The mouse spinal cord is a core component of the central nervous system, extending caudally from the brainstem. This structure functions as the primary communication pathway, relaying sensory information from the body to the brain and transmitting motor commands back to the periphery. Understanding this anatomy is valuable because mice serve as a primary model for studying human neurological diseases and spinal cord injury. Researchers use this model to investigate complex circuitry, movement coordination, and therapeutic strategies for nerve regeneration.
Gross Organization and Segmentation
The mouse spinal cord is a long, cylindrical structure encased within the vertebral column. It is segmented along its length, giving rise to 34 pairs of spinal nerves that connect the central nervous system to the body. These nerves define the segments:
- 8 cervical
- 13 thoracic
- 6 lumbar
- 4 sacral
- 3 coccygeal segments
The structure is safeguarded by three layers of protective membranes known as the meninges. The outermost layer is the dura mater, a tough, fibrous sheath that provides mechanical protection. Beneath this are the arachnoid mater and the innermost pia mater, separated by the subarachnoid space where cerebrospinal fluid circulates, providing cushioning for the neural tissue.
At each segment, dorsal and ventral rootlets emerge and merge to form the mixed spinal nerves. Dorsal rootlets carry afferent (sensory) information into the cord, while ventral rootlets carry efferent (motor) commands out to the muscles and glands. The cell bodies of the sensory neurons are located in the dorsal root ganglia.
The spinal cord features two distinct enlargements where the diameter is noticeably thicker. The cervical enlargement is in the upper segments, and the lumbar enlargement is in the lower segments. This increase in size accommodates the greater number of neurons required to innervate the forelimbs and hindlimbs, respectively.
The Central Architecture: Gray Matter
At the center of the mouse spinal cord lies the gray matter, which is shaped like a butterfly or the letter ‘H’ in cross-section. This region is primarily composed of neuron cell bodies, dendrites, unmyelinated axons, and glial cells, serving as the central processing hub. The gray matter is functionally segregated into projections known as horns.
The dorsal horn is the upper portion of the ‘H’ and is dedicated to receiving and processing incoming sensory information. Afferent fibers from the dorsal roots enter here, bringing signals related to touch, pain, temperature, and proprioception. Neurons within the dorsal horn are organized into distinct layers, known as Rexed laminae, which handle different modalities of sensation.
The ventral horn forms the lower portion of the ‘H’ and contains the cell bodies of motor neurons. These efferent neurons, including alpha and gamma motor neurons, project their axons out through the ventral roots to innervate skeletal muscles, controlling voluntary movement. The ventral horn is largest in the cervical and lumbar enlargements to accommodate the motor innervation of the limbs.
Between the dorsal and ventral horns is the intermediate zone, which includes the lateral horn in specific segments. The lateral horn is a small, lateral projection found mainly in the thoracic and upper lumbar regions. This region houses the preganglionic visceral motor neurons, which transmit signals related to the autonomic nervous system, controlling internal organs and glands.
The Peripheral Highways: White Matter
The white matter surrounds the central gray matter and is composed of myelinated axons, which form the long-distance communication pathways. These axons are bundled into tracts that run longitudinally, transmitting information up and down the spinal cord. In each half of the cord, the white matter is anatomically divided into three major columns, or funiculi.
The dorsal funiculus is situated between the dorsal horns and is dedicated almost exclusively to ascending tracts carrying sensory information toward the brain. These tracts primarily convey conscious proprioception and fine discriminative touch. The lateral funiculus lies lateral to the gray matter and contains a mix of both ascending sensory and descending motor tracts.
The ventral funiculus is located on the anterior side, between the ventral horns, and also carries a combination of ascending and descending pathways. Descending motor tracts originating from the brain travel through the lateral and ventral funiculi to synapse with motor neurons in the ventral horn. This structure ensures rapid and coordinated communication between the brain and the body, supporting movement and reflexes.