Sensory Receptors and the PNS Flashcards
The PNS is made up of visceral and somatic
- Describe the Somatic Nervous system
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
The PNS is made up of visceral and somatic
- Describe the visceral nervous system
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
TRANSDUCTION
- Define
Is the conversion of an external stimulus into electrical signal (ie photoreceptors: light → electrical signal)
TRANSDUCTION
=- explain mechanoreceptors
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
TRANSDUCTION
- explain G protein coupled receptors
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
What are the 2 properties of sensory receptors?
Receptive field
Adaptation
Describe adaptation
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)
What is the role of MERKELS DISCS & MEISSNERS CORPSUCLES in adaptation
Merkel disks – slowly adapting receptors (receptors continue to fire & convey info to CNS)
Meissner’s corpuscles – rapidly adapting receptors
There are two structural types of sensory receptors - what are these?
Non encapsulated (free nerve ending, merkel ending, endings around hairs) Encapsulated (pacinan corpsucle, meissners disc, muscle spindle
Explain some of the non capsulated ending sensory receptor
- 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
Explain some of the ecapsulated ending sensory receptor
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
What do PAIN RECEPTORS respond to?
- Intense mechanical stimuli
- Extreme levels of heat or cold
- Noxious chemicals
What are some high threshold mechanoreceptors?
- Sharp pain
- Small myelinated
- A-delta fibers
What are some polymodal nociceptor fibres (respond to different stimuli)
- Dull pain
- Small unmyelinated fiber
- C-fiber
Describe the nociceptor axon reflex
- 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
what are your receptors for proprioception?
MUSCLE SPINDLES
GOLGI TENDON ORGANs
Describe Muscle spindles
- 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
Muscle spindles- what are the types of intrafusal muscle fibres
- Nuclear bag fiber – wider nuclear region, nuclei are clustered
- Nuclear chain fibres – nuclei lined up in single file
Describe the sensory innervation of muscle spindles
primary and secondary
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
Describe the motor innervation of muscle spindles
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
Describe the golgi tendon organ
- 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
