Peripheral Sensory Systems
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Give an example of a type of excitatory and inhibitory neurone
- Excitatory - acetylcholine, glutamine, aspartate
- Inhibitory - glycine, gaba
Neurotransmitters can either be ...?
amino acids or monoamines
Nearly all senses go through which area of the brain?
Which sense does not go through the thalamus?
What are the a) special senses b) peripheral senses?
- a) Visual, auditory, vestibular, olfactory, taste
- b) Temperature, pain, touch/pressure/movment
What are the different types of sensory receptors? And what do they respond to?
- Mechanoreceptors - respond to mechanical deformation
- Chemoreceptors - respond to specific chemicals
- Thermal receptors - heat and cold receptors
- Photoreceptors - respond to light
- Pain receptors
There are no auditory receptors, what type of receptor does our sense of hearing come from?
What does overlap of receptive fields of sensory systems allow?
It aids in localisation of touch and pattern recognition as receptive fields get more complex
True or false: all afferent sensory neurones have receptive fields?
Describe the effect on sensitivity and localisation in a) Primary afferents which synapse onto a single secondary afferent b) Primary afferents each with their own secondary afferent
- a) There is reduced localisation as any two touches will be perceived as a single point by the secondary afferent. However, sensitisation is increased as the secondary afferent will pick up the sum of all the primary afferents.
- b) Good for localisation as there are many secondary afferent neurones
Explain the process of lateral inhibition. What is the advantage of lateral inhibition.
This is when stimulation of the secondary afferent neurone inhibits the neighbouring secondary afferents to sharped the signal going to the tertiary afferent. This improves localisation and pattern recognition.
What is a dermatome?
The map of an animal to show areas of skin all connected to the same dorsal root
What do the following mechanoreceptors detect: pacinian corpuscle, ruff ini, merkels, meissners?
- Pacinian - local pressure
- Ruffini - tension in collagen fibres
- Merkels - deformation of skin
- Meissners - sideways shearing
What are the two types of thermal receptors?
Cold and warm
What is sensory adaptation?
When you maintain the stimulus but the response declines within the sensory system. This reduces the amount of information being sent to the brain.
Which senses do not show adaption?
Pain and some postural senses
What is the difference between complete and partial adaption?
- Complete adaption - get a response when stimulus is applied, which quickly goes to zero and then you get another response when the stimulus is taken away.
- Partial adaption - stimulus is applied to receptor and there is an immediate response but over time this starts to decline.
How is the pacinian corpuscle stimulated?
The pacinian corpuscle has layers filled with fluid. Pressure stimulates the central neurone. The layers of fluid then redistribute themselves and remove pressure from the central neurone. When the initial stimuli is removed the fluid layers rearrange themselves and this stimulates the central neurone again.
What are the different types of skin afferent fibres?
I = A-alpha, II = A-beta, III = A-delta, IV = C
Which fibres are the largest/smallest?
Type I = largest. Type IV = smallest
Describe the route of dorsal columns in centrally projecting tracts. What type of fibres use these tracts?
- Information enters the spinal cord and projects straight up to the medulla. The first synapse is at the dorsal column nuclei (in the medulla). Information then crosses over to the contralateral side, synapses again near the thalamus and projects up into the somatosensory cortex.
- Type I and II fibres
Describe the route of ventro-lateral systems in centrally projecting tracts. What type of fibres use these tracts?
- Information enters the spinal cord, synapses and cross over to the contralateral side. It then projects right up to the thalamus, synapses and goes up into the somatosensory cortex.
- Type III and IV fibres
Which system is slower - dorsal columns or ventro-lateral systems?
Ventro-lateral systems as there are more fibres and synapses in this system
What side of the spinal cord (left or right) do touch, pain and temperature come up from? And which system do they use (dorsal columns or ventro-lateral systems)?
- Touch - left, dorsal columns
- Pain and temperature - right, ventro-lateral systems
Muscle spindles lie in series/parallel with muscle fibres? Golgi tendon organs lie in series/parallel with muscle fibres?
- Muscle spindles - parallel
- Golgi tendon organs - series
Muscle spindles are bigger/smaller than extrafusal fibres?
What is the innervation of muscle spindle?
From gamma motor neurones (to the ends of the fibre)
What type of sensory fibres do muscle spindle have?
Type Ia and II
What are muscle spindles sensitive to?
Which of the sensory muscle spindle firers are adaptive/non-adaptive?
Type Ia - adaptive. Type II - largely non-adaptive
How is the sensitivity of muscle spindle maintained?
If we only had alpha motor stimulation this would cause extrafusal fibres to contract. However the intrafusal fibres would become flaccid and lose their sensitivity. Therefore gamma and alpha co-activation stimulates both the extrafusal and intrafusal fibres.
How does gamma stimulation lead to sensory stimulation of Type Ia and II fibres?
Gamma stimulation contracts the two ends of the muscle spindle. this stretches the middle where the Type Ia and II fibres are which leads to sensory stimulation and increased activity of these fibres.
Describe the myotatic (stretch) reflex
Stretching muscle spindle stimulates sensory afferents. This goes to the spinal cord, through a single synapse, stimulates an alpha motor neurone leading to contraction of the muscle.
What does the golgi tendon organ respond to?
Tension in muscle (from stretch or contraction)
What type of fibres emerge from the golgi tendon organ?
Type Ib fibres
How is the golgi tendon organ stimulated?
Tension in the muscle causes tension in the tendon which crushes the endings of the Type Ib fibres leading to golgi tendon organ stimulation.
What tract do afferents from muscle spindles and golgi tendon organs use to reach the brain?
What area of the brain is important for correlating movement?
Where does information about limb proprioception come from?
Receptors in joints and muscles - position infer comes largely from joint receptors and limb velocity infer from muscle spindles.
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