Neuroscience at a Glance 3rd Edition
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Neuroscience Glossary

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Action potential is an all-or-nothing phenomenon and represents the basic electrical signal of the neuron. It is initiated at the initial segment of the axon hillock of the neuron and is propagated down the axon to the presynaptic terminals. It is typically in the order of 100 mV in amplitude

Active touch involves the corticospinal tracts and somatosensory system in the perception of shapes and textures by the manipulation of objects in the hand

Active zones are found in the presynaptic nerve terminal and are the sites where calcium enters the nerve terminal and the presynaptic vesicles containing neurotransmitter are released

Adaptation is defined as the decline in response of a sensory receptor to a continual steady stimulus

Afferent refers to a neuron or pathway that sends information into the CNS, typically sensory in nature

Agonist refers either to: (i) an agent that binds and activates a membrane receptor, and which in some cases defines that receptor type, e.g. the N-methyl-d-aspartate (NMDA) glutamate receptor; or (ii) a muscle that acts in a similar fashion to that performing the primary movement around a joint

Antagonist refers either to: (i) an agent that blocks the action of an agonist at its membrane receptor; or (ii) a muscle that opposes those muscles performing a specific movement around a joint. For example, the triceps muscle acts antagonistically to the biceps muscle in elbow flexion

Antidromic refers to the propagation of an action potential along an axon in a direction that is the reverse of the normal direction of transmission, and is typically only seen experimentally

Apoptosis an intrinsic cellular programme that once activated leads to cell death

Arachnoid membrane is a thin, transparent membrane which is avascular. It has trabeculae that communicate with the pia mater and in places pierces the dura to form arachnoid villi that are important in the absorption of cerebrospinal fluid

Association cortex are those areas that receive multiple inputs from sensory cortical areas subserving different modalities. They thus have a role in higher sensory processing as well as the formulation of motor responses to sensory stimuli. The posterior parietal, temporal and prefrontal cortical areas are typically described as association cortices

Astrocyte is a type of neuroglial cell that is found throughout the CNS and which has a number of important homeostatic and structural functions including the formation and maintenance of the blood-brain barrier

Autonomic nervous system (ANS) has a central and peripheral component and is concerned with the innervation of internal and glandular organs. It is made up of the sympathetic, parasympathetic and enteric nervous systems

Axoaxonic; axodendritic and axosomatic synapses refer to synapses made between an axon and another axon, dendrite or neuronal cell body (soma), respectively

Axolemma is the plasma membrane of the axon

Axon is the neuronal process that originates at the axon hillock and conducts information away from the neuronal cell body to the nerve terminal and synapses. There is only ever one axon per neuron, although that axon may have multiple branches

Axon collateral is a branch of the main axon, and originates at the node of Ranvier

Axon hillock is the site at which the axon originates from the neuronal cell body, and is in continuity with the initial segment of the axon. This is the most excitable part of a neuron as a result of its high density of sodium channels and is therefore the site of initiation of the action potential

Axoplasmic flow or axonal transport refers to the transport of macromolecules and membranous organelles along the axon. Typically this is away from the cell body (anterograde) and towards the nerve terminals, although retrograde transport does occur. Axonal transport can be either fast or slow in nature, and employs a number of motor proteins as well as filaments

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Basal ganglia refers to the following collection of subcortical structures which are found in each cerebral hemisphere: the caudate and putamen (neostriatum); the globus pallidum; the substantia nigra and the subthalamic nucleus.

Basilar membrane is found in the cochlea and on it sits the organ of Corti. Its width and flexibility changes with distance along the cochlea, and these properties allow it to perform an important function in frequency tuning within the cochlea

Blobs are cytochrome oxidase-rich areas in the primary visual cortex, found predominantly in cortical layers II and III and to a lesser extent layers V and VI

Blood-brain barrier (BBB) is formed by the very high resistance tight junctions between the cerebral capillary endothelial cells, which are maintained in this state by astrocytes. The BBB serves to prevent the passage of large molecules and cells into the CNS

Brainstem is that part of the brain that connects the spinal cord to the cerebral hemispheres and from which 10 out of the 12 cranial nerves originate. It is connected to the cerebellum via three pairs of cerebellar peduncles. It is composed of the medulla, pons and midbrain and controls many basic functions such as respiration, circulation and wakefulness

