Spinal Cord

The spinal cord is a long cylinder of nervous tissue housed within the vertebral canal, which it does not completely fill. It is a continuation of the brainstem and gives rise to spinal nerves on each side through a double row of roots: dorsal (sensory) roots and ventral (motor) roots. It extends from the foramen magnum to the sacral or lumbar region. The spinal cord is whitish in color, firm, and slightly elastic.

In a cross-section, one can observe a long, narrow central canal called the ependymal canal, filled with cerebrospinal fluid. This canal is located within a butterfly-shaped mass of gray matter, which is entirely surrounded by white matter. The spinal cord has two main functions:

• It acts as a link between the brain and all the organs connected to the spinal nerves. It receives information from peripheral receptors (such as pain, limb position, etc.), sends it to the brain where it is processed, and transmits motor commands from the brain to the effectors (muscles).
• It serves as an integration center for certain simple reflex functions, where information from the periphery generates a response without passing through the brain.

Spinal Reflexes

A reflex is an unconscious, involuntary, and stereotyped motor reaction that occurs in response to a stimulus. It allows the body to adapt, though it lacks fine control and precision. The physiological characteristics of this response are linked to a fundamental structural unit: the reflex arc.

Reflex Arc Organization

The reflex arc is the anatomical pathway essential for any reflex activity. The simplest reflex arc consists of:

• Afferent limb: made up of a sensory receptor (muscular or cutaneous) that responds to stimulation. This receptor sends information via afferent sensory fibers to the spinal cord.
• Reflex center: the spinal cord acts as the integration center where synaptic connections, simple or complex, occur between the afferent and efferent limbs.
• Efferent limb: consists of alpha motor neurons that innervate the effector muscle (flexor or extensor).

Reflex Classification

Reflexes are generally classified into:

Stretch Reflexes

These are triggered by stimulation of muscle receptors called proprioceptors, represented by neuromuscular spindles. They include two subgroups:

• Phasic reflexes: tendon reflexes that consist of a brief and sudden muscle contraction in response to a quick stretch of the tendon (e.g., patellar reflex at the knee).
• Myotatic reflexes: reflex muscle contractions in response to stretching of the muscle itself, aimed at maintaining a defined muscle length. This is a monosynaptic proprioceptive reflex found in all muscles but particularly in antigravity extensor muscles (such as the quadriceps, gastrocnemius, etc.), serving a postural function.

Flexion Reflexes

These are defense or withdrawal reactions involving flexor muscles in response to generally painful, nociceptive stimuli (such as from the skin). They result in an ipsilateral flexion movement and are polysynaptic reflexes involving several spinal interneurons.

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