Peripheral Sensory Systems

Question 1

what is the overview of the CNS

Answer 1

Question 2

what are the sensory systems

Answer 2

visual, auditory, vestibular, olfactory, taste, temperature, pain, touch/pressure/movement

some overlap between senses and stimuli (pain and temperature)

Question 3

what are the types of sensory receptors

Answer 3

1. mechanoreceptors
2. chemoreceptors
3. thermal receptors
4. photoreceptors
5. pain receptors

Question 4

what do sensory sytems have

Answer 4

sensory fields –> cover a defined area (ex. skin)

stimuli passed by sensory afferent –> overlap of fields can assist in localization of stimuli

Question 5

how is sensory information percieved with a primary afferent merge on a single secondary afferent

Answer 5

perceived as single point

good for sensitivity

graded potential

Question 6

how are stimuli percieved with a primary afferent associated with several secondary afferent

Answer 6

two distinct stimuli felt and localized

poor sensitivity but good localization

Question 7

what is lateral inhibition

Answer 7

five primary afferents and stringer stimuli to B

neuron B has a strong impulse onto the secondary neuron

secondary can have an inhibitory effect on its neighbours

this can suppress the impulse passing through A and C

produces a sharp response signal

improves localization and pattern recognition

Question 8

what are the skin senses

Answer 8

cold, heat, pain, pressure, kinesthesia (move joint)

dorsal roots are organized in orderly way to different areas (strips) of skin

Question 9

what are free nerve endings

Answer 9

mainly in hairy skin

not specific to a particular stimulus

Question 10

what is terminal encapsulations

Answer 10

hairless skin (ex. palmar/plantar surfaces)

mostly mechanoreceptors –> pacinian (local pressure), ruffini (tension in collagen fibres), merkels (deformation of skin), meissners (sideways shearing)

not all specific (shearing of skin will also stimulate pacinian and meissners)

Question 11

what are thermal receptors

Answer 11

free nerve ending encapsulated endings

two types –> hot and cold

two sensors required to differentiate temperature

at extreme temps activate pain receptors

also have paradoxical cold receptor

Question 12

what is sensory adaptation

Answer 12

maintain stimulus, response declines

most senses show adaptation and will be complete (smell, touch) or partial (temperature)

prevents overload

Question 13

what are the mechanism of adaptation in pacinian corpusle

Answer 13

most often adaptation is due to inactivation of ion channels

pacinian corpuscle and a little different

fluid within layers that surround the central neuron

pressure applied and passed to sensory neuron

redistribution of fluid to ease pressure and give adaption

Question 14

what are the two types of receptors in the muscle

Answer 14

1. muscle spindles –> in parallel with muscle fibres
2. golgi tendon organs –> in series with muscle fibres

Question 15

what are muscle receptors important for

Answer 15

proprioception

knowledge of position and movement of head/limbs and attitude and motion

through reflexes they are also important for posture

Question 16

what are muscle spindles

Answer 16

resemble extrafusal fibres but –> 1. smaller 2. innervation is from gamma motor neurons (to ends of fibre) 3. importantly they have sensory type Ia and II fibres

sensitive to muscle stretch

Question 17

what are the types of sensory fibres

Answer 17

type Ia fibres are adaptive

type II fibres are largely non-adaptive

muscle spindle important for postural reflexes so important not to have adaption

Question 18

what will gamma motor stimulation cause

Answer 18

contract the ends of the muscle spindle

alters sensitivity of spindle to stretch

alpha and gamme fibres act together

Question 19

what is alpha/gamma co-activation

Answer 19

important to maintain sensitivity to stretch if there is alpha motor stimulation

if alpha only spindle becomes flaccid and lose sensitivity

with both –> intra and extrafusal both contract and maintain sensitivity

Question 20

what does gamma stimulation also activate

Answer 20

sensory afferents and changes sensitivity of the muscle spindle

activation of the gamma neurons contracts the muscle spindle

this leads to stretch of the intrafusal fibres and firing of sensory afferents

Question 21

what is the myotatic reflex (stretch reflex)

Answer 21

muscle spindles also give rise to myotatic reflex

stretch muscle –> stretch spindle –> activates sensory afferent –> activates a motor neuron

simple but important postural reflex –> muscle stretch causes reflex contraction of that muscle

Question 22

what is the golgi tendon organ

Answer 22

situated within the tendon

responds to tension in muscle –> stretch or contraction

free nerve ending lie within the tendons

gives rise to type 1b sensory fibre

gives rise to inverse myotatic reflex –> also postural

Question 23

what are central projections

Answer 23

both the golgi and muscle spindle project in same way to CNS

enter at T1-T3 and synapse at Clarkes column

projects to cerebellum –> cerebellum is co-ordination centre for movement

sensory input to this pathway can originate from different levels of spinal cord