12. CNS (Sensory) (1) Flashcards
1
Q
- The nervous system is divided into the CNS: (2)
A
- The nervous system is divided into the CNS (brain and spinal chord) and PNS (neurones entering and exiting the CNS)
- Peripheral innervation is to somatic structures e.g. skin, muscles, visceral structures like the GIT
2
Q
Anatomical organisation: (4)
A
- In the periphery there are receptor that are stimulated by specific stimuli
- Stimuli are converted into electrical signals and transported via afferent/sensory neurones as action potentials to the CNS
- The information is then relayed via neurones in the CNS through various segments of the spinal chord depending on the origin of info
- Neurones pass through the brainstem and then to higher centres including the thalamus and somatosensory cortex
3
Q
Sensory systems:
A
- Sensory input to the CNS is generally from two major components —-> The general sensory input (somatic +visceral) and the special senses
4
Q
How is sensory input divided? (3)
A
5
Q
What are Neuroglia? (3)
A
- Cell types associated with the nervous system
- Make up 90% cells in CNS but occupy half the volume
6
Q
What is the function of astrocytes? (4)
A
- Act as a scaffold/frame that helps to hold nervous system together
- Repair
- K+ uptake
- Neurotransmitter uptake
7
Q
Astrocytes
- Two subtypes:
A
Two subtypes: fibrous astrocytes in white matter that contain many intermediate filaments and protoplasmic astrocytes in gray matter that have a granular cytoplasm Both induce capillaries to form tight junctions making up the blood-brain barrier
8
Q
What are Oligodendrocytes? (2)
A
- Form myelin sheath in the CNS (in the periphery, myelin is formed by Schwann cells
-Growth factors
9
Q
What are Microglia? (3)
A
- Immune defence through phagocytosis
- Removal of debris following infection, injury and disease
- Involved in brain protection and repair
10
Q
What are Ependymal cells?
A
- Formation of CSF
11
Q
- Neuroglia support neurones physically, metabolically and ______.
A
functionally
12
Q
Classification/properties of sensory receptors:
Source of stimulus: (7)
A
- Exteroceptors —-> External environment
- Mechanoreceptors (mechanical energy like pressure and touch)
- Visual receptors (light)
- Chemoreceptors (specific chemicals)
- Thermoreceptors (temperature) - Proprioreceptors —-> Position in space and time
- Interoceptors —> Internal environment
13
Q
What is the Threshold of response?
A
- Receptors can be classified by the level/ amount of stimulus required to activate them
14
Q
Threshold of response
* Low threshold —>
* High threshold —>
A
- Low threshold —> Low intensity stimulus elicits a response e.g light touch
- High threshold —> High intensity stimulus elicits a response e.g pain
15
Q
What is “an adaptation to stimulus”?
A
- Refers to how receptors respond to a constantly present stimulus.
16
Q
- Receptors that become insensitive to a constantly present stimulus is said to have ______
- A receptor that keeps responding to a constantly present stimulus is said to be ____-______ e.g receptors responding to noxious stimuli
A
- adapted
- on-adapting
17
Q
- Some receptors are slow adapting that in the presence of a long-term stimulus, after a while there is a reduced response to the _____ but there is still a response —–> Tonic receptors e.g proprioceptors
- _____ receptors are rapidly adapting —> Despite the presence of a stimulus they stop responding e.g when you first put clothes on you feel them but after a while, you stop feeling them
A
Stimulus
Phasic
18
Q
Modality-
* Unimodal:
* Polymodal:
A
- Unimodal —> Responds to 1 type of stimulus
- Polymodal —-> Responds to more than 1 type of stimulus
19
Q
Examples of receptors based on their stimuli
* thermoreceptors : (4)
A
- Detect thermal gradients
- Warmth thermoreceptors
- Cold thermoreceptors
- Extreme heat (>45 degrees) or extreme cold (<5) = pain
20
Q
Examples of receptors based on their stimuli
* photoreceptors:
A
- Rods & cones
21
Q
Examples of receptors based on their stimuli
* Osmoreceptors:
A
- Detect concentrations of solutes in fluids
22
Q
Examples of receptors based on their stimuli
* Osmoreceptors:
A
- Detect concentrations of solutes in fluids
23
Q
Examples of receptors based on their stimuli
* Proprioceptors: (4)
A
- Muscle spindle (amount and rate of stretch)
- GTO (Muscle/tendon tension)
- Joint receptors: position of joints
- Inner ear: semicircular canals, utricle, saccule
24
Q
Adaptation to stimulus:
- No adaptation
- Tonic
- Phasic
A
25
What are chemoreceptors?
