Mouse Spinal Cord Anatomy: Key Structures and Functions

The mouse spinal cord is a critical part of the central nervous system, transmitting sensory and motor information between the brain and body. Its structure provides insights into neural function, disease modeling, and potential therapies for spinal injuries and neurodegenerative disorders. Due to its similarities with other mammals, particularly humans, it serves as a widely used model in neuroscience research.

External Landmarks and Dimensions

The mouse spinal cord is a slender, elongated structure that extends from the brainstem through the vertebral column, terminating around the L6 vertebra. Unlike larger mammals, where the spinal cord extends further into the sacral region, the mouse spinal cord ends earlier, with the conus medullaris marking its tapered conclusion. The filum terminale, a delicate strand of fibrous tissue, extends beyond this point, anchoring the spinal cord and providing structural stability.

The overall length varies slightly depending on strain and age but generally measures between 25 to 30 mm in adults. Its diameter is not uniform, with two prominent enlargements: the cervical and lumbar intumescences. These regions correspond to increased neural input and output for forelimb and hindlimb function. The cervical enlargement, around C3–T1, is broader due to the high concentration of motor neurons innervating the forelimbs, while the lumbar enlargement, spanning L1–L6, supports hindlimb control. These enlargements are critical in experimental studies, particularly those investigating motor function recovery after spinal cord injury.

The dorsal and ventral surfaces exhibit distinct anatomical features that aid in orientation. The dorsal median sulcus runs along the midline of the posterior surface, dividing the cord into symmetrical halves. The dorsolateral sulci mark the entry points of dorsal root fibers carrying sensory information. On the ventral side, the ventral median fissure houses the anterior spinal artery, a major vascular supply route. This fissure also serves as a reference point for surgical procedures and histological sectioning.

Regional Segmentation

The mouse spinal cord is organized into cervical, thoracic, lumbar, sacral, and coccygeal regions, each corresponding to specific vertebral levels and functional roles. Spinal nerve roots emerge laterally and exit through intervertebral foramina. Due to the shorter spinal cord, there is a pronounced displacement of spinal nerves relative to vertebral levels, especially in the lumbar and sacral regions, where nerve roots extend caudally before exiting, forming the cauda equina.

The cervical region (C1–C8) is responsible for head, neck, and forelimb movement. It also houses the phrenic nucleus (C3–C5), which controls the diaphragm and is crucial for respiratory function. The thoracic region (T1–T13) contains preganglionic sympathetic neurons in the intermediolateral cell column, playing a central role in autonomic regulation. This region is narrower than the cervical and lumbar enlargements due to lower motor neuron density.

The lumbar and sacral regions (L1–S4) regulate hindlimb movement and pelvic organ function. The lumbar enlargement (L1–L6) contains motor neurons that innervate the lower limbs. The sacral region includes neurons essential for bladder, bowel, and reproductive system function, making it a frequent target in neurogenic dysfunction studies. The coccygeal region represents the terminal portion of the spinal cord and contributes to the cauda equina.

Gray Matter Organization

The gray matter of the mouse spinal cord is arranged in a butterfly-shaped structure within the central portion of the cord, consisting of neuron cell bodies, dendrites, and glial cells. Rexed’s classification divides it into ten laminae based on neuronal structure and function. These laminae are distributed across the dorsal, intermediate, and ventral horns, contributing to sensory processing, motor control, and interneuronal communication.

The dorsal horn (laminae I–VI) processes sensory inputs, including pain, temperature, and mechanoreception. Lamina I contains projection neurons that relay pain and temperature information to higher brain centers. Lamina II (substantia gelatinosa) integrates pain signals and is densely populated with inhibitory interneurons, making it a focal point for pain research. Deeper layers (laminae III and IV) process tactile and proprioceptive information.

The ventral horn (laminae VII–IX) contains motor neurons that directly innervate muscles. Alpha motor neurons, responsible for muscle contractions, are concentrated in lamina IX, while gamma motor neurons regulate muscle spindle sensitivity. Motor neurons are organized somatotopically: medial neurons control proximal muscles, while lateral neurons govern distal limb movements. The cervical and lumbar enlargements have a higher density of motor neurons due to increased innervation demands.

White Matter Pathways

The white matter surrounds the gray matter and consists of myelinated axons that facilitate rapid signal transmission. These axons form longitudinal tracts known as funiculi, divided into ascending and descending pathways. The dorsal funiculus carries sensory information, the lateral funiculus contains both sensory and motor tracts, and the ventral funiculus is primarily associated with motor control.

Ascending pathways relay sensory input to the brain. The dorsal column-medial lemniscus pathway (fasciculus gracilis and fasciculus cuneatus) transmits fine touch, vibration, and proprioception. The spinothalamic tract, in the anterolateral system, carries pain, temperature, and crude touch signals to the thalamus. Spinocerebellar pathways provide proprioceptive feedback to the cerebellum for motor coordination.

Descending pathways mediate voluntary and involuntary motor control. The corticospinal tract, originating from the motor cortex, is the primary conduit for voluntary movement, particularly skilled limb movements. The rubrospinal tract, from the red nucleus, influences limb flexion. Reticulospinal and vestibulospinal pathways regulate posture and reflexive motor responses.

Meninges and Cerebrospinal Fluid

The spinal cord is protected by three meningeal layers: dura mater, arachnoid mater, and pia mater. The dura mater is the outermost, forming a tough sheath. The arachnoid mater, beneath it, has a web-like structure allowing cerebrospinal fluid (CSF) passage. The pia mater, the innermost layer, adheres closely to the spinal cord and contains blood vessels supplying neural tissue.

CSF, produced by the choroid plexus, circulates through the subarachnoid space, cushioning the spinal cord and maintaining a stable extracellular environment. It plays a key role in clearing metabolic waste, with a relatively high turnover rate in mice. Experimental techniques such as lumbar puncture and cisterna magna injections exploit CSF dynamics for drug delivery and neuroprotection studies.

Vascularization and Blood Supply

The mouse spinal cord’s vascular network ensures a continuous supply of oxygen and nutrients. The vertebral arteries give rise to the anterior spinal artery and paired posterior spinal arteries. The anterior spinal artery, running along the ventral median fissure, supplies the anterior two-thirds of the spinal cord, including motor regions. The posterior spinal arteries, positioned dorsolaterally, nourish sensory pathways. These arteries receive reinforcement from segmental radicular arteries.

Venous drainage follows a structured pattern, with the anterior and posterior spinal veins directing deoxygenated blood to the vertebral venous plexus. Limited collateral circulation makes certain spinal segments more vulnerable to ischemic injury, a factor leveraged in research on neuroprotection. The blood-spinal cord barrier regulates molecular exchange between the bloodstream and neural tissue, impacting drug permeability and immune responses.

Key Differences From Other Mammals

While the mouse spinal cord shares fundamental similarities with other mammals, several anatomical and physiological differences set it apart. One of the most notable distinctions is its relative length compared to the vertebral column. In humans, the spinal cord terminates around L1–L2, whereas in mice, it extends to approximately L6. This difference affects the spatial organization of spinal nerve roots, which is important for translational research.

The corticospinal tract is less developed in mice than in primates. Instead, rodents rely more on the rubrospinal and reticulospinal tracts for voluntary movement. This divergence has implications for spinal cord injury models, as recovery mechanisms in mice may not fully replicate human responses. Additionally, mice have a higher density of microvasculature, possibly contributing to differences in ischemic tolerance. These anatomical and functional variations must be considered when extrapolating findings from mouse models to human applications.