Sensory systems Flashcards
What is the use of sensation?
• Codes information about the world, including our internal world
• Reconstructs and represents the world in the nervous system as a construct
o What you see isn’t necessarily what your brain sees
Is all sensation conscious?
• Conscious and unconscious reception of sensation
o Unconscious sensation- stuff that doesn’t get to cortex and hence don’t know about them
What is the role of sensory receptors?
• Sensory receptors detect specific physical stimuli
Describe a standard sensory pathway
• Information is relayed through modality specific pathways from spinal cord and brainstem, then to the thalamus and cortex
• Peripheral sensory cells have faithfully-mapped connections through these pathways
• Hierarchicial levels of sensory processing:
o Spinal cord/brainstem (primary neurons)-> thalamus (via specific sensory relay nuclei) (second order neurons) -> cerebral cortex (primary sensory cortical areas) (third order neurons)
o Primary somatosensory neuron cell body in dorsal root ganglion or cranial nerve ganglion
o Central process enters CNS, synapse in dorsal horn (spinal cord) or medulla (cuneate and gracile nuclei)
o Axon of secondary order neuron decussates
o Axon of secondary order neuron synapses on 3rd order neuron in thalamus
o Axon of 3rd neuron travels to primary somatosensory cortex (S1, post-central gyrus
What is the difference between conscious sensation and unconscious sensation?
- Conscious sensation involves activity of cortical neurons and requires input to the cortex
- Unconscious sensation (sensory information not perceived in the cortex) plays a fundamental role in behavioural responses to stimuli and in reflex activity and occurs at subcortical level
What is transduction?
• Transduction- the process by which an environmental stimulus causes electrical response in receptor cell.
o Receptors must provoke an action potential in sensory neurons in order to transmit the signal to the brain
What are the different types of sensory receptors according to stimulus type and their uses?
• Sensory receptors can be classified by stimulus type
o Chemoreceptors detect molecules (e.g. olfaction)
o Mechanoreceptors detect changes in pressure, position, stretch or acceleration (e.g. touch, hearing, equilibrium)
o Electromagnetic receptors are specialised for visible light (human vision), infrared radiation or magnetic fields (not in humans)
o Thermoreceptors detect hot or cold
o Nociceptors detect severe heat and pressure, as well as chemicals released by inflamed tissue. Nociceptors are a combination of mechanoreceptors and chemoreceptors
What are the types of sensory receptors by location?
• Sensory receptors can be classified by location
o Exteroreceptors- respond to stimuli in the environment external to the body
o Interoreceptors- respond to stimuli inside the body
Proprioceptors- respond to joint position, muscle length and contraction, head position
Where are the cell bodies of primary sensory neurons,and what is an exception to this rule?
- Primary sensory neurons have cell bodies in peripheral ganglia
a. Exception- retinal ganglion cells are not peripheral, but are located in a ganglion cell layer
Describe the possible receptive endings of sensory neurons
b. Receptive endings of sensory neurons
i. May have specialised sensory receptors at the receptive end
ii. May be free neuronal endings
iii. May contact non-neuronal sensory cells
iv. Receptive endings initiate action potential
Where are cell bodies of primary somatosensory neurons located
c. Cell bodies of primary somatosensory neurons are located in dorsal root ganglia or cranial nerve ganglia
Describe thermoreceptors in skin in terms of anatomy
i. In skin, thermoreceptors have encapsulated pressure receptors and free neuronal endings
What is the length of mechanoreceptors for limbs and trunk
More than 1 m
What is the axonal length of mechanoreceptors for jaw
100mm
Describe the size of olfaction axons
1mm
Describe the size of gustation axons
100mm
Describe the size of audition axons
100mm
Describe the size of vision axons
100mm
What sensation do free nerve endings usually convey?
Heat, light, pressure and pain
Describe the labelled line principle
• Labelled line principle- any stimulus sufficient to evoke an action potential will be interpreted as the pathway’s modality (what you perceive)
o Neuronal axons transmit only action potentials
o The sensation is determined by the point in the CNS on which the axon synapses
o The stimulus does not need to be the stimulus for which the receptor is specialised
How can a stimulus be localised?
• Mapping-spatial distribution of somatosensory neurons activated by a stimulus conveys information about the stimulus location
o Every sensory reception must be transmitted faithfully and accurately to localise the stimulus
What is sensory acuity?
• Sensory acuity: precision
o Precision- resolution and accuracy of the sensation
What variables affect sensory acuity?
Receptive field and field overlap
• Receptive field size
• Receptor density
Integration e.g. lateral inhibition and centre surround inhibition
What is receptive field, and what features of the receptive field enhance localisation of stimulus?
• Receptive field- area on the body surface or sensory organ that elicits a response
o Difference in neuronal excitation/inhibition in different parts of the receptive field enhance localisation of the stimulus
• Receptive field size
• Receptor density
Describe the relationship between receptive field size and sensory acuity
o The smaller the receptive field, the greater the acuity as more info goes to the cortex
Describe the relationship between receptor density and sensory acuity
• Receptor density
o The density of sensory receptors in the receptive field is a determinant of the acuity of a sensory system
o The more receptors per cubic mm, the higher the acuity
Greater density leads to increased resolution of sensation
Describe the relationship between axon number and acuity
o The more axons, the more acuity
Describe how differences in neuronal excitation/inhibition in different parts of the receptive field enhance the localisation of the stimulus
o More action potentials when the stimulus in the centre of the receptive field, fewer when on the edge of the field
More action potentials means more input to the brain and sharpens brain perception
How is shape of a stimulus determined?
• Lateral inhibition-
o Helps to sharpen the sensation, increases precision of stimulus location
o To determine the shape of a stimulus, the brain relies on detection of edges: the differences between maximum and minimum stimulation
Suppresses information from edges and enhances information in the centre
Inhibitory internerneuron next to centre suppresses activity of neighbouring neurons not centred on the stimulus
Excitation in middle, inhibition in edges
o Lateral inhibition in the CNS (secondary, tertiary neurons…) sharpens the stimulus
Strong signals are transmitted
Weak signals are suppressed
Where does lateral inhibition occur?
• At each level in the hierarchy, sensory signals are modified
o Lateral inhibition for sharpening detection of input
o Descending input may suppress or enhance sensory transmission
Describe the CNV trigeminal nerve pathway to the somatosensory cortex
• CNV-trigeminal nerve
o Skin receptors
o Synapse in the brainstem nuclei cuneate and gracile nuclei
o Synapse into VPL somatosensory thalamus
o Synapse in somatosensory cortex
Describe the CNVIII vestibulocochlear nerve pathway to the auditory cortex
• CNVIII-vestibulocochlear nerve
o Auditory receptors in cochlea synapse on bainstem nuclei
o These synapse on medial geniculate nucleus of the thalamus
o Synapse on auditory cortex
Describe the CNII optic nerve pathway to the visual cortex
• CNII-optic nerve o Photoreceptors in retina o Synpase onto other retinal neurons o Synapse onto lateral geniculate nucleis of the thalamus o Synapse onto visual cortex
What is a papilla made of?
• A papilla is composed of several hundred taste buds. Each taste bud has 50-150 taste receptor cells
What are taste receptor cells made of and what is their use?
o Taste receptor cells have microvilli at the apical end that project to the taste pore where taste cell is exposed to the end of the mouth
o Taste molecules bind to microvilli of taste cells
Describe the sensitivity of individual taste receptors cells to stimuli
• Individual taste receptor cells are often selectively sensitive to particular classes of stimuli according to different threshold levels
o Some of them, however, are more broadly tuned to stimuli- they are less specific in their responses
What are the 5 tastes?
