The orderly recruitment of α motor neurons, from smallest to largest, is referred to as the size principle. This principle ensures that weaker motor units, which are connected to smaller motor neurons, contribute to relative fineness of motor control during both weak and strong contractions.

A reflex is a simple and stereotyped motor response to a stimulus. The reflex arc consists of afferent fibers, interneurons, and motor neurons responsible for mediating the reflex.

Muscle spindles are complex sensory receptors present in skeletal muscle. They are located parallel to extrafusal muscle fibers and contain intrafusal muscle fibers known as nuclear bag and nuclear chain fibers. By being in parallel with the main muscle, the spindle can detect any changes in the length of the muscle.

Primary endings, formed by group Ia afferent fibers, are present on nuclear bag1, bag2, and chain fibers. On the other hand, secondary endings, formed by group II fibers, are found on nuclear chain and bag2 fibers.

Primary endings exhibit both static and dynamic responses that provide information about muscle length and the rate of change in muscle length. In contrast, secondary endings only display static responses and provide information solely about muscle length.

The γ motor neurons innervate the intrafusal muscle fibers associated with muscle spindles. Contraction of these intrafusal fibers does not directly cause significant changes in muscle tension or length. However, adjusting the tension level in these fibers allows γ motor neurons to influence the sensitivity of the muscle spindle to stretch.

Golgi tendon organs are located in the tendons of muscles and are arranged in series with the muscle. They receive innervation from group Ib afferent fibers. This in-series relationship allows tendon organs to detect the level of force generated by the muscle, whether it is due to passive stretch or active contraction.

The phasic stretch reflex, also known as the myotatic reflex, involves a monosynaptic excitatory pathway from group Ia afferent fibers in muscle spindles to α motor neurons that supply the same and synergistic muscles. Additionally, a disynaptic inhibitory pathway to antagonistic motor neurons is also present.

Golgi tendon organs evoke the inverse myotatic reflex. Afferent volleys in group Ib fibers from a particular muscle lead to disynaptic inhibition of α motor neurons to the same muscle and excitation of α motor neurons to antagonist muscles.

The flexion reflex is a crucial protective response that causes withdrawal of a limb from damaging stimuli. It is triggered by volleys in afferent fibers supplying various receptors, particularly nociceptors. Through polysynaptic pathways, these volleys result in excitation of flexor motor neurons and inhibition of extensor motor neurons on the same side. Simultaneously, the opposite pattern of action, inhibiting flexor and exciting extensor motor neurons, occurs on the opposite side, known as the crossed extension reflex.

Descending pathways can be categorized into the lateral system and the medial system. The lateral system includes the lateral corticospinal tract and a portion of the corticobulbar tract. These pathways influence the contralateral motor neurons supplying the musculature of the limbs, especially the digits, as well as the muscles of the lower part of the face and the tongue.

The medial system comprises the ventral corticospinal, lateral and medial vestibulospinal, reticulospinal, and tectospinal tracts. These pathways primarily affect posture and provide the motor background for movement of the limbs and digits.

While locomotion is triggered by commands relayed through the midbrain locomotor center, the detailed organization of locomotor activity is provided by central pattern generators formed by spinal cord circuits and influenced by afferent input.
The size principle refers to the orderly recruitment of α motor neurons according to their size, from smallest to largest. Because smaller motor neurons connect to weaker motor units, the relative fineness of motor control is similar for weak and strong contractions.
3.
A reflex is a simple, stereotyped motor response to a stimulus. A reflex arc includes the afferent fibers, interneurons, and motor neurons responsible for the reflex.
4.
Muscle spindles are complex sensory receptors found in skeletal muscle. They lie parallel to extrafusal muscle fibers, and they contain nuclear bag and nuclear chain intrafusal muscle fibers. By being in parallel to the main muscle, the spindle can detect changes in muscle length.
5.
Group Ia afferent fibers form primary endings on nuclear bag1, bag2, and chain fibers, and group II fibers form secondary endings on nuclear chain and bag2 fibers.
6.
Primary endings demonstrate both static and dynamic responses that signal muscle length and rate of change in muscle length. Secondary endings demonstrate only static responses and signal only muscle length.
7.
The intrafusal muscle fibers associated with muscle spindles are innervated by γ motor neurons. Contraction of intrafusal fibers does not directly cause significant changes in muscle tension or length; however, when the level of tension in these fibers is adjusted, γ motor neurons influence the sensitivity of the muscle spindle to stretch.
8.
Golgi tendon organs are located in the tendons of muscles and are thus arranged in series with the muscle. They are supplied by group Ib afferent fibers. Their in-series relationship means that tendon organs can detect the force level generated by the muscle, whether it is due to passive stretch or to active contraction of the muscle.
9.
The phasic stretch (or myotatic) reflex includes (1) a monosynaptic excitatory pathway from group Ia afferent fibers in muscle spindles to α motor neurons that supply the same and synergistic muscles and (2) a disynaptic inhibitory pathway to antagonistic motor neurons.
10.
The inverse myotatic reflex is evoked by Golgi tendon organs. Afferent volleys in group Ib fibers from a given muscle cause disynaptic inhibition of α motor neurons to the same muscle, and they excite α motor neurons to antagonist muscles.
11.
The flexion reflex is an important protective response because it acts to withdraw a limb from damaging stimuli. The reflex is evoked by volleys in afferent fibers that supply various receptors, particularly nociceptors. Via polysynaptic pathways, these volleys cause excitation of flexor motor neurons and inhibition of extensor motor neurons ipsilaterally. Concurrently, the opposite pattern of action (inhibition of flexor and excitation of extensor motor neurons) occurs contralaterally and is referred to as the crossed extension reflex.
12.
Descending pathways can be subdivided into (1) a lateral system, which ends on motor neurons to limb muscles and on the lateral group of interneurons, and (2) a medial system, which ends on the medial group of interneurons.
13.
The lateral system includes the lateral corticospinal tract and part of the corticobulbar tract. These pathways influence the contralateral motor neurons that supply the musculature of the limbs, especially of the digits, and the muscles of the lower part of the face and the tongue.
14.
The medial system includes the ventral corticospinal, lateral and medial vestibulospinal, reticulospinal, and tectospinal tracts. These pathways affect mainly posture and provide the motor background for movement of the limbs and digits.
15.
Locomotion is triggered by commands relayed through the midbrain locomotor center. However, central pattern generators formed by spinal cord circuits and influenced by afferent input provide for the detailed organization of locomotor activity.
16.