- Chemicals, gases, electrolytes
- Compound sensations = perception from central integration of various inputs e.g wetness = touch + pressure + thermal
26
What is Transduction? (2)
* The process of converting various stimuli/energy forms into electrical signals
*The various intrinsic and extrinsic stimuli in different energy forms in the body need to be converted to a ‘language’ which the nervous system can understand
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What is Transduction? (2)
* The process of converting various stimuli/energy forms into electrical signals
*The various intrinsic and extrinsic stimuli in different energy forms in the body need to be converted to a ‘language’ which the nervous system can understand
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Sensory receptors in the human skin: (2)
* The skin is the largest organ of the body and is subjected to various stimuli
* There are several types of receptors which can transduce those stimuli into electrical signals
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What is Receptive fields? (2)
* A receptive field is an area in which stimulation leads to response of a particular sensory neurone
* it is affected by convergence/divergence
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How are receptive fields tested? (5)
* If you place the tips of a calliper in the receptive field of a single neurone, the brain interprets the calliper tips as a single point
* When the tips are placed in two different receptive fields where there is no convergence of neurones, the tips are interpreted as two separate points ----> Two-point discrimination
* Receptive fields of receptors in the tips of the finger are smaller than those on the back
* The minimum distance between two calliper points that can be perceived as separate points is called two-point discrimination threshold
* It is also a measure of tactile acuity
31
What is the localisation of a stimulus aided by? (2)
* Localisation of a stimulus is aided by lateral inhibition where sensory neurones with receptors at the peripheral edge of a stimulus are inhibited compared to sensory info at the centre of the stimulus
* Lateral inhibition also allows for contrast
32
What is Chemesthesis? (2)
* Process whereby chemicals activate other sensations e.g pain, touch, temperature, texture
* Many chemicals activate transient receptor potentials (TRP) channels that are also involved in thermal sensation
32
What is Chemesthesis? (2)
* Process whereby chemicals activate other sensations e.g pain, touch, temperature, texture
* Many chemicals activate transient receptor potentials (TRP) channels that are also involved in thermal sensation
33
What is the Somatosensory system?
* Detects temperature, proprioception, head and body movement & touch
* Allows for the perception of pain
* Touch includes pressure, vibration and stereognosis (ability to recognise and identify object based on touch alone)
* Pain includes sense of itch & tickle
* Primary receptor surface for somatosensory system is located in the skin and peripheral component in muscles, tendons, and internal organs
34
What is the Somatosensory system?
* Detects temperature, proprioception, head and body movement & touch
* Allows for the perception of pain
* Touch includes pressure, vibration and stereognosis (ability to recognise and identify object based on touch alone)
* Pain includes sense of itch & tickle
* Primary receptor surface for somatosensory system is located in the skin and peripheral component in muscles, tendons, and internal organs
35
General arrangement/hierarchy of ascending neurones: (2)
* Generally, sensory info from one side of the body is processed in the Contralateral cerebral cortex hemisphere
* Therefore there has to be decussation (crossing over) somewhere in the pathway
36
What is the function of the primary afferents? (4)
* Primary afferents enter the spinal chord via the dorsal root
* Some afferents decussate immediately or close to the spinal region of the dorsal roots and project onto second order neurones
* These ascend in the Contralateral tracts to the thalamus and then to the post central gyrus of the somatosensory cortex
* Other pathways ascend ipsilaterally after entering the dorsal root and decussate at the level of the medulla oblongata then to the thalamus and sensory cortex
36
What is the function of the primary afferents? (4)
* Primary afferents enter the spinal chord via the dorsal root
* Some afferents decussate immediately or close to the spinal region of the dorsal roots and project onto second order neurones
* These ascend in the Contralateral tracts to the thalamus and then to the post central gyrus of the somatosensory cortex
* Other pathways ascend ipsilaterally after entering the dorsal root and decussate at the level of the medulla oblongata then to the thalamus and sensory cortex
37
What are Dorsal columns? (2)
* Associated with sensations of fine touch, pressure, vibrations and proprioception
* Pathway ascends ipsilaterally upon entrance in the dorsal root and decussate at the level of the medulla
38
Dorsal columns:
What are the nuclei involved? (2)
* Cuneate nuclei and fasciculus cuneatus are associated with info from superior parts of the body
* The gracile nuclei and fasciculus gracilis are associated with info from inferior parts of the body
39
What is the Spinothalamic tract? (3)
* Also know as the anteroposterior tract of the ventrolateral tract
* It decussate immediately in the spinal level
* Conveys pain, temperature and crude touch
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What is Spinocerebellar tract?
