Cranial Nerves Introduction & CN I PDF

Cranial Nerves Introduction & CN I Dr. A. Aktar Learning Objectives • • • • • • • • What are cranial nerves What types of functions in general do they serve What are the types of nerve fibers they carry Where are their nuclei located What is CN I, what is its function, what is the path it takes,...

Cranial Nerves Introduction & CN I Dr. A. Aktar Learning Objectives • • • • • • • • What are cranial nerves What types of functions in general do they serve What are the types of nerve fibers they carry Where are their nuclei located What is CN I, what is its function, what is the path it takes, how is it organized What is the olfactory bulb, where is it located What are the olfactory pathways, how and where are they organized What and where is the olfactory cortex, what are its connections Cranial Nerves • Bundles of motor or sensory fibers (axons) that innervate muscles or glands or • Carry impulses from sensory receptors or • Have a combination of motor & sensory fibers • Emerge thru foramina or fissures in cranium • Covered by tubular sheaths derived from cranial meninges • Numbered I – XII from rostral to caudal • Names reflect their general distribution or function • 6 categories of nerve fibers Cranial Nerves Motor (Efferent Fibers): • Motor fibers to voluntary (striated) muscle - Somatic motor (general somatic efferent) axons - Branchial motor – refers to muscle tissue derived from pharyngeal arches E.g. Muscles of mastication • Motor fibers to involuntary (smooth) muscles or glands - Visceral motor (general visceral efferent) axons - Constitute cranial outflow of PS division of the ANS - Presynaptic (preganglionic) fibers emerge from brain & synapse outside CNS in a PS ganglion - Postsynaptic (postganglionic) fibers continue to innervate smooth muscles & glands - E.g. Pupillary sphincter & lacrimal gland Cranial Nerves Sensory (Afferent Fibers): • Fibers transmitting general sensation - E.g. Touch , pressure, heat, cold from skin & mucous membranes - These include somatic sensory (general somatic afferent) fibers mainly carried by CN V - Also by CN VII, IX, X • Fibers conveying sensation from viscera - Include visceral sensory (general visceral afferent) fibers - From carotid body & sinus, pharynx, trachea, bronchi, heart, lungs, GI tract • Fibers transmitting unique sensations - Sensory fibers conveying taste & smell (special visceral afferent) - Sensory fibers serving vision, hearing, balance (special somatic afferent fibers) Sensations • Aristotle described over 2,000 years ago the 5 categories of sensory modalities: vision, hearing, taste, smell, touch • There are others, such as equilibrioception (sense of balance) & proprioception (our sense of our body’s position) that lie outside the original five described • Touch is a composite of multiple different senses that can be subdivided Illustration by Eva Bee for The Guardian, Feb 2023 Cranial Nerves Cranial Nerve Nuclei • Fibers of cranial nerves connect centrally to cranial nerve nuclei • Are groups of neurons - in which sensory or afferent fibers terminate or - from which motor or efferent fibers originate • Located in brainstem • Exception: CN I & II - These are extensions of forebrain • Nuclei of similar functional components generally aligned into functional columns in brainstem Cranial Nerves Cranial Nerve Nuclei • Cell columns of nuclei in brainstem not continous (like in spinal cord) • Are interupted & form a series of nuclei located at at longitudinal levels corresponding roughly to attachment points of the cranial nerves • No CN contains axons from all 6 categories Specialized Neuronal Detection of Chemicals • Using this info to affect autonomic funtion, behaviour or perception • 4 general categories: • Visceral chemoreceptors : Unconscious – O2 conc, glucose conc, neuroactive hormones etc/ • Gustatory receptor cells • Olfactory receptor neurons • Chemosensitive endings (common chemical sense) – e.g. Heat of chilli peppers, sting of ammonia, coolness of menthol (trigeminal endings in mucous membranes) • Internal & external chemistry monitoring • Rewarding & warning functions • • • • Spoilt food Smoke from fire Enjoyment of a meal/glass of wine Etc Olfactory Nerve ( CNI ) Olfaction: The sensation of odours that results from the detection of odorous substances aerosolized in the environment • Volatile chemicals drawn into nasal cavity (odoranst) • CN I Function: Special sensory (special visceral) afferent – the special sense of smell • Humans can detect >1,000,000,000, 000 odors • Ability to distinguish between odours is not analogous to being able to consistently identify them Olfactory Nerve ( CNI ) • Odorants can reach olfactory epithelium either through nostrils (orthonasal) or by way of the oropharynx (retronasally) Olfactory Nerve ( CNI ) • Cell bodies of receptor neurons located in olfactory organ ➔ olfactory part of nasal mucosa • Olfactory epithelium - Located in roof of nasal cavity, along nasal septum, medial wall of sup. nasal concha - Patch of cells, 1 – 2 cm2 - Each patch has app. 3 MM receptor cells, intersperced w/supporting cells & ducts of Bowman’s glands • Sensory endings of trigeminal n. fibers also found in olfactory epithelium • Not responsible for olfaction but for noxious sensations elicted by irritants (e.g. concentrated ammonia) Light sheet fluorescence microscopy – adult mouse olfactory epithelium Prof. Dr. Marc Spehr, RWTH Aachen University, Germany Olfactory Nerve ( CNI ) Olfactory receptor neurons : • 1st order neuron – bipolar neurons in olphactory mucosa • Both receptors & conductors • Mucous secreted by supporting cells & Bowman’s glands • Odorants diffuse over mucous layer – directly or bound to an odorant binding protein in the mucous ➔stimulate chemosensitive cilia of the olfactory receptors B: Tip of a cilium – cation channels Chemosensitive Cilia of the Olfactory Receptors High power scanning electron micrograph showing chemosensory cilia ( c ) emerging from a single olfactory vesicle ( v ) B: Tip of a cilium – cation channels Olfactory Nerve ( CNI ) • Axons (central processes) of olfactory receptors unmyelinated • Thinnest in CNS – 0.2 µm - & slowest conducting in entire nervous system ➔ collected into app. 20 olfactory filaments (L. fila olfactoria – filum = thread) constituting the R or L olfactory nerve (CN I) ➔ Pass thru holes in cribriform plate, surrounded by layers of dura mater & arachnoid mater ➔ End in olfactory bulb in ant. cranial fossa Ovoid structure • Olfactory bulb lies in contact w/inf (orbital) surface of frontal lobe • Contains mitral cells • 2nd order neurons Olfactory Bulb • Incoming olfactory n. fibers synapse w/ dendrites of mitral cells → form rounded areas: synaptic glomeruli • Olfactory bulb also contains tufted cells & granular cells (neurons) • These also synapse w/mitral cells Olfactory Bulb • Olfactory bulb receives axons from contralateral bulb thru olfactory tract • Also receives efferent fibers assumed to regulate or tune sensitivity in some way - mostly from anterior olfactory nucleus Olfactory Tract • Narrow band of white matter • Runs from post. end of olfactory bulb beneath inf. surface of frontal lobe • Consists of central axons of mitral & tufted cells & some centrifugal fibers from opposite olfactory bulb • Divides into: Medial Lateral Intermediate olfactory striae • Olfactory nerves are the only CNs to enter cerebrum directly Olfactory Pathways • As olfactory tract reaches anterior perforated substance, divides into medial & lateral olfactory striae • Lateral stria carries axons to olfactory area of cerebral cortex: periamygdaloid and prepiriform areas (primary olfactory cortex) • