Descending tracts and reflexes Flashcards
Within what fissure of the brain are the motor regions of the legs found?
Longitudinal fissure
Where is the primary motor cortex?
Pre-central gyrus
What lobe of the brain are the motor areas part of?
Frontal lobe
Would injury to the primary motor cortex result in ipsilateral or contralateral paralysis/paresis
Contralateral
What is a motor unit?
a lower motor neuron & the extrafusal muscle fibres it innervates
What is a myotome?
muscle fibres innervated by a single spinal nerve
What do alpha motor neurons innervate?
MOTOR UNITS of extrafusal fibres. Directly responsible for innervating and making a muscle contract!
What do gamma motor neurons innervate?
Intrafusal fibres of MUSCLE SPINDLES. Intrafusal muscle fibres are found inside the muscle spindle, and keep the muscle spindle under load during muscle contraction.
Describe the structure of alpha motor neuron axons
Large myelinated axons
Describe the structure of gamma motor neurone axons
Small diameter axons
What are intrafusal muscle fibres?
found inside the muscle spindle, and keep the muscle spindle under load during muscle contraction. Detect changes in muscle length.
How many neurons do descending tracts consist of?
Upper motor neuron and lower motor neuron (2-neuron chain)
Where can UMN lesions occur?
Cortex, corona radiata, internal capsule, descending tracts, brainstem, spinal cord
Where can LMN lesions occur?
Spinal cord (at the level of LMN cell body), spinal nerve, caudal equina, peripheral nerve damage
What are some causes of motor neuron damage?
Stroke, motor neuron disease, multiple sclerosis, CNS tumour, Meningeal tumour, spinal tumour, trauma, penetrating injury, fracture, dislocation, stenosis, IV disc prolapse
What are the symptoms of upper motor neuron lesion? (Hint: HENS)
Spastic paralysis, hyper-reflexia, no muscle wasting, extensor plantar response (Babinski reflex) - abnormal in adults
What are the symptoms of LMN lesion? (Hint: Form My Farm House)
Flaccid paralysis, hypo-reflexia, muscle wasting (atrophy), fasciculations (twitching)
What is monoplegia?
Paralysis of one limb
What is hemiplegia?
Paralysis of both limbs on one side
What is diplegia?
Paralysis of two limbs symmetrically eg. both arms
What is paraplegia?
Paralysis of both legs
What is quadraplegia?
paralysis of all four limbs
How many anterior spinal arteries are there and what do they supply?
1x anterior spinal artery supplies anterior 2/3 of spinal cord
How many posterior spinal arteries are there and what do they supply?
2x posterior spinal arteries supply posterior 1/3 of spinal cord
What are some causes of infarction?
Embolus, atheroma, dissecting AAA, tumour, trauma
What causes Amyotrophic Lateral Sclerosis: Lou Gehrig Disease (ALS)?
Degeneration of the corticospinal tract and ventral horn of the spinal cord
How does ALS present?
UMN and LMN symptoms are seen together. Often limb onset first and then spreads to other areas of body.
- Fasciculations
- Spasticity/cramps
- Weakness (limbs, neck, diaphragm)
- Dysartria, dysphagia, dyspnoea
What are the symptoms of a LMN disease?
muscle weakness, wasting, areflexia, fasciculations
Why do UMN lesions result in hyper-reflexia and spasticity?
In UMN lesions, the LMN is typically hyper-excitable due to the loss of the inhibitory input from the Reticulospinal Tract Renshaw cells…thus, hyper-reflexia & spasticity result.
LMN Disease is the opposite
What tract does Lateral corticospinal tract travel along?
Pyramidal tract
Where is the output for the lateral corticospinal tract?
Primary motor cortex, pre-motor cortex and supplementary motor area
What type of movement does lateral corticospinal tract carry?
Major voluntary skilled motor movement
What tract does the lateral vestibulospinal tract travel along?
Extrapyramidal system
Where is the output for the lateral vestibulospinal tract?
Vestibular nuclei of pons and medulla (brainstem)
What type of movement does lateral vestibulospinal tract carry?
Extensor control (anti-gravity action)
What tract does the ventral corticospinal travel along?
Pyramidal tract
Where is the output for the ventral corticospinal tract?
Primary motor cortex
What type of movement does the ventral corticospinal tract carry?
Minor motor movement
What do the reticulospinal tracts do?
Helps to INHIBIT LMN (so you don’t have overactive spastic / hyper-reflexive muscles)
Where do most neurons of the corticospinal tract decussate?
Majority (80-90%) of neurons decussate in caudal medulla, in the pyramids (pyramidal decussation) and become the LATERAL CST
Where do neurons of the ventral corticospinal tract decussate?
descend ipsilaterally and decussate close to termination (at the level of LMN synapse)
Where do UMNs synapse with LMNs in the corticospinal tracts?
In ventral grey horn
what is clonus?
rhythmic series of contractions (oscillations) caused by the alternate stretching & unloading of muscle spindles in a patient with spastic muscles
If you have hemi-section of the spinal cord, will you get UMN or LMN symptoms?
half the cord knocked out at a specific level - BOTH UMN & LMN signs in different parts of the body
What is the main motor pathway to the face?
Corticobulbar tract
Is most UMN innervation to the face ipsilateral or contralateral?
Mostly contralateral but minor innervation from ipsilateral. Exception is CN VII - lower part of face gets only unilateral, ipsilateral innervation.
What motor loss would there be in a brainstem lesion?
Brainstem lesion can cause ipsilateral LMN loss in head/neck. UMN loss in contralateral body – due to CST loss
Where does the lateral vestibulospinal tract decussate?
These tracts never decussate!!!
what do the lower motor neurons do in the lateral vestibulospinal tract?
Excite extensor muscles and inhibit flexor muscle
What does the reticulospinal tract do?
Activates Renshaw cells (interneurons that have an inhibitory effect on on LMNs)
What happens if you reduce reticulospinal tract function?
Will LOSE your ability to inhibit the hyper-excitable LMN and therefore will get muscle spasticity
What neurotransmitter do Renshaw cells secrete?
Glycine (inhibitory)
Where does the Reticulospinal tract originate?
Nuclei in the pons/medulla
Why can cervical cord damage lead to Horner’s syndrome?
The medullary tract and nucleus of the Reticulospinal tract help to control sympathetic chain. Reticulospinal neurons innervate preganglionic Sympathetic fibres in lateral grey horn
What is the myotatic reflex?
Muscle stretch reflex. Muscle spindles detect passive stretch in muscle and reflex acts to correct this. Stretched agonist muscle is contracted and inhibition of antagonist muscle causes it to relax.
What is the inverse myotatic reflex?
Golgi tendon reflex. Golgi tendon organs detect stretch in tendons and initiate a tendon-protective reflex. Agonist muscle is inhibited and antagonist muscle is stimulated to contract.
How many interneurons are there in the flexor reflex and the crossed extensor reflex?
More than 1
What is the flexor reflex?
Quick withdrawal of a limb from a painful (noxious) stimuli.
Mediated by free nerve endings
Synaptic connections span several spinal cord levels
Stimulates ipsilateral flexors of limb
Inhibits ipsilateral extensors of limb
What is the crossed extensor reflex?
Activation of the flexor reflex in a weight-bearing limb.
Ipsilateral flexor withdrawal
Contralateral extensor activation
