Flashcards in Lesson 5: General Sensory Receptors and Afferent Tracts Deck (72):
What is somatotopic representation?
Representation of the body on the cerebral cortex is distorted with more space being given to the head, neck and hands
In somatotopic representation, how are pathways organized?
Pathways start with neurons from same areas travelling together. Ascending information from the legs is joined in an orderly way by information from the arms then the head and neck. They remain in order and arrive at the cerebral cortex as a “homunculus”
Do peripheral nerves regenerate if the sheath is still aligned?
How is this different that a spinal cord lesion?
Spinal cord lesions - no spontaneous regeneration
Where do sensory neurons enter and synapse in the spinal cord? Where do motor neurons synapse and exit?
Sensory neurons enter and synapse in the dorsal horns while motor neurons synapse in the ventral horns and exit to peripheral nerves
Which side of the spinal cord is sensory/motor?
Dorsal side of the cord is sensory and the ventral side of the cord is motor
At the pyramidal decussation, what is formed?
Descending pyramidal fibres cross to from the lateral corticospinal tract
What is crossing in the closed medulla and what structure is formed in the medulla?
Where is this located in comparison to the 4th ventricle?
This is the important crossing of the dorsal columns pathway as they form the medial lemniscus (lemniscus = ribbon).
This is below the 4th ventricle
How is the open medulla distinguished from the closed medulla?
What is an important feature of the open medulla?
By the presence of the 4th ventricle and the inferior olivary nucleus
The important feature at this level is the inferior olivary nucleus
Why is the open medulla 'open'?
It is “open” because the 4th ventricle lies over the dorsal surface.
What is the main feature of the pons?
The huge bundle of fibres crossing within the “basal pons” that lies ventral to the core of the pons (tegmentum of the pons)
What type of fibres does the basal pons carry?
The basal pons carries motor fibres
Which fibres pass through the basal pons to the pyramids?
The corticospinal fibres descending from the crus cerebri above pass through
What fibres synapse and cross in the pons to enter the cerebellum?
The descending fibres from the motor cortex synapse and cross here to enter the cerebellum
What is the name for the sensory fibres that travel in the core (tegmentum) in a group of 3 ribbons of fibres?
Midbrain - Inferior Colliculus: What occurs at this level?
This is the level at which auditory reflexes are integrated with many associated pathways.
What does the crus cerebri do in the midbrain (inferior colliculus)?
The crus cerebri carries motor descending fibres.
In the core of the midbrain, which direction is sensory information travelling and where?
In the core of the midbrain, sensory information is ascending in the sensory lemniscus.
Is there an oculomotor nucleus at the inferior colliculus level of the midbrain?
What important connections cross at the level of the inferior colliculus of the midbrain?
Important connections with the cerebellum cross in the middle of this level
Midbrain-Superior Colliculus: what is a major activity in this area?
What is a major feature?
This is the level at which visual reflexes are integrated with many associated pathways.
A major feature is the oculomotor nucleus of cranial nerve III
In the superior colliculus of the midbrain, where is the crus cerebri carrying motor fibres?
Where is the sensory lemniscus in the midbrain?
The crus cerebri carries motor fibers on the ventral surface.
The sensory lemniscus is deep in the tegmentum of the midbrain
What type of proprioception does the cerebellum process?
Where does conscious proprioception terminate?
The cerebellum, a 2-neuron pathway from the limbs, processes unconscious proprioception.
Ends in the cerebral cortex, arriving through the medial lemniscus pathway.
Where do the anterior and lateral spinothalamic pathways cross?
Cross at the level they enter the spinal cord.
Where do the dorsal columns cross?
Dorsal columns don’t cross until they reach the medulla
Where do all modalities cross in the trigeminal thalamic tract?
All of the modalities cross right away in the trigeminal thalamic tract.
Where do conscious/unconscious pathways end?
They all end at either the conscious level in the cerebral cortex or unconscious level in the cerebellum
The Sensory Lemniscus: How do the anterior and lateral spinothalamic tracts travel?
What joins at the level of the medulla?
At the medulla, what is forming?
What joins in the pons and midbrain?
Collectively, what is this group called?
What does this organization eventually become?
The anterior and lateral spinothalamic tracts cross and travel together.
Medulla, the dorsal columns pathway crosses to join them; the trigeminal thalamic tract is forming (medulla and pons) and joins them
Pons and midbrain: (inferior colliculus/hearing level) the lateral lemniscus carrying auditory signals joins the group.
Collectively called: sensory lemniscus.
Organization eventually becomes the sensory homunculus
What does somatosensation begin with?
Begins with specialized receptors in the skin, muscles, joints and blood vessels that convert sensation to neural signal and transmit them through the spinal cord to the sensory cortex in the parietal lobe.
What is the job of the ascending fibres?
