20. Sensory Systems 🧠Flashcards
What receptors are responsible for the sensory modalities of touch and proprioception?
Mechanoreceptors
Describe the structure of mechanoreceptors involved in touch and proprioception.
The receptor is NOT a separate entity but is actually the peripheral terminal of the peripheral axon of the primary sensory neuron.
Describe the structure of a Pacinian corpuscle and explain how this structure relates to its function.
RA1
There is an axonal ending in the middle and it is wrapped around several concentric circles of epithelial cells – this allows the receptor to be very sensitive to vibration.
What is the difference between slow adapting and fast adapting receptors?
- Slow adapting receptors continue firing impulses for as long as the stimulus is present
- Fast adapting receptors tend to fire at the start of the stimulus and sometimes when the stimulus switches off but they tend to fade in the middle
What type of receptors are mechanoreceptors?
Mixture of slow and fast adapting receptors
Describe how sensory neurons vary in their properties.
They vary in SIZE and CONDUCTION VELOCITY
What are the two classifications of axons?
- Anatomical = based on axon diameter (labelled using LETTERS)
- A
- B
- C
- Physiological = based on conduction velocity (labelled using ROMAN NUMERALS) As axon diameter and conduction velocity are related, there is a lot of overlap in the classifications
Describe the general structure of sensory neurons that convey touch and proprioceptive information.
- They are LARGE and have a FAST conduction velocity
- Touch
- A beta (II)
- Mechanoreceptors
- A beta (II)
- Proprioception
- A alpha (Ia and 1b)
- Muscle spindle and Golgi tendon organs
- A alpha (Ia and 1b)
What is a receptive field?
An area of skin that is innervated by one sensory axon and its branches
Describe how the receptive fields in the lips and mouth vary from the receptive fields of the upper arm.
Lips and Mouth – high-density innervation with very small receptive fields Upper arm – larger receptive fields and thinner innervation
Describe how neurons can code for the intensity of a stimulus.
It is coded by the FREQUENCY of the action potentials going down the sensory fibres
Which part of the spinal cord carries sensory axons for touch and proprioception?
Dorsal columns as part of the dorsal column-medial lemniscus pathway
What are the bundles of axons within the spinal cord that have come from above and below the waist called? Describe their spatial arrangement within the spinal cord (somatosensation)
- Above the waist
- Cuneate Fasciculus
- Below the waist
- Gracile Fasciculus
- Axons from below the waist are packed more medially in the dorsal column and above the waist are more lateral
- Lower = Medial
- Higher = Lateral
Where do the cuneata and gracilis fasciuli synapse?
They synapse in the Cuneate and Gracile Nuclei in the medulla
Describe what happens after the gracile and fasciculus neurons synapse int the medulla and the tract that they run in (somatosensation)
- The second order neurons then cross the midline (decussation) where they are then known as internal arcuate fibres
- They continue up the brainstem in the medial lemniscus where they then synapse at the ventral posterolateral nucleus at the medulla now becoming third order neurones
- They then continue to the primary somatosensory cortex in the thalamus
Which thalamic nucleus is responsible for relaying somatosensory information from the neck down?
Ventral Postero-lateral
Ventral Postero-medial nucleus = face
Describe the passage of the third order sensory neuron (in the dorsal column-medial lemniscus pathway)
The third order neurone travels from the ventral postero-lateral nucleus in the thalamus to the primary somatosensory cortex
What is the main sensory nerve of the face?
Trigeminal Nerve (CN V)
Where does the trigeminal nerve enter the brainstem and where does it synapse with a second order neuron?
- Enters the braisntem at the pons
- It synapses at the trigeminal nucleus
Describe the passage of the trigeminal nerve as a second order neurone after synapsing at the trigeminal ganglion (somatosensation)
The second order neuron crosses the midline (decussation) and joins the medial part of the median lemniscus
Which thalamic nucleus is responsible for relaying sensory information from the face?
Ventral Postero-medial
What is lateral inhibition?