Broca's area is found in the posterior part of the dominant (usually left) prefrontal cortex and is important in the expression of language as speech. Damage to this area causes an expressive dysphasia

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Cauda equina refers to the descending ventral and dorsal roots of the lumbar, sacral and coccygeal nerves that lie in the subarachnoid space at the caudal tip of the spinal cord

Central nervous system (CNS) embraces those cells that lie within spinal cord and brain (brainstem, cerebellum and the cerebral hemispheres). It is composed of neurons and a number of different types of neuroglial cells

Central pattern generators refer to networks of neurons in the spinal cord (and brainstem) that are capable of generating their own outputs to motorneurons independently of any descending or peripheral sensory input. They are important in locomotion and respiration

Centre-surround (receptive field) organisation is found mainly within the visual system and describes a situation in which the stimulus at the centre of the receptive field elicits one response (e.g. a depolarisation or 'on' centre response), while an annulus of light around it produces the opposite effect (e.g. a hyperpolarisation or 'off' surround response)

Centrifugal nerve fibres project out of the CNS to the periphery. The term is generally reserved for sensory systems rather than motor pathways, and refers to projections down the sensory pathway which may include a CNS input to the sensory receptor itself (e.g. hair cells of the cochlea)

Cerebellum is found in the posterior fossa behind the brainstem to which it is connected by three pairs of cerebellar peduncles. It is important for learning and storing motor acts and in the coordination of voluntary movement

Cerebral cortex represents the outer layer of the cerebral hemisphere and contains neurons that are organised in terms of horizontal layers and vertical columns

Cerebral peduncle connects the midbrain to the thalamus and consists of the tegmentum, substantia nigra and crus cerebri that contains the descending motor and ascending sensory pathways that originate or relay to the cerebral cortex.

Cerebrospinal fluid (CSF) is found within the CNS ventricular system and subarachnoid space and is formed by the choroid plexus. It is reabsorbed by arachnoid granulations

Circadian rhythms refers to any naturally occurring, approximately 24 hour (daily), rhythm

Closed-loop (or reflexly controlled) movements are those that are guided by inputs from sensory systems

Cochlea is found in the inner ear and contains the receptors for hearing

Columnar hypothesis refers to conceptual organisation of the cerebral cortex in terms of vertical columns of cells with similar functional properties

Complex cells are found mainly in the primary visual cortex and were so-named by Hubel and Wiesel. These cells have large receptive fields that are maximally activated by a line or bar of illumination of a given orientation moving in a particular direction, that direction often being orthogonal to the line orientation

Contralateral on the opposite side

Corpus callosum is the largest commissure in the whole CNS and connects the two cerebral hemispheres

Corpus striatum consists of caudate nucleus, putamen and globus pallidus

Corticobulbar tracts refer to those pathways that originate in the cortical motor areas and terminate in the brainstem

Corticospinal or pyramidal tract takes its origin from the Betz cells in the primary motor cortex, as well as the primary somatosensory cortex and premotor cortical areas. Although it predominantly projects to the motorneurons, it also has inputs to other structures such as the dorsal column nuclei

Cotransmission refers to the release of more than one neurotransmitter at a synapse

Cranial nerves refer to the 12 sets of nerves that originate from the brainstem, retina and nose and which mediate the special senses as well as providing the motor and sensory innervation of the head and neck

Critical firing threshold is the membrane potential at which sufficient sodium channels are open to allow for the generation of an action potential

Critical period is that time in development when there is maximum plasticity in the evolving neural system, such that it can be modified by environmental inputs. It is best described in the developing visual system

Cyclic adenosine monophosphate (cAMP) is made by the enzyme adenyl cyclase and serves to trigger protein phosphorylation by cAMP protein kinases. It is thus an important secondary messenger in many cells, including neurons

Cytoskeleton refers to the network of microfilaments, microtubules, neurofilaments and an assortment of proteins which maintains neuronal architecture and allows it to modify its appearance during development, growth or injury

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Decussation refers to the crossing of a neural pathway in the spinal cord or brainstem

Dendrites are the neuronal cell processes that taper from the soma outwards, branch profusely and are responsible for conveying information to the neuron

