1. Intrafusal fibers and muscle spindles
Figure Intrafusal fibers are modified skeletal muscle fibers that do not stretch the length of the muscle, as extrafusal fibers (ef) do. Intrafusal fibers do not contribute to the force produced by skeletal muscle contraction but rather house muscle stretch receptors. Groups of intrafusal fibers are contained in a capsule called the muscle spindle.
A: Muscle spindles (green oblongs) are scattered throughout skeletal muscle.
B: A tangential view of a muscle spindle surrounded by extrafusal fibers is shown.
C: A cross-section through a muscle spindle shows that the diameters of intrafusal fibers are much smaller than those of extrafusal fibers.
D: Each intrafusal fiber contains an equatorial region surrounded by two polar regions. A mechanoreceptive sensory afferent called the Ia afferent weaves around the equatorial region of an intrafusal fiber. Stretch of this middle portion of the intrafusal fiber excites the Ia afferent. The polar regions of intrafusal fibers are contractile and contract in response to input from a γ-motoneuron (not shown).

2. Stretch reflex
Figure 22-2B: The essential circuits of the stretch reflex are diagrammed. A minimum of anatomical details are illustrated in order to most clearly convey the essential circuitry. Two types of neurons form the fundamental stretch reflex circuit. The endings of Ia afferents wrap around intrafusal fibers (if) and sense stretch. Ia afferents cross from the periphery into the central nervous system (dotted line) and directly contact α-motoneurons (α) that project into the periphery to innervate extrafusal fibers (ef) in the same or homonymous muscle, as well as in synergist muscles. Synergist muscles are those that work in concert to produce the same action. The fundamental stretch reflex circuit contains one central synapse, the synapse from the Ia afferent to an α-motoneuron.

3. Inhibition of antagonist muscles
Figure 22-2B: Muscle stretch also results in the inhibition of antagonist muscles. The inhibition of antagonist muscles is accomplished through activation of the glycinergic Ia inhibitory interneuron (Ia ii). The inset (blue box) shows an agonist–antagonist pair of muscles. The members of an agonist–antagonist muscle pair move a joint in opposing directions.

4. EMG testing: H reflex and M response
Figure The basic setup and ideal results of electromyography, or EMG, testing are illustrated.
A: Electrical stimulation of a peripheral nerve has the potential to excite the axons of Ia afferents (blue labeled Ia) and motoneurons (red labeled α). The effect of nerve stimulation on muscle activity is measured using EMG recording from the muscle. B–D: The height of the action potentials is proportional to the number of axons excited.
B: At the lowest stimulation intensities (small black arrow), only Ia afferents are excited. Electrical stimulation produces action potentials that travel away from the stimulation electrode toward either side. Action potentials that result directly from stimulation are denoted by a filled circle above. The action potentials in Ia afferents that travel toward the muscle have no effect. The Ia afferent action potentials that travel orthodromically toward the spinal cord lead to the excitation of α-motoneurons after a synaptic delay of about a half millisecond. Excitation of α-motoneurons leads to a muscle contraction after a delay of several milliseconds, depending on conduction distance to the muscle. This muscle contraction is termed the H reflex.
C: At moderate stimulation intensities (medium black arrow), more Ia afferent axons along with some motoneuron axons are excited. Action potentials are color coded according to whether they occur as a result of excitation of Ia afferent axons (blue) or of motoneuron axons (red). Electrical stimulation produces action potentials that travel away from the stimulation electrode on either side of both Ia and motoneuron axons. The orthodromically traveling action potentials in motoneuron axons reach the muscle and elicit a muscle contraction. This is termed the M response. The M response occurs much earlier than the H reflex because: (1) there is no synaptic delay; and (2) the conduction distance and time are far less. There are also action potentials that travel antidromically in the motoneuron axon. These action potentials collide with action potentials arising from the synaptic responses in motoneurons to Ia afferent input. Since action potentials that collide (x) do not continue on, only a portion of the action potentials continue to travel orthodromically in the motoneuron axons. The result is a small M response followed by an H reflex.
D: At the highest stimulation intensities (large black arrow), all Ia afferent and motoneuron axons are excited. Therefore, all of the action potentials arising from synaptic responses in motoneurons to Ia afferent input collide with action potentials traveling antidromically in the motoneuron axon (large x). Consequently, there is a large M response and no H reflex.