- Lateral inhibition takes place in the cuneate and gracile nuclei
- Each axon has lateral branches that are inhibitory on neighbouring axons
- So each axon will stimulate a second order neuron and inhibit neighboring first order neurons
What is the purpose of lateral inhibition?
Improves the resolution of localising the stimulus
Name the three parts of the somatosensory cortex.
- Primary somatosensory cortex (S1)
- Secondary somatosensory cortex (S2)
- Posterior parietal cortex
What is the posterior parietal cortex mainly involved in?
Spatial relationships
Damage to the touch and proprioception pathway will halt sensory information going up to the primary somatosensory cortex. What effects will this have?
- Anaesthesia
- Complete cessation of sensation
- Parasthesia
- Sensation is there but it isn’t normal
What is the most common cause of peripheral neuropathy?
Diabetes mellitus
What are the pain receptors called?
Nociceptors
Can very high intensity stimulation of mechanoreceptors cause the feeling of pain?
No – only nociceptors can cause pain sensation
Describe some features of nociceptors.
- Polymodal
- different types of nociceptor respond to different stimuli
- Free nerve endings
- usually just free axonal endings of neurones
- High threshold
- higher activation threshold than touch receptors
- Slow adapting
- this is good because it means you are constantly reminded of the presence of a potentially harmful stimulus
What are the two main types of sensory neurone that carries sensory information? State some characteristics of each.
- A
- Large
- Fast conducting
- Fast adapting
- Produces pain fast
- C-fibre
- Smaller
- Produces a dull, aching pain
- It reminds you of the injury so that you guard this part of the body
- SLOW conducting -UNMYELINATED
Compare the receptive fields of nociception to those of touch.
Receptive fields for nociception are much LARGER because the nociceptive pathway is phylogenetically older than touch and you don’t need to be able to localise pain as well as touch.
Describe the method of coding intensity in nociception.
Same as touch – increase in frequency of impulses
Describe the passage of the central pathway, which carries information about pain and temperature.
- First order neurone enters the spinal cord and synapses in the dorsal horn with a second order neurone
- Second order neurone decussates immediately and travels up the white matter of the spinothalamic tract
- It then goes up to the thalamus where it synapses with a third order neurone, which then goes to the primary somatosensory cortex.
Describe the somatotopic arrangement of the fibres in the spinothalamic tract.
Lower fibres = Lateral
Higher fibres = Medial (Opposite of dorsal columns)
Sharp point arrow head that would hurt to sit on upwards
Which thalamic nucleus relays sensory information from below the neck?
Ventral Postero-lateral
Where does decussation occur in the pain and temperature pathway?
- At the same level as the information coming into the spinal cord
- Dorsal root ganglion
Which nerve carries nociceptive information from the face?
Trigeminal Nerve (CN V)
Where does the trigeminal nerve enter the brainstem?
Pons
Describe the passage of the trigeminal nerve from entry into the brainstem (nociception)
- It enters the trigeminal ganglion in the pons and then it moves DOWNWARDS along the trigeminal nucleus
- ​Trigeminal nerve nucleus = largest of the cranial nerve nuclei (collection on neurones) that extend through the whole of the midbrain, pons and medulla oblongata
- It then synapses in the lower part of the trigeminal nucleus in the medulla
- The second order neurone then decussates and joins the medial end of the spinothalamic tract.
Which thalamic nucleus relays pain information from the face?
Ventral Posteromedial
Which part of the trigeminal nucleus does the first order nociceptive neurones from the face synapse in?
Spinal Trigeminal Nucleus
What is the role of the primary somatosensory cortex in processing the nociceptive stimulus?
It registers the LOCATION and INTENSITY of the stimulus
As the spinothalamic tract projects towards the primary somatosensory cortex, it gives off collateral branches. Which structures do these branches go to?
BT LH
- Brainstem
- Reticular formation
- Thalamus
- intralaminar nuclei
- Limbic structures
- Hypothalamus
What is the point of these collateral connections?
