peduncles Massive fiber bundles; one group links the brainstem with the cerebellum (cerebellar peduncles) and another is present on the ventral aspect of the midbrain containing descending fibers from the cerebral cortex to the brainstem and spinal cord (cerebral peduncle).
peduncular region Component of the midbrain containing the crus cerebri and substantia nigra.
pedunculopontine nucleus Nucleus of the pons that contains acetylcholine; associated with REM sleep.
peptide bond Formed when an amino group of one amino acid is joined with the carboxyl group of another amino acid. The amino acids are connected by peptide bonds to form a chain. Protein molecules are made up of different combinations of 20 amino acids.
perforant fiber pathway Fibers from the entorhinal cortex that form excitatory connections with granule cells of the dentate gyrus.
periamygdaloid cortex The more posterior regions of the pyri-form lobe underlying the amygdala.
periaqueductal gray (PAG) Gray matter surrounding the cerebral aqueduct of the midbrain.
pericallosal artery Supplies the precuneus (the portion of the parietal lobe caudal to the paracentral lobule).
periglomerular cell Particular inhibitory cell type of the olfactory bulb; y-aminobutyric acid (GABA)-ergic interneuron.
perilymph Clear extracellular-like fluid containing high sodium and low potassium concentrations filling all of the cavities of the bony labyrinth of the inner ear.
perikaryon Nerve cell body.
perineural oligodendrocytes Oligodendrocytes present in gray matter.
perineurium Connective tissue sheath that encloses each bundle of nerve fibers.
periosteal layer Layer of dura mater that adheres to the inner surface of the skull bone and is highly vascular and innervated.
peripheral nervous system (PNS) Spinal and cranial nerves that are present outside the central nervous system.
perivascular lining membrane Outer surface of blood vessels formed by the joining of astrocytic end-feet.
perivascular space A small space around a blood vessel.
petrosal sinuses Small sinuses located on the superior and inferior regions of the petrous portion of the temporal bone (known as superior and inferior petrosal sinuses, respectively); a network of veins from the cerebellum and medulla empty into the great cerebral vein of Galen and superior and inferior petrosal sinuses.
phagocytosis Transport of solid material into the cells that remove debris (e.g., microglia).
phantom limb pain Perceived sensation of pain from an amputated limb; overactivity of the dorsal horn neurons on the side of the amputated limb may create a false feeling that the pain is emanating from the amputated limb.
phencyclidine ([PCP] "angel dust") Drug of abuse taken for recreational purposes; blocks the N-methyl-D-aspartic acid receptor channel.
phenylethanolamine-N-methyl-transferase (PNMT) Enzyme present in the cytoplasm of a nerve terminal and in the adrenal medulla; catalyzes conversion of norepinephrine into epinephrine.
pheochromocytoma Neuroendocrine tumor of the adrenal medulla that originates in chromaffin cells and secretes excess adrenalin and norepinephrine.
phospholipids Long nonpolar chains of carbon atoms that are bonded to hydrogen atoms; a polar phosphate group is attached to one end of the molecule; abundant in the neuronal membrane.
photoreceptors Rods and cones located in the retina; consist of the following functional regions: an outer segment, an inner segment, and a synaptic terminal.
phototransduction Process by which light is converted into electrical signals in the retina.
phrenic motor nucleus (PMN) Located in the ventral horn of the cervical spinal cord (C3-C5); driven predominantly by the inspiratory neurons located in the rostral group of respiratory neurons located in the ventrolateral medulla; provides innervation to the diaphragm (one of the muscles involved in inspiration).
pia mater The innermost layer of the meninges, which lies closest to the brain.
pigment epithelium layer Outermost layer of the retina consisting of pigmented cuboidal cells that contain melanin.
pilomotor fibers Axons of postganglionic neurons located in the inferior and middle cervical paravertebral ganglia that innervate the erectile muscle of hairs.
pineal body One of the seven circumventricular organs, which have fenestrated capillaries, capillary loops, and large perivascu-lar spaces that permit the passage of larger circulating molecules into the adjacent brain tissue rather than a blood-brain barrier.
pineal gland Conical-shaped structure attached to the roof of the posterior aspect of the third ventricle; displays a circadian rhythm to light with respect to the release of several hormones; contains biogenic amines, including melatonin.