Depolarisation is a shift in the membrane potential of a cell to a less negative value. It increases the likelihood that a neuron will reach the critical firing threshold for an action potential and so is excitatory in nature

Desensitisation is the process in which the binding of a ligand to its receptor leads to a decreased response. It is also termed down regulation and in time there is an actual decrease in receptor number as a result of increased endocytosis of the ligand-bound receptors

Diencephalon refers to the thalamus, hypothalamus, subthalamus and epithalamus

Dorsal columns are those ascending sensory tracts in the spinal cord that relay information from the large myelinated sensory nerve fibres conveying light touch, joint position sense and vibration perception. They are preferentially damaged in tabes dorsalis, subacute degeneration of the spinal cord and Friedreich's ataxia

Dorsal column nuclei are found in the medulla and receive a synaptic input from the dorsal columns. They project to the thalamus as the medial lemniscus

Dorsal horn is that part of the spinal cord where the sensory afferent fibres enter. It contains a number of interneurons as well as receiving an input from descending pathways originating in the brainstem

Dorsal root ganglia are found just outside the spinal cord and contain the cell bodies of the sensory neurons in the PNS

Dorsolateral descending motor pathways refers to the corticospinal and rubrospinal tracts that preferentially innervate those motorneurons that control the distal musculature

Down regulation see Desensitisation

Dura mater is a thick tough membrane lying close to the cranium and vertebrae. It is separated from the arachnoid membrane by the subdural space. In the spinal cord the inner and outer layers of the dura mater are separated by the extradural space

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Efferent refers to a neuron or pathway that transmits signals away from the CNS to the periphery or some other CNS site

Electrical synapses allow the direct flow of current from a pre-synaptic to a postsynaptic cell. They typically consist of gap-junction channels

Endocytosis refers to the process in which a cell internally recycles part of its own membrane; for example, neurotransmitter receptors or fused vesicle membrane in the presynaptic nerve terminal

Endolymph is the fluid found within the scala media of the cochlea. It has a high potassium concentration, similar to intracellular fluid

Endorphins and enkephalins are naturally occurring opioid-like substances found in the nervous system. They are peptides that bind to specific receptors and are intimately involved in pain processing

End-plate is the region in muscle cells where the axon from the motorneuron contacts and makes a synapse

End-plate potential (epp) is the depolarisation seen at the neuromuscular junction when several vesicles of transmitter are released. It is made up of a number of miniature end-plate potentials, and if sufficiently large will reach threshold for action potential generation in the postsynaptic muscle fibre

Enteric nervous system controls the gut musculature and is part of the autonomic nervous system

Ependymal cells line the ventricular system of the brain and central canal of the spinal cord

Equilibrium potential is the membrane potential for a given ion species at which the electrical force driving it in one direction is countered by an equal chemical force in the opposite direction. The electrical force is determined by the charge on the membrane and the chemical force by the concentration gradient of the ion across the membrane. It can be calculated using the Nernst equation.

Excitatory postsynaptic potential (EPSP) is the depolarisation of a postsynaptic membrane in response to a synaptic input which increases the probability that a cell will fire an action potential

Exocytosis is the release of intracellular vesicles (which contain neurotransmitter) by a process involving the docking and fusion of the vesicular membrane with the membrane of the cell. This is the mode of release of neurotransmitter at chemical synapses

Extrafusal fibres are the main contractile elements of striated muscle and are found outside the muscle spindle (cf intrafusal fibres)

Extrapyramidal disorders are those conditions which are thought to be caused by a dysfunction of the basal ganglia (e.g. Parkinson's disease)

Extrapyramidal tracts refers to those descending motor pathways that do not take their origin from the cerebral cortex. It thus includes the rubrospinal, reticulospinal, tectospinal and vestibulospinal tracts

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Fasciculus is a tract or bundle of nerve fibres

Foramen magnum is the opening at the base of the skull where the brainstem ends and the spinal cord begins

Frequency coding (temporal) process by which the characteristics of a sensory stimulus are encoded in the form of patterns of action potentials

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G-protein is a GTP-dependent membrane protein that links a receptor for a neurotransmitter to an intracellular enzyme