The connections to the reticular formation (brainstem) and intralaminar nuclei (thalamus) allow the spinothalamic tract to increase your level of arousal to make sure that you are aware of potentially harmful situations
Which CNS structures are involved in signalling the unpleasantness of a stimulus?
- Limbic structures
- Hypothalamus
- Basal ganglia
- Cingulate gyrus
- Amygdala
- Hippocampus
- Hypothalamus
What are the two pathways that can reduce the amount of pain that you feel?
Central and Peripheral Inhibition Pathways
What is the main location of the central inhibition pathway?
Periaqueductal Grey Matter
Describe the arrangement and function of the central inhibition pathway.
- Increased brain activity will increase the impulses going down the central inhibition pathway, which goes to the dorsal horn at every level.
- These descending axons synapse with an interneurone and activate the interneurone
- The interneurone synapses with the first and second order nociceptive neurones and release ENKEPHALIN, which is inhibitory
- So enkephalin release will reduce the amount of information going up the spinothalamic tract hence you feel less pain.
What type of molecule is Enkephalin?
- Opioid
- Morphine mimics the action of this central inhibition system.
Other than a first order nociceptive neurone, what else has input into the second order nociceptive neurone?
- Non-nociceptive neurones
- Axons of non-nociceptive touch neurones will go into the dorsal horns
- Will also have collaterals that are capable of activating an inhibitory interneurone,
- ​can reduce the activity of the projecting neurone and hence reduce the activity going up the spinothalamic tract.
Describe the arrangement and function of the peripheral inhibition pathway.
Stimulation of touch receptors in the same area as the pain sensation will lead to increased activity of the non-nociceptive touch neurones meaning that there is increased activation of the inhibitory interneurone and hence reducing the activity going up the spinothalamic tract.
State two causes of loss of pain sensation.
-
Syringomyelia
- Cyst or cavity within the spinal cord
-
Charcot Joints
- due to peripheral neuropathy, you don’t realise that you are using your joints inappropriately or excessively – this leads to joint deformities
How can you get exacerbation of pain?
- Wind up in the dorsal horn If someone has chronic pain then certain peripheral nerves coming into the spinal cord will be carrying high levels of input for a long time
- The cells in the dorsal horn can lower their sensitivity or their synapses will change, which means that the information going into the spinothalamic tract is increased so this can actually increase the level of chronic pain.
Compare the receptive fields of nociception to those of touch.
Receptive fields for nociception are much LARGER because the nociceptive pathway is phylogenetically older than touch and you don’t need to be able to localise pain as well as touch.
Describe the method of coding intensity in nociception.
Same as touch – increase in frequency of impulses
Describe the passage of the central pathway, which carries information about pain and temperature.
- First order neurone enters the spinal cord and synapses in the dorsal horn with a second order neurone
- Second order neurone decussates immediately and travels up the white matter of the spinothalamic tract.
- It then goes up to the thalamus where it synapses with a third order neurone, which then goes to the primary somatosensory cortex
Describe the somatotopic arrangement of the fibres in the spinothalamic tract.
Lower fibres = Lateral Higher fibres = Medial (Opposite of dorsal columns)
Definition of rate coding
Signal strength is conveyed by impulse rates of the individual neurons
Template matching
Where a network of neurons can recognize a specific pattern of inputs from a population of presynaptic neurons
Extroception
Sensitivity to stimulu originating outside the body
What type of neurons are at the dorsal root ganglion?
- Pseudo-unipolar
- Axon of a dorsal root ganglion has two branches
- One projecting into the periphery
- One projecting to the CNS
- Axon of a dorsal root ganglion has two branches
TRP ion channels
- Non-selective cation receptor channels similar in structure to voltage-gated channels.
- They have four protein subunits each of which contain 6 transmembrane domains.