Carry info regarding pain, touch, temperature, and sense of position and travel through the spinal cord, brainstem, and thalamus before projecting to the primary sensory cortices in the parietal lobe.
What does LMN circuitry do and integrate from the higher CNS levels?
LMN circuitry distributes to local reflex output and integrates other ongoing output activity descending from the higher CNS levels.
Which neuronal terminal synapse with a LMN dendrite? Where do the rest of afferents terminate?
Only stretch afferent neuronal terminals synapse with a LMN dendrite or cell body and mediate the stretch reflex; all other afferents terminate in the dorsal horn association nucleus.
What is the type of somatic sensation that implies the displacement of the nerve endings and includes touch, pressure, vibration, and kinesthesia
What is the type of somatic sensation that includes sensation of cold and heat?
What do quickly adapting receptors do?
Respond strongly at the onset of the stimulus; as they adapt to the stimulus strength, their responses become weaker eventually dying out
What do non-adapting receptors do?
May not respond so vigorously to the stimulus onset. Once activated, they continuously provide signals to the brain as long as the stimulus remains present.
What is the type of sensory ending that is in subcutaneous tissues, skin, fingertips, palms, lips and external genitals, the most sensitive and rapidly adapting mechanoreceptors mediate sensations of vibration and fine discriminative touch.
What is the type of sensory ending that is in skin, cutaneous tissue, and visceral organs. Are nonadapting free nerve ending receptors that consist of fine branchings of fibre and mediate sensations of pain and temperature
Free nerve endings
What is the type of sensory ending that have Merkel receptors and Ruffini endings consist of nerve endings with knobs that mediate touch, temperature, and pressure.
Expanding tip endings
Three-neuron organization; Where do first-order neurons have their cell bodies, what info do the collect, and where do they transmit it?
Cell bodies in the spinal DRG, collect sensory info from the periphery and transmit it to the second-order neurons in the CNS
Three-neuron organization; Where are second-order neurons, where do they cross?
Are either in spinal cord or in the brainstem. These fibres consistently cross the midline and ascend to the opposite thalamus, which contains the third-order neurons
Three-neuron organization; Where are third-order neurons and where do they project?
Opposite thalamus, which contains the third-order neurons. These fibres project from the thalamus to the primary sensory cortex
Where do pathways mediating discriminative touch travel? Where is the decussation?
Have input axons proceeding up the ipsilateral side, and decussation is delayed to the medullary level.
Where do pathways which mediate pain, temperature and diffuse touch decussate?
Decussate in the spinal cord at multiple sites and proceed rostrally on the contralateral side.
What are the divisions of systems in the spinal cord?
-Somatosensory system: dorsal column-medial lemniscal system, anterolateral system - divided into lateral spinothalamic and anterior spinothalamic tracts
In the Medial Lemniscus (Dorsal Columns Pathway), what is the modality?
Light (fine discriminative) touch, conscious proprioception, 2-point discrimination, vibration, stereognosis.
In the Medial Lemniscus (Dorsal Columns Pathway), what is the course?
Where do they synapse?
Where do they cross?
Where do they ascend?
Where do they terminate?
Fibres from the primary neuron have ascended (without synapsing) in fasiciculus gracilis and fasiciculus cuneatus. In the closed medulla they now synapse on second order neurons in the nucleus gracilis and nucleus cuneatus, cross in a band called the medial lemniscus (without synapsing) and ascend in the core of the brainstem (tegmentum of the brainstem) to the VPL of the thalamus. (VPL is an allowed abbreviation). There they synapse on the third order neurons in the VPL to travel through the internal capsule, through the corona radiata to the parietal lobe, terminating according to their homunculus.
In the Medial Lemniscus (Dorsal Columns Pathway), what are the structures to note?
Cell bodies are: dorsal root ganglion, nucleus cuneatus (arms) or nucleus gracilis (legs) and finally VPL of the thalamus. At the level of the rostral medulla to pons the medial lemniscus slides laterally and joins the spinal lemniscus. From the pons the lateral lemniscus (hearing) joins them to form the sensory lemniscus.
In the Medial Lemniscus (Dorsal Columns Pathway), what are some features?
It doesn't cross in the spinal cord. It synapses and then crosses in the closed medulla in the internal arcuate fibres. Pathway ascends the spinal cord in the dorsal columns and forms the medial lemniscus in the medulla
In the Medial Lemniscus (Dorsal Columns Pathway), what are some deficits?
- Lesions interrupting the ascending fibres in dorsal column system affect fine discriminative touch sensation and position sense
- Usually both dorsal columns are affected by injury or disease and all modalities of epicritic sensation are impaired below the level of the lesion on the same side.
In the Medial Lemniscus (Dorsal Columns Pathway), What are tests for assessing the integrity of the system?
Two-point tactile discrimination, vibratory sense, position sense, stereognosis, and graphesthesia, Romberg test
In the Lateral Spinothalamic Tract, what is the modality?
