Page 329 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Proprioception, Stretch Reflex Monosynaptic stretch reflex (! C). Muscles
spindles are also affected by sudden stretching
Proprioception is the mechanism by which we of a skeletal muscle, e.g. due to a tap on the ten-
sense the strength that our muscles develop as don attaching it. Stretching of the muscle
well as the position and movement of our body spindles triggers the activation of type Ia affer-
and limbs. The vestibular organ (! p. 342) and ent impulses (! B2, C), which enter the spinal
cutaneous mechanosensors (! p. 314) assist cord via the dorsal root and terminate in the
ventral horn at the α motoneurons of the same
the propriosensors in muscle spindles, joints
Central Nervous System and Senses Muscle spindles (! A1) contain intensity (P) only one synaptic connection. The reflex time
muscle. This type Ia afferent input therefore
and tendons. Sensors of Golgi tendon organs
are located near muscle–tendon junctions.
induces contraction of the same muscle by
and differential (D) sensors for monitoring of
for this monosynaptic stretch reflex is there-
joint position and movement. The velocity of
fore very short (ca. 30 ms). This is classified as a
proprioceptive reflex, since the stimulation and
position change is reflected by a transient rise
in impulse frequency (D sensor; ! p. 315 D1,
response arise in the same organ. The mono-
spike), and the final joint position is expressed
synaptic stretch reflex functions to rapidly cor-
as a constant impulse frequency (P-sensor,
rect “involuntary” changes in muscle length
and joint position.
! p. 315 D2, plateau). Muscle spindles func-
tion to regulate muscle length. They lie parallel
Supraspinal activation (! B3). Voluntary
to the skeletal muscle fibers (extrafusal muscle
activation of α and γ neurons. The latter adjust
the muscle spindles (length sensors) to a cer-
trafusal muscle fibers). There are two types of
12 fibers) and contain their own muscle fibers (in- muscle contractions are characterized by co-
tain set-point of length. Any deviations from
intrafusal muscle fibers: (1) nuclear chain
fibers (P sensors) and (2) nuclear bag fibers (D this set-point due, for example, to unexpected
sensors). The endings of type Ia afferent neu- shifting of weight, are compensated for by re-
rons coil around both types, whereas type II adjusting the α-innervation (load compensa-
neurons wind around the nuclear chain fibers tion reflex). Expected changes in muscle
only (neuron types described on p. 49 C). These length, especially during complex movements,
annulospiral endings detect longitudinal can also be more precisely controlled by (cen-
stretching of intrafusal muscle fibers and re- trally regulated) γ fiber activity by increasing
port their length (type Ia and II afferents) and the preload and stretch sensitivity of the intra-
changes in length (Ia afferents) to the spinal fusal muscle fibers (fusimotor set).
cord. The efferent γ motoneurons (fusimotor
fibers) innervate both intrafusal fiber types, al- Hoffmann’s reflex can be also used to test the
stretch reflex pathway. This can be done by position-
lowing variation of their length and stretch ing electrodes on the skin over (mixed) muscle
sensitivity (! A1, B1). nerves and subsequently recording the muscle con-
Golgi tendon organs (! A2) are arranged in traction induced by electrical stimuli of different in-
series with the muscle and respond to the con- tensity.
traction of only a few motor units or more. Polysynaptic circuits, also arising from type II af-
Their primary function is to regulate muscle ferents complement the stretch reflex. If a stretch re-
tension. Impulses from Golgi tendon organs flex (e.g., knee-jerk reflex) occurs in an extensor
muscle, the α motoneurons of the antagonistic
(conveyed by type Ib afferents), the skin and flexor muscle must be inhibited via inhibitory Ia inter-
joints, and muscle spindles (some of which are neurons to achieve efficient extension (! D1).
type Ia and II afferent fibers), as well as de- Deactivation of stretch reflex is achieved by in-
scending impulses, are jointly integrated in hibiting muscle contraction as follows: 1) The muscle
type Ib interneurons of the spinal cord; this is spindles relax, thereby allowing the deactivation of
referred to as multimodal integration (! D2). type Ia fibers; 2) the Golgi tendon organs inhibit the
Type Ib interneurons inhibit α motoneurons of α motoneurons via type Ib interneurons (! D2); 3)
the muscle from which the Ib afferent input the α motoneurons are inhibited by the interneurons
(Renshaw cells; ! D4) that they themselves stimu-
originated (autogenous inhibition) and activate lated via axon collaterals (recurrent inhibition; ! D3;
316 antagonistic muscles via excitatory inter- p. 321 C1).
neurons (! D5).
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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