L10 Sensory Receptors Sensory Pathways and Peripheral Sensation Flashcards Preview

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Flashcards in L10 Sensory Receptors Sensory Pathways and Peripheral Sensation Deck (31):
1

Learning Outcomes (for general perusal)

  • General principles of sensation
  • Receptor types, coding, general properties
    • Mechanoreceptors
    • Chemoreceptors
    • Thermoreceptors
    • Nociceptors
  • Describe nerve pathways involved in somatrosensory perception
  • Explain the role of the somatosensory cortex in sensory processing
  • Have an overview of common somatosensory defects

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What are sensory receptors?

•Specialized cells providing CNS with info about world outside or inside our bodies

•Respond to several types of stimuli

Stimuli converted to action potentials in the process of TRANSDUCTION that carry information to CNS

•CNS interprets this info as various sensations

3

  1. What is a 'somatic sense'?
  2. What are the somatic senses?
  3. What are the 'special senses?

  1. General sensory mechanisms that collect information from all over body

  2. •pain

    •Temperature

    •Touch

    •Pressure

    •Vibration

    •proprioception

  3. •Vision

    •Hearing

    •Smell

    •Taste

    •Equilibrium

4

What are the four basic receptor types classified according to transduction mechanism?

 

 

 

What other way can they be classified? Give examples

  • Mechanoreceptors
    • compression/stretching of receptor
  • Thermoreceptors
    • Separate hot/cold receptors
  • Electromagnetic
    • Light in retina
  • Chemoreceptors
    • O2/CO2/H+, osmolarity, taste, smell, etc

 

on basis of the purpose they serve

Nociceptors –give rise to pain sensations – mechano-, thermo- and chemoreceptors

Proprioceptors – mechanoreceptors that give info about joint position, but can also include vision and vestibular information

 

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Describe the specificity and modality of the receptors

  • Receptors show high specificity to one type of stimulus
    • Touch receptor very sensitive to pressure, relatively insensitive to chemical stimuli
  • Action potentials in specific nerves always interpreted according to the receptor that nerve connected to (ie, modality)
    • –photo stimulation of the retina gives visual sensation

      –stimulation of optic nerve with other stimuli (pressure, electrical) also gives visual sensation

 

 (will respond to other stimuli when strong enough) 

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  1. What is the receptive field?
  2. What are the receptive fields like in highly sensitive areas?
  3. Where has larger receptive fields?
  4. How can this be tested?

  1. Area monitored by single receptor 
  2. Small (Eg, finger tips, tongue, lips < 1mm) close together, with little overlap, to allow fine DISCRIMINATION, ie., the ability to distinguish when two separate stimuli are applied close together.
  3. General body surface has larger receptive fields (7 cm) (Eg, skin on torso, legs, arms)

  4. By two-point discrimination

    1. ‘2 point discrimination’ brush – essentially a tooth brush with 2 fine bristles, and the separation can be altered

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Use the Pacinian Corpuscle as an example to describe Receptor Potential and Sensory Coding

This receptor consists of a bare neurone tip surrounded by concentric tissue layers

Local pressure causes deformation of tissue

Transferred to unmyelinated fibre tip

Deformation of fibre tip allows local depolarizing Na+ entry

This receptor potential is graded with stimulus strength

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Receptor potential and sensory coding

  1. When is an action potential generated?
  2. When is the sensation percieved?
  3. What happens as the receptor is stimulated?
  4. What is the strength of a stimulus coded by?

  1. If the stimulus and receptor potential is great enough for the amount of sodium ion entry to reach threshold for depolarization
  2. when the stimulus is sufficient to cause a large enough receptor potential and threshold is exceeded.
  3. As receptor stimulated, more action potentials propagated

  4. an increase in the frequency of action potentials in the primary afferent nerve

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Adaptation

  1. What are the types of responses possible when there is a constant stimulus at a receptor?
  2. Describe both of these responses

  1. Phasic or tonic responses possible

2.Phasic 

  • adapts rapidly, receptor potential and resultant action potentials diminish.
  • Gives transient information
  • Eg pacinian corpuscle

Tonic

 

  • adapts very slowly, receptor potential and action potentials maintained
  • Gives constant sensation
  • Eg some proprioceptors

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Mechanoreceptors

  1. What type of membrane channels do they have?
  2. What do they respond to?
  3. Outline the 3 groups of Mechanoreceptors

  1. All have stretch-sensitive membrane channels

  2. Respond to distortion this membrane

  3. Tactile receptors

    Proprioceptors

    - Muscle spindles, golgi tendon organs, joint receptors

    Baroreceptors

    - Carotid sinus, aortic arch

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Mechanoreceptors

1) Tactile Receptors

Can be fine or crude touch receptors (different ascending tracts). Range in complexity from free nerve endings to sensory complexes.

