Motor Pathways Flashcards
1
Q
Pyramidal System
(consists of corticospinal tracts)
A
- 2 neurone, contralateral relay: (although there’s several v.short neurones linking the 2 in the spinal cord)
- Upper motor neurone (UMN) relays impulses from cerebral cortex–> Spinal cord, forms descending tract from cerebral cortex to spinal cord level, crossing the midline (technically an association fibre, not a motor fibre as it relays info within the CNS)
- Lower motor neurone (LMN) cell bodies within the spinal cord, transmit impulses from SC to periphery. Motor neurons going from spinal cord to muscles on the right side of the body by appropriate peripheral nerve
CAREFUL:
-Motor neuron has its cell body in the CNS and runs to a muscle (true motor neuron)
-The upper motor neuron does not leave the CNS and falls under the group of association motor neurons. Not an ACTUAL motor neuron
2
Q
Corticospinal Tracts of Pyramidal System
A
- First general system involved with motor control –> Pyramidal System (the general system of controls)
- made of corticospinal tracts (anatomical term actually describing the tracts in this system). Relay from Cerebral Cortex to Spinal Cord in the CNS –> ergo corticospinal
- Conveying messages down to spinal cord level
- out to the muscles on the right side of the body (ex: right limb)
- at one point they decussate from left to right (and of course vise versa)
** all will decussate: will either be before the level of the spinal cord or spinal cord level (termination level)
- May synapse to spinal cord by more than one interneuron
3
Q
Effects of Lesions
(Pyramidal System)
A
- LMN disease-Paralysis of muscles innervated & no reflex activity (e.g. knee jerk)
- Lower motor damage is a damage that occurs to a motor neuron in the PNS
- would damage reflexes
- could also result in a significant atrophy of these muscles after its innervation is lost
- think about dissections (ex: radial nerve in forelimb)
- some damage to nerves in a limb can have very minimal effects because the other nerves/muscles will compensate. Atrophy of the muscles may still occur, can use this as a clinical diagnosis
- UMN disease (ex: spinal cord lesion by compression):
- LMNs still functional- no paralysis & reflexes can occur
- But lack of control from higher centres –> abnormal in magnitude of reflexes
- Would not be paralyzed or undergo muscle atrophy as they are still innervated,
- But there may be a lack of control from higher centers. May have a lesser or exaggerated response
- HUGE importance in man – basis of “skilled” activity, much less in domestic species - Little deficit in motor activity of cats with severed corticospinal tracts.
- Some of the lesions have a LARGE effect on man and lesions of this type can have truly profound effects in man if this pyramidal system is affected- In man the belief is that the spinothalamic tract is responsible for all pain reception
- Minimal effects when it is destroyed in a cat, there are some minimal noticeable effects in cats, but nothing like man. So there must be some sort of compensation occurring in these animals to allow them to function properly
- SO – other systems must exist given high degree of sophistication of locomotor activity in domestic mammals
4
Q
Extra Pyramidal System
A
Comprised of a number of descending tracts:
- (listed are the 4 most important ones)*
- **Named based on where they start and where they finish
(Prefix tells you where they originate)
- Rubrospinal tract- begins at red nucleus in midbrain (large mass of well vascularised, grey matter). Rubro-= red. Red part of the brain? Red nucleus in the brain.
- Reticulospinal tract- begins in the reticular formation of the brainstem
- Vestibulospinal tract- begins in vestibular nuclei
- Tectospinal tract- begins in tectum (roof) of midbrain. Tectum = roof
5
Q
Motor Pathway Summary
A
- “Command centres” = where the descending pathway originates from in the brain
- Majority of the projections from the command centres= contralateral, one exception to this is the vestibular nuclei which can project to both sides of the SC via the vestibular spinal tract (paired projection on ispilateral & contralateral sides). Command centre of vestibular nuclei is In pons and medulla oblongata
- Command centres themselves receive projections from higher levels. Cerebral cortex indirectly controls extrapyramidal system via the globus pallidus (one of the basal nuclei in the cerebral hemisphere)
- The Corticospinal tract starts off in the cerebral cortex (cerebrum) which is a direct control where as the extrapyramidal tracts appear to originate closer to the midbrain (globus pallidus) –> “Command centers” are all somewhat within the brain stem
- In the basal nuclei, dopamine= the main NT
(not acetylcholine)- disease processes involving lack of dopamine transmission through the basal nuclei–> Parkinson’s
- interferes with motor control
6
Q
Decussation
(contralateral relay)
A
Decussation (contralateral relay)
- Relay of ascending & descending information is very largely contralateral (although bilateral would seem more advantageous)- in higher mammals, contralateral relay has become progressively greater
Tract decussation is clinically useful for determining sites of lesions
- Above decussation- deficit is on opposite side (of lesion)
- Below decussation- deficit is on the same side (of lesion)
- Useful to know where these pathways decussate because then you can find the site of possible lesions more efficiently
7
Q
Command Centres: Extrapyramidal System
A
- Command centers are under control of other higher centers still –> the cerebral cortex, but unlike pyramidal system, it is not in a direct way, but an indirect way.
