Sensory and locomotor circuits Flashcards
What are the 2 different modalities of sensory information and what do they code for? (2)
Localised: cranial nerve transmission
* Vision
* Hearing
* Taste
* Smell
* Balance
General: spinal nerve transmission
* Pain
* Touch
* Pressure
* Temperature
* Proprioception
* Vibration
Spinal nerve pathway (3)
1) Convey sensory info (mechanically stimulated/touch) = depolarises => AP sent to peripheral receptors.
2) Have their soma in the dorsal root ganglion = axonal projections transmitted from here
3) Synapse with neurons in the dorsal horn of the spinal cord
Describe the somatotopic organisation of the spinal cord (6)
- Somatosensory info is transmitted in an orderly manner to the brain by the axons of sensory neurons that conform the dorsal columns tract
- 4 divisions of the spinal cord: cervical, thoracic, lumbar, sacral
- each region further divided into spinal segment + receives bilateral sensory info
- axons belonging to each spinal cord + corresponding dermatome is organised according to spinal cord divisions
- tract located b/w Dorsal horn + spinal cord
- topographically organised info to corresponding dorsal column tract
Explain the sensory receptor to primary somatosenory cortex pathway (5)
Three step relay process along the dorsal column tract:
‒ 1st order neuron (dorsal horn – dorsal columns) -> enters spinal cord
‒ 2nd order neuron (dorsal columns – thalamus) -> nucleus gracilis or cuneatus
DECUSSATES to the other side of the NS -> project to thalamus via medial leminscus (bundle of axonal fibres carry SS info)
‒ 3rd order neuron (thalamus – cortex)
– nucleus gracilis (lower trunk +limbs) and nucleus cuneatus (upper trunk +limbs)
1) Trigeminal tract pathway (5 +1)
- at level of thalamus, axonal projections converge into trigeminal nerve: Somatosensory information from face, mouth, tongue and dura mater
- Three steps
1. (Trigeminal tract/cranial nerve 5 -> enters CNS @ level of PONS) - Trigeminal nucleus
axons from trigeminal nucleus DECUSSATE
- Thalamus (thalamic secondary nuclei)
- Cortex (S1) - axonal projections to S2 + M1
2) Thalamus (6)
*nucleus located within diencephalon (just above midbrain)
*Gateway to the cortex because it’s responsible for transmission of most sensory info
- Relay station for all sensory information (except olfaction -> straight to cerebral cortex).
- Can screen out irrelevant information -> according to behavioural demands
- Specific nuclei for certain functions e.g. ventral posterior (VP) nucleus for SS info (not homogenous)
- Extensive connections with the rest of the CNS (especially cerebral cortex) + receives extensive cortical feedback
3) Primary somatosensory cortex S1 (3)
- The primary somatosensory cortex S1(Broadman’s area 3b) is (in the postcentral gyrus) on the parietal lobe
- Receives direct projections (dense afferent) from the VP nucleus of the thalamus (highly responsive => lesions = impairment of somatic sensation)
- It is flanked by other areas involved in ss processing (3a = body position processing, 1 (+2) = processing texture + shape, etc)
Explain the cortical columns found in primary somatosensory cortex (3)
- Layered structure: layers I to IV (from dorsal to ventral)
- Axonal projections from the thalamus arrive to L6-> then transmitted to upper + lower layers = processed + sent to other cortical areas for further analysis + integration
- Cells organized in columns according to function and connectivity
-(1st area where cortical columns were defined -> fingers) : neurons organised according to the finger they process info from + type of somatosensory stim. => fingers have either slowly or rapidly adapting neurons depending on the location
Explain the somatotopic representation of the body with an example of a model (3)
- Point-to-point representation of each area of the body
- Body areas are represented in proportion to degree of innervation, not size
- eg Sensory homunculus:
- Topographic representation of the body parts along the postcentral gyrus of the parietal lobe.
- corresponds to the contralateral side of the body.
- Areas are represented according to density of innervation, not size
- eg lips more innervated than hips = bigger on model
Somatosenory info + Plasticity -> animal + human eg (4)
Due to decussation of SS info @ medulla: responds to info at contralateral side of the body!
The representation of somatosensory info at the level of the cortex is plastic and can adapt depending on sensory stimulation.