- Pore between the 5th and 6th segment
TRPA1
- Only expressed in low-threshold cold receptor terminals
- Threshold below 17°C
TRPV4
Threshold above 27°C
TRPV3
Threshold above 35°C
TRPV1 & TRPV2
Threshold above 45°C
Thermal nociceptors
Activated by extremes in temperature greater than 45°C (
- TRPA1 = < 17°C
- TRPM8 = < 25°C
- TRPV4 = > 27 °C
- TRVP3 = > 35°C,
- TRPV1 & 2 = > 45°C
Mechanical nociceptors
Respond to excess pressure / mechanodeformation
Polymodal nociceptors
Can be activated by high-intensity, mechanical, chemical or thermal stimuli
Silent nociceptors
Found in the viscera, not normally activated by noxious stimuli instead by inflammation and various chemical agents which dramatically reduce their firing threshold resulting in secondary hyperalgesia and central sensitization
Nociceptive pain
Activation of nociceptors in the skin or soft tissue in response to tissue injury
Neuropathic pain
Pain caused by damage or disease affecting the somatosensory nervous system
Dysesthesia
Abnormal sensations
Allodynia
Pain from normally non-painful stimuli
Lamina 1
Many neurons in this most superficial lamina respond to noxious stimuli conveyed by A-delta and C fibers
Lamina 2
- Substantia gelatinosa
- Receives input from C fibers that are activated selectively by the cold
- Site of nociceptive modulation
- Densely spaced layer that contains many different classes of local interneurons, some excitatory and some inhibitory
Lamina 3 & 4
- Contains a mixture of local interneurons and supraspinal projection neurons
- Many of these neurons receive input from A-beta afferent fibers that respond to innoccous cutaneous stimulation such as the deflection of hairs and light pressure
Lamina 5
- Contain neurons that respond to a wide variety of noxious stimuli and project to the brain and thalamus
- Neurons in lamina 5 also receive input from nociceptors in visceral tissue
- The convergence of somatic and visceral nociceptive inputs onto individual lamina 5 neurons provides one explanation for referred pain
Lamina 6
- Receive inputs from large diameter fibers that innervate muscles and joints
- 1 - A alpha
- Muscle spindle
- Golgi tendon organ
- 1 - A alpha
- These neurons are activated by innocuous joint movement and do not contribute to the transmission of nociceptive information
Lamina 7
- Respond to stimulation on either side
- Activation contributes to the diffuse quality of many pain conditions
What are the main landmarks of the visual pathway?
- Eye
- Optic nerve
- Optic chiasm
- Optic tract
- Lateral geniculate nucleus
- Optic radiation
- Primary visual cortex (striate cortex)
Where do retinal ganglion axons coming down the optic nerve synapse?
Lateral Geniculate Nucleus
Where is the lateral geniculate nucleus found?
Thalamus
What are the fibres leaving the lateral geniculate nucleus called?
Optic Radiation
Which order neurones are these and where do they terminate?
4th Order Neurones They terminate in the primary visual cortex
What are the first, second and third order neurones in the visual pathway?
First Order – photo-receptors (rods and cones) Second Order – bipolar cells Third Order – retinal ganglion cells
What happens as the retinal ganglion cells enter the optic nerve, which improves the transmission of the signal?
They become myelinated
What percentage of retinal ganglion cell fibres crosses the midline at the optic chiasma?
53%
Describe the convergence and receptive field sizes of rods and cones.
Rods have high convergence and large receptive fields Cones have low convergence and small receptive fields
Describe how the convergence of the rod system differs across different parts of the retina.
The rod system near that macula has lower convergence than in the peripheral retina
What is the benefit of having high convergence and a large receptive field?
High light sensitivity
What is the benefit of having low convergence and a small receptive field?
Fine visual acuity
Retinal ganglion cells can be described as on-centre or off-centre. What do these two terms mean?
On-centre – they are stimulated by light falling on the centre of the receptive field and inhibited by light falling on the edge of the receptive field Off-centre – they are stimulated by light falling on the edge of the receptive field and inhibited by light falling on the centre This is important in contrast sensitivity and enhanced edge detection
Where do the fibres that decussate at the optic chiasma originate?
