Download Week 2 Section Handout

Proprioception, Vestibular System, and Mechanoreception
COGS107B – Week 2 Section Handout
TA: Tim Mullen
Disclaimer: These handouts are intended only to supplement the lecture and reading. While I will try to be as factually accurate as
possible, I am not responsible for any errors that might be made (although please let me know if there are mistakes). If there is a
conflict between what is said here and what you learned in lecture or the text, then always err on the side of the lecture and text.
Principles:
Topographic Representation: a systematic arrangement of neurons in the brain such
that neurons representing similar features of a single sensory modality (e.g., frequency in
audition or spatial location of a stimulus in vision) are grouped in the same space in the
brain. A key feature of this is high interconnectivity between members of each group.
These groups are, in turn, organized in a systematic fashion.
(Some) Key Terms:
topographic representation, homunculus, otolith organs, utricle, saccule, proprioception,
dorsal root ganglion, generator potential, muscle spindle, golgi tendon, intrafusal fibers,
Pinocchio effect, vestibular system, hair cell receptors, semicircular canals, vestibularocular reflex, knee-jerk reflex, mechanoreceptor, Meissener’s corpuscle, Pacinian
corpuscle, Merkel disks (SA1 afferents), Ruffini (SA2) receptors, response fields
(surround inhibition (complete vs. partial), small vs. large, whole vs. patchy),
sustained/persistent/static vs. transient/dynamic response, slow vs. rapid adaptation, twopoint discrimination test, “Braille” test, somatosensory cortex (primary vs. secondary),
serial vs. parallel processing, cortical column, thalamus.
Proprioception
This week’s lectures covered the proprioceptive, vestibular, and mechanoreceptive
systems. Proprioception is the awareness of the position of the body and limbs. The major
functions of proprioception (broadly defined) include:




joint-protecting reflexes (e.g., knee-jerk)
adjustment of muscle contraction / recruitment
kinesthesia: detection of body position and movement
coordination of motor commands
The two main proprioceptors discussed in lecture are muscle receptors and tendon
receptors. Muscle spindle afferents (muscle receptors) are arranged in parallel with the
muscle fibers (terminating on the intrafusal muscle fibers) and respond when the muscle
is stretched. A key point is that the response sensitivity of these receptors is related to the
relative (rather than absolute) elongation of the muscle. Golgi tendon organs (tendon
receptors) terminate on the collagen fibers of tendons, which are arranged in series with
muscles. These receptors respond to tendon stretch and therefore signal muscle
contraction (since tendons connect individual muscles, muscle contraction leads to
tendon stretch). A key point here is that Golgi tendon organs respond persistently as the
tendon is stretched. Muscle spindles respond transiently to changes in relative elongation
of the muscle (returning to baseline once changes in muscle elongation cease). Golgi
tendon organs respond in a sustained fashion the entire time the muscle is contracted
(tendon is stretched). As such, muscle spindles register changes in the dynamic state of
the muscle system while Golgi tendon organs register the persistent or static state of the
musculature.
The knee-jerk reflex is a classic
example of a simple
proprioceptive circuit. Striking
the patellar ligament causes
stretching of the quadriceps
tendon which causes
patella
depolarization of Na+ channels in
muscle spindle afferents within
the quadriceps muscle (recall,
muscle spindles register muscle
elongation). Action potentials
propagate up the axon of the
connected dorsal root ganglion
cell. This axon splits in the
dorsal (posterior) horn of the
(inhibitory)
spinal cord. One branch synapses
directly onto a motor neuron which conducts an efferent (excitatory) impulse back to the
quadriceps muscle causing it to contract (this is called a monosynaptic reflex arc). The
second branch synapses onto an inhibitory interneuron which in turn synapses onto a
motor neuron connected to the antagonistic hamstring muscle. Inhibition of this motor
neuron (by the interneuron’s release of glycine) causes relaxation of the hamstring
muscle. The simultaneous contraction of the quadriceps muscle and relaxation of the
hamstring muscle causes the lower leg to kick outward.
muscle
spindle
The Vestibular System
The vestibular system performs the following functions:
 postural reflexes
 gaze adjustments
 assessment of self-motion (kinesthesia)
The hair cell receptor is the primary receptor underlying vestibular system function.