Pain and temperature
In the Lateral Spinothalamic Tract, what is the course?
Beginning with receptors for pain and temperature, the primary neuron, with its cell body in the dorsal root ganglion enters the dorsal horn, synapsing in the dorsal horn. The second order neuron, with its cell body in the dorsal horn, crosses the spinal cord at the level it enters, and ascends in the lateral white column (lateral fasciculus) to the VPL of the thalamus. There it synapses on the third order (tertiary) neuron that travels through the internal capsule to the parietal lobe (areas 1,2 and 3) ending according to its source (arm or leg) in the homunculus. From the pons it forms part of the sensory lemniscus.
In the Lateral Spinothalamic Tract, what are the structures to note?
Receptors and the three cell bodies (dorsal root ganglion, dorsal horn of the spinal cord, VPL of the thalamus).
In the Lateral Spinothalamic Tract, what are some features?
It is a 3-neuron pathway, crosses immediately, and ends in the parietal lobe.
In the Lateral Spinothalamic Tract, what are some deficits?
- Damage to peripheral (spinal) nerves or the DRG subsequent to compression or reduced local circulation results in reduced pain sensitivity.
- Damage to spinothalamic fibres also affects transmission of pain and temperature to the sensorimotor cortex.
-Brainstem lesion affects contralateral half of the body. If DRG, substantia gelatinosa damaged, sensation is affected on ipsilateral half of body
In the Lateral Spinothalamic Tract, what are some test for assessing the integrity of the system?
- By pricking the body surface with a pin or by pinching the skin. Patient asked to describe sensation. Repeated until an area of deficit is mapped.
In the Anterior Spinothalamic Tract, what is the modality?
Crude touch and pressure
In the Anterior Spinothalamic Tract, what is the course?
This pathway is the same as the lateral spinothalamic except it ascends a little more anterior in the cord. It then joins the lateral spinothalamic to become the spinal lemniscus.
In the Anterior Spinothalamic Tract, what are some structures to note?
Same as lateral spinothalamic
In the Anterior Spinothalamic Tract, what are some features to note?
Same as lateral spinothalamic
In the Anterior Spinothalamic Tract, what are some deficits?
Interruption causes no obvious clinical deficit.
In the Anterior Spinothalamic Tract, what are some test for assessing the integrity of the system?
Diffuse touch is tested using a piece of cotton or a wisp of wool. Patient relying on this backup system notices a touch but cannot determine its location or describe its quality. Can be assessed effectively only when the pathway for fine discriminative touch is not functioning
In the Trigeminal Thalamic Tract, what is the modality?
Pain and temperature, crude touch and pressure, fine touch, 2-point discrimination, conscious proprioception, vibration and stereognosis.
In the Trigeminal Thalamic Tract, what is the course?
The pain and temperature travel downward briefly, then synapse in the spinal nucleus of the trigeminal nerve, cross and then ascend in the sensory lemniscus. The second modality of light touch and pressure, enter the pons, synapse in the chief sensory nucleus, cross and ascend in the sensory lemniscus. The third modality (proprioception) is mediated by primary sensory neurons whose cell body is in the mesencephalic nucleus. These neurons synpase at the chief sensory nucleus. From there, the pathway differs between conscious and unconscious proprioception.
In the Trigeminal Thalamic Tract, what are structures to note?
Note that the trigeminal ganglion (analogous to dorsal root ganglion) is not a synapse, but the cell bodies of the primary neuron.
In the Trigeminal Thalamic Tract, what are some features?
This is a combination of three pathways, all coming from the face. It is the sensory input from the Trigeminal nerve, cranial nerve V.
In the Trigeminal Thalamic Tract, what are some deficits?
- Common disease that affects sensory ganglion is herpes zoster
- Tic douloureux is characterized by episodes of intense pain that is paroxysmal
- Damage to either trigeminal ascending tract causes partial to complete anesthesia in the face and head.
In the Trigeminal Thalamic Tract, what are some tests for assessing the integrity of the system?
Pain and temperature are assessed by pricking with a pin or pinching the skin and using warm and cool objects as stimuli. Patient asked to report the quality and location of sensation to map distribution
In the Spinocerebellar Tract, What is the course?
There are only 2 neurons in this pathway. That is because it does not reach the conscious level. From the cerebellum, it will eventually provide a last neuron that "informs the cerebral cortex". So, a kind of unfinished pathway.
In the Spinocerebellar Tract, what are structures to note?
This pathway enters the dorsal horn and synapses as did the spinothalamic pathways. It then ascends ipsilateral to where it entered, in the lateral white column (fasciculus). At the medulla, it leaves through the inferior cerebellar peduncle to enter the cerebellum.
In the Spinocerebellar tract, what are some features?
It has only two neurons, it doesn't cross, it doesn't reach conscious level and "it" is really three pathways combined.