Ganglion refers to a group of functionally related cell bodies in the PNS

Gate theory refers to the hypothesis put forward by Melzack and Wall in the mid 1960s that postulated that the passage of nociceptive information across the dorsal horn could be inhibited by large fibre synaptic inputs. It forms the rationale for using transcutaneous nerve stimulation as a treatment for pain

Gating is the active transition of an ion channel from an open to a closed state

Generator potential is the depolarisation induced in the terminal of a sensory receptor, which if sufficiently large can initiate an action potential in the afferent axon

Glia or neuroglia form the other major cell class in the nervous system with neurons. There are several different types of glial cell within the CNS but only one in the PNS (Schwann cells)

Golgi tendon organs are sensory receptors found in the tendons of muscles. They thus lie in series with the muscle fibres and so primarily signal tension within the muscle. They relay information to the spinal cord in the form of a Ib afferent nerve

Gray or Grey matter is that part of the nervous system that contains neuronal cell bodies

Gustatory system are those neurons and pathways involved in taste

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Habituation is a learning process in which a subject decreases his/her behavioural response to a repetitive stimulus

Hair cells are the sensory receptors of hearing in the cochlea of the ear as well as of balance in the vestibular apparatus of the inner ear

Hippocampal complex is found in the medial temporal lobe and consists of the dentate gyrus, CA1-CA4 subfields of the hippocampus proper and the subiculum. It is part of the limbic system and is especially susceptible to hypoxic and ischaemic damage. It is thought to be important in memory and has been the site of most research on long-term potentiation

Hyperalgesia is the evocation of pain by a weakly noxious stimulus

Hypercolumn is defined in the primary visual cortex as that area of cortex which contains an ocular dominance column for each eye and a complete set of orientation columns, covering 180° of orientation

Hypercomplex or end-stopped cells (after Hubel and Wiesel) are found in the visual cortex and have large receptive fields which respond maximally to a line or bar of illumination of given orientation and length

Hyperpolarisation is a shift in membrane potential to a more negative value. It reduces the probability of an action potential being generated and is thus inhibitory

Hypothalamus is part of the diencephalon. It lies just above the pituitary gland and has a number of important neural and endocrine functions

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Iatrogenic disorders are those caused inadvertently as a consequence of medical treatment

Inhibitory postsynaptic potential (IPSP) is the hyperpolarisation of a postsynaptic membrane in response to a synaptic input which decreases the probability that a cell will fire an action potential

Initial segment of the axon is that part of the axon, adjacent to the axon hillock, which has the highest density of sodium channels and which is the most excitable part of a neuron. It is thus the site of action potential generation

Internal capsule large mass of white matter running between the basal ganglia and thalamus containing many ascending and descending pathways to and from the cortex

Interneurons are neurons with either small axons that project locally within the CNS (so-called local interneurons) or long axons that relay to distant CNS targets (relay interneurons)

Intrafusal fibres are the specialised muscle fibres found within the muscle spindle

Ion channel is a transmembrane pore that allows ions to flow across a membrane. It exists in at least an open and closed state and is regulated by either a change in membrane potential (voltage-gated channels) or the binding of a specific neurotransmitter or chemical substance (ligand-gated or chemically activated channels)

Ipsilateral on the same side

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Junctional fold is the specialisation seen in the postsynaptic muscle fibre at the neuromuscular junction

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Kindling is an experimentally induced process in which a repeated focal application of an initially subconvulsive electrical stimulus ultimately results in a partial or generalised seizure

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Labyrinth is made up of the utricle and sacculus and together with the semicircular canals forms the peripheral vestibular system of the inner ear

Lateral inhibition is the reciprocal suppression of submaximally excited inputs by neighbouring neurons in a sensory pathway. It serves to increase contrast within the sensory pathway

Ligand is a substance that binds to a membrane receptor or ion channel. Most ligands are either neurotransmitters, hormones or drugs

Limbic system is a collection of structures that lie along the medial aspect of the temporal (and to a lesser extent frontal and parietal) lobe and includes the cingulate gyrus, parahippocampal structures, entorhinal cortex, hippocampal complex, septal nuclei and amygdala. It is involved in emotional responsiveness as well as having an important role in memory acquisition. Many people object to the use of this term, complaining that it is too vague to be of value

