Transcript Somatic sensory neurons

SENSORY PHYSIOLOGY
LECTURE 14
CH 10
What is a sensory receptor?
-They keep us aware of our internal body or external world
-A specialized cell (not a protein) or a specialized dendritic ending
(e.g. pacinian corpuscle) or free nerve ending
Characteristics of Sensory Receptors
Dendrites ‘transduce’ (convert) the ‘adequate’ stimulus into a
membrane potential.
E.g. photoreceptors
Na+ channels open up, Na+ flows in, depolarization (receptor
potential)occurs, and this stimulates voltage-gated channels to open.
Classification of Sensory Neurons
1. Somatic sensory neurons –
-- information reaches conscious awareness
-- two major subgroups:
exteroceptors – provide info about the external
environment
proprioceptors – provide info about the position of the
head and body in space
2. Visceral sensory neurons –
-- information does not reach consciousness or is poorly
localized
-- interoceptors –
Somatic sensory neurons: Exteroceptors
1. Exteroceptors:
a) photoreceptors
b) chemoreceptors
1. olfactory receptors
2. taste buds
3. nociceptors- pain
- stimulated by chemicals released from damaged tissue
c) mechanoreceptors
1. touch and pressure receptors
2. vibration, tickle, itch receptors
3. auditory (cochlear) receptors
d) thermoreceptors
Student Activity
Turn to your neighbor and talk about how you think Ben Gay
works.
Active ingredients: menthol, methylsalicylate (aspirin), which
does not go very far into the body. But Ben Gay feels
hot/cold/hot/cold
Somatic sensory neurons: Proprioceptors
1. Proprioceptors: provide info about the position of the head and
body in space (kinesthesia, which is the sensation of movement).
a) muscle stretch receptors (muscle spindle fibers)
b) Golgi tendon organs – detect
c) joint receptors
d) vestibular apparatus (balance and equilibrium)
Visceral sensory neurons: Interoceptors
1. chemoreceptors: O2, CO2, H+, visceral nociceptors
2. baroreceptors
3. osmoreceptors
4. visceral stretch receptors
5. irritant receptors
NEURAL PATHWAYS FOR SENSORY INFO
FIG. 8.24
All somatesthetic information goes to the somatosensory cortex
-- information from the same area of
the body projects to the same area
of the somatosensory cortex
--disproportionately
large areas for hands
and feet
FIGURE 8.7
The sense of “hurt” resulting from pain
-- Why do we feel sad from pain?
FIGURE 8.18
-- probably a result of impulses
passing from the thalamus to the
cingulate gyrus, which is part of the
limbic system (emotional center).
Referred Pain
-- Referred pain is pain that is felt
in a somatic location, but instead
is the result of damage to an
internal organ.
-- referred pain is due to the
synapsing of visceral and somatic
sensory neurons at the same
interneuron as they enter the
spinal cord.
Angina pectoris – pain resulting from
damage to the heart.
Angina pectoris is often felt as stimulation
of a nociceptor in the left arm.
TONIC AND PHASIC RECEPTORS: Sensory Adaptation
PHASIC -After
awhile you
no longer sense
the effect.
Neurons send
many action
potentials at first,
but then they
send fewer or no
action potentials.
e.g. spray perfume, or e.g. putting your glasses on your head
TONIC RECEPTORS – E.G. Nociceptors (pain) do not adapt.
Law of Specific Nerve Energies
The law says that no matter what ending is stimulated, you will
always feel it by its original modality (adequate stimulus). E.g.
stimulation of photoreceptors results in the person seeing light.
Why? Because that neuron goes to a specific location in the brain.
Example: a person with an amputated leg often has a “phantom” limb
because the dendrite ending remains in the spinal cord after
amputation.
How Does the Receptor Work?
It creates a “generator” potential (or receptor potential), which is
similar to an EPSP.
It is a partial depolarization (graded); nearby are voltage-regulated
gates.
Sensory Dendrites contain the voltage-regulated gates
-- interneurons and motor neurons do not have voltage regulated
gates on their dendrites.
Two-Point touch Threshold and Receptive Field
Receptive field – an area of the body that, when stimulated by a
sensory stimulus, activates a particular sensory receptor.
- depends on the density of receptors in that area
Two point threshold – minimum distance at which 2 points of touch
can be perceived as separate.
Lateral Inhibition
Receptors that are most strongly
stimulated inhibit those around them.
This allows us to perceive welldefined sensations at a single
location instead of a “fuzzy” border
This process occurs in the CNS,
where interneurons that pass
between sensory neurons are
inhibitory.