The nasal part of the retina These fibres are responsible for the temporal half of the visual field
What effect do lesions anterior to the optic chiasm have on vision?
Affects only ONE eye
What effect do lesions posterior to the optic chiasm have on vision?
- Affects BOTH eyes
- Right-sided lesion = left homonymous hemianopia
- Left-sided lesions = right homonymous hemianopia
What is the effect of a lesion at the optic chiasm?
Bitemporal hemianopia
Which part of the brain does the upper division of the optic radiation travel through and which parts of the visual field is it responsible for?
- Parietal lobe
- Responsible for the inferior visual quadrants
Which part of the brain does the lower division of the optic radiation travel through and what part of the visual field is it responsible for?
- Temporal lobe
- Responsible for the superior visual quadrants
The lower division loops inferiorly and anteriorly before going posteriorly towards the primary visual cortex. What is this loop called?
Meyer’s Loop
What would be the consequence of a lesion in Meyer’s loop?
Superior homonymous quadrantopia
What would be the consequence of a lesion of the upper division of the optic radiation?
Inferior homonymous quadrantopia
What is homonymous hemianopia typically caused by?
Strokes and other cerebrovascular accidents
Where is the primary visual cortex located?
Along the Calcarine Fissure in the occipital lobe
Describe which parts of the primary visual cortex are responsible for the different fields of vision.
- The macula has a disproportionate representation in the primary visual cortex
- The left primary visual cortex is responsible for the right visual field from both eyes
- The right primary visual cortex is responsible for the left visual field from both eyes
- Visual cortex above the calcarine fissure is responsible for the inferior visual field
- Visual cortex below the calcarine fissure is responsible for the superior visual field
How is it possible for the macula to be spared by a stroke in the primary visual cortex leading to homonymous hemianopia?
The area representing the macula in the primary visual cortex has adual blood supply (from both right and left posterior cerebral arteries) meaning that it is less vulnerable to ischaemia
What are the two pathways of the extrastriate cortex and what are they responsible for?
- Dorsal Pathway – deals with motion detection
- Ventral Pathway – handles detailed object recognition and face recognition
Describe what happens to the eyes in the light.
Iric circular muscle contracts Constriction of pupillary aperture Reduced rate of photopigment bleaching Increased depth of field
Describe the pathway that is responsible for the consensual lightreflex.
Retinal ganglion cells from the retina send they axons back via the optic nerve The fibres that are responsible for the pupillary reflex will get passed the optic chiasm and then leave the posterior 1/3 of the optic tract before it reaches the LGN The axons then go to the pretectal nucleus in the dorsal brainstem The afferent pathways from each eye then synapse on the Edinger-Westphal nuclei on both sides of the brainstem. A parasympathetic nerve from the Edinger-Westphal nuclei to the ciliary ganglion forms the efferent pathway Short ciliary nerves travel from the ciliary ganglion to the pupillary sphincter Summary: Retinal Ganglion Cell –> Pretectal Nucleus –> Edinger-Westphal Nucleus –> Ciliary Ganglion –> Short Ciliary Nerves –> Sphincter Pupillae
What would the consequences be of a right afferent defect?
Light shone in right eye: no direct or consensual response Light shone in left eye: direct and consensual response present
What would the consequences be of a right efferent defect?
Light shone in right eye: no direct response, consensual response present Light shone in left eye: direct response, no consensual response
What does RAPD mean?
Relative Afferent Pupillary Defect A partial pupillary response is still present despite damage to an eye and its pupillary reflex pathway – there is some degree of constriction
What test would you do to identify RAPD? What would you expect to see in a patient showing a RAPD?
Swinging Torch Test When the light is shone on the good eye, there will be a direct and consensual response When the light is then swung and shone at the bad eye, there will be a paradoxical dilation of the iris in the bad eye This is because the constriction response elicited by the bad eye is weaker than the consensual response elicited by the good eye