Long latency or transcortical reflexes refer to the delayed and smaller electromyographic changes that are seen following the sudden stretch of a muscle. The dorsal column-medial lemniscal system and corticospinal tract are thought to relay the afferent and efferent limbs of this reflex, that may have a role in load compensation

Long-term depression (LTD) is defined as a decrease in strength of synaptic transmission with repetitive use that lasts for more than a few minutes

Long-term potentiation (LTP) is defined as an increase in strength of synaptic transmission with repetitive use that lasts for more than a few minutes. In the hippocampus LTP can be triggered by less than 1 second of intense synaptic activity and lasts for hours or more. It may, with LTD, underlie memory acquisition

Lower motoneurons (LMNs) are those motorneurons that directly innervate the muscles. They are therefore found in some of the cranial nerve nuclei as well as the anterior horn of the spinal cord. Damage to the LMN results in muscle weakness, wasting with a loss of reflexes

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Major histocompatibility complex (MHC) is important in regulating the activity of all immune responses involving T-lymphocytes, as the T-lymphocyte receptor requires the presentation of foreign antigen with a class of MHC molecule. All cells express MHC class I but only a few specialised cells express MHC class II. These latter cells are termed antigen presenting cells and are important in controlling the extent to which an immunological reaction can be mounted. In the CNS, MHC class II expression is low and probably restricted to microglial cells

Medial lemniscus is the fibre tract connecting the dorsal column nuclei to the ventroposterior nucleus of the thalamus

Medial longitudinal fasciculus connects the oculomotor nuclei as well as projecting down into the upper cervical cord. A lesion to the brainstem portion of the fasciculus commonly occurs in multiple sclerosis and produces an internuclear ophthalmoplegia

Medulla forms the lower part of the brainstem, lying between the cervical spinal cord and pons. It contains the nuclei of the lower cranial nerves (IX-XII), as well as many critical cardiorespiratory centres

Meissner's corpuscles are cutaneous receptors found close to the surface of hairless (glabrous) skin which have small receptive cells and rapidly adapt to an applied sensory stimuli. They are thus classified as rapidly adapting type I (RAI) receptors and are sensitive to low-frequency mechanical stimuli or flutter

Memory is the storage of learned information

Meninges are the membranes separating the skull and vertebral column from the brain and spinal cord (see dura mater, arachnoid membrane and pia mater)

Merkel's disc is a cutaneous receptor of the slowly adapting type I phenotype (SAI) that lies close to the surface of glabrous skin. They are sensitive to sustained pressure

Metabotropic receptors respond on activation with glutamate binding by initiating a number of intracellular biochemical events which modulate synaptic and neuronal activity. They are not directly linked to any specific ion channels

Microglial cells are a type of neuroglia that are similar to the peripheral macrophage. They probably play an important part in the immune responses of the CNS

Midbrain lies at the top of the brainstem between the pons and cerebral peduncle. It contains the third and fourth cranial nerve nuclei, as well as a number of other important structures such as the red nucleus, substantia nigra, periaqueductal grey matter and colliculi

Miniature end-plate potential (mepp) represents the post-synaptic membrane potential in the muscle fibre in response to the release of one vesicle of neurotransmitter from the motorneuron nerve terminal. The release of several vesicles results in a summation of mepps to give an end-plate potential (epp)

Modality is the type of preferred sensory stimulus transduced and relayed by a sensory system

Motorneurons (MNs) are those neurons that are directly involved in the activation of muscles. They can broadly be defined in terms of those directly innervating muscles-the lower motorneurons (LMNs)-and those that innervate the LMN, the upper motorneurons (UMNs). The LMN can be further subdivided into those that innervate the muscle spindle (gamma-motorneuron) and those that innervate the extrafusal (force-generating) muscle fibres (alpha-motorneuron)

Motor unit refers to the relationship of a single motorneuron axon to the number of muscle fibres it innervates

Muscle spindle is an encapsulated sensory organ that lies within muscles and conveys information to the spinal cord as well as receiving a specific input from the anterior horn gamma-motorneuron. It is important in setting muscle tone, mediating the tendon stretch reflex, as well as providing useful proprioceptive information to the CNS