5. α-γ co-activation
Figure 22-4A-C: In the relaxed state (left), intrafusal fibers (if) are taut and sensitive to stretch. However, if an -motoneuron were to be activated alone (middle), the extrafusal fibers (ef) would contract but the intrafusal fibers would go slack. In this situation, a load would have no effect on an intrafusal fiber. Consequently, Ia afferents would go “off-line” as they could no longer sense stretch. - coactivation (right) resolves this problem by maintaining the intra- and extrafusal fibers at matching lengths.
The basic setup and ideal results of electromyography, or EMG, testing are illustrated. A: Electrical stimulation of a peripheral nerve has the potential to excite the axons of Ia afferents (blue labeled Ia) and motoneurons (red labeled α). The effect of nerve stimulation on muscle activity is measured using EMG recording from the muscle. B–D: The height of the action potentials is proportional to the number of axons excited. B: At the lowest stimulation intensities (small black arrow), only Ia afferents are excited. Electrical stimulation produces action potentials that travel away from the stimulation electrode toward either side. Action potentials that result directly from stimulation are denoted by a filled circle above. The action potentials in Ia afferents that travel toward the muscle have no effect. The Ia afferent action potentials that travel orthodromically toward the spinal cord lead to the excitation of α-motoneurons after a synaptic delay of about a half millisecond. Excitation of α-motoneurons leads to a muscle contraction after a delay of several milliseconds, depending on conduction distance to the muscle. This muscle contraction is termed the H reflex.
C: At moderate stimulation intensities (medium black arrow), more Ia afferent axons along with some motoneuron axons are excited. Action potentials are color coded according to whether they occur as a result of excitation of Ia afferent axons (blue) or of motoneuron axons (red). Electrical stimulation produces action potentials that travel away from the stimulation electrode on either side of both Ia and motoneuron axons. The orthodromically traveling action potentials in motoneuron axons reach the muscle and elicit a muscle contraction. This is termed the M response. The M response occurs much earlier than the H reflex because: (1) there is no synaptic delay; and (2) the conduction distance and time are far less. There are also action potentials that travel antidromically in the motoneuron axon. These action potentials collide with action potentials arising from the synaptic responses in motoneurons to Ia afferent input. Since action potentials that collide (x) do not continue on, only a portion of the action potentials continue to travel orthodromically in the motoneuron axons. The result is a small M response followed by an H reflex.
C: At the highest stimulation intensities (large black arrow), all Ia afferent and motoneuron axons are excited. Therefore, all of the action potentials arising from synaptic responses in motoneurons to Ia afferent input collide with action potentials traveling antidromically in the motoneuron axon (large x). Consequently, there is a large M response and no H reflex.

6. The gamma loop
Figure 22-4D: The γ loop starts with γ-motoneuron (γ-mn) activation (brown arrows marked 1). The effect of γ-motoneuron activation is contraction of the polar ends of the intrafusal fibers (see inset in blue box). Contraction of the polar ends of the intrafusal fibers stretches the equatorial region of the intrafusal fibers, resulting in Ia afferent activation (blue arrows marked 2). Ia afferent activation leads to excitation of -motoneurons (α-mn). Activity in -motoneurons (red arrows marked 3) leads to extrafusal fiber contraction and muscle tension. Thus, the γ-loop starts with γ-motoneuron activation and culminates in muscle tension that is dependent on sensory activity in Ia afferents.

7. Ib reflex
Figure Ib afferents weave in and out through the Golgi tendon organs located at the junction of tendon and muscle. During active contraction, but not during passive stretch, the Golgi tendon organs are stretched and Ib afferents are excited. Ib afferents excite inhibitory interneurons, which in turn inhibit homonymous α-motoneurons.

8. Learning somatosensation
Figure A: Sensory afferents from the foot synapse on reflex encoder neurons (RE) in the deep dorsal horn. The input from different areas of the foot arrives via synapses of different weights (proportional to the width of the line). The most heavily weighted inputs to reflex encoders are from areas of skin that are withdrawn by the contraction of the motoneurons targeted by those reflex encoder neurons. In this way, the receptive field of a reflex encoder neuron maps onto the withdrawal field produced by activation of the same reflex encoder neuron.
B: At birth, inputs from wide areas of the skin make equally weighted synapses on reflex encoder neurons. During sleep, a spontaneous burst of activity in a reflex encoder neuron leads to contraction of the targeted muscle. The synapses from afferents that increase their activity after muscle contraction are weakened or eliminated. In contrast, synapses from afferents that decrease their activity after muscle contraction are strengthened. This process of somatosensory imprinting leads to mature circuits in which receptive and withdrawal fields overlap.
Modified from Schouenborg, J. Action-based sensory encoding in spinal sensorimotor circuits. Brain Res Rev 57: 111–17, 2008 with permission of the publisher, Elsevier; and from Sonnenborg, F.A., Andersen, O.K., and Arendt-Nielsen, F. Modular organization of excitatory and inhibitory reflex receptive fields elicited by electrical stimulation of the foot sole in man. Clin Neurophysiol 111: 2160–9, 2000, with permission of the publisher, Elsevier.

9. Walking cycle
Figure The walking cycle of a young infant is illustrated. The cycle proceeds in a clockwise direction. This child’s gait is not fully mature, but shows the essential features of human locomotion. Each leg alternates between stance (solid lines in the circle) and swing (dotted lines). Stance starts at heel strike and swing starts with lift off. For example, the left leg (red) begins the stance phase with a heel strike as the right leg (blue), is still in contact with the ground. Weight transfers to the left leg as the body is propelled forward. As the left leg assumes the weight of the body, the right leg begins the swing phase and so on. Between the left and right legs’ swings, both legs are in contact with the ground, a period of double support (black arcs). The immaturity of the gait is evident in the minimal arm swing, the downward head posture, and less ankle flexion than is typically present in a healthy adult.
Inspired by the work of Muybreadge, E. The human figure in motion. New York: Dover, 1955.

10. Stretch reflex during locomotion
Figure 22-8A–B: Feedback from Ia afferents innervating the iliopsoas (red) and quadriceps (blue) muscles signals the stretch caused by hip extensors and excites the homonymous muscles contributing to the hip flexion needed for lift off. In A, both muscles are short and neither is stretched. When the leg extends back (B), extending the hip joint, both muscles are stretched. Stretching these muscles facilitates the hip flexion needed for take off.

11. Ib reflex excites during walking
Figure 22-8C: During stance, the quadriceps (blue) and gastrocnemius (tan) muscles contract to bear the weight of the body and load. The stronger the contraction, the more excitation of Ib afferents occurs. During locomotion, Ib activation leads to excitation of the homonymous muscles disynaptically. Note that the excitatory effect of Ib afferents on the homonymous muscle only occurs during locomotion and is exactly opposite to the effect mediated by the Ib reflex at other times.