intro to neuro exam 2

sensation uses ascending divergent hierarchical circuits to carry…

….to carry environmental info → spinal cord and brainstem → sensory cortex

Divergence

• divide into many parts

• 1 → 4 → 2 each

• increasing from one circuit to many

Convergence

• go from many to one circuit

• 2 each → 4 → 1

Sensory neurons, receptor neurons, and sensory afferents are

Interchangeable terms - first neurons in each sensory pathway

• perform sensory transduction

Sensory transduction

conversion of stimulus energy to electrical energy

• first step

no receptor for stimulus then

then no detection!

Pain, temp, pressure sensors

= pseudo-unipolar

• 1 long axon

• cell body

• NO dendrites!

example of sensory neurons

rods, cones, olfactory receptors, taste buds, hair cells, pain/temp/pressure sensors

Somatosensory system

info about what the body is doing, where it is, and what is going on in the environment

• encodes and processes sensory information from the skin, muscles, and viscera

sensations from somatosensory system include:

• touch

• pressure

• vibration

• limb position (proprioception)

• heat

• cold

• pain

• itch

Somatosensory Afferents

pseudounipolar neurons - soma and axon with NO dendrites

• axon splits in 2 directions: one towards spinal cord and one towards skin/muscle

why do pseudo-unipolar (somatosensory afferents) have no dendrites?

information comes from environment not other neurons so it doesn’t need dendrites/spines to receive signals from other neurons

type of receptor ending determines…

…determines what type of stimulus activates the neuron

size of receptor ending (branching area) determines..

..determines the size of the receptive field (area) of the neuron

Types of receptor endings

1. Mechanoreceptors - touch, vibration, pressure and proprioception

• highly specialized for specific feelings

1. Free Nerve endings - pain, temp, itch

• similar endings

Touch and Pain are ____ different systems

2 different systems

Mechanoreceptors

• Merkel cells

• Ruffini afferents

• Meisner corpuscles

• Pacinian corpuscles

Receptive Fields

area of skin surface over which stimulation results in a significant change in the firing rate of action potentials

size of receptive field depends on…

…depends on the type and size of the receptor ending

Spatial accuracy

ability to discern the presence of two tactile stimuli separated by a small distance

small receptive fields increase the ability…

….the ability to discern two stimuli that are close together - greater sensory sensitivity

______ have the smallest receptive fields

fingers - most accuracy

Leg and Back =

= largest receptive fields

• weaker sensory sensitivity and less ability to identify two tactile stimuli that are close together

The size of somatosensory receptive fields determines…

…determines spatial sensitivity

we use touch to interpret

• texture

• shape

• size

• weight

• perform tasks

• use tools

touch requires the multiple classes of

classes of mechanoreceptor

the somatosensory system has a variety of receptor-ending types to encode…

…to encode various types of sensory stimulation

• such as touch, vibration, pressure - all examples are mechanoreceptors

mechanoreceptors respond to _____

____ responds to mechanical stimulation

mechano-sensitive channels open with

open with deformation or stretch of skin or muscle

• when stretched → Na+ and Ca2+ enter and depolarize skin or muscle

receptor potential

change in membrane potential of receptor afferent by cation influx

_____ are graded with the strength of stimulation

receptor potentials are ____

How we detect the difference between stimuli of differing intensities:

1) frequency of AP firing

2) Number of sensory neurons recruited

MORE AP is generated

A strong stimulus generates AP _____ often then weak stimulus

our brains interpret AP frequency as _____

our brains interpret _______ as stimulus strength

smallest receptive fields provide..