5 tastes • Sweet • Sour • Salty • Bitter • Umami
Describe the neurotransmitter and pathway of salty taste
• Salty (Na+) passes through permanently non-gated sodium channel
o Taste cells release serotonin on gustatory axons
Describe the neurotransmitter and pathway of sour tastes
• Sour (acid H+) passes through Na+ channel, blocks K+ channel and activates a type of ion channel from transient receptor potential channels
o Taste cells release serotonin onto gustatory axons
Describe the neurotransmitter and pathway of sweet, bitter and umami tastes
• Sweet, bitter, umami: G-coupled receptors T1R and T2R
o Release ATP
How do taste cells identify unique tastes?
Taste cells tend to be receptive to particular taste categories
• Each food activates a different combination of basic tastes, helping make it unique
• Most foods have a distinctive flavour as a result of their combined taste and smell occurring simultaneously
• Other sensory modalities such as texture and temperature also contributed to a unique food-tasting experience
Describe the taste pathway
o Taste molecules bind to microvilli of taste cells
o Action potential along nerves CNVII, CNIX, CNX- the axons of primary afferent neurons
Primary neuron has peripheral cell body in the ganglia of CNVII, CNIX, CNX
• CNVII-facial nerve (anterior 2/3 of the tongue and palate send axons into this nerve)
• CNIX-glossopharyngeal nerve (posterior 1/3 of the tongue is innervated by this nerve)
• CNX-vagus nerve (Regions around the throat send tasste axons to a branch of cranial nerve X)
o Taste impulse relays through brainstem cranial nerve nuclei and synapses onto secondary neuron in brain stem: the gustatory nucleus, a part of the solitary nucleus in the medulla
o Then secondary neuron synapses onto ventral posterior medial nucleus of the thalamus third order neuron
o 3rd order neuron relays through thalamus to primary gustatory cortex (area 36)- gustatory- insular and opercular cortex for conscious perception
o 3rd order neuron relays through brainstem to hypothalamus for unconscious sensation (autonomic, hunger, satiety, food seeking)
Describe the olfaction pathways
o Exception to most of the rules
Doesn’t go through the thalamus
o Molecules bind to microvilli/cilium of receptor cells in olfactory epithelium
o Primary neurons in CNI- synapse on secondary neurons in the olfactory bulb
CNI- shortest primary neuron in sensory system
o Secondary neuron sends axons into the CNS- directly to olfactory cortex and amygdala
o Amygdala and olfactory cortex interact with hypothalamus for unconscious olfactory processing- feeding behaviour, homeostasis, satiety
Describe the somatosensory homunculus and its implications for brain mapping
• Lots of sensation in the tongues, lips and hands and not much in torso
• The primary somatosensory cortex is mapped according to body plan: somatotopic-more sensitive regions occupy more cortex compared to less sensitive areas
o Area of brain dedicated to face and hands is massive compared to rest of the body
Describe the two types of skin and their relative sensitivity
• Types and layers of skin
o Hairy and glabrous (hairless)
Hairy skin less sensitive to glabrous
Describe the two layers of skin and their relative sensitivity
o Epidermis (outer) and dermis (inner)
Epidermis- small and sensitive receptors
Dermis-less sensitive receptors
What is the function of skin
• Functions of skin o Protective function o Prevents evaporation of body fluids o Provides direct contact with world Contains somatosensory receptors (mechanoreceptors)
What are mechanoreceptors?
