The Spinal Cord: The Intermediary between Nervous Systems

By Frank Amthor

The spinal cord is a key part of the central nervous system. Specifically, within the spinal cord are the connections between the central nervous system (such as motor neurons from primary motor cortex) and the peripheral nervous system (skin, muscle, and tendon receptors going to the spinal cord, and alpha motor neurons relaying motor commands from the spinal cord to the actual muscles).

The sensory neurons and alpha (lower) motor neuron axons constitute the peripheral nervous system, which mediate voluntary behavior and sensing.

The spinal cord also has integration and coordination functions, although it is close to the lowest level of the hierarchy of controllers. It mediates feedback between sensory and motor pathways for each limb and coordinates limb movements for locomotion.

Each level of the spinal cord contains a local processing module for the area of the body controlled from that segment, plus connections to other spinal cord segments and to and from the brain (through the medulla all the way to neocortex). The top of the spinal cord deals with muscles and sensory information from the neck, while the bottom spinal cord segments deal with the toes.

The spinal cord segments are designated as 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 3 coccygeal. These are numbered from the top down, so the highest cervical spinal segment is C1, while the lowest sacral segment is S5.

Looking at the spinal reflex

The fundamental unit of coordinated action mediated by the spinal cord is the spinal reflex; you can see the anatomy of this reflex in Figure 2-6. Striking the kneecap stretches the quadriceps muscle, as though one’s knees were buckling. Muscle spindle receptors in the quadriceps project to the spinal cord, and, through interneurons, activate alpha motor neurons that contract the muscle to maintain upright posture.

Spinal-Reflex
The spinal reflex.

Alpha motor neurons are the neurons that innervate muscle cells, which in turn move the limbs. The motor neurons have their cell bodies in the ventral (toward the stomach side of the body) part of the spinal cord. The axons that drive the muscles exit the spinal cord via a tract called the ventral root.

On the dorsal side of the spinal cord are the axons of sensory cells, such as stretch receptors, that enter via the dorsal root.

When the doctor taps just below your kneecap with a rubber hammer, the tap slightly stretches the tendon from your knee to your foot, stretching the quadriceps muscle and spindle, which causes your leg to extend in compensation. This test checks the integrity of your peripheral nervous system (sensory and motor nerves acting through the spinal cord).

What doesn’t the figure show? It doesn’t show several pathways connecting a spinal cord segment to other segments, and to the brain:

  • The original message gets sent to the brain. The sensory input coming in the dorsal root synapses on spinal neurons that relay the sensory message all the way to the brain (somatosensory cortex, just posterior to the central sulcus).
  • Command messages come down from the brain. Cortical (upper motor) neurons from the primary motor cortex come all the way down the spinal cord and synapse on the same alpha motor neurons that innervate muscles for the stretch reflex to allow you to voluntarily extend your leg.
  • Communication occurs between segments. Motor events occurring in one segment send messages to other segments to coordinate body actions. If, for example, you were standing while your left leg started to buckle at the knee, your right leg would probably stiffen in compensation, and you would probably extend your left hand up, among other things. These connections also are involved in the central pattern generator control of gait mentioned previously.

Getting your muscles moving

How do muscles work? Muscles are groups of cells connected to each other in long parallel chains the ends of which are connected to bones by tendons. The muscle cells contain chains of proteins called actin and myosin. When these proteins are stimulated, they slide over each other, causing the cell to contract along its length.

Alpha motor neurons provide the stimulation by releasing the motor neuron neurotransmitter acetylcholine. When the acetylcholine is received by the receptors on the muscle cells, it causes a muscle cell action potential (an electrical pulse in the cell that causes it to momentarily contract) that causes the actin-myosin to slide (through the raising of intracellular calcium concentration), which in turn causes the contraction.