…provide high spatial resolution

• form and texture coding

• such as valleys of fingerprints - merkel receptors

Larger receptive fields therefore…

….therefore lower spatial resolution

• light touch, vibration, slipping grip

• ex peaks of fingerprints - meissner corpusle

receptors can also tell us two important characteristics of the stimulus:

• when it occurs (how often)

• how long it occurs

Slowly adapting receptors

continue to respond throughout the entire duration of the stimulus

Rapidly adapting receptors

respond to the on-off of the stimulus, not to its continued duration

• will respond again when stimulus is removed

Proprioception

information about limb and body position in space

• essential for accurate performance of complex movements

• proprioceptors

Proprioceptors

“Receptors for self”

• class of mechanoreceptors

Types of Proprioception

• Skeletal Proprioception

  • Muscle Spindles

  • Golgi Tendon Organs

  • Joint Receptors

Muscle Spindles

• found in striated muscle

• sensory afferents wrap around the central part of the spindle

• stretching the muscle fibers activates mechanoreceptors ON the sensory afferents

Spindle

specialized muscle fibers surrounded by a capsule

Golgi Tendon Organ

• found in tendons

  • between muscle fibers and tendons

• sensory afferents and collagen fibers interweave in the capsule

• contracting the muscle fibers activates mechanoreceptors on the sensory afferents

• encodes muscle TENSION

Proprioceptors provide…

… provide sensory input from the muscles to the spinal cord

Muscle Spindle

• integrated within the muscle

• provides information about muscle length

• DECREASE AP firing rates when muscle contracts

• AP firing rate codes length of muscle

Golgi Tendon Organ

• found in the tendon

• provides info about the force of muscle contraction

• INCREASE AP firing rates when muscles contract

• AP firing rate codes force of muscle contraction

Muscle spindles and golgi tendon organs both…

…both fire AP during passive stretch but when muscle contracts

• Spindles DECREASE AP

• Golgi tendon organs INCREASE AP

somatosensory cortex is located in

located in post central gyrus

Somatotopic maps

• do not represent the body in actual proportions

• circuitry is devoted to functions that are most important

  • hand manipulation, facial expressions, speech

• Humunculus - little man - grossly enlarged face, lips, hand

Cortical remapping

• amputations changes cortical representations

• map expands the remaining limbs into the area of the amputated limbs

  • can be source of phantom limb syndromes or lack of sensory feedback/proprioception to brain

Nociception

• “to hurt” - Nociception

• pain receptors

Nociception - pain receptors

are free nerve endings in the skin (no specializations)

Nociception is defined as

the encoding and processing of noxious stimuli by the CNS and PNS

• sensory processing

Pain is define as

unpleasant sensory (physical) and emotional (psychological) experience associated with actual or potential tissue damage

• sensory and higher brain processing

Analgesia

relief from pain

Noxious stimuli

are the stimuli that are actually or potentially damaging to tissue

• damage to tissue can lead to release of variety of substances that act on specialized pain sensing sensory neurons

• can recruit immune cells to area

tissue damage can also recruit immune cells to the area

this releases their own substances onto pain-sensing sensory neurons to alert them to presence of noxious stimuli

• this can cause swelling in the area

visceral tissue damage (ex. pancreas, lungs, liver)

damage to skin/muscle causes pain but ______ does not always lead to pain

Nociceptors

the neurons responsible for sensing noxious stimuli from the external and internal environment

examples of noxious stimuli

• pungent natural compounds

• noxious cold temperature

• noxious mechanical stimuli (hit with hammer)

• Inflammatory peptides

• Environmental irritants

Characteristics of nociceptors

• have free nerve endings that have innervate target

• broken into two main groups: A and C fibers

• Express channels and receptors that help transduce mechanical, thermal, chemical stimuli.

Ion channels and GPCRs

Nociceptors express various __________ that activate in response to mechanical, thermal and/or chemical stimuli

thermal/mechanical stimuli

Primarily detected through a family of ion channels called “TRP (transient receptor potential) channels”

• influx of Ca++ and/or Na+ when activated

chemical stimuli

detected through TRP channels and GPCRs

Aδ- fibers

First pain

• Myelinated

• responsive to mechanical and thermal pain stimuli

• fast and intense pain

C- fibers

Second pain

• not myelinated

• responsive to mechanical, thermal and chemical stimuli

• lingering, throbbing, chronic pain

express TRP or other mechanoreceptors

Noxious stimuli are transduced at the periphery by C and Aδ fibers which _______

• this leads to withdraw from painful stimulus, pain avoidance and an emotional reaction

  • reflux

somatic nociceptive pain is more localized then visceral nociceptive pain

______ pain is more localized then ______ pain

Visceral pain

• sparser innervation

• diffuse pain

  • larger less specialized receptive fields

• referred pain

• mainly mechanical nociceptors

somatic pain

• dense innervation

• localized pain

  • smaller receptive fields

• thermal, chemical, mechanical nociceptors

Hyperalgesia

tissue damage can increase sensitivity to painful stimuli near the site of the injury

inflammation causes…

…causes release of prostaglandins that interact with pain receptors to decrease threshold for depolarization and AP

• therefore less stimulation is needed to get the pain neurons to fire

non-steroidal anti-inflammatory drugs (NSAIDs)

block prostaglandin - decrease pain

• how advil/asprin works

they release chemicals that increase the activity of nociceptor afferents

damaged tissue, inflammatory and tumor cells _______

• “inflammatory soup”

• acts to sensitize pain

Neuropathic pain

Due to damage to the receptors or the spinal cord/brain regions that process pain

• difficult to treat with analgesics/NSAIDs bc not “chemical soup” mediated

• complication of diabetes, shingles, AIDS, MS, stroke, amputation

Neuropathic pain examples

due to disease/illness

• stabbing sensation

• electric shock like sensation

• numb sensation

• shooting sensation

• burning sensation

• throbbing sensation

• pins/needles

• stabbing sensation

Nociceptive pain

nociceptors in tissues send pain signals to the CNS

Neuropathic pain

damage to the nerve itself causes typical pain symptoms

purpose of nociceptive pain

useful indicator that tissue damage has occurred

Sensory transduction

through activation of transient receptor potential (TRP) channels, in the free nerve endings of pain afferents

• channel activation by temp, chemical, mechanical stimuli that allows Ca2+ and Na+ influx to depolarize the sensory afferent

why do hot wings and cool mint herbs taste like burning?

same TRP channels that respond to temp also respond to chemicals in spicy food or “cool” herbs - brain can’t tell difference

Dorsal Root Ganglion (DRG)

contains the somas sensory afferents from both somatic and visceral structures

Dermatome

at each segment of the spinal cord, each DRG and its associated spinal nerve innervate a territory of the body

first order neurons are

______ are sensory neurons

enter spinal cord at the same DRG

pain afferents from viscera (organs like heart) and pain fibers from the skin (dermatome) enter the spinal cord at _______

why this results in confusion in interpretation in source of pain due

• visceral afferents synapse have same second order neurons in the spinal cord as the pain afferents from skin

• second order neurons are not specific to somatic or visceral inputs

referred pain

confusion causing brain to interpret visceral pain as cutaneous pain

• ex. pain of heart attack is often felt in left arm

Pharmacological treatments for pain

NSAIDs and Opioids

NSAIDs

• block prostaglandin, a pro-inflammatory molecule, synthesis

• over the counter, low-moderate pain

Opioids

• act on variety of G-protein coupled opioid receptors

• endorphins and opioids extremely important for analgesia (relief from pain)

• by prescription, moderate-severe pain

Opiate receptors

• both pre and postsynaptic, located in many areas of the brain and spinal cord

• all are metabolic

• activation of opiate receptors leads to decreases in neuronal activity

examples of opiate receptors

1. Mu receptors

2. Delta receptors

3. Kappa receptors

why do opiates (morphine and oxycontin) relieve pain?

• endogenous opioid and cannabis receptors are found in synapses between 1st and 2nd order neurons in the pain pathway

• activation of these receptors prevent NT release

  • prevents signal from being passed to brain

Opioids bind to endogenous opioid receptors to produce inhibition (pre and post synaptic receptors to reduce pain signal)

2 mechanisms:

• (Pre) reduce NT release at first order to second order neuron synapse

  • reduce excitation of post-synaptic neuron

• (Post) Hyperpolarize the post synaptic neuron

  • less likely to respond to pain afferent

Cornea

transparent tissue - outer area of eye

• majority of light refraction - not adjustable → would need lasik to adjust it

Aqueous humor

produced by the vascular part of the ciliary body - replaced 12x a day

• very watery

Glaucoma

Poor drainage of the aqueous humor

lens

adjustable refraction for focusing at various distances

• zonal fibers hold lens in place to connect to ciliary muscles

• ciliary muscle shapes lens

• transparent tissue