Muscle spindle structure
in mid-portion of spindle is a connective tissue capsule which expands
in its central region to form a fluid-filled space
two types of muscle fibers in spindle
2-3 bag fibers
3-5 chain fibers (Figure 4.1, page 87)

Muscle spindle structure
three types of nerve endings on spindles
primary
secondary
plate (motor)
primary (Ia) afferent - large diameter neurons
secondary (II) afferent - smaller diameter neurons
motor (y - gamma) - efferent - small diameter neurons

Muscle spindle afferent responses
cannot distinguish between Ia & II responses during sustained
contraction since receptors fire at same rate
Ia fibers conduct at 80-120 ms-1 (in cat)
II fibers conduct at 20-70 ms-1 (in cat)
humans never conduct > 80 ms -1
spindles are more plentiful in the hand than in more proximal muscles

Muscle spindle afferent responses
Ia receptor is very sensitive to dynamic phase of stretch
at onset of stretch the discharge frequency jumps to new value and
continues to increase throughout stretch (Figure 4.3, page 90)
Dynamic Index - at end of stretch there is a decline is firing rate for
about 500 ms until a new steady state is reached - the difference
between the rate at the end the stretch and the new steady state is the
dynamic index

Muscle spindle afferent responses
Ia is considered velocity-sensitive due to large increase in firing
rate when stretch is detected
II is considered more length-sensitive (Figure 4.3, page 90)
notice that Ia is also length-sensitive based on its change in firing
rate in response to stretch (Figure 4.3, page 90)

Muscle spindle afferent responses
it is possible to record from nerve fibers in awake humans -
advantages?
spindles may respond to arterial pulse & respiratory movements
Ia can signal vibration 1 to 1 up to 200 Hz
II can signal vibration 1 to 1 up to 100 Hz

Muscle spindle response to gamma motoneuron activity
Ia responds in two ways to gamma stimulation
ys - > resting discharge rate and static sensitivity, < dynamic
sensitivity
yd - > resting discharge rate but less than ys stimulation, primary
effect is to produce large dynamic response of Ia

Muscle spindle response to gamma motoneuron activity
in response to ys stimulation, II endings respond with > resting
discharge rate & > static sensitivity
II endings are unaffected by yd stimulation
contraction of any type of spindle fiber occurs in the capsular sleeve
- this contraction results in a stretch of the central region causing an
> in resting discharge rate

Responding to small displacements & after-effects
spindle responds to very small displacements
much less sensitive to larger displacements
thus, spindle is a non-linear receptor (Figure 4.8, page 97)
when given a large stretch, spindle sensitivity <, when new length is
reached, spindle is rapidly reset

Responding to small displacements & after-effects
spindle output to a given amount of stretch is not always constant
series of rapid stretches, the response to a slow test stretch depends
on the length at which muscle was held after stretches
ex. if muscle is elongated for several seconds then returned to initial
length, the response is smaller with longer latency when given test
stretch compared to when the muscle is returned to its initial length
after series of stretches - more slack in sensory region

Golgi tendon orgrans
first described in detail by Golgi in 1880
GTOs lie in series with the fibers located in the musculotendinous
junction
spindle-shaped connective tissue capsule which encloses collagen
strands and terminals of the afferent fiber (Fig. 4.10 page 101)
innervated by group Ib large diameter afferent fibers
usually more spindles than GTOs (2:1)

Golgi tendon organ - function
stimulus for GTOs is mechanical deformation generally in the form of a
muscle contraction
originally thought to be high threshold receptors
ery sensitive to actively generated muscle force (threshold - 0.1g)
current view is GTOs important in signalling variations in contractile
force rather than steady-state force level

Golgi tendon organ - afferent responses
during a tendon tap spindles respond to initial stretch when GTOs are
quiet
GTOs respond during active contraction when spindles are quiet

Paciniform corpuscles & free nerve endings
Paciniform corpuscles often in musculotendinous junction - innervated
by group II (large diameter) fibers
rapidly adapting organs with sensitivity to high frequency vibration
may number 30% of the number of spindles in a muscle

Paciniform corpuscles & free nerve endings
free nerve endings are plentiful throughout all muscle structures
all non-medullated & group III small diameter medullated fibers
terminate as FNEs
activated by high threshold mechanical stimulation, called
åpressure-painÇ receptors
also respond to nociceptive stimuli ò ischemia

Joint receptors
three main types of joint receptors
free nerve endings - most numerous of joint receptors - located in
connective tissue - group III & small non-medullated fibers
Golgi endings - located in joint ligaments - group I (large) fibers
Ruffini endings - located in joint capsule - group II fibers

Joint receptors
vast majority do not signal static joint angle
majority have no response in the angular mid-range but fire at the
extremes of joint rotation
many receptors signal extremes ranges of both flexion and extension
thus, precise role of joint receptors is unclear

Cutaneous mechanoreceptors
three types of cutaneous receptors
thermoreceptors
nociceptors
mechanoreceptors
mechanoreceptors have large role in movement control, particularly in
the hands & feet
unclear how mechanoreptors are used in movement control

Cutaneous mechanoreceptors - types
four types of cutaneous mechanoreceptors
Merkel discs - upper layers (of skin)
Meissner corpuscles - upper layers
Ruffini endings - deeper layers
Pacinian corpuscles - deeper layers (Figure 4.15, page 111)

Cutaneous mechanoreceptors - function
Merkel discs - responds to localized vertical pressure - does not
respond to lateral stretch
Meissner corpuscles - responds to maintained pressure, rapidly adapting
Ruffini endings - responds to stretch over a wide area - slowly
adapting - may signal limb position

Cutaneous mechanoreceptors - function
Pacinian corpuscles - responds to mechanical deformation - acts as high
pass filter - thus, signals only high frequency stimuli (e.g. vibration)
Humans can detect firing of single impulse of afferent fiber caused by
mechanical deformation at finger tips

Sensations of limb position
Does spindle activation contribute to conscious sensation of limb
position and/or movement?
stimulation of single spindle fibers does not reach consciousness
evidence that activation does contribute to sensation
vibration (Figure 4.17, page 121)
tendon pulls

Sensations of limb position
joint receptors provide limited information about joint position -
middle finger evidence (page 122-3)
important to consider whether you are testing sense of position or
sense of movement