Spinal cord & periphery - physiology Flashcards
Are the dorsal/ventral/lateral horns white or grey matter
grey
Function of corticospinal (pyramidal) tract in simple terms
Carries MOTOR impulses from motor area in brain to skeletal muscles, controlling voluntary movements
The corticospinal tract consists of projection nerve fibres which project down from the primary motor cortex and pass through… (2)
the POSTERIOR LIMB OF THE INTERNAL CAPSULE
then project down from the IC and pass through the brainstem where they CROSS OVER in the medulla
Corticospinal tract sit in which column of the spinal cord
Lateral
Where do LMNs begin
Ventral horn
How many neurons are involved in the corticospinal pathway
2
-UMN and LMN
Pathway of UMN in the lateral corticospinal tract
starts at the motor cortex and travels down the brainstem, crossing over, then continuing down the spinal cord as the lateral corticospinal pathway until it has to leave the lateral column and enter the ventral horn where it synapses
Pathway of LMN in the lateral corticospinal tract
picks up the synapsed impulse in the ventral horn and leaves spinal cord via ventral root –> mixed spinal nerve –> respective muscle
Ascending spinal tracts involve how many neurons
3
Ascending spinal tracts involve 3 neurons between peripheral receptor and cortex, describe the role of these neurons
- 1st order
- 2nd order
- 3rd order
1st order (pseudounipolar) -picks up impulse from peripheral receptor and synapses as soon as it enters the spinal cord (spinothalamic) or in the medulla (dorsal column)
2nd order
-picks up the synapsed impulse either where it entered the spinal cord (spinothalamic) or at the medulla (dorsal column) and carries it to the thalamus where it synapses
3rd order
-picks up the synapsed impulse in the thalamus and carries it to the internal capsule where it then radiates out as many fibres to the somatosensory cortex
Name the 2 ascending tracts
Posterior/dorsal column
Lateral spinothalamic tract
Posterior/dorsal column function
Carries sensations of fine touch, tactile localisation, vibration sense, proprioception
Describe the role of the 1st order neuron in the posterior/dorsal column
picks up impulse from periphery, enters spinal nerve –> dorsal root –> spinal cord at the spinal level corresponding to the nerve root value of the structure being sensitised
Once entered spinal cord, the 1st order neuron then carries these sensations into the posterior column (white matter) and rises up the spinal cord to the medulla
The 1st order neuron ends by synapsing with the cell body (gracile and cuneate nuclei) of the 2nd neuron in the lower part of the medulla
Describe the role of the 2nd order neuron in the posterior/dorsal column
crosses over (decussates) in the medulla and forms a tract
The tract now is called the medial lemniscus (still the posterior column), and travels to the thalamus where it synapses
Describe the role of the 3rd order neuron in the posterior/dorsal column
Picks up the synapsed impulse in the thalamus and its axons pass through the IC and radiate to the post-central gyrus
Function of lateral spinothalamic tract
Carries sensations of PAIN + TEMP
Describe the role of the 1st order neuron in the lateral spinothalamic tract
Picks up impulse from periphery, enters spinal nerve –> dorsal root –> spinal cord at the spinal level corresponding to the nerve root value of the structure being sensitised
1st order neuron synapses the moment it enters into the spinal cord at the dorsal horn (grey matter) and terminates there at the same level it entered
Describe the role of the 2nd order neuron in the lateral spinothalamic tract
Picks up synapsed impulse at the level of entry and crosses over to the lateral column of spinal cord at the same level and starts rising up the lateral spinothalamic tract
Synapses at the thalamus where the cell body of the 3rd order neuron is
Describe the role of the 3rd order neuron in the lateral spinothalamic tract
Picks up the synapsed impulse from 2nd order neuron and passes through the IC then radiates out to the post-central gyrus
purpose of spinal cord level reflexes
to decrease the time needed to generate the response, e.g. where an immediate response is needed like touching something burning hot (we need to immediately contract muscle)
Define a reflex
involuntary pattern of response brought about by a SENSORY STIMULUS
Define a reflex arc
neural pathway that controls a reflex
-pathway followed by nerves which carry sensory fibres from the receptor to the spinal cord, and then carry the response generated by the spinal cord to effector organ
Components of a reflex arc (3)
sensory neuron
motor neuron
(+/- interneurons)
Define a monosynaptic reflex
One without interneurons, just sensory and motor
Define a polysynaptic reflex
One that involves interneurons as well as sensory and motor neurons
Is the stretch reflex (i.e. knee jerk reflex) monosynaptic or polysynaptic
Mono
Describe the stretch reflex
- what is it caused by
- what sensory fibres are activated
- what is subsequently activated
- what contracts
- what relaxes + how