Lateral inhibition allows us to
distinguish different pitches, closely
related odors, or the borders of light
and darkness, for example.
Taste buds
Taste buds have 50 – 100
different taste cells in them
Each cell responds to a different
taste
5 types: salt (sodium), sour (H+),
Sweet (sugar), bitter (quinine), umami (savory; responds to amino acids,
such as glutamate).
Taste
SMELL (Olfaction)
SMELL (Olfaction)
The olfactory receptors have cilia,
which project into the nasal cavity.
Detect 380 different smells; originally
we had 1000 genes. Many are now
pseudogenes (mutated).
Nasal stem cells divide every 1-2 months
G proteins (up to 50) may be associated
with each receptor, which may
account for olfactory sensitivity.
Olfactory neuron axons synapse with 2nd order neurons in the olfactory
bulb.
They do not go to the thalamus, but go directly to the cerebral cortex.
Questions for students:
Which receptors are used by both gustatory and olfactory senses?
Why do Ramen noodles taste meaty even though there’s no meat in them?
Vestibular Apparatus: Equilibrium
Two parts:
1) semicircular canals
-- rotational, or angular, acceleration
-- oriented in 3 planes
-- help us maintain balance when
turning our head, spinning, tumbling
2) otolith organs (utricle and saccule)
--provide information about linear
acceleration
-- utricle: horizontal acceleration
--saccule: vertical acceleration
Vestibular Apparatus: Mechanism
Bending of hair cells
results in the production
of action potentials
Bending opens up
ion channels 
depolarization
Bent towards
kinocilium: depolarization
bent away from kinocilium: hyperpolarization
Vestibular Apparatus: The otolith organ
The hair cells project in an
endolymph-filled membrane
the membrane contains
CaCO3 crystals
horizontal acceleration bends
utricle hair cells; vertical
acceleration bends saccule hair
cells
Vestibular Apparatus: Semicircular canals
Three different semicircular canals
detect movement in 3 orientations:
front-back; up-down; side-to-side
hair cells at the base of each canal
detect endolymph movement
How does the brain know which
direction you’re spinning? Hair
cells send action potentials at a constant rate when you’re not moving. But
when you rotate your head in one direction it increases the frequency of
action potentials.
Neural Pathways
VESTIBULOOCULAR REFLEX
Click here for a 30 second you tube video.
Vestibular nystagmus –
visual tracking to keep a
field of visual space within
view
vertigo – a loss of
equilibrium resulting
in a sense of spinning
Student Activity
Meniere’s disease is thought to be due to pressure changes in the inner
ear.
Why do patients show up at the doctor complaining of dizziness and
exhibiting Nystagmus?
Hearing: The Cochlea
Sound waves enter the
auditory canal and vibrate
the tympanic membrane
Middle ear: malleus,
incus, stapes
oval window
Hearing: The Middle Ear
This is a close up of the middle ear
The stapes is the last bone in
the sequence
It vibrates the oval window,
which mechanically vibrates the
endolymph inside the cochlea
The eustachian tube (auditory tube) is connected to the nasopharynx.
It equalizess the pressure of the middle ear with the changing atmospheric
pressure.
Hearing: The Cochlea
Hearing: Cross Section through Cochlea
the membrane that the
hair cells are stuck into is
called the tectorial membrane
Fluid makes the basilar membrane
bounce and this bends the
hair cells and causes
depolarization
Action potential is sent to
cranial nerve VIII
Hearing: PITCH
High pitch wiggles
the basilar membrane
closer to the oval window
medium pitch wiggles the
basilar membrane further
along the path
low frequency wiggles the
basilar membrane further
away
In the figure to the left, this is shown
as 20,000 Hertz (the lowest frequency).
Neural Pathways for Hearing
Sensory neurons from the vc
nerve synapse in the medulla
oblongata
neurons extend from medulla
to midbrain
neurons extend from midbrain
to auditory cortex of temporal
lobe
G.Hearing Impairment
Conduction deafness: Sound waves are not
conducted from the outer to the inner ear.
a.May be due to a buildup of earwax, too much
fluid in the middle ear, damage to the
eardrum, or overgrowth of bone in the middle
ear
b.Impairs hearing of all sound frequencies
c.Can be helped by hearing aids
Hearing Impairment, cont
2.Sensorineural/perceptive deafness: Nerve
impulses are not conducted from the cochlea to
the auditory cortex.
a.May be due to damaged hair cells (from loud
noises)
b.May only impair hearing of particular sound
frequencies and not others
c.May be helped by cochlear implants
3.Presbycusis – age-related hearing impairment