• Mechanoreceptors- ion channels which are sensitive to mechanical stimulation
Describe the axons of mechanoreceptors of the skin
• Mechanoreceptors of the skin all have unmyelinated axon terminals, and the membranes of these axons have mechanosensitive ion channels that convert
Describe the 4 types of skin mechanoreceptors and where they are located, their size and their sensitivity
• Superficial/small/sensitive (epidermis) o Meissner’s corpuscles- o Merkel’s disks • Deep/larger/insensitive (dermis) o Pacinian corpuscles o Ruffini’s endings
Talk about Meissner’s corpuscles:
- Location
- Size
- Composition
- Adaptation
- Receptive field size
- Vibration response
o Meissner’s corpuscles-
On ridges of glabrous skin only
Small structure
Nerve terminal fibres with Schwann cells surrounding them in layers which provides insulation to the nerve fibres from the mechanical stimulus
Rapid adaptation with small receptive field size
Respond to vibrations of about 50HZ
Talk about Merkel’s disks:
- Location
- Composition
- Adaptation
- Receptive field size
o Merkel’s disks
Nerve terminal that synapses with an epithelial cell
• Epithelial (merkel) cell contains mechanoreceptors and detects mechanical stimulus
• Single cell synapsing with single terminal
Found on glabrous and hairy skin
Slow adaptation with small receptive field size
Talk about Pacinian corpuscles:
- Location
- Size
- Composition
- Adaptation
- Receptive field size
- Vibration response
o Pacinian corpuscles
Largest structure (2mm in size)
1mm diameter
Highest density in fingers
Multiple layers of epithelial cells (with fluid in between layers) wrapped around nerve terminal
Structure is well insulated from external stimulus pressure
• Due to insulation, does not continuously fire when stimulus is applied
Rapid adaptation with large receptive field size
Deep in dermis
Respond to vibrations of about 200-300 Hz
Talk about Ruffini’s endings
- Location
- Size
- Composition
- Adaptation
- Receptive field size
o Ruffini’s endings
Very large
Found in both hairy and glabrous skin
Stiff, collagen fibres wrapped around nerve terminals
Not well insulated from mechanical stimulation
Likely to fire action potentials on entire duration that pressure is perceived
Slow adaptation with large receptive field size
Describe Ake Vallbo et al’s 1970 experiment
• Got recording electrode and poke it into median nerve of specimen
• Recorded action potentials in different nerve axons (individually)
• At same time, would move stimulus probe in palm of hand while recording with electrode
• By repeating this process, worked out that sometimes they were recording from nerve axon which would fire an action potential in only a small area of palm of hand, but sometimes some axons fired action potentials in large areas of the hand
o Some axons had large or small receptive fields
What influences mechanoreceptor adaptation?
Adaptation-
• Some mechanoreceptors have insulation from mechanical stimulus, and hence are fast adapting
o Amount of insulation influences adaptability-no insulation means slow adaptation and vice versa
What do rapidly adapting skin receptors detect?
• Rapidly adapting receptors detect vibration
o Onset and offset of stimulus detected- mechanoreceptors stop firing during the pressure, only when it’s on or off
What do slow adapting skin receptors detect?
• Slowing adapting receptors detect pressure
o Continuous pressure of stimulus detected-mechanoreceptors fire during the pressure
What are the primary afferent fibres for the skin?
o Aa, Aβ, Aδ,C
Describe Aa fibres in terms of:
- Muscle group innervations
- Myelination
- Diameter
- Speed
- Function
- Axons from muscle group 1
- Most highly myelinated
- Diameter of 13-20um
- Speed of 80-120m/sec
- Proprioceptors of skeletal muscle
Describe AB fibres in terms of:
- Muscle group innervations
- Myelination
- Diameter
- Speed
- Function
- Axons from muscles group II
- Medium myelination
- Diameter of 6-12um
- Speed of 35-75m/sec
- Mechanoreceptors of skin
Describe Aδ fibres in terms of:
- Muscle group innervations
- Myelination
- Diameter
- Speed
- Function
- Axons from muscle group III
- Small myelination
- Diameter of 1-5um
- Speed of 5-20m/sec
- Pain and temperature
Describe C fibres in terms of:
- Muscle group innervations
- Myelination
- Diameter
- Speed
- Function
- Axons from muscles group IV
- No myelination
- Diameter of 0.2-1.5um
- Speed of 0.5-2 m/sec
- Temperature, pain and itch
Where do AB fibres project?