What are the following?

  1. Free nerve endings
  2. Root hair plexus 
  3. Tactile (Merkel's) Discs
  4. Tactile (Meissner's) Corpuscles
  5. Laminated (Pacinian) Corpuscles
  6. Ruffini Corpuslces

 

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  1. Bare dendrites between epidermal cells. Tonic discharge, small fields. Sole cornea receptors 

  2. sensory dendrites surrounding hair follicles. Rapidly adapting

  3. very sensitive tonic touch receptors. Dendrites closely associated with large epithelial cells. Very small fields

  4. fine touch and low freq. vibration. Fast adapting. Dendrite network surrounds Schwann cell. All contained in capsule in dermis. Many in sensitive areas – finger tips, eyelids, lips, etc

  5. deep pressure, high freq. vibration. Single dendrite surrounded by fluid-filled layers. Pressure distributed around lamellae quickly, so rapidly-adapting, wide distribution, esp. fingers and viscera

  6. skin pressure and distortion. Dendrites intertwined with collagen fibres in capsule. Tonic discharge

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Mechanoreceptors

2) Proprioceptors

  1. What do these monitor?
  2. Name the three types and what they do

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  1. monitor position of joints, tendon and ligament tension, muscle contraction

2. Muscle Spindle - monitor skeletal muscle length, trigger muscle stretch reflexes

Receptors in central region with sensory afferents, contractile regions either end with ɣ motor fibres

Muscle stretch causes intrafusal stretch, sends info. to spine, synapses with α motor neurone to trigger muscle contraction to oppose stretch. Also inhibits muscles opposing contraction

Contraction of extrafusal fibres (via α-motorneurones) usually accompanied by contraction of intrafusal fibres of spindle to maintain sensitivity

Gogli Tendon Organ - Located in tendon, in series with muscle. Sense muscle tension and Initiate inhibitory reflex

Joint Receptors -  Free nerve endings in joint capsules. Detect joint pressure, movement and tension

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Mechanoreceptors

3) Baroreceptors

  1. What do these do?
  2. Describe them
  3. What is their role in major arteries?

  1. Monitor changes in pressure. Produce different affects, according to tissue. Rapidly adapting

  2. Free nerve endings in elastic tissue of some distensible organs, including blood vessels, portions of respiratory, digestive and urinary tracts

  3. Baroreceptors monitor blood pressure in walls of major arteries, including carotid sinuses, aortic bodies

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Temperature Receptors

  1. What are they?
  2. What are they sensitive to?
  3. Where are they, what do they do?
  4. What underlying mechanism do these receptors rely upon?

  1. Free nerve endings

  2. either hot or cold

3. Hypothalamus - the major determinant of temperature regulation

Dermis - also contribute to temperature regulation, and also provide temperature sensation

Liver, Spine, Skeletal Muscle - provide smaller contributions to temperature sensation and thermoregulation

4. 

  • the activity of a specific set of sensory nerve membrane proteins, the transient receptor potential, TRP channels.
  • Different TRP channels are active (produce receptor potentials) at different temperatures.
  • For example, moderate cold sensation is caused by activation of the TRPM8 channel at temperatures increasingly below 27˚C, whereas for moderate warm sensation, by temperatures increasingly exceeding above 25˚C and 31˚C in TRPV3 and TRPV4 channels, respectively. Some of these channels may transduce noxious temperatures and are involved in pain perception as well (see later).

The TRPM8 channel is also activated by menthol, thus dental products (tooth paste, creams, etc), gum, mints, shaving products, liniments, etc that contain menthol give a cool sensation.

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Chemoreceptors

  1. Give examples
  2. What do they do?

1. 

  • Carotid and aortic bodies monitor blood pH, CO2, O2 (involved in control of respiration and blood gasses )
  • Medulla oblongata surface monitors CSF pH and CO2

 

2. Elicit respiratory, cardiovascular and behavioural responses

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Nociceptors

  1. How is pain detected?
  2. What are pain receptors?
  3. What do they respond to?
  4. How can nociceptor activity be modified (seeing as it doesnt adapt)?
  5. Which axon types are they carried by?

  1. is a sensation that indicates damage or potential damage to body tissues, detected by specific receptor, not just intense stimulation of other receptor types.
  2. Free nerve endings, relatively dense concentration in the skin

3. 

  • Chemicals often associated with tissue damage or inflammation such as potassium, prostaglandins, ATP, adenosine, 5-HT

  • Mechanical stress associated with tissue damage

  • Temperature extremes above 43˚C or below 18˚C, again mediated by TRP channels. 

