cell surface. Second messengers diffuse within the cell and alter cell behavior. They amplify signals, allow a single first messenger to control several cellular processes, and help integrate the many signals that cells receive.


A state of increased muscular tone in which abnormal stretch reflexes intensify muscle resistance to passive movements.

Spinal cord segments

Divisions of the spinal cord along its length. Each spinal cord segment sends a pair of motor and sensory nerves to the body. Higher segments control movement and sensation in upper parts of the body, whereas lower segments control movement and sensation in lower parts of the body.


The functional connection between a nerve cell axon and target cells, which may be other nerve cells, muscle cells, or gland cells. At the synapse the axon releases a chemical neurotransmitter that diffuses across a tiny gap and binds to receptors (molecules on the surface of the target cell) that then change the target cell’s behavior. Synapses may be excitatory (which increases the probability that the target cell will be activated) or inhibitory (which reduces the probability that the target cell will be activated) or may have more complex influences (such as adjusting the sensitivity of cells to other signals).

Tight junction

A junction formed between adjacent cells, which prevents the passage of large molecules.


Molecular process by which the genetic material deoxyribonucleic acid (DNA) is used as a template to become messenger ribonucleic acid (mRNA), which is subsequently used as a template for protein production.


Molecular process by which messenger ribonucleic acid (mRNA), in conjunction with ribosomes, forms proteins.

Trophic factor

A natural cell growth and survival molecule. Neurotrophic factors are trophic factors that affect nerve cells.


Relating to the belly or front of the body.

White matter

Areas of the brain and spinal cord that primarily contain myelinated axons. White matter is located in the outer portion of the spinal cord and surrounds the gray matter.

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