• Aβ fibres project into the dorsal root to the dorsal horn of the spinal cord
Describe what type of relationship dermatomes have with spinal cord segments and what disease
o Each segment of the spinal cord receives information from a single dermatome (skin territory)
Dermatomes have a 1 to 1 correspondence with segments
• Shingles-herpes zoster virus
o Restricted to a single dorsal root
o Causes hyperactivity of sensory neurons, leading to burning and stabbing pain
o Useful in mapping dermatomes
Describe what happens when somatosensory axons first reach the spinal cord in the dorsal column tract
• Sensory organisation of the spinal cord for dorsal column
o Division of spinal gray matter- dorsal horn
Goes from mixed spinal nerve into dorsal root and dorsal horn of spinal cord- does not synapse in between
Collateral branches innervate spinal interneurons terminates for spinal reflexes, and another branch ascend in dorsal column to medulla
o Myelinated AB axons (touch-sensitive)
What area of the body and function is the gracile fasciculus concerned with?
o Gracile fasciculus- touch and proprioception below T6 (lower limbs)
What area of the body and function is the cuneate fasciculus concerned with?
o Cuneate fasciculus- touch and proprioception above T6 (upper limbs)
Cuneate fasciculus only appears in T6 up in terms of spinal cord cross sections
Describe the dorsal column tract
o Process-
Touch and proprioceptive information(in AB axons) ascends to the dorsal nuclei and terminates in the dorsal column nuclei
• Cuneate nuclei-more lateral
• Gracile nuclei-more medial
Second order neuron decussates in the medulla (sensory decussation) at the medial lemniscus
Following decussation the medial lemniscus projects to the ventral posterior nucleus of the thalamus
Third order neuron projects from the ventral posterior nucleus of the thalamus up to the primary somatosensory cortex (S1)
Describe the function and pathway of the trigeminal touch pathway
• Sensory pathway for the face
• Trigeminal nerve (cranial nerve V) has AB fibres, mechanoreceptor ion channels
• There is trigeminal ganglion
• Process
o Information from the face goes to the pons and synapses onto the principle sensory trigeminal nucleus, where it decussates and goes to the medial part of the ventral posterior nucleus of the thalamus to the primary somatosensory cortex
What parts make up the somatosensory cortex?
• Postcentral gyrus (areas 3b, 3a,1 and 2) in parietal lobe behind the central sulcus
What areas is the posterior parietal cortex and what is the function of this cortex as well as its composition?
• Posterior parietal cortex (areas 5 and 7)
o Somatosensory association areas
o Integrate somatosensory information
o Thalamic inputs to S1 terminate mainly in layer IV
Neurons of layer IV project to cells in the other layers
o Involved in somatic sensation, visual stimuli and movement planning
What is neglect syndrome?
o Neglect syndrome- usually contralateral to lesion
Loss of compounding of sensations
Part of body or world is ignored or suppressed, and its very existence is denied
Describe Penfield’s experiments
o Penfield
Electrical stimulation evokes somatosensory experiences in patients undergoing surgery for epilepsy (studied 1065 patients-107 female)
Didn’t publish any work on females-seems to be discrepancy between male and female maps after looking at further publications
Is the cortical map plastic or rigid?
Plastic
• Adjust depending on amount of sensory experience
What happens to the cortical map if you remove a body part?
• Leads to dynamic reorganisation of cortical maps -> phantom limb sensations
o If body part is lost, responsible cortex part gets invaded by neighbouring cortices
What happens to the cortical map if you overstimulate a body part?
• Overstimulate body part- you can increase body map for that body part
Describe Brodmann area 3b in terms of:
- Input
- Function
- Lesions
- Organisation in terms of limbs and receptors
o Receives dense input from ventral posterior nucleus of the thalamus
o Neurons selectively responsive to touch
o Lesions impair somatic sensations from particular body part which has been lesioned
o Organisation
Each finger would be in separate column of neurons within layer 4 of the cortex area 3b
• Each finger represented by an adjacent area of cortex
• Thalamic projections terminate in layer IV
Information that comes from rapidly adapting receptors kept separate from information coming from slowly adapting receptors
• Cells that respond to similar inputs stacked vertically across cortical layers
• Labelled-line from individual mechanoreceptors maintained
Describe Brodmann area 3a in terms of input and function
• Area 3a also receives dense input from the thalamus but concerned with proprioception
Describe the function of area 1 and its input
o Area 1 receives a dense input from area 3b and is concerned with texture
Describe the function of area 2 and its input
o Area 2 receives input from area 3b and 3a, but this time sensation of shape and size
What happens if you lesion areas 1 and 2?