Myelin sheath is the fatty layer of insulation that is wrapped around most large nerve fibres and which allows for rapid, unattentuated conduction of the action potential

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Neostriatum refers to the caudate nucleus and putamen and is the major receiving area of the basal ganglia

Nernst equation is used to calculate the equilibrium potential for a particular ion

Neuromuscular junction (NMJ) is the point of communication between the lower motorneuron axon nerve terminal and the muscle fibre it innervates

Neural Stem cell These cells are found in the developing brain (embryonic neuro stem cells) as well as at certain sites in the adult CNS. They are capable of dividing to form neurons, astrocytes, olyodendrocytes, as well as more neural stem cells. In the adult brain they may have a role in repair and memory

Neuron (or neurone) is one of the two major classes of cell within the nervous system. Neurons can be classified into one of three major types: sensory, motor and interneurons. They generate and propagate action potentials and communicate with each other through billions of connections termed synapses

Neurotransmitter is a chemical substance released by a presynaptic nerve terminal that diffuses and binds to specific postsynaptic receptors which leads to an alteration in current flow in that postsynaptic cell

Neurotrophic factors are specific molecules that appear to promote the survival and/or mitogenesis of neural cells

Nitric oxide (NO) is a short-lived gas that readily passes through cell membranes and which may mediate long-term potentiation. It is known to activate a number of intracellular secondary messengers and so may have a number of other roles within the CNS

N-methyl-d-aspartate (NMDA) glutamate receptor is a type of glutamate receptor that may be important in a number of different synaptic processes, including long-term potentiation

Nociceptors are sense organs detecting damaging stimuli which are often perceived as being painful

Nodes of Ranvier are the gaps in the myelin sheath where the axolemma is directly exposed to tissue fluid. They contain ion channels and are responsible for propagating the action potential down the axon by saltatory conduction. They are also the site of origin for axon collaterals

Nucleus is a collection of functionally related cell bodies in the CNS (cf ganglia in PNS)

Nystagmus is an eye movement in which there is a biphasic oscillation of the eyes. It can be seen normally under some circumstances (such as the extremes of gaze) as well as in a number of diseases affecting the brainstem, vestibular apparatus and cerebellum

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Ocular dominance columns were first described by Hubel and Wiesel in the primary visual cortex. Each ocular dominance column represents a vertical collection of neurons that have a dominant input from one eye. This ocular dominance is determined by the thalamic input to layer IV of the primary visual cortex

Oculomotor nuclei are the third, fourth and sixth cranial nerve nuclei located in the midbrain and pons responsible for controlling the extraocular muscles and linked to each other by the medial longitudinal fasciculus

Oligodendrocyte is a type of neuroglial cell found in the CNS that creates the myelin sheath around the axon

Open-loop (or volitional) movements are those triggered by a sensory cue or some internal desire to move

Organ of Corti is found in the cochlea and contains the hair cells important in auditory transduction

Orientation selective columns were first described by Hubel and Wiesel in the primary visual cortex, and represent a vertical collection of neurons that have a preferred orientation of visual stimulus

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Pacinian corpuscle is a cutaneous (and visceral) receptor that is classified as being rapidly adapting type II (RAII). It lies deep in the dermis of the skin and is sensitive to high-frequency mechanical stimuli or vibration

Parallel processing is the relaying and analysis of sensory information down a number of parallel pathways, each performing some relatively specific processing. This is best seen in the visual system

Parasympathetic nervous system is one of the parts of the autonomic nervous system. It uses acetylcholine as its postganglionic neurotransmitter

Peripheral nervous system (PNS) is defined as those nerves that lie outside the brain, brainstem or spinal cord. The PNS consists of nerve trunks made up of both afferent fibres or axons conducting sensory information to the spinal cord and brainstem and efferent fibres transmitting impulses primarily to the muscles

Phosphorylation is the covalent addition of a phosphate group to a protein by a protein kinase with an alteration in the activity of the protein

Pia mater is a vascular membrane that covers the surface of the CNS

Plasticity can occur at the level of synapses (see Long-term depression and Potentiation) as well as within neural systems (e.g. visual cortex). It refers to the ability to change the efficacy of synaptic transmission and neuronal connections in the face of altered afferent activity