- Goes to the globus pallidus which is one group of nuclei (basal nuclei) that lie in the depths of the cerebral hemisphere.
- They have not become part of the cerebral cortex but stayed in mantle layer.
- Main output station from these basal nuceli is the globus pallidus.
- Primarily Dopamine involved in this system! not ACh
- From the spinal cord: the tracts are running out to innervate muscles on the right side o the body, lower motor neuron. Heading out to reach the appropriate limb muscles. (vise versa)
8
Q
Sensory
A
- Touch, conscious proprioception (dorsal columns)- decussation occurs in medulla oblongata (brainstem)- Sensory Pathways
- Unconscious proprioception (Spinocerebellar Tracts)- largely an ipsilateral relay so no decussation. Some cross multiple times but they END on the same side
- Pain (ARF)- more primitive system & is bilateral- so a lesion on one half of the spinal cord will not produce a sensory deficit on either side
- in man, the spinothalamic pathway decussates
- Don’t decussate until they ascend as far as the brain stem (medulla oblongata)
- Ipsilateral input, final input is into the same side, cross once then back over
9
Q
Motor Pathways
(Decussation)
A
- nearly all decussation of these pathways occurs at the brainstem level (pons/medulla oblongata)
- Corticospinal Tracts
- Much of these actually occur in the brain stem. They decussate within the brain, in the brain stem
10
Q
Tract location is clinically useful for determining severity of spinal injury
A
- Dorsal region of the white matter is largely occupied by The dorsal columns (relaying ascending info)- Sensory- Dorsal white columns (gracile and cuneate tracts). Mainly ascending tracts
- Ventral region of white matter is largely composed Of descending pathways (motor control)- Ventral white matter is composed of Descending tracts. Can be damage through compression to these descending tracts. Contralateral.
- Lateral white column is a mixture of both- ascending and descending info
- The spinocerebellar tracts (unconscious proprioception- ipsilateral) are located very superficially in the lateral region (responsible for unconscious proprioception information)
- The spinoreticular tracts are deeply located, Close to the grey matter (bilateral relay)- PAIN-To prevent transmission through it would Require extensive damage on both sides of spinal cord. Spinoreticular tract system: would need damage on each side and rather extensive for these animals to feel no pain (bilateral)
11
Q
Cerebellum
A
- 3 pairs of cerebellar peduncles attach it to the brainstem
- There’s 3 general regions of the cerebellum, separated on an evolutionary basis:
- Archicerebellum
- Palaeocerebellum
- Neocerebellum
3 regions or territories of the cerebellum.
shows how it started off and how the cerebellum has changed through evolution
12
Q
Archicerebellum
(cerebellum)
A
Archicerebellum-
- caudally located,
- earliest evolved region-
- major input= vestibular system of internal ear –> CN VIII (vestibulocochlear nerve) –>brainstem nuclei–> archicerebllum
- “ancient”: evolution of internal ear or vestibular system. This is much smaller now in our domestic species
- flocculus/nodulus/balance/posture brainstem inputs (vestibular, visual etc.)
- Receiving from vestibular system. Originating from internal ear
13
Q
Neocerebellum
A
Neocerebellum- evolved first in mammals & is larger in higher mammals (size= associated with size of cerebral hemisphere)
- major input= cerebrum, cerebral cortex! down spinal cord and into the pons before inputting into the neocerebellum
- caudal hemispheres, voluntary motor- cortical inputs
- Neo- newest. Domestic animals and primates, this is the biggest territory of the three
- biggest of the cerebellum
- When the cerebral cortex sends down motor information through the corticospinal tracts (pyramidal system) it also relays information into the cerebellum at the level of the pons and into the neocerebellum
- carbon copy of the cerebral cortex’s information down to the lower motor neurons. does that to control them but also “informs” the cerebellum
- Relays info from the cerebral cortex back into the extrapyramidal system
14
Q
Palaeocerebellum
A
Palaeocerebellum
- rostrally located
- evolved next, in parallel with the evolution of limbs
- major input=musculoskeletal system via spinocerebellar tracts (unconcious proprioception)
- Found in rostral part. Next bit of cerebellum that evolved. Evolved at a later stage with the evolvement of the limbs. This part is quite large as the limbs are rather large in domestic animals
- Main input Is from the limbs. Bringing information to the cerebellum about locomotion
- vermis/rostral, tone/posture, cord inputs (spindles/Golgi’s)
15
Q
Function of the Cerebellum and signs of cerebellar disease
A
- Function: Integrates info, relays back to brainstem Nuclei & any info going to cortex is relayed via Thalamus–> Coordination of phasic & postural muscle activity (not initiation)
- Doesn’t directly control the muscles themselves. Both these effects are coordinated by effects of cerebellum
- Clinical signs of cerebellar disease:
- Dysmetria (incoordination)
- Hypermetria (over-contraction)
- Not paralysis
- Can be quite common in domestic animals in different forms
- Disease of cerebellum shows up as showing some lack of coordination of muscle activity but NOT paralysis!
- upper and lower motor neurons could still be working fine!
