Neural Control: Motor Tracts

tracts aka

bundled axons

2 main tracts are found in the spinal cord

• ascending (sensory)

• descending (motor)

4 generalizations of neuronal pathways

• decussation

• relay

• somatotopy

• symmetry

decussation

most pathways cross over from one side of the CNS to other

relay

most pathways consist of a chain of 2-3 neurons

somatotopy

most pathways have a precise spatial mapping (tract fibers reflecting the body)

symmetry

pathways are paired (left and right)

descending tracts function

provide motor signals from UMNs to LMNs (& local circuit neurons) in the brainstem or spinal cord

descending tracts 2 types

• pyramidal (direct) tracts

• extrapyramidal (indirect) tracts

pyramidal (direct) tracts travel

input to LMNs directly from the primary motor cortex

pyramidal (direct) tracts function

regulate skilled movements

extrapyramidal (indirect) tracts travel

input to LMNs from the brainstem

extrapyramidal (indirect) tracts function (3)

• regulate eye, head, and neck mvm

• aids in balance and posture

• unconscious movements

pyramidal (direct) 2 tracts

• corticospinal tract (CST)

• corticobulbar tract

corticospinal tract

from primary motor cortex → spinal cord (spinal nerves)

corticobulbar tract

from primary motor cortex → brainstem (cranial nerves)

corticospinal tract travel

axons from UMNs in the cerebral cortex descend to the spinal cord

• synapse w a local circuit neuron or LMN

corticospinal tract function

controls movement of the body

corticospinal tract 2 types

• lateral CST

• anterior CST

corticospinal tract originates

from primary motor cortex (UMN)

corticospinal tract runs

through brainstem

CST lateral and anterior divides where

at the medulla

lateral CST function

most imp pathway for controlling voluntary movement

lateral CST descends

in lateral column of spinal cord

lateral CST synapses with

local circuit neurons or LMN in ventral horn of spinal cord

lateral CST function

control movements of the distal limbs

fractionation def

ability to activate individual ms independently from others

lateral CST is important for controlling voluntary mvm via

fractionation

• due to activation of inhibitory interneurons (aka prevents unwanted ms from contracting)

anterior CST descends

in the anterior column of the spinal cord

anterior CST decussate at

spinal cord level

anterior CST synapses with

local circuit neurons / LMNs in the ventral horn of the spinal cord

anterior CST function

controls muscles of the trunk and proximal limbs

lateral CST pathway 6 steps

1) starts primary motor cortex, cell body of motor neuron

2) travels down through brainstem

3) splits at medulla

4) whatever level splits lateral side of spinal cord

5) synapses w LMN in ventral horn

6) controls distal limb

anterior CST pathway 6 steps

1) starts primary motor cortex, cell body of motor neuron

2) travels down through brain stem

3) keeps travelling down ant spinal cord

4) whatever level, splits

5) synapses w LMN in opp ventral horn

6) controls trunk/postural stuff

extrapyramidal (indirect) includes

all other motor tracts

extrapyramidal (indirect) travel

axons from UMNs in the brainstem descend to the spinal cord & synapse on LMNs

extrapyramidal (indirect) includes (5)

• rubrospinal

• tectospinal

• vestibulospinal

• lateral reticulospinal

• medial reticulospinal

the neurological exam

clinical assessment used to determine what specific parts of the CNS are affected by damage or disease

the neurological exam: 5 sections

• mental status exam

• cranial nerve exm

• coordination

• sensory exam

• motor exam

mental status exam evaluates

frontal cortex/cerebrum

cranial nerve exam evaluates

brainstem

coordination evaluates

cerebellum

sensory and motor exam evaluate

spinal nerves & reflexes

upper motor neuron lesions due to (7)

• cerebrovascular accidents

• ALS

• MS

• TBI

• cancer

• neurodegeneration

• infections

UMN lesions S/Sx (3)

• hemiplegia (one side paralysis)

• paraplegia (waist dow paralysis)

• quadriplegia (all paralysis)

UMN lesions clinical manifestation

• bilateral S/Sx

• spasticity → inc tone

• loss of voluntary control

• sensory loss

• abnormal reflexes → hyperreflexia of deep tendon reflexes

LMN lesions due to (2)

• trauma

• viruses

LMN lesions S/Sx (7)

• unilateral

• dec ms tone → hypotonia

• paralysis of involved ms

• sensory loss

• progressive ms atrophy

• diminished or absent spinal reflex

• fasciculations → ms twitching

if there is an injury below the medulla

S/Sx will be on the same side as injury

if there is an injury above medulla

S/Sx are on opp sides of injury