Ascending Pathways Flashcards
What is the modality and rate of adaption of Hair Follicle Receptors?
- Touch
* Rapidly adapting
What is the modality and rate of adaption of Merkel Endings?
- Pressure, low frequency vibration
* Slowly adapting
What is the modality and rate of adaption of Meissner Corpuscles?
- Light Touch
* Rapidly adapting
What is the modality and rate of adaption of Pacinian Corpuscles?
- Vibration, joint position sense
* Rapidly adapting
What is the modality and rate of adaption of Ruffini Endings?
- Skin stretch, joint position sense, pressure
* Slowly adapting
What is the modality and rate of adaption of Nociceptors?
- Pain
* Free nerve endings
What is the modality of muscle spindles?
- Muscle length
* Proprioception
What is the modality and rate of adaption of Golgi Tendon Organs?
- Joint position sense (JPS)
* Slowly adapting
Describe the sensor response
- Increased permeability to all ions
- Sodium predominates because of the max difference between intra/extracellular concentrations, therefore similar in ion movements to APs
- Generator potential in the sensor region causes a spike in the adjacent membrane then propagated as an action potential
Describe Aα axons
- Fastest - 270mph
- Proprioceptors of skeletal muscle
- Largest diameter - 13-20µm
Describe Aβ axons
- 167mph (second fastest)
- 6-12µm diameter
- Mechanoreceptors of the skin
Describe Aδ axons
- 1-5µm diameter
- 67mph
- Pain, temperature, localised pain
Describe C axons
- 0.2-1.5µm diameter (smallest)
- 5mph (slowest)
- Temperature, pain, itch
Describe lateral inhibition
- A way of improving localisation
- Each sensory neurone has a receptive field, dendrites are dense at the centre of this field, closer to the cell body and more diffuse at the periphery
- Stimuli firing at the centre of the receptive field will activate more dendrites than at the periphery resulting in faster firing
- The receptive fields of neurones overlap, the stimulation will be more intense for the neurone experiencing the stimulus closer to the cell body
- The neurones will have branches that will inhibit the neighbouring neurone, the one experiencing a stronger stimulus will inhibit the neighbouring neurone to a greater extent
- The result is a bigger difference in the firing rate of the neurones, helping the brain to discriminate between 2 points
What is a dermatome?
- Area of skin with a sensory supply from a single spinal nerve
- The 1st neuron in the ascending pathway sense and exits from their cognate dermatome then feeds into a chunk of spinal cord that belongs to that body segment
Where do motor neurones exit the spinal cord?
Ventral root
Where do the sensory neurones enter the spinal cord?
Dorsal root
What are the 2 pathways for conscious sensation?
- Anterolateral tract (spinothalamic)
* Dorsal column (medial lemniscal pathway)
What sensation is carried by the anterolateral tract?
- Pain
- Temperature
- Itch
- Crude touch
What sensation is carried by the dorsal column (medial lemniscal tract)?
- Proprioception (conscious)
- Vibration
- Light touch/ fine touch - 2 point discrimination
Where does the anterolateral (spinothalamic) tract cross the midline?
Close to the entry level, in the white commissure
Where does the dorsal column (medial lemniscal) tract cross the midline?
In the medulla
Describe the route of the dorsal column pathway
- The first neuron (DRG) enters the spinal cord and ascends on the same side to the medulla
- The second neurone crosses over in the medulla and ascends to the thalamus
- The third neurone in the thalamus ascends to the cerebral cortex (the post central gyrus)
What are the differences in the dorsal column pathway for the arm vs the leg?
- For the leg, the first neuron fibres travel in the gracile tract, whereas the arm travel in the cuneate tract
- The leg fibres cross in the gracile nucleus, the arm in the cuneate nucleus
- In the spinal cord the arm fibres are more lateral than the leg
- In the brainstem, the arm fibres are more medial than the leg fibres
Where is the motor cortex
• Precentral gyrus
Where is the primary somatic sensory cortex?
• Postcentral gyrus
What is the information from the dorsal column (medial lemniscal) pathway used to do?
• Determine the shape of an object without sight - Stereognosis
- proprioception from the finger joints and fine discriminating touch provides an accurate model of an object that is handled
• Determine the texture of an object without sight - Astereognosis
- texture is determined by vibration and slip (Meissner) receptors as well as fine discriminating touch
- pleasure can also be associated with this pathway
What area of the brain must be functioning in order for the dorsal column pathway to function?
The somatosensory association area in the parietal lobe
Describe the route of the anterolateral (spinothalamic) pathway
- 1st neurone (DRG) enters the spinal cord
- 2nd neurone in the posterior horn crosses to the opposite side close to the entry level in the white commissure and ascends to the thalamus
- 3rd neurone in the thalamus projects to the cerebral cortex
What are the two types of pain?
