Neuroscience Terms Starting With M
Neuroscience Glossary: M
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Medulla Oblongata
/ meh-DUL-ah ob-lon-GAH-tah / · Latin medulla (marrow) + oblongatus (elongated)
Medulla Oblongata is the lowest segment of the brainstem, connecting the pons above to the spinal cord below, and housing control centers for breathing, heart rate, blood pressure, and protective reflexes.
The medulla contains the cardiac center controlling heart rate and force, the vasomotor center regulating blood vessel tone and blood pressure, and the respiratory center rhythmically generating breathing. Its protective reflex circuits coordinate coughing, sneezing, swallowing, and vomiting. Nearly all ascending sensory and descending motor tracts between the brain and spinal cord pass through the medulla, and most cross the midline here in the pyramidal decussation, which is why damage to the left side of the brain produces weakness on the right side of the body.
Opioid overdose kills mainly by suppressing the medulla's respiratory center, causing apnea. The reversal drug naloxone works by rapidly displacing opioids from receptors in medullary respiratory neurons, restoring breathing within minutes of intravenous administration.
Respiratory System Fun Facts →The medulla oblongata is not just a relay station. It performs autonomous life-sustaining functions that continue even during deep anesthesia or coma.
In the common carp (Cyprinus carpio), chemoreceptors on the ventral surface of the medulla detect rising carbon dioxide levels in the blood and trigger an increase in breathing rate within seconds. These sensors respond to the drop in cerebrospinal fluid pH that accompanies a rise in dissolved CO2, not to oxygen levels directly, which is why breathing a low-oxygen mixture does not immediately produce the same urgent drive to breathe.
Membrane Potential
/ MEM-brayn poh-TEN-shul / · Latin membrana (skin) + potentia (power)
Membrane Potential is the electrical voltage difference between the inside and outside of a neuron's plasma membrane, resulting from the unequal distribution of charged ions across the membrane and typically measured in millivolts.
At rest, the membrane potential is about negative 70 millivolts, maintained by the differential permeability of the membrane to potassium and sodium and by the sodium-potassium ATPase pump. Graded changes in membrane potential toward zero or more negative encode the strength of incoming stimuli. Meanwhile, action potentials are rapid, stereotyped membrane potential changes that propagate the signal along the axon without decrement.
The patch-clamp technique, developed by Erwin Neher and Bert Sakmann, who shared the Nobel Prize in Physiology or Medicine in 1991, uses a tiny glass pipette to measure current through a single ion channel. Their work revealed that individual channels open and close in milliseconds and that their conductance is measured in picosiemens, roughly a billion times smaller than the resistance of a household lightbulb.
Plasma Membrane Functions →Membrane potential is not simply a voltage measured across the outer surface of a cell. It is a dynamic quantity that fluctuates continuously as ion channels open and close in response to incoming signals, neuromodulators, and metabolic state.
Astrocytes in the rat hippocampus maintain a resting membrane potential near negative 85 millivolts, about 15 millivolts more negative than a typical neuron, because their membranes are dominated by potassium leak channels rather than voltage-gated sodium channels. Nearby neuronal firing can shift astrocyte membrane potential by several millivolts as potassium is taken up from the extracellular space.
Motor Cortex
/ MOH-tor KOR-teks / · Latin motor (mover) + cortex (bark)
Motor Cortex is the region of the frontal lobe immediately anterior to the central sulcus that generates voluntary movement commands transmitted through the corticospinal tract to lower motor neurons controlling skeletal muscle.
The primary motor cortex is organized somatotopically as the motor homunculus, with disproportionately large representations of the hand, face, and tongue reflecting the precision demands of those body parts. Neurons in the motor cortex begin firing about 500 milliseconds before a voluntary movement is consciously initiated, as captured by the readiness potential recorded with electroencephalography. The premotor and supplementary motor areas anterior to the primary motor cortex plan and sequence movements before their execution.
Transcranial magnetic stimulation applied over the motor cortex can produce involuntary muscle twitches by directly activating corticospinal neurons, allowing precise mapping of the motor cortex in conscious human subjects without surgery. A single pulse lasting less than one millisecond is sufficient to trigger a measurable muscle response detectable by electromyography.
The motor cortex is not the only brain region controlling voluntary movement. The cerebellum, basal ganglia, brainstem, spinal cord, and sensory feedback pathways all contribute to coordinated action.
Fun Facts About the Nervous System →Recordings from motor cortex neurons in rhesus macaques (Macaca mulatta) have shown that the activity of roughly 100 simultaneously recorded neurons can predict the direction and speed of an upcoming arm movement up to 300 milliseconds before it begins. This signal can drive robotic limbs through brain-machine interfaces with update rates of 20 to 100 commands per second.
Motor Neuron
/ MOH-tor NYOOR-on / · Latin motor (mover) + Greek neuron
Motor Neuron is a nerve cell in the central or peripheral nervous system that transmits signals from the brain or spinal cord to muscles or glands, directly controlling voluntary and involuntary movement.
Upper motor neurons originate in the motor cortex and descend through the corticospinal tract to synapse on lower motor neurons in the spinal cord or brainstem. Lower motor neurons exit the spinal cord through the ventral roots, travel in peripheral nerves, and form neuromuscular junctions with skeletal muscle fibers. Loss of lower motor neurons produces the flaccid weakness and muscle atrophy characteristic of motor neuron diseases including amyotrophic lateral sclerosis.
Each alpha motor neuron in the human spinal cord innervates anywhere from a few muscle fibers in the extraocular muscles to more than a thousand fibers in the large muscles of the leg, a ratio called the innervation number that determines how finely graded a muscle's force output can be.
Fun Facts About the Nervous System →Motor neurons are not identical across the body. Alpha motor neurons directly control muscle contraction while gamma motor neurons regulate the sensitivity of muscle spindle stretch receptors.
In the African clawed frog (Xenopus laevis), spinal motor neurons controlling the laryngeal muscles fire in precise rhythmic bursts to produce mating calls, with male neurons firing at roughly 60 Hz and female neurons at about 30 Hz. The 2-fold sex difference in firing rate is set partly by intrinsic membrane properties of the motor neurons themselves.
What Do Gorillas Eat? →Myelin Sheath
/ MY-eh-lin sheeth / · Greek myelos (marrow) + Old English sceath
Myelin Sheath is the lipid-rich insulating layer wrapped around many axons by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system, dramatically increasing the speed of electrical signal conduction.
By insulating the axon between exposed nodes of Ranvier, myelin forces the action potential to jump from node to node in saltatory conduction, increasing conduction velocity from under one meter per second in unmyelinated fibers to over 70 meters per second in the largest myelinated axons. Myelination also reduces the metabolic cost of action potential propagation by limiting ion exchange to the small area of each node. Destruction of myelin in multiple sclerosis disrupts neural conduction and produces the variable neurological deficits characteristic of the disease.
Each oligodendrocyte in the central nervous system can myelinate up to 50 separate axon segments simultaneously, wrapping its processes around segments roughly 150 micrometers long. Schwann cells in the peripheral nervous system, by contrast, each myelinate only a single axon segment.
Myelin simply insulates axons like rubber coating on a wire. Myelin also provides metabolic support to axons by transferring lactate and other nutrients through specialized monocarboxylate transporters, and axons deprived of this support degenerate even when electrical conduction remains intact.
In the shrimp-like crustacean Penaeus monodon, myelinated axons in the peripheral nervous system conduct signals at speeds exceeding 200 meters per second, among the fastest recorded in any animal. Each myelin segment in this species spans up to 500 micrometers, roughly three times longer than a typical mammalian internode, contributing to the exceptional conduction velocity.