-> seen in monkeys: cutting of finger = reorganisation or cortical map + SS cortex
-> also seen in humans = phantom pain
This plasticity is also present in other sensory areas of the cortex (e.g. visual, auditory
What is the sensorimotor integration? (3)
It is the sensory hierarchy and motor hierarchy interacting with each other.
There are differing levels of integration.
Sensory info is received (parietal + temporal inputs) and then transmitted to cortical association areas (frontal lobe) = generation of a motor command (behavioural response to a stimulus)
Describe cortical connectivity and its role in transmission (2)
These patterns come about from the joining of axonal projections that join into fibres and travel from one cortical area to another.
They have either association or commissural fibres
Describe and explain association fibres (2)
Short range:
- link nearby cortical areas of cortex by arching beneath adjacent cerebral sulci
Long range:
- link distant cortical areas, travelling through white matter, travelling underneath the cerebral cortex
Define commissural fibres (1)
They connect corresponding cortical regions between hemispheres = coordinated pattern activity/coordinated movement of both sides of body
What 3 cortices are involved in the planning of movement? (3)
Prefrontal cortex- integrates sensory info + evaluates need for motor action
Posterior parietal cortex - spatial relationship b/w the body and environment
Motor cortex - SMA +PMA (area 6) M1(area 4-activates specific muscle groups involved in the execution of the desired movement) -@ level of pre central gyrus
What is the motor homunculus? (3)
M1 (primary motor cortex) can be delineated by anatomical distribution of motor supply = creates the motor homunculus.
- there’s one motor homunculus on each hemisphere
- controls the movement if anatomical structures on opposite side of the body
Motor cortex and plasticity (3)
The somatotopic organisation of the motor cortex is plastic.
= areas in the primary motor cortex can be reorganised eg after injury = preserves/generates movement
Reorganisation of the motor cortex is responsible for learning of fine motor skills
Corona radiata and internal capsule anatomy (3)
- m1 command has to be transmitted to motor neurons in the spinal cord
- corona radiata (L5) group to form = internal capsule, which connect the cerebral cortex with subcortical structures:
— thalamus
— brain stem
— spinal cord
This pathway = main outputs pathway towards the spinal cord
What are the spinal cord motor pathways? (3+3)
Axonal projections from upper motor neurons are organised into 2 groups of fibres tracts:
Lateral pathway:
- carry motor info = controls voluntary movement
- lateral column of spinal cord -> further divided into corticospinal tract + rubrospinal tract
Ventromedial pathway:
- controls postural muscles + locomotion
- travels along the ventral column of spinal cord -> further divided into tectospinal tract, vestibulospinal tract, pontine reticulospinal tract, medullary reticulospinal tract
Describe the corticospinal tract - DESCENDING PATHWAY (3)
Largest tract in CNS -> DESCENDING PATHWAY
2 step relay rocess:
- L5 upper motor neuron travel from cortex to ventral horn of spinal cord -> through corona radiation + internal capsule to cerebral peduncles @ level of midbrain -> got to medulla
DECUSSATE to contralateral side @ level of medullary pyramids (= medullary tract)
- lower motor neuron (ventral horn of spinal cord - muscle fibre -> convey the motor command
Explain the Rubrospinal tract pathway + info (4+1)
- Originates in red nucleus of midbrain
- receives (afferent projections)direct connections from the motor cortex
- axons along tract decussate immediately @ pons => travel down the lateral columns alongside the corticospinal tract + terminate in ventral horns
- function in fine motor movement but in humans = less prominent that corticospinal tract
Explain lower motor neurons + their distribution (4+3)
- in ventral horns of the spinal cord
- transmit motor signal from CNS to muscle fibres in target muscles
- axons of these neurons group = form ventral root of spinal nerves
- spinal nerves are mixed nerves - sensory and motor component
- unevenly distributed in ventral horns of spinal cord
- neurons innervating axial muscles in body core = more medial than neurons innervating distal muscles
- neurons innervating flexor muscles = dorsal to neurons innervating extensor muscles
Lower motor neurons and skeletal muscle link (3)
Skeletal muscles are not evenly distributed along the body
=> muscles at level of legs and arms than in the middle section of the body
= more lower motor neurons in cervical and lumbar regions than the thoracic segments
What are myotomes?(1)
A myotome is the group of muscles innervated by all lower motor neurons in a single spinal nerve.
- motor counterparts of dermatome