- elicited by a sharp tap to the inelastic tendon, and so the force is transferred to the muscle fibres which stretch and activate the 1a afferents innervating spindles, increasing the number of APs in the 1a afferents projecting in to the spinal cord through the dorsal horn
- -> this then activates the alpha motorneurones and the agonist muscle contracts - antagonist muscle must also relax (stretch) during this reflex while the agonist muscle contracts (shortens)
- -> the sensory fibres from the stretched spindle also connect indirectly with antagonist muscles by activating inhibitory interneurons to the antagonist muscle which decreases alpha motorneurone transmission to the antagonist causing it to relax (RECIPROCAL INHIBITION)
Muscle then shortens back to previous length
While the stretch reflex is occurring, spindle afferent information also ascends to the brain for what reason
ascends in the posterior column, connects with somatosensory cortex (parietal lobe) to tell the brain about length of muscles
Golgi tendon reflex also known as
inverse stretch reflex or clasp-knife reflex
Is the Golgi tendon reflex mono or polysynaptic
polysynaptic
Describe the Golgi tendon reflex
1) As the agonist muscle contracts (shortens), it pulls on the tendons and the Golgi tendon bodies increase their discharge (= 1b sensory (afferent) nerves from GTOs increase firing of APs)
2) The 1b afferents activate inhibitory interneurons to the agonist muscle and decrease the contraction strength (relaxes it), while also activating excitatory interneurons to the antagonistic muscle to contract it
During the Golgi tendon reflex, what communication is made with the brain
information about muscle tension ascends in the POSTERIOR COLUMN to the somatosensory cortex
Purpose of the Golgi tendon reflex
prevents over contraction and prevents the muscle from tearing the tendon insertion points away from the bones
The flexor/ crossed extensor reflex uses information from what receptors
Nociceptors
Purpose of flexor/ crossed extensor reflex
withdraw the affected part of the body away from the stimulus and towards the body
Describe the flexor reflex
- what sensory fibres are activated
- what neurons do they activate
Small diameter Aδ nociceptive fibres (SMALLER DIAMETER THAN SPINDLE AFFERENTS SO CONDUCT SLOWER THEREFORE THIS REFLEX SLOWER THAN STRETCH REFLEX) that trigger pain enter spinal cord and BRANCH A LOT
- -> activate excitatory interneurons in several spinal segments which activate α motorneurones controlling all the flexor muscles of the affected limb
- -> contraction of affected limb
Purpose of the crossed extensor reflex
If one leg is withdrawn/flexed from a sharp object, we need the other leg (CONTRALATERAL limb) to extend to keep us upright
Describe the crossed extensor reflex
-what neurons are involved
1) Several excitatory interneurons cross the spinal cord and excite the contralateral extensors to contract it
2) While other interneurons inhibit the contralateral flexors
How can the Golgi tendon reflex be over-ridden by higher centres, e.g. when holding a baby for long
descending voluntary excitation of alpha motoneurons can override the inhibition from the GTOs and maintain contraction.
Excess load such as holding heavy objects would activate what reflex
Golgi tendon reflex
How can the stretch reflex be over-ridden by higher centres
strong descending inhibition hyperpolarises α-motoneurones therefore the stretch reflex cannot be evoked
In the stretch reflex, spindle input is highly localised and affects only alpha motoneurones of…
whereas
In the flexor withdraws reflex, pain fibre input is diffuse and spreads through…
one or two spinal segments
several spinal segments
UMNs are above the level of…
decussation
If lesion is above level of decussation, signs and symptoms are ipsilateral or contralateral
If lesion is below level of decussation, signs and symptoms are ipsilateral or contralateral
contralateral
ipsilateral
A left UMN lesion at the internal capsule in the corticospinal tract would present with (3)
- R or L paralysis
- hyper/hyporeflexia
- increased/decreased tone
R sided paralysis
Hyper-reflexia
Increased tone
A left UMN lesion at the upper cervical spinal cord in the corticospinal tract would present with (3)
- R or L paralysis
- hyper/hyporeflexia
- increased/decreased tone
L sided paralysis
Hyper-reflexia
Increased tone (spastic)
A left LMN lesion at the upper cervical cord in the corticospinal tract would present with
- R or L paralysis
- hyper/hyporeflexia
- increased/decreased tone
L sided paralysis
Hyporeflexia/absent reflexes
Decreased tone - flaccid
A lesion above the level of decussation in the dorsal column would present with
Contralateral sensory loss (of fine touch, pressure, vibration, proprioception)
A lesion below the level of decussation in the dorsal column would present with
Ipsilateral sensory loss (of fine touch, pressure, vibration, proprioception)
A lesion wherever in the lateral spinothalamic tract will always result in pain and temp sensation loss in the ipsilateral or contralateral side + why
CONTRALATERAL SIDE because decussation occurred right when it entered the spinal cord