pinealocytes Specialized secretory cells of the pineal gland.
pinna (external ear, auricle) Directs sound vibrations in the air to the external auditory canal.
pinocytosis Process that involves transport of liquid material into the cells that remove debris (e.g., microglia).
piriform cortex (paleopallium) Region of temporal cortex adjoining the amygdala; receives olfactory information and transmits it to amygdaloid nuclei. Also called primary olfactory cortex.
pitch Highness or lowness of the sound; dependent on the frequency of the pressure wave generated by the sound.
pneumotaxic center Group of respiratory neurons located in the dorsolateral pontine tegmentum, just caudal to the inferior colliculus; considered to be essential for maintaining a normal breathing pattern.
polypeptide A protein made of a single chain of amino acids.
pons Part of the brainstem extending from the midbrain to the medulla.
pontine arteries Arise from the basilar artery and supply portions of the pons and ventrolateral pons.
pontine (medial) reticulospinal tract Pathway arising from the nucleus reticularis pontis caudalis and nucleus reticularis pon-tis oralis. These neurons project ipsilaterally to the entire extent of the spinal cord, and their principal function is to facilitate extensor spinal reflexes.
pontine gaze center Region of the pons adjacent to the abducens nucleus that integrates inputs from the cerebral cortex and vestibular nuclei in order to regulate horizontal eye movements.
pore loop Hydrophobic regions connecting helical segments in voltage-gated channels.
positron emission tomography (PET) Imaging technique involving injection of a radiolabeled substance into the circulation. Excess protons are converted into neutrons by radioactive decay, and a positron (a positively charged electron) is emitted; the positron collides with an electron, releasing two photons that are sensed by detectors. Blood flow and glucose utilization in the desired region of the body are then monitored.
postcentral gyrus Region of cerebral cortex that has the central sulcus as its anterior border and the postcentral sulcus as its posterior border. It is the primary receiving area for kines-thetic and tactile information from the periphery (trunk and extremities).
postcommissural fornix Component of the fornix that is distributed to the diencephalon.
posterior Above the midbrain, posterior means "toward the back of the brain," and below the midbrain, posterior means "toward the dorsal surface of the body."
posterior cerebral arteries Arteries that arise at the terminal bifurcation of the basilar artery and supply the occipital lobe and most of the midbrain, thalamus, and subthalamic nucleus.
posterior cerebral artery syndrome Interruption of the calcar-ine branch of the posterior cerebral artery, which supplies both the upper and lower banks of the visual cortex, results in hemianopsia.
posterior chamber Space between the lens and the iris in the eye.
posterior commissure Commissure of the brain located on the border of the diencephalon and midbrain; associated with functions of the oculomotor complex.
posterior communicating arteries Arteries that arise from the internal carotid arteries at the level of the optic chiasm; travel posteriorly to join the posterior cerebral arteries.
posterior inferior cerebellar artery (PICA) Arises from the vertebral artery and supplies the regions of the lateral medulla that include the spinothalamic tract, dorsal and ventral spi-nocerebellar tracts, descending sympathetic tract, descending tract of cranial nerve V, and nucleus ambiguus.
posterior lobe Largest lobe of the cerebellum associated, in part, with regulation of coordinated movements of the distal musculature. Also called the neocerebellum.
posterior neuropore A temporary opening of part of the neural tube of the early embryo extending from the central canal to its posterior aspect. posterior parietal arteries Supply different regions of the parietal lobe.
posterior parietal cortex (PPC) Region of the superior aspect of the parietal cortex that integrates different modalities of sensory inputs and projects its axons to area 6 of the frontal lobe. This region is essential for programming and sequencing of motor responses, and damage to this region results in apraxia and sensory neglect. posterior pituitary gland See neurohypophysis.
posterior spinal artery (PSA) This artery typically arises from the posterior inferior cerebellar artery and descends on the dorsolat-eral surface of the spinal cord slightly medial to the dorsal roots.
posterior temporal artery Supplies different regions of the temporal lobe and lateral portions of the occipital lobe. posterolateral fissure Major fissure of the cerebellum separating the flocculonodular from posterior lobes. posterolateral spinal vein See spinal veins.