    TRPV1 >43 ºC & capsaicin

      TRPV2 >52 ºC

      TRPA1 <18 ºC & several chemical, incl. menthol

4. prostaglandins sensitize nociceptors (prostaglandin inhibitors (eg asprin) are analgesic)

5. 

 

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Somatosensory system pathways

  1. What are 1st order neurones?

1st order synpase with 2nd order neurones

2, What are 2nd order neurones?

  1. nerves with receptor endings

    -  cell bodies in dorsal root ganglia

    -  enter spinal cord via dorsal roots

  2. travel to brain via two primary ascending tracts

    -  connect to cerebral cortex – 3rd order neurones

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Nerve classification

  1. How are Primary afferent fibres carrying action potentials away from the receptors generally classified?
  2. Which fibres have the fastest conduction velocities and what are they associated with?
  3. What are the unmyleinated axons, and what do they carry?

  1.  generally classified on their conduction velocities. The larger the diameter of the axon, and the amount of insulating myelin, the faster the conduction velocity. 
  2. in fibres are associated with the receptors for the sending information about movement associated with GTOs and spindles, to allow integration with motor command and to alter that motor command as the action takes place
  3.  C fibres - carry crude touch sensation, aching (slow) pain and temperature.

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Primary afferent neurones

What are the 4 types and what are their roles? What are these in order of?

  • Aα - muscle spindle, golgi tendon organ, touch & pressure

  • Aβ - touch, pressure & vibration

  • Aδ - touch & pressure, pain & temperature

  • C - pain & temperature

These are in order of decreasing diameter and conduction velocity 

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Path to the Spine

  1. How does afferent information from the receptor travel to the brain?
  2. Where does the primary afferent nerve enter the spine?
    1. Where is the cell body for the nerve?
  3. Where do the slower axons travel to?
  4. Where do the faster-conducting axons travel to?
  5. Then where do they go?

 

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  1. via defined ascending pathways, depending on the receptor and type of axon it travels along.
  2. outside the spinal cord in the dorsal root ganglion.
  3.  to defined layers (lamina I & II) in more dorsal regions of the cord
  4. Faster-conducting axons travel into deeper/more Ventral lamina (III-IX) 
  5. They may then synapse with secondary nerves that then travel up the cord to the brain in defined tracts, or they may ascend in these tracts without synapsing. Different receptors are associated with different ascending tracts. 

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Dermatomes

  1. Where do primary afferents enter the spinal cord? What do they carry?
  2. What is a dermatome?
  3. Describes these regions
  4. When can these dermatomes become apparent?

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  1. via spinal nerves each SN carries information from receptors from specific regions of the body. 
  2. an area of skin sensation associated with the spinal nerve entering the spine at a particular spinal level. These have been mapped, and it is possible to predict at what spinal nerve sensory information from different regions of skin enters the cord.
  3. These regions appear in fairly discrete transverse bands across the trunk and head, but appear to travel longitudinally down the legs and arms. (However, if you consider the human standing on all fours (hands and feet), the banding pattern corresponding with the transverse bands of the trunk becomes apparent again.
  4. in shingles. Here, the herpes zoster virus (associated with chicken pox) resides in the primary afferent cell bodies in a dorsal root ganglion of a particular spinal nerve. Sometimes the virus becomes active and travels antidromically, down the primary afferent to cause pain and blistering at the ends of the afferent nerves. Thus the receptor regions associated with a particular spinal nerve (dermatome) become visible.

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Referred Pain

  1. Explain the relevance of dermatomes
    1. Give an example
  2. How can we get clues about underlying disease?

  1. afferents arising from several internal organs (e.g. heart) enter the spinal cord at a specific level(s), along with the somatosensory input from the dermatomes associated with that level of entry. These organs do not have somatotrophic representation in the cortex (see below), and so when activity is stimulated in these afferents, they are interpreted as pain arising from the somatosensory regions of the dermatomes associated with that spinal level.
    1. when narrowed arteries to cardiac tissue result in hypoxic myocardium, afferents arising from the myocardium are stimulated. These enter the spinal cord at levels T1-T4. This is interpreted as pain arising from the dermatomes associated with T1-T4, i.e. diffuse pain arising classically from the left pectoral region and inferior regions of the left arm. This is REFFERED PAIN.
  2. Different organs are associated with pain arising from specific dermatomes

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Give an overview of a section through the spinal cord

  • Sensory axons travelling up spinal cord (green)t
    • Dorsal column
    • Spinocerebellar tract
    • Lateral spinothalamic tract
    • Ventral (ant) spinothalamic tract
  • Motor axons going down spinal cord (pink)

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Ascending paths

SPINOTHALAMIC TRACT (anterolateral tract)

  1. What information is carried by this tract?
  2. What is the conduction velocity of these afferents?
  3. What happens after synapsing in the spine?
  4. Where does it ascend to?
  5. What happens after a 3rd synapse?