* Associated with proprioception and the information is conveyed to the cerebellum ipsilaterally
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What is Spinocerebellar tract?
* Associated with proprioception and the information is conveyed to the cerebellum ipsilaterally
41
* We also need to consider the _____ nerves with sensory functions
* They have their nuclei in the _________
cranial
brainstem
41
* We also need to consider the _____ nerves with sensory functions
* They have their nuclei in the _________
cranial
brainstem
42
Tracts in the spinal chord:
42
Tracts in the spinal chord:
43
Somatosensory pathways summary:
43
Somatosensory pathways summary:
44
What is the thalamus?
* Info from ascending neurones project to the thalamus
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Main sensory functions of the thalamus: (4)
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The somatosensory cortex
* S1 =
* SII/S2 =
* S1 = somatic sensory area 1/ 1* somatosensory cortex in the post- central gyrus
* SII/S2 = somatic sensory area 2 or 2* somatosensory cortex in the wall of the Sylvian fissure
47
S1 also projects to S2
What is the function of S1?
* S1 processes and encodes type and intensity of the sensory inputs
48
S1 also projects to S2
What is the function of S2?
* S2 is believed to perform higher order functions e.g sensorimotor integration, integration of info from two body halves, attention, learning and memory
49
* There are specific areas mapped on the brain that receive specific inputs from different regions of the body -----> _______ ______
* Not all body areas are equally represented on the brain map
* Area with high sensitivity have a ______ representation
sensory homunculus
greater
50
Visceral sensation
* Viscera =
organs within the body cavity e.g GIT, uterus
51
Visceral sensation
What are the visceral receptors?
- Many visceral receptors e.g chemoreceptors, baroreceptors, mechanoreceptors, nociceptors
* no proprioceptors, few temperature and touch receptors
52
What are the reflexes involved in visceral receptors? (3)
* Mostly involved in reflexes which are not consciously perceived
* Sensations are relayed by autonomic afferent fibres and include hunger, nausea, visceral pain
* Conscious sensations are poorly localised
53
What is pain?
* An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage
54
Pain receptor =
* Receptor = nociceptor
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What is a nociceptor?
A receptor preferentially sensitive to a noxious stimulus or to a stimulus which could become noxious if prolonged.
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What is a nociceptor?
A receptor preferentially sensitive to a noxious stimulus or to a stimulus which could become noxious if prolonged.
56
* Pain threshold:
The least experience of pain which a subject can recognise
57
* Pain tolerance:
The greatest level of pain which a subject is prepared to tolerate
58
* Analgesia:
Pain suppression
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* Analgesia:
Pain suppression
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* Allodynia:
Pain due to a stimulus that does not normally provoke pain
60
* Hyperalgesia:
An increased response to a stimulus that is normally painful
61
* Hypoalgesia:
Reduced response to a normally painful stimulus
62
* Gender-specific pain:
Menstrual, parturition, cancer of the cervix
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What are the other types of pain?
64
How is pain activated? (4)
65
* Tissue damage due to trauma of cutting off blood supply (ischaemia) results in...
* These then activate the _____ which sends action potentials up the afferents ( C or A§ fibres)
...cells dying and releasing their contents K+, inflammatory cytokines (5HT, prostaglandins, histamine)
nociceptor
66
What is the function of prostaglandins? (2)
* Prostaglandins which play an important role in inflammation are synthesised from arachidonic acid via the enzyme Cox-2.
