the spinal cord is greatly influenced by...
Descending projections from cells in the brainstem and cerebral cortex. These brainstem and cortocal neurons are referred to as Upper Motor Neurons, and, unlike Lower Motor Neurons, they have no direct synaptic link with muscles.
The Motor system can be organized into 2 major descending systems and what are each of their components (2 and 4)?
1) Corticospinal Tracts (Pyramidal tracts, Pyramidal Motor System)
-Includes the Corticopontine and Corticobulbar tracts to the brainstem. These 2 plus the corticospinal tracts = Pyramidal Motor System
2) Extrapyramidal Motor System (Extracorticospinal tracts)
-Include Rubrospinal, Reticulospinal, and Vestibulospinal and Tectospinal tracts
The major distinction between the two major descending systems is clinical (otherwise they function as an integrated whole)
Lesions to the pyramidal motor system result in paralysis while lesions to the extrapyramidal motor system result in profound changes to motor behavior, but not paralysis.
Origin of Corticospinal Tracts (CST)
Neurons - including the large pyramidal-shaped neurons (Betz cells) - located in the Precentral Gyrus (Brodmann area 4), Premotor Cortext (Area 6), and Postcentral Gyrus (Areas 3, 1, 2).
Course of the Axons from Cortex: Internal Capsule - Posterior Limb
Axons from cortex leave via the white matter and collect in posterior limb of the Internal Capsule
Locations of CST in Brainstem (important to know diff locations to understand differing symptoms that occur with lesions in different locations) - Midbrain
Axons descend through the midbrain and from the portion of the Cerebral Peduncle called the Crus Cerebri
Locations of CST in Brainstem - Pons
Axons pass through the pontine region as the Pyramidal Tracts (Corticospinal Tracts)
Locations of CST in Brainstem - Medulla
1) Pyramidal tracts in the medulla form an elongated bulge on the ventral surface of the medulla known as the Medullary Pyramids
2) In the lower medulla, about 85-90% of the axons of the pyramidal tracts cross to the contralateral side as the Pyrmidal Decussation
Cortical projections also go to midbrain, medullar (Corticobulbar Tract), and pons (Corticopontine Tract); all of these collectively are now being called the...
and what do these tracts synapse with and what do these axons control (5)
Corticonuclear Tracts
-These tracts synapse with certain cranial nerve motor nuclei in the brainstem
-These axons control eye movement (cranial nevers III, IV, and VI), facial muscles (CN VII), muscles of mastication (CN V), muscles of swallowing (CN IX and X), and tongue musculature (CN XII)
Spinal Cord Location and Termination of Corticospinal Tracts - 2 types of CST
1) The axons which decussate in the Lower Medullar (85-90%) descend in the spinal cord as Lateral Coricospinal Tract (Lateral CST)
2) The axons which remain ipsilateral (15%) descend as the Ventral (Anterior) Corticospinal Tract
Lateral CST - what does its axons synapse with, functions, and injuries
These axons synapse with spinal cord neurons for the initiation of voluntary muscle activity, especially in distal muscles (hands, etc.)
Injuries to the Lateral CST in the Spinal Cord (actually, a lesion anywhere alons its course and also including the corticopontine and corticobulbar tracts) are Classified Clinically as UPPER MOTOR NEURON LESIONS
Ventral CST - what does its axons synapse with, functions, and injuries
-These synapse primarily w/ neurons controlling axial musculature to help maintain balance and posture.
-It is not possible clinically to recognize lesions specifically involving the Ventral CST
The corticospinal tracts synapse with (2)
1) Ventral Horn Motor Neurons
2) Also interneurons of the dorsal horn nucleus proprius
-this connection plays a role in modulation of sensory input
-much of this input is inhibitory
Background on Lower Motor Neurons - found where, include what
Include all neurons that directly innervate skeletal muscles.
Found in both the spinal cord and brainstem (certain cranial nerve motor nuclei that control skeletal muscle are also classified as lower motor neurons)
Injury to neurons that innervate skeletal muscle is classified clinically as Lower motor neuron injury
Background on Upper Motor Neurons - includes what, refer to what
Includes all of the descending fiber systems that can influence and modify the activity of lower motor neurons.
Usually refer to the axons of the corticospinal and corticobulbar tracts
Injury to the corticospinal tract is classified clinically as an Upper motor neuron injury (doesn't mean higher up in spinal cord, means in the CST)
Lower Motor Neuron Lesion (results from and characteristics (4)
Results from damage to ventral horn motorneurons, ventral roots, and the spinal nerve itself, ie: peripheral nerve injury
1) Flaccid Paralysis (or paresis - weakness)
2) Hypotonia (decreased muscle tone)
3) Hyporeflexia (diminished reflexes) or Areflexia (no reflexes)
4) Muscle Atrophy - loses its CT
More info on Flaccid Paralysis and Hypotonia
-The combination of decreased muscle tone and a paralysis is called a Flaccid Paralysis
-Muscle tone is defined clinically as the degree of resistance imparted by a limb or body part that is being passively manipulated by the examiner
Upper Motor Neuron Lesion (results from and characteristics (5)
Results from spinal cord injury.
*All characteristics occur ipsilateral from the site of lesion and below*
1) Spastic Paralysis (or paresis)
2) Hypertonicity (Spasticity)
3) Hyper-reflexia - hyperactive myotatic reflexes (exaggeraed knee-jerk and other deep tendon reflexes)
4) Babinski Sign (Reflex)
5) Clonus
Spastic Paralysis
The combination of increased muscle tone and paralysis. Increased muscle tone occurs from no inhibition to the descending control of gamma neurons
Babinski Sign (Reflex)
1) Pathological reflex resulting from an upper motor neuron lesion
2) When the sole of the foot is firmly stroked there is a dorsiflexion of the big toe and a fanning of small toes. However, the normal response is a plantar flexion of the toes
3) Be aware that infants will show a Babinski sign until the nervous system has matured and has produced all of its myelin (about 18 months)
Alternating contraction of antagonistic muscles resulting in a series of flexion and extension movements
Common locations of upper motor neuron lesions (4)
1) Cortical Stroke
2) Internal Capsule Stroke
3) Brainstem Lesions
4) Spinal Cord Injury