This is the same cell type that is used in audition, although the function is different in the
vestibular system. Hair cells consist of a series of hair-like stereocilia with a single large
kinocilium (the tallest sterocilium). Mechanically-gated ion channels (permeable to Ca+
and K+) on a stereocilium are connected to channels on a neighboring stereocilium by a
spring-like process. Bending the hair cell towards the kinocilium increases the proportion
of open channels, depolarizing the cell, while bending it away from the kinocilium
decreases the proportion of open channels, hyperpolarizing the cell. We saw that hair
cells in the semicircular canals respond transiently to dynamic changes in head rotation
about the trunk. In contrast, hair cells in the otolith organs (the utricle and saccule)
respond in a sustained fashion to static head position. Remember that rotating the head in
one direction excites hair cells in the semicircular canals ipsilateral to (in the direction of)
head movement while inhibiting hair cells in the contralateral (opposite) semicircular
canals. The vestibulo-ocular reflex (VOR) is based on this antagonistic
excitation/inhibition.
Mechanoreceptors
Mechanoreceptors are responsible for tactile sensation. As the name suggests, these
receptors are mechanically activated. Pressure, vibration, and stretch activate these
receptors allowing detection of form, texture, object movement across the skin, etc. In
lecture we discussed the 5 major types of mechanoreceptors: Pacinian corpuscles,
Meissner’s corpuscles, Merkl disks (SA1 afferents), hair receptors, and Ruffini cells
(SA2 afferents). The first 4 are the most important to know about. You should know the
properties of these receptors and what stimuli they respond to. Table 1 is a reproduction
of the properties table from Prof. Nitz’s lecture. Try to fill in all the blanks (the
completed table is in the slides). If you can do this without looking at the answers, you’re
doing well.
Table 1. Mechanoreceptor properties. (Fill in the blanks).
Mechanoreceptors
type
RA/SA
depth
response field
sensitivity
info. processed / best
stimulus
Pacinian
----------------------
Meissner
Merkl
SA2
SA
deep
small (12-25
mm)
----------------------
hand shape / stretch
hair
Mechanoreceptors can be classified as rapidly adapting (RA) or slowly adapting (SA).
RA receptors respond transiently to a stimulus, generally firing a burst of action
potentials only at the onset and offset of a stimulus. SA receptors exhibit a sustained
response to a stimulus, showing an increased firing rate for the entire duration of the
stimulus.
We discussed a few experiments which are used to evaluate the properties of
mechanoreceptors. One of these was the two-point discrimination test. This test
involves indenting the skin simultaneously at two points and measuring the distance
between the two points required for the subject to identify these as two separate points
(rather than a single point). This two point test is repeated all over the body providing us
with information about the density of pressure-sensitive mechanoreceptors over the body
and the number and type of connections these receptors make in the brain. In order to
discriminate between two nearby points, receptors must be packed tightly enough for at
least two different receptors to be innervated. But this is not enough; these receptors must
also connect to different groups of neurons in the somatosensory cortex (S1). This means
that areas of the body with higher receptor density will generally correspond to enlarged
areas of representation in S1. The minimum distance at which two points can be
distinguished from each other is approximately 1mm. Do you recall where on the body
sensitivity is the highest and where it is the lowest? As a hint, think about how the
somatosensory homunculus is distorted – what parts of the body are enlarged and what
parts are shrunken? Also, what mechanoreceptor responds well to points, and what does
this experiment tell us about the density (over the body) and receptive field size of this
type of mechanoreceptor?