* Drugs that inhibit this enzyme have anti-inflammatory properties as well as being analgesics e.g Aspirin
67
Ascending pathways for noxious information and the roles of the higher centres
* Once the afferents enter the spinal cord there are 3 key pathways
-----> Spinothalamic tract:
carries sensation of pain from the spinal chord to
the thalamus and then to somatosensory cortex
67
Ascending pathways for noxious information and the roles of the higher centres
* Once the afferents enter the spinal cord there are 3 key pathways
-----> Spinothalamic tract:
carries sensation of pain from the spinal chord to
the thalamus and then to somatosensory cortex
68
Ascending pathways for noxious information and the roles of the higher centres
* Once the afferents enter the spinal cord there are 3 key pathways
-----> Spinoreticular tract:
carries sensation from spinal chord to reticular
formation in medulla (alertness/consciousness)
69
Ascending pathways for noxious information and the roles of the higher centres
* Once the afferents enter the spinal cord there are 3 key
---> Branch from Spinothalamic tract to hypothalamus and limbic system affecting the ________ nervous system ( increased Sympathetic output + increased HR)
autonomic
69
Ascending pathways for noxious information and the roles of the higher centres
* Once the afferents enter the spinal cord there are 3 key
---> Branch from Spinothalamic tract to hypothalamus and limbic system affecting the ________ nervous system ( increased Sympathetic output + increased HR)
autonomic
70
What is referred pain? (4)
* Sometimes pain can be felt at a site other than where the nociceptors were activated Referred pain
* Deep pain is felt in somatic structures some distance away
* E.g Myocardial infarction causes chest, shoulder, left jaw, upper left arm pain
* This is a result of convergence of neurones from structures in similar developmental dermatomes
71
What is peripheral sensitisation? (2)
* When tissue damage occurs resulting in inflammation, it can also activate nociceptors that release substances to enhance inflammation (CGRP, substance P)
* These cause an increased activity in C fibres
72
What is central sensitization?
73
Central sensitisation:
What is the long-term activation?
* Long term activation of nociceptors due to untreated injuries or lack of use of analgesics causes C fibres to repeatedly cause long-term changes in dorsal horn neurones
74
* The increase neurotransmitter release from primary afferents causes sensitisation of receptors on post-synaptic _______
* Hyperalgesia and AB inputs can activate pain pathways causing ________
neurones
allodynia
75
What is an itch & tickle?
76
Descending inhibition of pain (battlefield analgesia): (4)
* When someone is hurt but only perceives the pain a while after it was initially inflicted
* Due to activation of descending pathways that inhibit the transmission of ascending neurones
* There is production of endogenous opioids, enkephalins and noradrenaline amongst others
* PAG and NRM are involved as well
77
What is counter irritation? (3)
* Sometimes when you have a painful site, rubbing the surrounding areas can cause some pain relief
* Rubbing action activates mechanoreceptors which send collaterals to the substantia gelatinosa area of the dorsal horn where interneurones are stimulated
* interneurones release inhibitory transmitters that inhibit the transmission of sensory information up the Spinothalamic tract
78
Pharmacology and therapeutics of sensory disturbances:
78
Pharmacology and therapeutics of sensory disturbances:
79
Lesions to the sensory tract:
* Lesions to post central gyrus =
= loss of sensation contralaterally
80
Lesions to the sensory tract:
* Lesion to brain stem (above the medulla) =
= if unilateral, loss of Contralateral touch, vibration and proprioception and pain and temperature below the lesion
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* Lesion to spinal chord = if bilateral,
loss of sensation below the lesion
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Lesions to sensory tract
* Lesion to spinal chord = if unilateral,
loss of ipsilateral touch, vibration and proprioception and loss of Contralateral pain and temperature below the lesion.
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What is a transverse cord section?
* With complete transaction of the spinal chord, there is loss of sensation bilaterally below the level of the lesion
83
What is a transverse cord section?
* With complete transaction of the spinal chord, there is loss of sensation bilaterally below the level of the lesion
84
What is Brown-sequard syndrome? (3)
* In cases where there is damage to one half of the spinal cord e.g the right half, spinothalamic tracts on the right side conveying info from the left side of the body are damaged below the lesion
* The right posterior column conveying info from the same side are also damaged
* Spinocerebellar tracts are also damaged
85
What is Posterior cord syndrome?