Pons is found in the brainstem between the medulla and midbrain. It contains part or all of the fifth to eighth cranial nerve nuclei as well as relaying the major cortical input to the cerebellum

Positive feedback refers to the process whereby a disturbance about a homeostatic set-point results in an increase in that disturbance. This form of feedback is rarely used in biological systems because of its explosive and damaging nature, but such a system operates in the generation of the upstroke of the action potential

Posterior fossa refers to that part of the skull that lies below the tentorial membrane and above the foramen magnum, and which contains the brainstem and cerebellum

Postsynaptic cell is that neuron or cell that has an altered excitability as a result of binding a neurotransmitter released by an afferent presynaptic nerve terminal

Premotor cortex refers to the lateral part of Brodmann's area 6 which is just anterior to the primary motor cortex. It is involved in the planning and initiation of movement as well as the control of proximal musculature. It is to be distinguished from the premotor cortical areas which refers to all prefrontal cortical areas that project to the primary motor cortex

Presynaptic cell is that neuron which synapses on to another neuron or cell

Primary afferent (primary sensory neuron) is the first neuron in the sensory pathway; it thus refers to the sensory receptor, its afferent axon and cell body together with the synaptic contacts in the spinal cord

Principle of recruitment corresponds to the order in which different muscle fibres are activated (see Size principle)

Proprioception the sense of joint/body position

Pyramidal tract see Corticospinal tract

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Quantal transmission refers to the vesicular release of neurotransmitters at the chemical synapse. The quantum of transmitter is thought to be that contained in a single presynaptic vesicle

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Receptive field of a sensory receptor represents the space within the receptive sheet (e.g. skin, retina, etc.) where the receptor is located and able to respond to stimuli

Receptor potential refers to potential change in a sensory receptor to an appropriate stimulus

Refractory period refers to that period of time after the generation of an action potential when the membrane is either inexcitable or only activated to submaximal responses by suprathreshold stimuli

Resting membrane potential refers to the equilibrium potential for the neuronal membrane in the non-activated state and approximates to the equilibrium potential for potassium

Reticular formation refers to a series of poorly defined structures in the brainstem that have diffuse projections both rostrally to the cerebral hemispheres (particularly the thalamus) and caudally to the spinal cord

Retinotopic map or projection refers to the orderly projection of retinal axons to the tectum and thalamus and the subsequent projection from the lateral geniculate nucleus of the thalamus to the visual cortex

Rexed laminae refers to a series of morphologically different areas of the spinal cord grey matter

Ruffini ending is a cutaneous receptor found deep within the dermis. It is classified as a slowly adapting type II receptor (SAII) and is sensitive to lateral stretching of the skin

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Saccades are the rapid movement of the eyes to a new visual target or fixation point

Saltatory conduction is that seen in myelinated nerve fibres where the action potential is generated only at the nodes of Ranvier

Sarcoplasmic reticulum (SR) is found in striated skeletal muscle and envelops the myofibrils and is important as an intracellular store of Ca2+

Schwann cells are found in the PNS and are responsible for providing the myelin sheath to axons in peripheral nerves

Secondary messengers are substances produced within a cell subsequent to the binding of some chemical to its membrane receptor. This substance then acts in a number of different ways within that cell to produce its effect. Examples of secondary messengers include cAMP, cGMP, diacylglycerol (DAG) and inositol triphosphate (IP3)

Secretion refers to the release of substances by vesicular exocytosis

Semi-circular canals are found in the inner ear and are part of the vestibular system

Serial processing refers to the relaying and analysis of sensory information along a single pathway, with each higher stage of the pathway performing some relatively more complex processing. This is well demonstrated in the visual system

Simple cells (after Hubel and Wiesel) are neurons found in the visual cortex that have small receptive fields which respond maximally to a line or bar of illumination of a given orientation

Size principle is the order of recruitment of motorneurons to a given input by virtue of the size of their cell bodies

Somatotopy or somatotopic representation refers to the organisation of the ascending dorsal column-medial lemniscal pathway in terms of the location of the sensory fibres from cutaneous receptors. This ensures that the input from receptors in neighbouring areas of skin is maintained in the ascending projection to the cortex

Spatial summation is performed in the postsynaptic cell and refers to the integration of all synaptic inputs at a given point in time at the trigger zone