The Braille test is another experiment which involves scanning a plate embossed with a
Braille pattern beneath the stationary fingertip of a human. In the experiment mentioned n
the slides, action potentials (APs) are recorded from four different types of
mechanoreceptors as the plate is scanned over the surface of the fingertip. At each point
on the plate, if an AP is generated there, we make a dot at the corresponding point. Thus,
mechanoreceptors that respond well to fine texture and points (what type of receptor does
this?) will track the Braille pattern faithfully – the AP pattern for this cell will look very
much like the Braille pattern. Mechanoreceptors that are not pressure-sensitive will not
show any response (e.g., SA2 afferents) and those with broad receptive fields will show a
diffuse activation pattern (e.g., Pacinian corpusles).
A key point from this week’s lecture was the difference between transient/dynamic and
sustained/persistent/static response properties of sensory receptors. We’ve seen that
across the vestibular, proprioceptive, and mechanoreceptive systems, some cells respond
transiently, encoding for dynamically changing information while others respond
persistently (sustained), encoding for static information. Table 2 gives a breakdown of
this information.
Table 2. Response properties of sensory receptors: transient/dynamic vs. persistent/sustained/static
Proprioceptive
Vestibular
Mechanoreceptive
Golgi
muscle
otolith
semicircular SA
RA
tendon
spindle
organs
canals
receptors
receptors
organs
afferents
persistent/sustained/
X
X
X
static
transient/dynamic
X
X
X
From Sensation to Integration (Sensory Pathways to the Cortex)
A final important point from this week is the pathways from sensory receptors through
dorsal root ganglia to the cortex. Stimulation of sensory receptors causes action potentials
to propagate up the axon of the connected dorsal root ganglion cell to the dorsal horn of
the spinal cord where the axon splits. One branch directly innervates motor neurons in the
ventral horn of the spinal cord (remember the knee-jerk reflex?) The other branch
projects all the way up to the medulla
oblongata in the caudal (towards the feet)
portion of the brainstem. There it creates its
first synapse onto neurons in the gracile
nucleus. The axons of these second-order
sensory neurons then decussate (cross over)
to the contralateral (opposite) side of the
medulla and form a column projecting up to
the thalamus where they in turn synapse
onto third-order sensory neurons in the
ventral posterolateral nucleus of the
thalamus. These neurons then project to the
somatosensory cortex in the postcentral
gyrus, where information from multiple
sensory pathways is integrated.
What is particularly important to note is that
information from different receptor types is
largely segregated until it reaches the
somatosensory cortex. For example, rapid
adapting (RA) and slowly adapting (SA)
mechanoreceptors project information to the cortex through independent pathways. This
information is then combined in the cortex in (at least) two ways. The first is through
parallel cross-region connectivity in primary and secondary somatosensory cortex (S1
and S2, respectively). For example, sub-region 2 of S1 receives input in parallel from
sub-region 3a (which in turn receives input from SA receptors) and sub-region 3b (which
receives input from RA receptors). Thus information from SA and RA receptors may
become integrated in sub-region 2. S2 also receives parallel input from multiple regions
of S1. Another mode of integration is through within-region connectivity in the cortical
columns of the somatosensory cortex. Vertical connections between different layers in a
cortical column allow information projecting to distinct layers to become integrated. For
example information from RA receptors which projects to cortical layer IV is relayed to
layer II which also receives input from SA receptors. Horizontal connections in layer II
may then allow information from SA and RA receptors to be combined.
Good luck on your quiz next week! Listen to the podcasts, review the lecture slides,
and consult the textbook for additional information and you will do well.
References
Patellar Reflex. Wikipedia. retrieved Jan 20, 2009 from http://en.wikipedia.org/wiki/Patellar_reflex
Squire, L. R., Berg, D., Floyd, B. E., du Lac, S., Ghosh A., Spitzer, N, C. (2008) Fundamental
Neuroscience. 3rd ed. Elsevier. Burlington, MA.
Nitz, D., COGS 107B Lecture Slides. http://www.cogsci.ucsd.edu/~nitz/