o Lesioning areas 1 and 2 leads to predictable deficiencies in discriminating texture, size and shape
Describe the somatosensory cortex in rats and why it is like this
• S1: Rat
o Vibrissae to navigate around environment due to poor eyesight- so whiskers very sensitive for navigation
o Area 3b in a rat-barrel cortex
Specific stack of cells specific for vibrissae
What is 2 point discrimination?
• The minimum distance necessary to differentiate between two points touching the body simultaneously
What happens in lesions above the decussation in the CNS in lateral corticospinal tract?
o Lateral corticospinal tract
Decussates in the caudal medulla
Effect is contralateral, whole side of the body neck down
What happens in lesions above the decussation in the CNS in dorsal columns?
o Dorsal columns
Decussates in the medulla
Effect is contralateral, whole side of the body neck down
What happens in lesions above the decussation in the CNS in spinothalamic tracts?
o Spinothalamic tract
Decussates in the spinal cord
Crosses (almost) immediately in spinal cord at approximate level of entry
Effect of lesion to tract is contralateral, from region of body innervated by the lesioned segment down
What are nociceptors and what neurotransmittors do they release? What is their function?
• Nociceptors (free unmyelinated nerve endings)- sense organs that respond to noxious stimuli
o Release substance P peptide and glutamate
o Aim of these receptors is not to determine grade or location of damage, but to incite person to get away from damage
What are 3 types of nociceptors, their function and components?
o 3 types of nociceptors
Mechanical nociceptors- activated by strong stimuli such as pinch, and sharp objects that penetrate, squeeze and pinch the skin
• Sharp or pricking pain, via fast A-delta fibers
Thermal nociceptors- activated by noxious heat (temperature above 45 degrees) or noxious cold (temperature below 5 degrees)
• Hot pain, via fast A-delta fibers
Chemical nociceptors- show selective responses to histamine and other chemicals
Polymodal nociceptors- activated by noxious mechanical stimuli, noxious heat, noxious cold and irritant chemicals
• Slow dull burning pain or aching pain, via slow lightly-myelinated C fibers
• Percept persists long after the stimulus is removed
What is noxious stimuli and what can they cause the release of?
• Noxious stimuli- those that either threaten or actually produce damage
o Noxious stimuli can cause the release of proteases which digest peptides to activate nociceptors, ATP and potassium ions to activate nociceptors
What is nociception?
• Nociception- transmission of electrical signal from a peripheral nociceptor to the central nervous system
o Can get a noxious stimulus without feeling pain
o Sensory activation
What is pain?
• Pain- perception of pain is a product of brain’s abstraction and elaboration of nociceptive input
o Pain is not simply a sensation: it is an unpleasant sensory and emotional experience and involves sensory components and emotional components
Have to have emotional component to make sure stay away from pain in the future
What are two types of pain and are they both beneficial?
- Acute (short-lasting) pain is beneficial
* Chronic (long-lasting) pain has no benefit
What is the use of acute pain?
• Acute (short-lasting) pain is beneficial
o Homeostatic mechanism
Drives behaviour
o Important for survival, protect from damage: pain is a signal that aims to alter your behaviour to remove yourself from potential danger
o Fight or flight response when get acute pain stimulus
Describe when an organism will fight and when it will take flight
If organism has noxious stimulus on face, it tends to try to fight
If organism has noxious stimulus on rear, it tends to want to flee
Does the fight or flight response need the cortex?
Initial behavioural response doesn’t need cortex at all
Is acute pain from all parts of the body the same?
o Different acute pain stimuli will feel extremely different-
Pain arising from skin produces a distinctively different responses when compared with pain arising from deeper structures
• In skin, usually sharp pain
• In muscles and deeper structures, usually dull pain
o No nociceptors in brain and insensitive ones in hollow organs
Why do we behave differently at acute cutaneous pain to other acute pain from deeper structures?
