Download SOMATOSENSORY SYSTEM

SOMATOSENSORY SYSTEM
The senses collect information about the modality,
intensity, duration, location of events in the world. The
initial contact with the external world occurs through
specialized neural structures: sensory receptors. Each
receptor is sensitive to a form of physical energy, e.g.
mechanical, chemical, electromagnetic energy. The
receptors transform energy into electrical energy (common
language for all sensory systems; a series of action
potentials; neural encoding) which is called stimulus
transduction.
Sensory Receptors Convert Different Forms of Energy
into Electrical Signals
1- We have sensors in our bodies that can detect heat,
pressure, stretch, acceleration, sound, light, smells, tastes
and other forms of energy
2- Sensory receptors act as transducers, converting many
forms of energy into action potentials that the brain can
interpret
3- Many receptors, such as those in the skin, are simply
constructed; others, such as the eye and ear, are very
elaborate
The Type of Sensation Perceived Depends Upon
Connection in Brain
1- Almost all sensory input passes through the thalamus
but sensory action potentials in the thalamus look alike-it
is impossible to tell the type of sensation by looking at the
action potential
2- The type of sensation felt depends upon which part of
the cortex the sensory nerve goes to
3- Presumably, if you were able to cut the auditory and
optic nerves and reattach them to the wrong stumps, you
should be able to hear the lightning and see the thunder
A Stronger Stimulus Gives More Action Potentials per
Minute (Frequency Coding)
1- Stimuli cause sensory receptors to depolarize and
produce a voltage called a generator potential- this is not
an action potential, it is not transmitted
2- The generator potential causes the sensory nerve to
produce a series of action potentials that are sent to the
brain
3- When a stimulus gets stronger you do not get bigger
action potentials- that would violate the all-or-none
principle
4- Instead you get more action potentials per minute
5- The brain knows that a higher frequency of action
potentials (more per minute) means a stronger stimulus
If a Stimulus is Continued Sensory Receptors Adapt &
Become Less Sensitive
1- If a stimulus is maintained at a constant intensity for a
long time the nerve seems to lose interest in it- the nerve
has adapted and become less sensitive
2- This allows us to tune out background noise, to ignore
the touch sensation from our clothing , to lose awareness
of the temperature of the room, etc..
3- Some nerves, such as those for pressure and touch,
are fast-adapting; others, such as those for muscle stretch
and some types of pain, are slow-adapting- the sensation
lasts a long time
4- Example: temperature receptors
1- Two types: warm & cold receptors
2- If one hand is placed in warm water and the other
is placed in cold water, the temperature receptors will
adapt and become less sensitive
3- After adaptation, if both hands are placed in
lukewarm water, the hand originally in warm water will feel
cold, and the hand originally in cold water will feel warm
Skin Sensations are Usually Perceived at the Location
of the Receptor
1- Somatic (body) senses are perceived to be coming from
the location of the sensory receptor
2- Sometimes the body is fooled: phantom limb painperson feels a limb which is no longer present
3- Skin sensations are quite complicated:
1- Merkel cells and Ruffini endings respond to steady
pressure
2- Pacinian corpuscles and Meissner's corpuscles
give the sense of vibration
3- There are separate warm and cold receptors
4- Receptors associated with skin hairs allow you to
feel the displacement of hairs
5- Several types of pain receptors respond to
mechanical trauma or very high or low temperatures
4- Uneven distribution of receptors: close together on
finger tips & face; far apart on back, legs, arms, belly
RECEPTORS
RECEPTORS cover the surface of the body, and different
dimensions of the world leave traces on them. Receptors
convert energy into electrical activity.
For different kinds of sensations, different kinds of
receptor cells. Rod and cone cells of the eye's retina are
specialized to respond to the electromagnetic radiation of
light. The ear's receptor neurons are topped by hair
bundles that move in response to the vibrations of sound.
