49 Functional Systems
Anatomy & Organization of the Medulla Spinalis
Fig. 49.1 Anatomy of a spinal cord segments
Three dimensional representation, oblique anterior view from upper left. The substantia grisea of the medulla spinalis is found internally, surrounding the canalis centralis in an H-shaped, or butterfly-like, configuration. This is the reverse of what was seen in the encephalon where the substantia grisea was on the external aspect in a cortical configuration. The primary function of the medulla spinalis is to conduct impulses to and from the encephalon and to facilitate this, both substantiae grisea and alba are organized into longitudinal groupings.
Fig. 49.2 Organization of the substantia grisea
Left oblique anterosuperior view. The substantia grisea of the medulla spinalis is divided into three columnae (cornua).
- Columna anterior (cornu anterius): contains motor neurons
- Columna intermedia (cornu laterale): contains sympathetic or parasympathetic (visceromotor) neurons in selected regions
- Columna posterior (cornu posterius): contains sensory neurons
Afferent (blue) and efferent (red) neurons within these columnae are clustered in nuclei according to function.
Fig. 49.3 Innervation of muscles
Motor neurons that innervate specific muscles are arranged into vertical columns in the cornu anterius of substantia grisea, the columns themselves can be called nuclei, in a fashion similar to that seen in motor nuclei in truncus encephali. Most muscles (intersegmental muscles) receive innervation from numerous motor nuclei spanning several segmenta medullae spinalis. Monosegmental (or indicator) muscles have their motor neurons located entirely within a single segmentum medullae spinalis.
Fig. 49.4A Organization of the substantia alba
Left oblique anterosuperior view. The columnae substantiae griseae partition the substantia alba analogously into funiculi anterior, lateralis, and posterior. The substantia alba of the medulla spinalis contains ascending and descending tractus which are the CNS equivalent of peripheral nervi.
Fig. 49.4B Overview of sensorimotor integration
Schematic illustrates the pathway of incoming primary afferent (sensory) neuron impulses, the axon of which ascends to synapse with the secondary and tertiary afferent (sensory) neurons in the truncus encephali and cerebrum ending in a synapse on a neuron in the sensory cortex. An interneuron links this with an upper motor neuron in the motor cortex which then descends through the funiculi substantiae albae of the medulla spinalis to a motor neuron, which then synapses with a lower motor neuron, the axon of which passes out the n. spinalis to the effector organ.
Fig. 49.5A Principal intrinsic fasciculi of the medulla spinalis (shaded yellow)
Left oblique anterosuperior view. The majority of muscles have a multisegmental mode of innervatin that necessitates axons to ascend/descend multiple segmenta medullae spinalis to coordinate spinal reflexes. The neurons of these axons originate from interneurons in the substantia grisea forming intrinsic reflex pathways of the medulla spinalis. These axons are collected into intrinsic fascicles which are arranged chiefly around the substantia grisea. These bundles make up the intrinsic circuits of the medulla spinalis.
Fig. 49.5B Intrinsic circuits of the medulla spinalis
Afferent neurons are shown in blue, efferent neurons in red. The neurons of the spinal reflex circuits are in black. These chains of interneurons, which are entirely contained within the medulla spinalis, comprise the intrinsic circuits of the cord. The axons of these intrinsic circuits pass to adjacent segments in intrinsic fascicles located along the edge of the substantia grisea.
Sensory & Motor Pathways
Fig. 49.6 Sensory pathways (ascending tracts)
Fig. 49.7 Motor pathways (descending tracts)