  1. temperature, pain, crude touch and pressure receptors. Also tickle and itch (thought to have specific very sensitive cutaneous receptors).
  2. This information has poor spatial discrimination and is transmitted with slower conduction velocities (0.5-40 m/s) in smaller fibres.
  3. the secondary neurone CROSSES IN THE SPINE, to ascend contralaterally in the spinothalamic tract
  4. to the thalamus of the mid-brain.
  5. the tertiary neurone travels to the sensory cortex and sensation potentially becomes conscious.

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Ascending paths

DORSAL COLUMN (Medial leminiscal pathway)

  1. Where does this carry information from?
  2. What are the conduction velocities like?
  3. Do they synpase in the spine? How do they travel up the spinal cord?
  4. Where do they ascend to?
  5. Where do they travel to then?
  6. Where is the 3rd synapse?

  1. proprioceptors, fine touch and vibration, with high discrimination
  2. slower conduction velocities (40-100 m/s). larger myelinated fibres.
  3. NO. ascend ipsilaterally, without crossing in the spine.
  4. the dorsal column nucleus in the brainstem.
  5. synapses with a 2nd neurone that crosses to the other side within the medulla, to synapse with a 3rd neurone in the thalamus, and then to the sensory cortex.
  6. The thalamus

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Ascending Paths

Spinocerebellar tract

  1. What information does it convey?
  2. Outline this tract
    1. Where does it originate?
    2. How do the 2nd order neurons ascend?

  1. conveys information to the cerebellum about limb and joint position. especially important in control of movement and balance. 
  2. Spinocerebellar Tract
    1. in the spinal cord
    2. some ipsilaterally, some contralaterally

    3. in the cerebellum

(Pain information also ascends in discrete tracts - see pain lectures)

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Somatosensory Cortex

  1. Where does somatosensory information arrive in the brain?
  2. How does the cortex show SOMATOTROPHIC REPRESENTATION?
  3. What reflects the density of receptors and the sensitivity of a region of the body?            
  4. What is the SENSORY HOMUNCULUS?
  5. Where are the special senses processed?
  6. Where are the somatic senstations from one particular side of the body processed?

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  1.  in the sensory cortical areas in the postcentral gyrus of the parietal lobe, where potentially activity results in conscious sensation
  2. Different parts of the body are represented in different parts of these cortical areas
  3. The amount of cortex given to processing sensation from a particular region of the body
    1.  e.g sensitive skin of the fingers occupies a relatively large amount of cortex vs. the skin of the back which is relatively insensitive.
  4. The map of sensations associated with different areas of the cortex, where the proportion of cortex devoted to each body part is represented by the amount of cortex associated with it.
  5. These lie in other regions of the cortex.
  6. the somatic sensations from one side of the body are processed by cortex on the opposite side of the brain.

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Special Senses

What is in the

  1. Occipital lobe?
  2. Frontal lobe?
  3. Temporal lobe?

 

What is the role of the somatic sensory association areas?

  1. Visual Cortex
  2. Gustatory cortex
  3. Auditory and olfactory cortex

 

Monitors activity in primary sensory cortex

Allows recognition of somatic senses

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Somatosensory Defects

 

  1. What do Peripheral lesions lead to?
  2. What do Brain/spine lesions lead to?
  3. What happens when a specific peripheral nerve is cut (trauma)?
  4. What can point to the nerve roots affected?
  5. What are common causes of NEUROPATHY (impaired conduction)?
  6. What does neuropathy lead to?

  1. Localised symptoms
  2. more widespread symptoms
  3. sensation will be lost from the receptive fields associated with the primary afferent axons in the damaged nerve
  4. (trauma or tumour) The areas of skin in which there is sensory deficit (ie the dermatome affected) can point to the spinal root or roots affected.
  5. Diabetes

    Multiple sclerosis

    Chronic excessive alcohol consumption

    Chemotherapy

  6. pins and needles (paraesthesia) and then numbness (anaesthesia) and all modalities of sensation are usually affected.

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Somatosensory Defects

  1.  

31

  1. What can happen when trauma occurs in the spine?
    1. Describe the features
    2. When can a unilateral lesion occur?
      1. Describe this
  2. Give a common cause of Somatosensory damage within brain
    1. What does it cause?

  1. Ascending tract damage:
    1. bilateral (normally) and sensory loss in all modalities below level of lesion
    2. Slowly growing spinal tumour
      1. joint position sense lost on same (ipsilateral) side of lesion, temp. & pain senses lost on opposite (contralateral) side
  2. usually caused by stroke (bleed), damaging tracts from thalamus to cortex (internal capsule)

    1. causes sensory loss from opposite side of body and motor deficits from nearby affected areas common (potentially paralysis)

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