Specificity of a sensory receptor refers to its capacity to respond selectively to a particular type of stimulus

Spinocerebellar tracts originate in the spinal cord and project to the cerebellum. The dorsal spinocerebellar tract preferentially relays information from the muscle spindle and projects through the inferior cerebellar peduncle. In contrast, the ventral spinocerebellar tract relays information on spinal cord interneuronal activity and projects to the cerebellum through the superior cerebellar peduncle

Spinothalamic tract forms part of the anterolateral system relaying information on pain and temperature from the cutaneous receptors to the thalamus. The spinoreticular and spinomesencephalic tracts are the other two pathways making up the anterolateral system, although for simplicity these two tracts are often grouped together to form the spinoreticulothalamic pathway

Substantia nigra is found in the midbrain. There are two parts to it: the pars compacta provides a dopaminergic input to the neostriatum, while the pars reticulata component provides a GABAergic input to the thalamus and brainstem

Sudomotor refers to the autonomic innervation, and thus activity, of sweat glands

Sulcus refers to the groove between two gyri in the cerebral cortex

Supersensitivity is the process in which the binding of a ligand to its receptor leads to a greater than normal response. It is also termed upregulation and in time there may be an actual increase in receptor number. It typically occurs in situations where there has been reduced synaptic activity

Supplementary motor area (SMA) corresponds to the medial part of Brodmann's area 6. It is important in the planning and initiation of movement

Sympathetic nervous system is one of the three major subdivisions of the autonomic nervous system. It uses noradrenaline as its postsynaptic neurotransmitter

Synapse is the specialised site of communication between two cells, typically neurons

Synaptic bouton is the presynaptic nerve terminal enlargement that contains the vesicles and apparatus for neurotransmitter release

Synaptic cleft is the extracellular gap between the presynaptic nerve terminal and the postsynaptic nerve process. It is typically in the region of 50 nm

Synaptic vesicles are found in the nerve terminal and contain the neurotransmitter. They fuse with the presynaptic nerve terminal membrane in response to a depolarising input, and by so doing release their neurotransmitter which can then bind to receptors in the postsynaptic (and at some synapses, presynaptic) nerve process

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Tectorial membrane is found in the scala media of the cochlea and overlies the hair cells of the organ of Corti

Tentorial membrane is a fold of the dura mater that has its free margin surrounding the midbrain and a fixed margin attached to the skull at the top of the posterior fossa. It separates the cerebellum from the cerebral hemispheres

Temporal summation is performed in the postsynaptic cell and refers to the integration of synaptic inputs over time

Tetanic stimulation refers to high frequency stimulation of the presynaptic neuron

Thalamus is part of the diencephalon and is important not only in relaying information to the cerebral cortex but in controlling the degree of arousal and attention within CNS neural systems

Transcortical reflexes see Long latency reflexes

Transduction in sensory receptors involves the conversion of a stimulus from the external or internal environment into an electrical signal for transmission through the nervous system

Tranverse or T-tubules are specialised invaginations of the sarcolemma in striated muscle, which are important in conveying the action potential deep into the muscle fibre

Triad refers to the arrangement in skeletal muscle of a T-tubule between two terminal cisternae of the sarcoplasmic reticulum

Trigger zone refers to that part of the neuron where an action potential is initiated. In neurons, other than sensory receptors, this is usually the initial segment of the axon

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Upper motorneurons (UMNs) are those motorneurons that both directly and indirectly innervate the lower motorneurons of the brainstem and spinal cord. Damage to the UMN and its caudal projection results in muscle weakness, increased muscle tone, increased tendon reflexes but no muscle wasting

Up regulation see Supersensitisation

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Ventromedial descending motor pathways refers to the reticulospinal, tectospinal, vestibulospinal tracts that preferentially innervate those motorneurons that control the proximal/axial musculature

Vestibulo-ocular reflex maintains visual fixation with head movement

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Weber-Fechner law describes the relationship that predicts that the ability to detect a change in intensity stimulus at a given intensity is constant

Wernicke's area is that part of the dominant (usually left) parietal cortex that is concerned with the comprehension of speech; damage to this area results in a receptive dysphasia

White matter are those parts of the CNS that primarily contain nerve fibres and glial cells