Материал: Kaplan USMLE-1 (2013) - Anatomy

Section IV • Neuroscience

In A Nutshell

Motor end plate of skeletal muscles

UMN, upper motoneuron; LMN, lower motoneuron

Intermediate Zone

Theintermediate zone ofthe spinal cord from Tl to L2 contains preganglionic sym­ pathetic neuron cell bodies and Clarke nucleus, which sends unconscious proprio­ ception to the cerebellum.

NEURAL SYSTEMS

There are 3 major neural systems in the spinal cord that use neurons in the gray matter and tracts or fasciculi of axons in the white matter. These neural systems have components that can be found at all levels of the CNS from the cerebral cortex to the tip ofthe spinal cord. An understanding of these 3 neural systems is essential to understanding the effects oflesions in the spinal cord and brain stem, and at higher levels of the CNS.

Motor System

Voluntaryinnervation ofskeletal muscle

Upper and Lower Motoneurons

Two motoneurons, an upper motoneuron and a lower motoneuron, together form the basic neural circuit involved in the voluntary contraction of skeletal muscle everywhere in the body. The lower motoneurons are found in the ven­ tral horn ofthe spinal cord and in cranial nervenuclei in thebrain stem. Ax­ ons of lower motoneurons of spinal nerves exit in a ventral root, then join the spinal nerve to course in one of its branches to reach and synapse directly at a neuromuscular junction in skeletal muscle. Axons of lower motoneurons in the brain stem exit in a cranial nerve.

To initiate a voluntary contraction of skeletal muscle, a lower motoneuron must be innervated by an upper motoneuron (Figure IV-4-3). The cell bodies ofupper motoneurons are found in the brain stem and cerebral cortex, and their axons descend into the spinal cord in a tract to reach and synapse on lower motoneurons, or on interneurons, which then synapse on lower motoneurons. At a minimum, therefore, to initiate a voluntary contraction of skeletal muscle, 2 motoneurons, an upper and a lower, must be involved. The upper motoneuron innervates the lower motoneuron, and the lower motoneuron innervates the skeletal muscle.

The cell bodies of upper motoneurons are found in the red nucleus, reticular for­ mation, and lateral vestibular nuclei of the brain stem, but the most important

location ofupper motoneurons is in thecerebralcortex. Axons of these cortical neurons course in the corticospinal tract.

Both ends ofthe muscle spindle are connected in parallel with the extrafusal fibers, so these receptors monitor the length and rate of change in length of extrafusal fibers. Muscles involved with fine movements contain a greater density of spindles than those used in coarse movements.

Commonlytested muscle stretch reflex.es

The deep tendon (stretch, myotatic) reflex is monosynaptic and ipsilateral. The afferent limb consists of a muscle spindle, Ia sensory neuron, and efferent limb

(lower motor neuron). These reflexes are useful in the clinical exam.

Muscle stretch (myotatic) reflex

1be muscle stretch (myotatic) reflex is the stereotyped contraction of a muscle in response to stretch of that muscle. The stretch reflex is a basic reflexthat oc­ curs in allmuscles and is the primary mechanism for regulating muscle tone.

Muscle tone is the tension present in all resting muscle. Tension is controlled by the stretch reflexes.

The best example of a muscle stretch or deep tendon reflex is the knee-jerk re­ flex. Tapping the patellar ligament stretches the quadriceps muscle and its muscle spindles. Stretch of the spindles activates sensory endings (Ia afferents), and af­ ferent impulses are transmitted to the cord. Some impulses from stretch recep­ tors carried by Ia fibers monosynaptically stimulate the alpha motoneurons that supply the quadriceps. This causes contraction ofthe muscle and a sudden exten­ sion of the leg at the knee. Afferent impulses simultaneously inhibit antagonist muscles through interneurons (in this case, hamstrings).

Inverse muscle stretch reflex

The inverse muscle stretch reflex monitors muscle tension. This reflex uses Golgi tendon organs (GTOs). These are encapsulated groups of nerve endings that ter­ minate between collagenous tendon fibers at the junction of muscle and tendon. GTOs are oriented in series with the extrafusal fibers and respond to increases in force or tension generated in that muscle. Increases in force in a muscle increase the firing rate of lb afferent neurons that innervate the GTOs, which, in turn, polysynaptically facilitate antagonists and inhibit agonist muscles.

Muscle tone and reflex activity can be influenced by gamma motoneurons and by upper motoneurons. Gamma motoneurons directly innervate the muscle spindles and regulate their sensitivity to stretch. Upper motoneurons innervate gamma motoneurons and also influence the sensitivity of muscle spindles to stretch. Stimulation of gamma motoneurons causes intrafusal muscle fibers lo­ cated at the pole of each muscle spindle to contract, which activates alpha moto­ neurons, causing an increase in muscle tone.

Chapter It • The Spinal Cord

MEDICAL 365