Spinal Cord

Keywords

spinal cord, ventral horn, dorsal horn, dermatomes, posterior columns, autonomics, spinothalamic tract, lower motor neurons, upper motor neurons, dorsal root ganglia

Chapter Outline
The Spinal Cord Is Segmented, 56
- Each Spinal Cord Segment Innervates a Dermatome, 56
All Levels of the Spinal Cord Have a Similar Cross-Sectional Structure, 56
The Spinal Cord Is Involved in Sensory Processing, Motor Outflow, and Reflexes, 57
Spinal Gray Matter Is Regionally Specialized, 57
Reflex Circuitry Is Built Into the Spinal Cord, 58
- Reflexes Are Modifiable, 58
Ascending and Descending Pathways Have Defined Locations in the Spinal White Matter, 59
- The Posterior Column–Medial Lemniscus System Conveys Information About Touch and Limb Position, 59
- The Spinothalamic Tract Conveys Information About Pain and Temperature, 59
- Additional Pathways Convey Somatosensory Information to the Thalamus, 60
- Spinal Information Reaches the Cerebellum Both Directly and Indirectly, 60
- Descending Pathways Influence the Activity of Lower Motor Neurons, 60
The Autonomic Nervous System Monitors and Controls Visceral Activity, 61
- Preganglionic Parasympathetic Neurons Are Located in the Brainstem and Sacral Spinal Cord, 61
- Preganglionic Sympathetic Neurons Are Located in Thoracic and Lumbar Spinal Segments, 61
- Visceral Distortion or Damage Causes Pain That Is Referred to Predictable Dermatomes, 61
A Longitudinal Network of Arteries Supplies the Spinal Cord, 62
Spinal Cord Damage Causes Predictable Deficits, 62

The spinal cord is pretty small, but its importance is out of proportion to its size. It’s the home of all the motor neurons that work your body, and of a large percentage of the autonomic motor neurons as well. It’s also the recipient of nearly all the sensory information taken in by your body. Beyond that, many of the organizing principles of spinal cord reflexes and pathways apply to other parts of the CNS.

The Spinal Cord Is Segmented

Key Concept

The spinal cord is shorter than the vertebral canal.

Segments of the spinal cord ( Fig. 10.1 )—eight cervical , twelve thoracic , five lumbar , five sacral , and one coccygeal —are defined by the spinal nerves formed from dorsal and ventral roots attached bilaterally to each segment. The spinal cord has a cervical and a lumbar enlargement , serving the needs of the arms and legs respectively, and ends at the pointed conus medullaris . The conus medullaris is located at vertebral level L1/L2, even though the dural sac surrounding the spinal cord extends to vertebral level S2. So dorsal and ventral roots from progressively more caudal levels need to travel progressively longer distances through spinal subarachnoid space before reaching their intervertebral foramina of entry or exit. The collection of spinal nerves in subarachnoid space caudal to the conus medullaris is the cauda equina (Latin for “horse’s tail”).

Spinal nerves C1–C7 use the foramen above the corresponding vertebra, C8 uses the foramen between vertebrae C7 and T1, and all others use the foramen below the corresponding vertebra.

Each Spinal Cord Segment Innervates a Dermatome

The mesoderm and ectoderm that develop adjacent to a given spinal cord segment go on to form bones, muscles, and skin in predictable locations. The resulting systematic relationships between cord segments and different muscles and areas of skin ( dermatomes ) are tremendously important in clinical neurology.

All Levels of the Spinal Cord Have a Similar Cross-Sectional Structure

The gray matter core of the spinal cord is roughly in the shape of an H with a dorsal-ventral orientation at all levels ( Fig. 10.2 ). The dorsally directed limbs of the H are the posterior (or dorsal ) horns , and the ventrally directed limbs are the anterior (or ventral ) horns . The posterior horn, derived from the alar plate of the neural tube, is a sensory processing area that receives most of the afferents that arrive in ipsilateral dorsal roots. The anterior horn is derived from the basal plate and contains the motor neurons whose axons form the ventral roots. The intermediate gray matter between the anterior and posterior horns is a mixture of interneurons and tract cells in sensory and motor circuits; at the thoracic and upper lumbar levels the intermediate gray matter also contains sympathetic autonomic motor neurons, while the sacral level intermediate gray matter also contains the parasympathetic autonomic motor neurons. The axons of the autonomics leave the spinal cord by way of the ventral roots.