posteromarginal nucleus Located in the lamina I of the spinal cord. Terminals of dorsal root fibers mediating pain and temperature sensations synapse on these cells; axons of these cells cross to the opposite side and ascend as the lateral spinotha-lamic tract.
postganglionic parasympathetic fibers Fibers arising from ganglia that lie close to their target organs and that receive inputs from preganglionic parasympathetic fibers.
postictal period The time following a seizure. postrotary nystagmus See nystagmus.
postsynaptic junctional folds Formed in depressons of the muscle fiber membrane and lined by a basement membrane (connective tissue consisting of collagen and glycoproteins).
posttraumatic stress disorder (PTSD) Psychiatric disorder resulting from a life-threatening event in the distant past of the individual; such an event reappears in the patient’s memory during the daytime or in the evening in dreams and may relate to the disruption of mechanisms that control rapid eye movement sleep. postural hypotension See orthostatic hypotension.
pre-Botzinger complex An area just caudal to the Botzinger complex in the medulla implicated as the site of respiratory rhythm generation.
precentral arteries Supply different regions of the frontal lobe.
precentral gyrus Region of the cerebral cortex bounded posteriorly and anteriorly by the central and precentral sulci, respectively. Integrates motor signals from different regions of the brain and serves as the primary motor cortex for control of contralateral voluntary movements; neurons within the pre-central gyrus are somatotopically organized. precommissural fornix Component of the fornix that passes rostral to the anterior commissure and that is distributed to the septal area.
prefrontal cortex Region of the far rostral cerebral cortex, which includes inferior (orbital gyri), medial, and lateral aspects of the frontal lobe; this region plays an important role in the processing of intellectual and emotional events. prefrontal lobotomy A neurosurgical procedure (not used anymore) involving the undercutting of the afferent and efferent connections of the prefrontal cortex in order to control manifestations of hostility, anger, and violent behavior. preganglionic parasympathetic fibers See parasympathetic division of autonomic nervous system. premotor area (premotor cortex) Region of cerebral cortex immediately rostral to the precentral gyrus and immediately ventral to the supplemental motor area (area 6 of Brodmann); this region regulates movements associated with the contralat-eral side of the body by playing an important role in the initiation and sequencing of movements. preoptic region Anterior aspect of the basal forebrain containing lateral and medial preoptic nuclei; concerned with endocrine functions and temperature regulation. pre-propeptides Protein precursors.
prepyriform area Region of cortex extending from the lateral olfactory stria to the rostral aspect of the amygdala. prestriate area An association area related to functions of the visual system.
pretectal region (or area) Most rostral region of midbrain bordering on the diencephalon; it comprises a component of the pathway mediating visual reflexes.
prevertebral ganglia Sympathetic ganglia, separate from the sympathetic chain ganglia. Neurons in these ganglia receive preganglionic axons that pass uninterrupted through the para-vertebral ganglia. primary fissure Major fissure of the cerebellum separating the anterior from the posterior lobe. primary somatosensory cortex Postcentral gyrus (areas 3, 1, and 2 of Brodmann).
primary auditory cortex Processes auditory signals; located in the transverse temporal gyri. primary motor cortex Located in the frontal lobe; processes the planning and execution of movements. primary olfactory cortex (piriform or pyriform cortex) Processes olfactory signals; located in the temporal lobe.
primary visual cortex Located on both sides of the calcarine sulcus; receives visual signals from the temporal half of the ipsilateral retina and nasal half of the contralateral retina; concerned with perception of signals from the contralateral half of the visual field.
principal neurons (projecting, type I, or Golgi type I neurons) Possess axons that form long fiber tracts in the brain and spinal cord.
prion An infectious agent consisting primarily of protein. The misfolded form of prion protein has been implicated in several diseases including Creutzfeldt-Jakob Disease, Kuru and bovine spongiform encephalopathy (mad cow disease). procedural memory Refers to a form of memory that differs from declarative memory in that the individual may not be consciously aware of the procedures involved in carrying out a task (e.g., shooting a basketball, skiing, hitting a baseball).
prodromal phase First phase of schizophrenia, which typically precedes the onset of the first psychotic episode, consisting of physical complaints, such as muscle pain and headaches; an increased interest in religion, philosophy, and the occult; and bizarre thoughts, distorted perception, and unusual speech. projecting neurons See principal neurons.