Behavioural reaction changes whether noxious stimulus is cutaneous or coming from deep structure
• Cutaneous- reactions to invigorate fight or flight reaction to get away from pain
o Activation of skin nociceptors produces two distinct perceptions of pain: a sharp first pain (A fibres) followed by a duller, longer lasting second pain (C fibres)
• Deeper structures- reactions for rest and recuperate
How do we behave under acute cutaneous pain?
Brisk movements
Rise of pulse rate and blood pressure
Sense of invigoration
Hypersensitivity
How do we behave under pain from deeper structures?
Quiescence/quietness Slowing of the pulse Falling of the blood pressure Sweating Nausea
What is chronic pain?
o Pain lasting at least 3 months
What is the cause of chronic pain, impact of chronic pain and examples?
o Significant impact on sufferer’s life
Has high suicide rate
o Pain results from changes in brain activity- which may or may not result from a change in nociceptor activity
o For example: phantom pain, post stroke pain, spinal cord injury pain
What are 2 types of chronic pain and their causes?
o Types of chronic pain
Nociceptive chronic pain
• Results from constant activation of nociceptors
o E.g. arthritis, temporomandibular disorder
Neuropathic chronic pain
• Results from damage to the nervous system
o E.g. post stroke pain, phantom limb pain, lower back pain, trigeminal neuralgia, post-herpatic pain
o Lesions in the ascending pain pathways can result in chronic neuropathic pain
o Peripheral lesions can also result in neuropathic pain
• Neuropathic pain is associated with loss of thalamic GABA
• Neuropathic pain may result from altered thalamo-cortical activity
o Using EEG studies
Describe Melzack and Loeser’s 1978 observations
• 1978-Melzack and Loeser
o Did laminectomy and then had cauda equina decompressed, then sympathetic chain novocain block, a lumbar sympathectomy, a posterior rhizotomy and finally a cordotomy were performed without affording any relief of his pain
o But it was brain itself giving a constant perception of pain
Describe the cause of phantom limb pain and a solution to it
• S1 cortical reorganisation and phantom limb pain
o Cortical remapping correlates with phantom limb pain
Brain plasticity looked at in the 1990’s
o Mirror box therapy
Subjects think they have all limbs
Results in a decrease in phantom limb pain
Only works for a handful of input- visual input can change the way in which you perceive noxious stimuli
Describe where nociceptor afferents synapse on the spinal cord and face
• Nociceptor afferents synapse in dorsal horn of spinal cord
o Projecting neurons in lamina I receive A-delta and C fibers information
o Neurons in lamina II receive input from C fibers and relay it to other laminae
o Projecting neurons in lamina V (wide-dynamic range neurons) receive A-delta, C and A-beta (low threshold mechanoceptors) fibers information
o Run with the zone of Lissauer and terminate within the substantia gelatinosa
• Nociceptors for the face synapse in the spinal trigeminal nucleus and decussate to the thalamus in the trigeminal lemniscus
Describe the spinothalamic pathway in terms of role and pathway
o Spinothalamic pathway
Pain location, intensity, sensory quality, affective component
Starts in dorsal horn of spinal cord and decussates through the ventral white commissure
Then travels through lateral medulla and pons to terminate in the thalamus (central lateral nucleus and ventral posterior lateral nucleus)
• Also projections up to medial dorsal thalamus, which project to cingulate cortex and insular cortex responsible for affective component
Then goes up to primary and secondary somatosensory cortices
The more the noxious stimulus hurts, the more activation in this pathway
How could you study the spinothalamic pathway?
