Neuroscience Terms Starting With P
Neuroscience Glossary: P
Parasympathetic Nervous System
/ pair-ah-sim-pah-THET-ik NER-vus SIS-tem / · Greek para (beside) + sympathetikos + Latin nervosus + systema
Parasympathetic Nervous System is the division of the autonomic nervous system that promotes rest, digestion, and recovery by slowing heart rate, increasing digestive activity, and conserving energy through cranial and sacral nerve outflows.
Parasympathetic fibers originate in the brainstem nuclei of cranial nerves III, VII, IX, and X and in the sacral spinal cord segments S2 to S4. The vagus nerve carries about 75 percent of all parasympathetic fiber traffic, supplying the heart, lungs, and abdominal viscera. At cardiac muscarinic receptors, parasympathetic activation slows heart rate, while stimulation of smooth muscle and glands in the gut promotes secretion and motility.
Unlike sympathetic neurons, whose preganglionic fibers synapse in ganglia close to the spinal cord, parasympathetic preganglionic fibers travel long distances to synapse in ganglia located within or immediately adjacent to their target organs.
Vagal nerve stimulation devices implanted in the chest deliver electrical pulses to the vagus nerve and are FDA-approved for drug-resistant epilepsy and treatment-resistant depression, demonstrating that directly activating parasympathetic pathways can produce meaningful therapeutic effects in neurological and psychiatric conditions.
Fun Facts About the Nervous System →The parasympathetic system does not simply oppose everything the sympathetic system does. The two branches have complementary rather than strictly antagonistic roles, and many organs receive only one type of autonomic innervation.
Immersing the face in cold water triggers the diving reflex in humans, a parasympathetically mediated response in which heart rate can drop by 10 to 25 percent within seconds. Trained competitive breath-hold divers exploit this reflex, with some elite divers recording heart rates below 20 beats per minute during deep dives.
Parietal Lobe
/ pah-RY-eh-tul lohb / · Latin parietalis (of the wall) + Greek lobos
Parietal Lobe is the region of each cerebral hemisphere between the frontal and occipital lobes, housing the primary somatosensory cortex and association areas that integrate sensory information, spatial awareness, and attention.
The primary somatosensory cortex in the postcentral gyrus processes touch, proprioception, pain, and temperature from the contralateral body, organized as a somatotopic map in which the hands and face occupy disproportionately large cortical territories. Behind this primary strip, the posterior parietal cortex integrates sensory information with spatial context to guide action and directs attention across the visual field. Damage to the right parietal lobe produces hemispatial neglect, in which patients ignore everything on their left side despite intact vision, demonstrating that this region constructs a representation of space rather than merely receiving sensory input.
Neuroimaging studies of skilled tool users show selective activation of the left inferior parietal lobule, a region substantially expanded in humans relative to other great apes.
Patients with right parietal damage and hemispatial neglect may eat only the food on the right side of their plate, draw only the right half of a clock face, and deny ownership of their left limbs, a condition called somatoparaphrenia.
The parietal lobe only processes touch sensation. It is a major hub for multisensory integration, spatial navigation, body ownership, numerical cognition, and the online control of reaching and grasping movements.
Tool use in humans activates a specific region of the left inferior parietal lobe that is disproportionately large relative to other primates, suggesting this parietal expansion is associated with the evolution of complex manipulative skills. Comparative neuroimaging studies estimate that the human inferior parietal lobule is roughly two to three times larger, relative to total brain size, than the homologous region in chimpanzees (Pan troglodytes).
Peripheral Nervous System
/ peh-RIF-er-ul NER-vus SIS-tem / · Greek periphereia (circumference) + Latin nervosus + systema
Peripheral Nervous System is all neural tissue outside the brain and spinal cord, including cranial nerves, spinal nerves, sensory ganglia, autonomic ganglia, and the enteric nervous system of the gut.
The peripheral nervous system connects the central nervous system to the body’s sensory receptors, muscles, and glands, transmitting sensory information inward and motor commands outward. Unlike the CNS, the peripheral nervous system has significant regenerative capacity after injury because Schwann cells guide regenerating axons and the inhibitory myelin environment present in the CNS is absent. Somatic fibers control voluntary movement and conscious sensation, while autonomic fibers regulate involuntary functions including heart rate, glandular secretion, and smooth muscle tone.
Peripheral axons can regenerate at roughly 1 to 3 millimeters per day following injury, a rate that explains why recovery from nerve damage in the hand or foot can take many months.
The enteric nervous system, embedded in the walls of the gastrointestinal tract, contains approximately 500 million neurons, roughly five times the number in the spinal cord, and can coordinate peristalsis and secretion without any input from the brain or spinal cord.
Fun Facts About the Nervous System →The peripheral nervous system is not a passive relay system. Peripheral sensory neurons contain local protein synthesis machinery and can modify their own excitability through locally translated proteins in response to injury or inflammation.
Translation Biology →Guillain-Barré syndrome, in which the immune system attacks peripheral nerve myelin following certain infections such as Campylobacter jejuni gastroenteritis, can cause ascending paralysis that reaches peak severity within two to four weeks. Approximately 80 percent of patients eventually recover the ability to walk independently, reflecting the regenerative capacity of peripheral nerve fibers.