The anterior and posterior horns divide the surrounding spinal white matter into anterior , lateral , and posterior funiculi (funiculus is Latin for “string”).

As dorsal roots approach the spinal cord, the afferent fibers sort themselves out so that large-diameter fibers enter medial to small-diameter fibers. The large-diameter fibers, carrying touch and position information, have some branches that travel rostrally in the posterior funiculus and others that end in deeper portions of the posterior horn. The small-diameter fibers, primarily carrying pain and temperature information, travel in the dorsolateral fasciculus ( Lissauer tract ) to termination sites in a superficial zone of the posterior horn called the substantia gelatinosa .

The Spinal Cord Is Involved in Sensory Processing, Motor Outflow, and Reflexes

The wiring principles discussed in Chapter 3 are pretty apparent in the spinal cord. Central processes of primary afferents (cell bodies in dorsal root ganglia ) are the only routes through which sensory information from the body can reach the spinal cord. They give rise to branches that feed into reflex circuits, into pathways to the thalamus, and into pathways to the cerebellum. These primary afferents end in ipsilateral gray matter, mostly but not entirely in the posterior horn. Lower motor neurons in the anterior horn receive inputs from reflex circuits, as well as through descending pathways (i.e., axons of upper motor neurons ), and project to ipsilateral muscles. They are the only routes through which the spinal cord can tell skeletal muscles to contract, and loss of lower motor neurons or their axons is followed by flaccid paralysis of the muscles they innervate—profound weakness, loss of tone and reflexes, and atrophy.

Spinal Gray Matter Is Regionally Specialized

Key Concepts

The posterior horn contains sensory interneurons and projection neurons.
The anterior horn contains motor neurons.
The intermediate gray matter contains autonomic neurons.

Some parts of the spinal gray matter (e.g., the substantia gelatinosa) are present in all segments. Others are either present at only some levels or are emphasized at some levels in ways that make functional sense. The most prominent examples of the former are preganglionic sympathetic neurons , present in the T1–L3 intermediate gray and forming a pointy lateral horn in thoracic segments; preganglionic parasympathetic neurons , present in the S2–S4 intermediate gray; and Clarke nucleus , at the base of the posterior horn from T1–L2 and particularly prominent at lower thoracic levels. As an example of level-specific emphasis, an anterior horn is present at all levels but is enlarged laterally in the cervical and lumbar enlargements to accommodate all the lower motor neurons for distal muscles (limbs) supplied by these levels.

Reflex Circuitry Is Built Into the Spinal Cord

Key Concepts

Muscle stretch leads to excitation of motor neurons.
Painful stimuli elicit coordinated withdrawal reflexes.
Reflexes are accompanied by reciprocal and crossed effects.

Reflexes are involuntary, stereotyped responses to sensory inputs, and every kind of sensory input is involved in reflex circuitry of one or more types. The simplest kind of reflex imaginable (other than the axon reflex; see Chapter 9 ) involves a primary afferent and a lower motor neuron, with a synapse in the CNS connecting the two. This is the circuit underlying stretch reflexes ( Fig. 10.3 ), through which a muscle contracts in response to being stretched. (Stretch reflexes are tested by tapping tendons and so they’re often called deep tendon reflexes , even though the receptors that initiate them are located in muscle spindles and not tendons.)

Stretch reflexes are the only monosynaptic reflexes. All others involve one or more interneurons. An example is the flexor reflex (or withdrawal reflex ), through which a limb is removed from a painful stimulus ( Fig. 10.4 ). This reflex is considerably more complex than a stretch reflex because all the muscles of a limb, and therefore motor neurons in several spinal segments, come into play.