* Bilateral posterior cord syndrome or damage to dorsal columns results in bilateral loss of vibration, pressure, touch and proprioception sensation
86
What is Anterior cord syndrome?
* Affects temperature and nociception
87
What is the somatosensory homunculus?
88
...
89
90
What is syringomyelia?
91
What is Romberg's test?
92
* The ear sensory functions have two main roles ---->
Hearing (organ of corti in the cochlea) and proprioception (vestibular apparatus)
92
* The ear sensory functions have two main roles ---->
Hearing (organ of corti in the cochlea) and proprioception (vestibular apparatus)
93
Pinnae funnel sound waves: (2)
* Pinnae funnel sound waves into the external auditory meatus to the eardrum
* This causes movement of the auditory ossicles to transmit the sound wave energy to the oval window of the cochlea
94
What is the cochlea? (4)
* Cochlea Coiled tube 2 3/4 turns, 35 mm long + transforms sound pressure waves to action potentials
* Basilar membrane & vestibular
membrane divide cochlea into 3
chambers (scalae)
* S vestibuli, s media, s tympani
* Perilymph has a similar composition to plasma however endolymph is rich in K+ ions
94
What is the cochlea? (4)
* Cochlea Coiled tube 2 3/4 turns, 35 mm long + transforms sound pressure waves to action potentials
* Basilar membrane & vestibular
membrane divide cochlea into 3
chambers (scalae)
* S vestibuli, s media, s tympani
* Perilymph has a similar composition to plasma however endolymph is rich in K+ ions
95
What is the origin of corti?
*Transverse section view of the organ of corti
* Hair cells in the scalp media are the sensory cells
96
What are hair cells? (2)
* Hair cells are the sensory receptors in the cochlea and vestibular apparatus.
* They have cilia on their surface
97
What are hair cells? (2)
* Hair cells are the sensory receptors in the cochlea and vestibular apparatus.
* They have cilia on their surface
98
* Small cilia =
* Large cilia =
* Small cilia = stereocilia and large cilia = kinocilia
99
What are alex of the cilia?
* Alex of the cilia are connected by tip links which are associated with ion channels
* The hair cells have both sensory and motor innervation
100
What happens when stereocilia are bent in the direction of kinocilia? (5)
* When sterocilia are bent in the direction of kinocilia, tip links cause the opening of apical ion channels which result in K+ ions from surrounding endolymph entering the cell
* Ca2+ ions also enter the cells
* These ions depolarise the cells causing the release of neurotransmitters
* Unlike other extracellular fluid which is rich in Na+, endolymph is rich in K+ ions which is why it enters the cells causing depolarisation
* When stereocilia are bent away from kinocilia, ion channels close and the cells hyperpolarise
101
How is a sound wave transmitted? (2)
* Sound wave is transmitted through the outer and middle ear and through the inner ear endolymph inside the cochlear duct
* The basilar membrane vibrates, which cause the organ of corti to move against the tectorial membrane
102
What are inner hair cells?
* Inner hair cells are the primary sensory cells which generate action potentials and are stimulated by fluid movements
103
What are outer hair cells ? (2)
* Outer hair cells also respond to sound waves but depolarisation makes them shorten while hyperpolarisation makes them lengthen
* Theses actions increase the amplitude and clarity of sounds
104
What is the basilar membrane? (2)
* Wider and more flexible at apical end near helicotrema and narrow & stiff at basal end near window
* As waves move up cochlea it’s amplitude reaches a maximum, then drops off
105
What is the basilar membrane? (2)
* Wider and more flexible at apical end near helicotrema and narrow & stiff at basal end near window
* As waves move up cochlea it’s amplitude reaches a maximum, then drops off
106
* High frequency =
* Low frequency =
* High frequency = maximum height near base
* Low frequency = maximum height near apex
107
* Frequency (pitch) is coded by
* Frequency (pitch) is coded by hair cell position in cochlea
108
* Intensity (loudness) is coded by
amplitude of wave ---> increased APs generated in hair cells (higher rate) and more hair cells are activated