Pituitary Gland
/ pih-TOO-ih-tair-ee gland / · Latin pituita (phlegm, mucus)
Pituitary Gland is a pea-sized endocrine gland at the base of the brain that secretes hormones controlling growth, reproduction, thyroid function, adrenal function, water balance, and lactation under the direction of the hypothalamus.
The anterior pituitary synthesizes and secretes growth hormone, luteinizing hormone, follicle-stimulating hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, and prolactin, each regulated by specific releasing or inhibiting hormones delivered from the hypothalamus through the pituitary portal blood vessels. Vasopressin, produced in the supraoptic nucleus of the hypothalamus, is transported down axons to the posterior pituitary, where it is released into the bloodstream to regulate water reabsorption in the kidneys. Oxytocin, produced in the paraventricular nucleus, follows the same axonal transport route and triggers milk letdown and uterine contractions during labor.
This arrangement makes the pituitary subordinate to higher brain control rather than an autonomous master gland, as was once assumed.
Certain teleost fish, including the Mozambique tilapia (Oreochromis mossambicus), can shift pituitary prolactin secretion dramatically within hours of moving between fresh water and salt water, using the same hormone that triggers milk production in mammals to regulate gill ion transport instead.
Endocrine System Fun Facts →The pituitary gland is an independent master gland that directs the rest of the endocrine system on its own. Releasing and inhibiting hormones from the hypothalamus govern pituitary output at every step, placing ultimate control one level higher in the brain.
Acromegaly, caused by excess growth hormone from a pituitary adenoma in adulthood, produces progressive enlargement of the hands, feet, and facial features over years. Affected individuals may show hand circumference increases of several centimeters and shoe size increases of two or more sizes before diagnosis, with mean diagnostic delay reported at approximately 7 to 10 years after symptom onset.
Postsynaptic Potential
/ pohst-sin-AP-tik poh-TEN-shul / · Latin post (after) + Greek synapsis + potentia (power)
Postsynaptic Potential is the transient change in membrane potential at the postsynaptic neuron following neurotransmitter binding, either depolarizing the membrane in an excitatory manner or hyperpolarizing it in an inhibitory manner, without necessarily triggering an action potential.
Excitatory postsynaptic potentials arise when glutamate opens cation channels, allowing sodium influx that depolarizes the membrane by a few millivolts, while inhibitory postsynaptic potentials arise when GABA or glycine opens chloride or potassium channels. Unlike action potentials, postsynaptic potentials are graded in amplitude and decay with distance from the synapse. Temporal summation of repeated EPSPs and spatial summation of simultaneous EPSPs from different synapses allow the postsynaptic neuron to integrate multiple inputs before reaching the threshold needed to fire.
A typical cortical neuron receives thousands of synaptic inputs and must integrate their combined effect at the axon initial segment, where voltage-gated sodium channel density is highest.
The axon initial segment, where the axon emerges from the cell body, is the site of action potential initiation because its high density of voltage-gated sodium channels gives it the lowest firing threshold of any part of the neuron, roughly 10 to 20 millivolts above the resting potential.
Every signal arriving at a synapse triggers an action potential in the postsynaptic cell. Postsynaptic potentials are graded signals that must summate from many synapses before the membrane reaches the threshold needed to fire an action potential.
Shunting inhibition occurs when IPSPs do not hyperpolarize the membrane but instead open ion channels that clamp the membrane near the resting potential, reducing the amplitude of concurrent EPSPs and preventing summation. In cerebellar Purkinje cells, inhibitory input at the axon initial segment can suppress firing even while more than 100 excitatory synapses are active on dendrites.
Prefrontal Cortex
/ pree-FRUN-tul KOR-teks / · Latin pre (before) + frontalis + cortex (bark)
Prefrontal Cortex is the anterior region of the frontal lobe that mediates executive functions including working memory, decision-making, impulse control, goal-directed planning, and the regulation of emotional responses.
The prefrontal cortex is disproportionately expanded in humans compared to other primates and is the last brain region to fully mature, with myelination and synaptic refinement continuing into the mid-twenties. Top-down projections from the prefrontal cortex modulate emotional processing in the amygdala, attention systems in the parietal lobe, and behavioral responses generated by basal ganglia circuits. Damage to the prefrontal cortex impairs judgment, social behavior, and the ability to plan for the future while often leaving performance on standard intelligence tests intact, as documented in the case of Phineas Gage following his 1848 railroad accident.
Functional imaging studies show that working memory tasks activate the dorsolateral prefrontal cortex bilaterally, with signal intensity correlating with the number of items held in mind.
Rhesus macaques (Macaca mulatta) with lesions of the dorsolateral prefrontal cortex fail delayed-response tasks in which they must remember the location of a hidden food reward for as little as 5 to 10 seconds, a deficit first documented by Carlyle Jacobsen in the 1930s that established the prefrontal cortex as the neural substrate of working memory.
The prefrontal cortex is simply a seat of rationality that opposes emotional limbic regions. It contains significant emotional processing circuitry of its own, and its connections with the amygdala are bidirectional, meaning emotion shapes prefrontal activity just as prefrontal activity shapes emotion.
Patients with ventromedial prefrontal cortex damage perform normally on standard intelligence and logic tests but make catastrophically poor real-world decisions, as documented by Antonio Damasio and colleagues in studies of patients like Elliot in the 1990s. On the Iowa Gambling Task, such patients continue choosing high-risk card decks even after accumulating repeated losses, a pattern healthy participants abandon within roughly 40 to 50 card selections.