Chapters 16 & 17

Appendix 3 Overview

1. Introduction
   
   
2. Spinal cord circuitry


3. Descending pathways to the brainstem & spinal cord

Introduction

The goal of this Appendix is to review the organization of the pathways by which the upper motor neurons of the motor cortex governs the lower motor neuronal circuits of the spinal cord and brainstem.  The neuroanatomical organization of these pathways is covered here.  The more detailed account of their neurophysiological roles in motor control is provided in Purves et al., Neuroscience 5th Ed., Chapters 16-17 (Sinauer Assoc., Inc.).

Spinal cord circuitry

The basic reflex circuit that is responsible for muscle tone is illustrated in Figure A3-1.  Use your reflex hammer (or your hand or some other appropriate object) to demonstrate the patellar tendon reflex on a willing subject.  If the reflex is difficult to elicit, it may help to have the subject hold both hands together and pull hard.  A tendon tap just when the subject starts to pull may elicit a more lively reflex. (Can you think of an explanation for this phenomenon?)

Descending pathways to the brainstem & spinal cord

The corticospinal and corticobulbar pathways are illustrated in Figures A3-2 and A3-3.  Neurons in layer V of the motor cortex give rise to axons that descend through the internal capsule, the cerebral peduncle and the medullary pyramids to the caudal end of the medulla where most of them cross in the pyramidal decussation.  These crossed fibers descend through the lateral corticospinal tract to terminate on motor neurons and interneurons in the lateral aspect of the ventral horn.  A small number of fibers do not cross in the pyramids but remain on the same side.  These axons form the anterior corticospinal tract and innervate the medial aspects of the ventral horn bilaterally.

Descending cortical axons that innervate the cranial nerve motor nuclei for the muscles of the face and head are known as the corticobulbar tract1. Most of these axons innervate bilaterally premotor interneurons associated with brainstem reticular formation. The interneurons of the reticular formation, in turn, supply the motor nuclei. Some of the cranial nerve motor nuclei also receive direct bilateral input from the motor cortex. The consequence of the bilateral corticobulbar innervation is that damage to the fibers on only one side does not result in dramatic deficits in function. There are three notable exceptions to the pattern of symmetrical, bilateral cortical innervation of the local circuits controlling cranial nerve motor nuclei. For each of these motor nuclei, corticobulbar inputs arise from both cerebral hemispheres, but there is some bias in favor of inputs from the contralateral motor cortex:

• the hypoglossal nucleus receives primarily contralateral corticobulbar input;
• the trigeminal motor nucleus receives primarily contralateral corticobulbar input;
• the part of the facial motor nucleus that innervates the lower face receives primarily contralateral corticobulbar input. (The part of the facial motor nucleus that innervates the upper face is innervated bilaterally; see Box 17A in Neuroscience, 5th Ed.)

Essentially, functions involving muscles of the face that may be performed unilaterally (i.e., on one side of the face, such as pushing the tongue against one cheek, biting on one side with the lateral dentition, or raising one corner of the mouth) are governed by upper motor neuronal control signals from the contralateral motor cortex.

There are a number of other important descending pathways to the spinal cord.  The vestibulospinal and reticulospinal pathways are the most important, providing descending fibers from the vestibular nuclei and the reticular formation, respectively, that innervate lower motor neurons located medially in the spinal cord.  You will also learn about a “tectospinal” pathway from the superior colliculus to the cervical spinal cord, but most of the descending output from the superior colliculus is actually mediated via indirect connections through reticulospinal projections.  Here’s what’s important to know about these brainstem-to-spinal cord pathways:

• vestibulospinal tracts—feedback adjustments of posture in response to head movements and disturbances of postural stability
• reticulospinal tract—feedforward adjustments of posture that anticipate instability associated with voluntary movements
• tectospinal tract (mediated by projections to reticular formation and reticulospinal projections)— feedback adjustments of head and neck posture that support a change in direction of gaze.