Spinal cords, tracts, UMN and LMN Flashcards
What is the denticulate ligament?
The ligament that suspends the spinal cord in the canal at the lateral aspect
What is the denticulate ligament formed from?
Formed from pial and arachnoid tissue and attaches to the dura at points along the length of the cord
What does the white matter of the spinal cord consist of?
Axons
What does the grey matter of the spinal cord consist of?
Cell bodies of neurons
What are the three components of blood supply of the spinal cord?
- One anterior and two posterior longitudinal arteries that originate from the vertebral arteries and run the length of the spinal cord
- Segmental arteries that originate from vertebral, intercostal and lumbar arteries
- Radicular arteries that travel along the dorsal and ventral roots
Where in the brain is the primary somatosensory cortex located?
Postcentral gyrus of the parietal lobe
What signals are transported in the dorsal column/medial lemniscus system?
Fine touch and proprioception
Where do fibres ascending in the dorsal column decussate?
Medulla
Where do the 2nd order axons of the dorsal column originate?
Medulla - synapses THEN crosses
What signals are transported in the spinothalamic tract?
Pain, temperature and deep pressure
Where do the fibres of the spinothalamic tract cross?
Segmentally - cross at the level they enter the spinal cord
Where do the second order neurones of the ascending tracts synapse?
Thalamus
Which ascending tract is being shown here?
Spinothalamic tract
Label these structures.
Where is the primary motor cortex located?
The precentral gyrus of the frontal lobe
What signals are transmitted by the corticospinal tract?
Fine, precise movement
What forms the ‘pyramids’ located on the anterior aspect of the medulla?
Corticospinal tracts
Where do most of the corticospinal tract fibres cross?
Pyramids of the medulla
Which section - lateral or ventral corticospinal tract - is formed from fibers crossed at the medullary pyramids?
Lateral
Which section - lateral or ventral corticospinal tract - is formed from fibers that did not cross at the medullary pyramids?
Ventral - 15% of fibres
At what level do fibres from the ventral corticospinal tract cross?
Segmentally
Which descending tract is being shown here?
Corticospinal tract
What clinical signs would be seen in a stroke affecting the internal capsule and why?
Spastic paralysis with hyperflexion of the upper limbs
Corticospinal tract intermingles with other fibres in the internal capsule, so CVA here results in loss of descending control
What is decorticate posture?
An abnormal posturing in which a person is stiff with bent arms, clenched fists, and legs held out straight
The arms are bent in toward the body and the wrists and fingers are bent and held on the chest
Where does the tectospinal tract supply?
Mostly cervical area
Thought to mediate head and neck movement in response to visual input
What is the function of the vestibulospinal tract?
Excitatory input to antigravity extensor muscles
Originates in the pons and medulla and so recieves input from cerebellum and vestibular apparatus
What is the clinical picture of Brown-Sequard syndrome and what causes it?
Lateral hemisection of the spinal cord
Ipsilateral paralysis, hyperreflexia & extensor plantar reflex and ipsilateral loss of vibratory sense and proprioception
Contralateral loss of pain and temperature
What are upper motor neurons?
Motor neurons that originate either in the motor region of the cerebral cortex or in the brain stem and carry motor information down to the lower motor neurons
What are lower motor neurons?
Neurons that innervate all skeletal muscles and thus allow all voluntary movement
Act as a link between upper motor neurons and muscle
What do cranial nerve lower motor neurons control?
Movements of the eyes and tongue, and contribute to chewing, swallowing and vocalization
Where do upper motor neurons and lower motor neurons meet?
In the spinal cord
What is the internal capsule?
The internal capsule is a white matter structure situated in the inferomedial part of each cerebral hemisphere of the brain, carrying both ascending and descending tracts past the basal ganglia
Which motor tract makes up a large part of the internal capsule?
Corticospinal tract
What conditions can cause an upper motor neuron lesions?
Stroke
Multiple sclerosis
Traumatic brain injury
Cerebral palsy
What are the signs and symptoms of an upper motor neuron lesion?
Muscle weakness
Increased tone
Increased relfexes
Decreased motor control including decreased speed, accuracy and dexterity
Spasticity
What are the signs and symptoms of a lower motor neuron lesion?
Decreased tone
Reduced relfexes
Muscle weakness and atrophy
Fasiculation
What is hyperreflexia?
Hyperreflexia is the amplification of the “muscle stretch-contraction” reflex, aka the myotatic reflex, the tendon reflex, or the stretch reflex
What happens to the motor neurons when a muscle is abruptly stretched i.e. when the tendon is tapped?
The lower motor neuron is activated and causes contraction of the muscle
How do the cerebellum and basal ganglia affect motor control?
The motor control circuitry of the basal ganglia and cerebellum do not input directly to the LMN
. They influence the speed, timing, coordination, and precision of movements by their input to the motor cortex, not directly but via the thalamus
Why does spasticity occur?
Spasticity is hypertonia as a result of damage to the corticospinal tract and damage to the inhibitory descending motor pathways, therefore contraction is unopposed
Because the stretch reflex is a short-latency reflex, spasticity appears 40-60 msec after a stretch stimulus
How does the contraction in spasticity come to an end?
Initially the contraction is unopposed because of damage to the CST and the inhibitory descending motor pathways
Eventually, the contraction weakens as the Golgi tendon organ is stimulated, allowing relaxation of the muscle
By what mechanism does rigidity differ from spasticity?
It is mainly due to an exaggeration of the long-latency reflex (LLR), as opposed to spasticity which is the stretch reflex or short-latency reflex (SLR)
What is the pathway of the long-latency reflex?
The LLR pathway starts in the muscle stretch receptors, enters the cord, projects all the way to the somatomotor cortex, and returns to the cord activating the LMN
What impact does the basal ganglia have on the long latency reflex?
The basal ganglia exerts its influence on the LLR via its projections to the thalamus which then projects to the somatomotor cortex
How does the long latency reflex and the basal ganglia cause cog wheel rigidity?
The repetitive activation of the LLR as the muscle is being stretched (e.g. by flexing a joint) results in intermittent increase in muscle tone during the course of the motion
What are nociceptors?
First order peripheral sensory neurones activated by intense stimuli that are noxious or damaging
Where are the cell bodies of nociceptors located?
Dorsal root ganglia
Trigeminal ganglia
What are Ad nocieptors?
Ad-fibres are mechanical/thermal nociceptors that are thinly myelinated and respond to noxious mechanical and thermal stimuli, causing ‘fast’ pain
What are C-fibre nociceptors?
C-fibres are nociceptors that are unmyelinated and collectively respond to all noxious stimuli, causing ‘slow’ pain
Which part of the dorsal column does sensation from the lower limb travel in?
Gracile fasiculus
Which part of the dorsal column does sensation from the upper limb travel in?
Cuneate fasiculus