prolactin Pituitary hormone essential for milk production from the lactating mammary gland. pro-opiomelanocortin A large protein that serves as a precursor for a number of peptides that have been identified within the hypothalamus, including P-lipotropin, melanocyte-stimulating hormones, and P-endorphin.
propeptide Pre-propeptide from which the signal sequence of amino acids is removed. proper sensory nucleus See nucleus proprius.
proprioception Sensory modality that provides feedback solely on the status of the body internally; indicates whether the body is moving with required effort as well as where the various parts of the body are located in relation to each other. In conscious proprioception, the receptors located in the joints and joint capsules (proprioceptors) provide sensory information to the cerebral cortex, which, in turn, uses this information to generate conscious awareness of kinesthesia (i.e., the joint position, direction, and velocity of joint movements). In non-conscious proprioception, the impulses arising from the proprio-ceptors mediating this type of sensation (muscle spindles and Golgi tendon organs) are relayed to the cerebellum rather than to the cerebral cortex.
proprioceptors Sensory receptors associated with the stretch of a muscle, tendon, or bodily position.
prosencephalon Developing forebrain that will become the diencephalon and telencephalon.
prosopagnosia Disorder of the inferotemporal cortex in which there is a loss of the ability to recognize familiar faces even though the patient is able to describe the physical features of such individuals. prostaglandin E2 One of the endogenous substances known to sensitize nociceptors causing hyperalgesia; cyclo-oxygenase metabolite of arachidonic acid. protoplasmic astrocytes Present in the gray matter in close association with neurons; serve as metabolic intermediaries for neurons; give out thicker and shorter processes that terminate in expansions called end-feet.
pseudobulbar palsy Mild form of weakness in muscles that regulate breathing, swallowing, speech, and chewing after damage to corticobulbar fibers.
pseudo-unipolar neuron Neuron from which a single process arises that divides into two branches; one branch projects to the periphery, and the other projects to the central nervous system (CNS). Information collected from the peripheral end is transmitted to the CNS via the axonal terminals (e.g., sensory cells in the dorsal root ganglion).
pseudoptosis Partial drooping of the upper eyelid. psychotic stage Second phase of schizophrenia in which the patient loses touch with reality; this loss is reflected by positive symptoms, which include hallucinations (such as hearing voices) and delusions (such as a feeling of persecution), and negative symptoms, which include disorganized thinking (expressed by the absence of normal association of thoughts, poor speech, and loss of affect).
ptosis Drooping of the eyelid resulting from damage to the nerves innervating the levator palpebrae superior muscle. pulmonary stretch receptors Located in the smooth muscles of the lung; activated during inspiration.
pulvinar nucleus Large nucleus of the posterior thalamus that projects to the inferior parietal lobule and is believed to be concerned with mediating complex auditory and visual discriminations.
pupil Central circular opening in the iris of the eye.
pupillary constriction The result of shining light into the eye, causing a reflex (pupillary light reflex) mediated through the parasympathetic component of cranial nerve III, resulting in contraction of the pupillary constrictor muscles with a reduction in the size of the pupil.
purines New class of neurotransmitters that includes adenosine triphosphate.
Purkinje cell Large inhibitory neuron of the gray matter of the cerebellar cortex that projects to deep cerebellar nuclei.
Purkinje cell layer One of three histological layers of the cere-bellar cortex; contains Purkinje cells and is one-cell thick. putamen Major component of the basal ganglia, along with the caudate nucleus and globus pallidus.
pyramid Protuberance on the ventromedial aspect of the lower brainstem containing corticospinal and corticobulbar fibers.
pyramidal cell layer Layers III (external) and V (internal) of the cerebral cortex containing pyramidal cells. One of three layers of the cerebellar cortex; major source of output neurons of the hippocampus.
pyramidal cells Prominent cells of the cerebral cortex and hip-pocampal formation that form the main output neurons of these regions.
pyramidal decussation Region of the caudal medulla where the corticospinal tract crosses from one side of the brainstem to the other.
pyriform cortex (paleopallium) Region of temporal cortex adjoining the amygdala; receives olfactory information and transmits it to amygdaloid nuclei. Also called primary olfactory cortex. pyriform lobe Cortical mantle that provides significant input to the amygdala.