- Discriminative - localising pain, well localised and brief
- Affective -‘suffering pain’ less localised and longer lasting
What type of fibres carry discriminative pain?
• A delta fibres
Where is discriminative pain location perception
Post central gyrus
What type of fibres carry affective pain?
Unmyelinated C fibres
Describe the path of the fibres carrying affective fibres in the Brain
- Arrives at the reticular formation in the brainstem (parabrachial nucleus)
- Then goes either to the thalamus (intralaminar nucleus) or the hypothalamus (physiological response)
- From the thalamus, fibres travel to the amygdala deep in the temporal lobe then to either the cingulate gyrus (emotion) or the insula (avoidance)
Why can pain prevent sleep?
- Affective pain
* Synapse with brainstem nuclei to increase arousal, preventing sleep
Why can pain make you sweat/feel sick?
- Indirectly stimulates the autonomic circuitry
* Hypothalamus
What provides us with the pain basis for avoidance behaviours?
- The insula
* it integrates discriminative and affective pain, co-ordinating pain avoidance
What are the cortical areas of pain perception?
- Discriminative pain = post central gyrus
* Affective pain = cingulate gyrus
Opioids
- Treat pain
- Reduce the fMRI signals in the insula and the thalamus
- Work at spinal, brainstem and cortical levels
What is the spinal cord gate theory?
- Interneurones downstream of the touch pathways inhibit pain pathways
- e.g. rubbing helps with the pain
- TENS trans-electrical nerve stimulators (applied near to the damaged area)
What is the brain stem to spinal cord pathways - descending control of pain?
- The periaqueductal matter is a continuation of the periventricular grey matter, located around the cerebral aqueduct in the midbrain
- It sends descending projections to the spinal cord and ascending projections to the thalamus and frontal lobes
- Terminate on inhibitory interneurones in the dorsal horn of the spinal cord
What is the thalamus?
A collection of nuclei on diverse pathways to the cerebral cortex
What are the functions of the thalamus?
- Conscious sensation relay and processing
- Motor circuits - basal ganglia to motor cortex
- Emotions/memory
- Special sense haring and vision
What are the connections of the cortex and the thalamus?
- White matter bundles
- Thalamic connections forming peduncles (anterior, inferior, posterior, inferior)
- Thalamic connections form part of the internal capsule (anterior thalamic radiation, superior thalamic radiation, auditory thalamic radiation, optic radiation)
What are the main pathways of the thalamus?
- Sensory relay
- Motor circuits
- Limbic systems
- Special sense
Sensory relay pathway of the thalamus
- Sensory info from the head through the VPM- ventral posteromedial nucleus
- Sensory info from the body via the VPL - ventral posterolateral nucleus
Motor circuit pathway of the thalamus
- To motor cortices from the basal ganglia and cerebellum
* Ventral lateral nucleus and ventral anterior nucleus
Limbic system pathway of the thalamus
• Nuclei on emotion, pain and memory pathways
Special sense pathway of the thalamus
- Vision through the lateral geniculate body (LGB)
* Auditory through the medial geniculate body (MGB)
What occurs in a anterolateral tract lesion?
• Decreased perception of pain and temperature on the contralateral side of the body, one or two dermatomes below the lesion
How can anterolateral tract pathway lesions be utilised?
- In surgery to offer pain relief of a terminal disease and pain
- Pain returns approx 1 year after procedure
What are anterolateral fibres susceptible to?
- Central cord pathology e.g. swelling can obliterate fibres
- They are susceptible because they cross early
What is the result of a hemi-section of the spinal cord?
Results in loss of pain perception on the contralateral side and a loss of proprioception on the ipsilateral side
What conditions affect the dorsal spinal cord?
- Vitamin B12 deficiency
- HIV myelopathy
- Tabes Dorsalis (syphilis)
- Multiple Sclerosis
Describe the spinocerebellar pathway
- Unconscious proprioception
* Does not cross the midline - always ends up on the same side of the Brain
What part of the brain issues correction information to achieve smooth accurate movements?
The cerebellum
Fasciculus gracilis
Carries information from the lower body extremities (touch, vibration, conscious proprioception)
Fasciulus cuneatus
Carries information from he upper body extremities (touch, vibration, conscious proprioception)
Dorsal and ventral spinocerebellar tracts
Proprioceptive information from the muscle spindle (dorsal) and the Golgi tendon organs (ventral)
Anterolateral tract
Tocu, pain, itch, temperature
what does testing the joint position sense do?
Exclusively dorsal columns
What does testing pain and temperature sensation do?
Exclusively lateral spinothalamic tracts
What does testing light touch do?
Localises lesion
What does testing vibration sense do?
Good for screening several pathologies
