Download SOMATOSENSORY SYSTEM

Point Localization
•Receptor density
•Size of receptive
fields
•Dermatomes
•Somatosensory
topography
•Cortical area
Unipolar (DRG) cell: Axonal Fibers
Fiber Classifications
GENERAL Classification system
Fiber Type
Aalpha
Abeta
Agamma
Adelta
B
Diameter
3-20 (um)
6-12
3-6
2-5
<3
C(nonmyelinated)0.1-1.5
Speed
75-120 m/s
30-75
15-30
12-30
Spike Duration
0.4-0.5 ms
Abs. Ref. Per.
0.4-1 ms
3-15
1.2
1.2
0.5-2.3
2
2
Sometimes used for SENSORY neurons.
Number
Ia
Ib
II
III
IV
Fiber Type
Aalpha
“
Abeta
Adelta
C (dorsal root)
Origin
Muscle spindle, annulospiral ending
Golgi Tendon Organ
Muscle spindle, flowerspray ending, touch, pressure
Pain and cold receptors; some touch receptors
Pain, temperature, and other receptors
12.9
Homunculus at Cortex
12.16
Which side of the body?
Cortical columnar organization and receptive fields
Each area (Brodmann’s) receives
input from one type of receptor
from a specific body part.
12.17
Cells in column share same
central location on the skin.
RA= Rapid adaptation
SA= Slow adaptation
Note overlapping
receptive fields of
receptors.
Clinical symptoms
Consider the following:
1. What symptoms would occur
at each site of injury?
4
2. Conversely, if you were given
a set of symptoms, could you
deduce the injury site, which
side, etc?
3
Sites of injury:
1. Lumbar (L5)
2. Cervical (C2-C3)
3. Brainstem at tectum
4. Cortical injury
1
2
12.30
Proprioception: sensation of body position and movement
using sensory signals from muscles, joints, and skin.
Neural information needed by CNS to regulate a muscle contraction
1. Initial length of the muscle
2. Velocity (rate) of length change
3. Loads (mass) acting in opposition
Primary Proprioceptors:
1. Muscle spindle organ - two basic types of receptors in the muscle
a. Nuclear bag ending or annulospiral ending or primary ending.
b. Myotube or flowerspray or secondary ending
2. Golgi tendon organ
Input from these receptors (via unipolar neurons) reaches all levels of CNS.
1. At spinal cord, sensory input is used to control reflexes.
2. At cortex, this information is used
a. to detect location of a limb
b. for controlling voluntary movements.
Muscle spindle organ: Nuclear Bag ending
• Parallel to extrafusal muscle fibers
• Nuclear Bag ending or Primary ending*
– Detects instantaneous length and
speed of stretch
• Dynamic: responds during dynamic
phase of stretch
• Fast adapting
• AP rate increases when there is an
increase in length of muscle or
rate of change in length.
– Resets to new length or rate quickly
so it can respond to next change.
– Innervation:
• Aα (Ia – see Fiber table)
• One fiber/ spindle
• Central portion of nuclear bag.
Muscle spindle organ: Nuclear Chain
• Nuclear Chain ending or Secondary
ending*
• Detects instantaneous length during static
phase
– “Static” response best but also responds
to onset of stretch.
– Slow adapting
– Increase in AP rate when there is increase
in length of muscle
• Innervation:
– Aβ(Ib – see Fiber table)
– Several Chains/spindle
– Several fibers/spindle
– Peripheral portion of sensory zone
• Movement of muscle, limb, reflexes,
voluntary motion, etc., etc., etc.
Golgi Tendon Organ
• Series with extrafusal muscle fibers
• Detects change in tension:
–Either stretch or contraction.
–Contraction: stronger response (in
stretch, muscle fibers absorb more
change in tension; contraction- muscles
pull on GTO).
• Innervation: Aβ fiber (Ib)
• Spinal cord connection:
–Interneurons
–Inhibition of agonistic muscle
–Excitation of antagonistic muscle
Example: strength of grip (steady),
compensation for fatigue, maintaining
posture, maintaining sensitivity of
spindle.
GTO and autogenic inhibition
•
•
•
Ib afferent Action Potential
(AP)
Synapse on Interneuron
Interneuron inhibits alpha
motor neuron to the same
muscle.
Reflexes: Myotatic reflex (stretch)
Reflexes: rapid, involuntary, stereotypical movement
Myotatic reflex: increase in contraction and shortening of muscle to compensate for
sudden increase in weight stretching the muscle.
Patellar reflex I
Reflexes
1. Least affected by voluntary control
2. Graded response related to intensity
of stimulation
The tap:
1. Monosynaptic
2. Tap lengthens quadriceps, stretches
spindle.
3. AP in Ia fiber to spinal cord.
4. Excitation of alpha motor neuron.
5. ACh released at endplate.
6. Extrafusal skeletal muscle fibers
contracted, shortens spindle.
Patellar reflex II: reciprocal inhibition
Then:
1. Contraction of muscle causes increased
tension of tendon.
2. Exceeds threshold of GTO and generates
action potentials.
3. Inhibits alpha motor neurons of
quadriceps, excites antagonist.
4. Movement in opposite direction.
Flexor Reflex
•
•
•
Multiple segments
Polysynaptic
Normally inhibited by descending influences overridden by nociception
13.24
Cross-extensor Reflex
•
Flexor reflex of ipsilateral
limb to the nociception.
•
Adds excitation of
contralateral limb
(quadriceps) via
interneuron to help support
body.