Sensory Receptors and the PNS

Question 1

The PNS is made up of visceral and somatic

- Describe the Somatic Nervous system

Answer 1

o Receptors on skin (stimulation)→ DRG → brain, terminates in somatosensory cortex.
o Communication with motor cortex → elicits voluntary motor activity in response to this sensory information

Question 2

The PNS is made up of visceral and somatic

- Describe the visceral nervous system

Answer 2

o Visceral afferent supplying lining of GIT → pseudounipolar cell (ganglion) → spinal cord → hypothalamus → autonomic response (ie increased gastric secretion).

IE the HYPOTHALAMUS rather than the THALAMUS recieves all of the information and the THALAMUS rather than the cerebral cortex is a major source of descending pathways
- Also autonomic transmission to the periphery involves an intermediate synapse in the autonomic ganglia

Question 3

TRANSDUCTION

- Define

Answer 3

Is the conversion of an external stimulus into electrical signal (ie photoreceptors: light → electrical signal)

Question 4

TRANSDUCTION

=- explain mechanoreceptors

Answer 4

Tactile receptors, auditory & vestibular hair cells, some taste cells

• Stimulus triggers opening of ion channels → ionotropic mechanism (direct effect from mechanical stimulus)
• Membrane deformation (ie force) → cytoskeletal displacement → opening of channel

Question 5

TRANSDUCTION

- explain G protein coupled receptors

Answer 5

Photoreceptors (rods & cones), olfactory neurons, some taste cells, pain fibers
- Stimulus binds to receptor
- G -protein linked to enzyme that alters concentration of intracellular second messengers → ion channels opening = metabotropic mechanism
o Change in intracellular environment
- Stimulus is converted into a receptor potential (brief alteration in the membrane potential)
o Depolarising (most cases)
o Hyperpolarising (photoreceptors)
o In vestibular & auditory receptors – they may be depolarizing or hyperpilarising depending on the stimulus

Question 6

What are the 2 properties of sensory receptors?

Answer 6

Receptive field

Adaptation

Question 7

Describe adaptation

Answer 7

Loss of sensitivity during course of maintained stimulus
- Slow → eg muscle spindle (very low adaptation)
- Rapid → eg nerve endings around the hair
o i.e. after a brief period of hair being deflected, you no longer feel it (high degree of adaptivity - you lose sensation very quickly)

Question 8

What is the role of MERKELS DISCS & MEISSNERS CORPSUCLES in adaptation

Answer 8

Merkel disks – slowly adapting receptors (receptors continue to fire & convey info to CNS)

Meissner’s corpuscles – rapidly adapting receptors

Question 9

There are two structural types of sensory receptors - what are these?

Answer 9

Non encapsulated (free nerve ending, merkel ending, endings around hairs)
Encapsulated (pacinan corpsucle, meissners disc, muscle spindle

Question 10

Explain some of the non capsulated ending sensory receptor

Answer 10

• Free nerve ending (may serve several modalities)
  o Formed by branching terminations of sensory fibers in the skin, with no obvious specialization around them other than partial ensheathment by schwann cells.
• Merkel endings
  o The ending is a disk-shaped expansion of a terminal sensory fiber, applied to the base of a Merkel Cell
  o Merkel cells are located in the basal layer of the epidermis
• Endings around hairs

Question 11

Explain some of the ecapsulated ending sensory receptor

Answer 11

Layers of cells (a capsule) around the nerve ending that acts as a filter & can modify input. Barrier function.

• Pacinian corpuscle – vibration -> = dorsal column/medial lemniscal pathway (hypodermis – deeper than meissner’s)
• Meissner’s corpuscle– fine tactile discrimination
• Muscle spindle

Question 12

What do PAIN RECEPTORS respond to?

Answer 12

• Intense mechanical stimuli
• Extreme levels of heat or cold
• Noxious chemicals

Question 13

What are some high threshold mechanoreceptors?

Answer 13

• Sharp pain
• Small myelinated
• A-delta fibers

Question 14

What are some polymodal nociceptor fibres (respond to different stimuli)

Answer 14

• Dull pain
• Small unmyelinated fiber
• C-fiber

Question 15

Describe the nociceptor axon reflex

Answer 15

• Impulses travel towards & away from cell body
• Release of neurotransmitter in spinal cord & periphery
• Release of glutamate & neuropeptides
  o Dilation of blood vessels – flare response
  o Histamine → increase in capillary permeability – accumulation of tissue fluid (oedema)
• Hyperalgesia → sensitization of peripheral nociceptors

Question 16

what are your receptors for proprioception?

Answer 16

MUSCLE SPINDLES

GOLGI TENDON ORGANs

Question 17

Describe Muscle spindles

Answer 17

• In the muscle proper
• Detect muscle stretch
• Consists of intrafusal muscle fibers, presence of capsule
  o Intrafusal muscle fibers are skeletal muscle fibers that comprise the muscle spindle, and are innervated by GAMMA-motor neurons.
  o These fibers are a proprioceptor that detect the amount & rate of change of length of a muscle
  o These fibers are walled off from the rest of the muscle by a collagen sheath (fusiform/spindle shaped)
  • This capsule surrounds the middle third of the fibers
  o All intrafusal muscle fibers are multinucleated, & the central (non contractile) region contains the nuclei
• Lies in parallel with extrafusal muscle fibers
  o Innervated by alpha-motor neurons & contract
• Ends of capsule are attached to extrafusal muscle fibers
• Hence if a large skeletal mm is stretched, so are the intrafusal muscle fibers

Question 18

Muscle spindles- what are the types of intrafusal muscle fibres

Answer 18

• Nuclear bag fiber – wider nuclear region, nuclei are clustered
• Nuclear chain fibres – nuclei lined up in single file

Question 19

Describe the sensory innervation of muscle spindles

primary and secondary

Answer 19

o	Primary (1a; annulospiral structure)
•	Nerve fiber enters the capsule & branches, supplying every intrafusal fiber in a given spindle
•	More sensitive to the onset of stretch, adapts more rapidly

o	Secondary (II; “flower-spray” structure)
•	Formed by a few smaller nerve fibers that branch & primarily innervate nuclear chain fibers on both sides of the primary ending

Question 20

Describe the motor innervation of muscle spindles

Answer 20

o Gamma motor neurons
• Much smaller than normal motor neurons
• Cell bodies in spinal cord, axons innervate the contractile portion of intrafusal muscle fibers
• When a muscle is contracted (ie biceps), this relieves tension on the nuclear region of the intrafusal fiber → low sensitivity (to muscle stretch starting from this contracted state)
• Gamma motor neurons can cause spindle to fire → part of intrafusal fiber (one either side of nuclear region) to contract → “pre-stretches” muscle spindle → controls sensitivity of the spindle to stretch

Question 21

Describe the golgi tendon organ

Answer 21

• Detects muscle tension
• Spindle shaped receptors found in the junction of muscles & tendons
• Encapsulated → Interwoven collagen bundles surrounded by a thin capsule
• Lie in series with muscle fibers
• Stimulated by tension on capsule as a result of muscle contraction