Studying this pathway-
• Put thermode on lip or hand for about 10 seconds and put person in scanner to analyse tract
• Inject 5% salty water under skin or muscle, which hurts and lasts about 5 minutes
Describe the use and pathway of the spinoreticular pathway
o Spinoreticular
Arousal-hypersensitivity : increases overall awareness of environment
From dorsal horn, decussates at ventral white commissure to synapse on reticular formation
• Area of brainstem that is relatively defuse
Synapses on thalamus and goes to somatic sensory cortex
Describe the use and pathway of the spinomesencephalic pathway
Defensive behaviours
From dorsal horn, decussates at ventral white commissure and synapses on midbrain periaqueductal grey
• All neurons necessary for fight or flight are in midbrain periacqueductal brain matter
o Will be responsible for vocalisation, high heart rate, blood to muscles and away from gut
o Has dampening system after initial signal activation
o Modulated by hypothalamus and other systems
Has columns in it
How do we know so much information about noxious information and pathways?
• Animal studies- tract tracing techniques, neural activation (c-fos electrophysiology, autoradiography) immunohistochemistry
o Primarily rodents and cat work
• Human studies- lesions, EEG, brain imaging (PET, fMRI)
o Brain imaging
Look for areas in the brain which show an increase in activation each time noxious stimulus is increased or decreased
In animals and humans get very similar patterns of activation
In brain imaging, which brain regions activate with increase noxious stimulus?
Get increases in areas such as the thalamus, cingulate cortex, insula, and a primary somatosensory cortex
Is there a specific pain cortex?
• No pain area- combination of activation of multiple areas that give pain sensation and emotional quality
What cortex is responsible for the localisation of pain?
• S1 is most probably involved in conscious body localisation of pain
What is involved in the direction of behaviour after pain?
o Periacqueductal gray has somatosensory map but is more involved in subconscious behaviour direction
Describe the evidence that cingulate and insulate cortex are involved in processing of pain emotionally
• Cingulate and insulate cortex are involved in processing of pain emotionally
o Lesion studies suggest this
Berthier, Starkstein, Leiguarda
• Individuals have had a stroke- given noxious stimulus but it does not bother them (no emotional relevance) but can tell sensory quality of it
• Associated with lesion in posterior insula cortex
o Rainville et al. 1997
People hypnotised
• Changed perceived unpleasantness whilst keeping intensity the same
Cingulate cortex perceived the change unpleasantness
S1 changed if intensity was changed
o When look at pain, cingulate and insula light up when watching other people’s pain
What parts of the brain are responsible for emotional pain processing?
Cingulate and insula cortex
Can pain be suppressed? When?
• Pain reflexes and behavioural responses can be altered or suppressed if not appropriate. Pain can be suppressed if not needed for survival
True or false: all tissue injury results in pain and vice versa
False
• Not all tissue injury results in pain and not all pain is associated with tissue injury
Describe what can modulate pain perception
• Pain perception can be modulated by all kinds of factors, including behavioural states (stress, sex), cognitive states (hypnosis), mental states (trance), social norms and drugs
Describe Melzack and Wall’s gate control theory
• Melzack and Wall gate control theory-afferent regulation
o If stimulate large diameter fibre (like a normal mechanical fibre) then can modulate noxious information at spinal cord level
Pain evoked by activity in nocioreceptors can also be reduced by simultaneous activity in low threshold mechanoreceptors (AB fibers)
o Gate theory of pain suggests that certain neurons of the dorsal horns, which project an axon up the spinothalamic tract, are excited by both large-diameter sensory axons and unmyelinated pain axons
Projection neuron is also inhibited by an interneuron, and the interneuron is both excited by the large sensory axon and inhibited by the pain axon
Activity in the pain axon alone maximally excites the projection neuron, allowing nociceptive signals to rise to the brain. However, if the large mechanoreceptive axon fires concurrently, it actives the inhibitory interneuron and suppresses nociceptive signals
Describe how endogenous analgesia occurs
o Interaction at level of spinal cord gives pain relief
Endogenous analgesia-descending regulation
• Midbrain Periaqueductal grey projects down into nucleus raphe magnus of medulla which will send axons down to dorsal horn and trigeminal nucleus which will inhibit incoming noxious information
o Opiates work this way
