King Ch 8, Pt I - Medial lemniscal system Flashcards
What is the medial lemniscal system?
A major pathway for afferent (sensory) information that converges from the gracile–cuneate and spinothalamic systems.
How many conscious sensory modalities can be recognized?
At least 12.
What are the four groups of conscious sensory modalities based on their mode of central transmission?
- Touch, pressure, and joint proprioception, transmitted by cuneate and gracile fascicles
- Pain (superficial and deep), heat, cold, transmitted by the spinaothalamic tract (in humans)
- Deep pain, transmitted by the spinaothalamic tract and the ARAS
- Special senses (vision, hearing, balance, taste, olfaction)
What is nociception?
The conscious perception of harmful and obnoxious stimuli.
What are nociceptors?
Specialized sensory receptors that detect and transduce noxious stimuli.
True or False: All mammals have the capacity to experience pain.
True.
What is kinaesthesia?
The awareness of the precise position and movements of body parts.
What are the three types of nociceptors based on structure?
- Free nerve endings
- Neurite-receptor cell complexes
- Complex laminated receptors (encapsulated receptors)
Describe the anatomy and function of free nerve endings
- Abundant in the skin
- Mainly small diameter unmyelinated axons that branch liberally after losing their Schwann cell covering, although there are also some finely myelinated axons
- Respond to pain (e.g. nociceptors), temperature change and mechanical distortion of the skin
Describe the anatomy and function of neurite-receptor cell complexes (Merkel cells)?
- Aka Merkel cells, aka epithelioidocytus tactus.
- Each cell is intimately associated with an axonal ending from a myelinated fibre
- Occur immediately beneath the epidermis
- Is a slowly adapting mechanoreceptor, discharging continuously when the skin is distorted
Describe the anatomy and function of complex laminated receptors?
- Aka encapsulated receptors
- Have a lamination structure
- Varying degrees of size and complexity
- Occur in subcutaneous tissues, joint capsules, ligaments
- All tension receptors transmitting information about the tension in the capsule and ligaments
What is the role and pathway of the primary afferent neuron in the touch, pressure, and joint proprioception pathways?
Cell of the primary afferent neuron in a dorsal root ganglion > axon enters the dorsal horn > divides, forming short branches and a long collateral > long collateral ascends in the dorsal funiculus, in the gracile or cuneate fascicle > synapses in the gracile or medial cuneate nucleus
Describe the function and pathway pf the short branches from the afferent neuron after entering the dorsal horn
The short branches synapse with interneurons, some forming part of reflex arcs and also with the ascending reticular formation
Describe the arrangement of axons of the gracile and cuneate fascicles
In the gracile and cuneate fascicles, axons are arranged somatotopically.
The axons relating to the hindlimb pass as near to the midline as possible, before turning cranially to ascend within the gracile fascicle.
The axons coming in subsequently from the trunk and forelimb also pass as far towards the midline as they can and then ascend, those from the forelimb and neck travelling in the cuneate fascicle i.e. FC is stacked on top of FG, so becomes more lateral.
NB: hindlimbs = gracile, forelimbs = cuneate
What happens to the axon of neuron 2 in the medial lemniscal pathway?
Cell in the gracile nucleus or medial cuneate nucleus in the medulla oblongata > axon decussates to the opposite side of the neuraxis in the deep arcuate fibres > continues rostrally in the medial lemniscus > synapse in the thalamic nuclei
Medial lemniscus is a bundle of axons
Where does neuron 3 in the medial lemniscal system project?
Cell in the ventral group of thalamic nuclei (ventrocaudal nucleus) > corona radiata > contralateral somatosensory cortex
What are the effects of a unilateral lesion in the dorsal funiculus?
Ipsilateral deficits of cutaneous sensation and joint proprioception.
What is the common feature in cases of tabes dorsalis in humans?
Degeneration of the dorsal funiculus.
What does the term somatotopic localization refer to?
The orderly arrangement of axons corresponding to body segments.
What is the significance of the sensory homunculus?
It represents the cortical area allocated to each body part based on sensory nerve supply.
What happens when the dorsal funiculi in the cervical region are experimentally destroyed in monkeys?
Severe ataxia of the forelimbs, but less pronounced ataxia of the hindlimbs
This indicates a complexity in proprioceptive pathways.
What is the result of bilateral section of the dorsal funiculi at C4 in dogs?
High stepping gait of the forelimbs, but not of the hindlimbs
Similar results have been observed in cats and monkeys.
Where do the majority of proprioceptive fibres from the hindlimb joints synapse?
In the dorsal horn (in the thoracic nucleus) with neurons that ascend in the dorsal spinocerebellar tract
This occurs after leaving the gracile fascicle in the first lumbar segments.
What is nucleus Z in relation to proprioceptive pathways?
A special nucleus where fibres end ipsilaterally near the gracile nucleus and travel to join the medial lemniscus
It may be a displaced fragment of the gracile nucleus.