Olfactory neurons at the back of the nose respond to
odorant chemicals that bind to them. Taste receptor cells
on the tongue and back of the mouth respond to chemical
substances that bind to them. Meissner's corpuscles are
specialized for rapid response to touch, while free nerve
endings bring sensations of pain.
Modality
Stimulus
Receptor types
Receptors
-----------------------------------------------------------------------------------------------------------------Vision
Light
Photoreceptor
Rods, Cones
Audition
Sound
Mechanoreceptor
Hair cells (cochlear)
Balance
Head motion
Mechanoreceptor
Hair cells (semicircular canals)
Somatic
Mechanical,
Mechanoreceptor,
eg. Meissner
thermal,
thermoreceptor,
chemical
chemoreceptor ,
pain receptor
free nerves
Taste
Chemical
Chemoreceptor
Taste buds
Smell
Chemical
Chemoreceptor
Olfactory sensory neurons
Each of the senses activates a separate area of the
cerebral cortex, the sheet of neurons that makes up the
outer layer of the brain's hemispheres.
RECEPTIVE FIELD
Receptive fields (RF). Receptors transfer their stimulation
to the sensory areas of the brain through several neuronal
connections. Neurons monitor the world by parceling it
into small regions of space, known as receptive fields. RFs
are defined in terms of single neurons.
RF location is related to stimulus modality. Receptive
fields of primary somatosensory cortical neurons are
smallest on the fingers and become larger on the hand
and forearm.
RF size is defined by convergence
convergence, and receptor density affects sensitivity and
acuity (see figure below)
[[[Name of the figure: convergence & receptive field_1]]]
[[[Name of the figure: convergence & receptive field_2]]]
[[[Description of the figure below: Two spots of light can be
discriminated easily by non-converging pathways (right),
while they might stay fused in one even at large interspot
spacing when convergence occurs (left).]]]
[[[Name of the figure : RF size varies]]]
TWO-POINT DISCRIMINATION
GENERAL PRINCIPLES
general principal 1
all sensory info except for olfaction crosses over in
the brain ·this info then passes to the lateral geniculate
nucleus (LGN: part of the thalamus) which is the first
processing relay in the brain where info from different
types of receptors is split up and passed on to the primary
cortex- area 17 also known as the primary visual or striate
cortex where the first simplistic analysis of information
takes place
general principal 2
all senses except olfaction first relay in the thalamus
before padding onto the cortex ·this information is then
passed onto surrounding areas of the cortex, secondary
and tertiary visual areas (areas 18 and 19) where
processing of more complex visual info takes place (color,
movement, shape, binocularity(
General Principal 3
all sensory info first passes to the primary cortex
before being passed to higher cortical areas for assembly
and then on to association areas ·where it is associated
with other sensory information in order to form a complete
representation of an object (looks like ice cream, smells
like ice cream, tastes and feels like ice cream- and then it
is identified, it must be ice cream) . Of course, as with
other senses, there are other (older) pathways than the
principal cortical one, but we will focus on the cortex
General Principal 4
Transduction (Receptors)
1- The first step in turning a specific type of energy into an
electrical signal differs between modalities, after that all
systems follow similar principals.
2- in most sensory systems, physical energy (e.g. light) is
changed (transduced) into neural impulses. The way that
the system does this is dictated by the type of energy that
is being transduced
3- transduction occurs in the sensory receptors
4- the receptor potential is produced by an opening of
cation (Na+ and K+) channels similar to the synaptic
potential. If sufficient these generate an action potential
through the process of encoding, which is transmitted to
second order projection neurons in the CNS.
General principal 5 :Parallel pathways convey
information
1- higher order sensory processing occurs in the cortex
2- with the exception of olfaction, sensory systems project
to the thalamus prior to the cortex
3- the path by which sense information is transmitted to
the cortex is called the sensory pathway or sensory tract
4- synapses along the way are called sensory relays
5- all sensory tracts (except olfaction) relay in the
thalamus
6- all sensory systems terminate in a primary sensory
region of the cortex
================= FINIS ===============
Dr Mahmoud Ahmad Fora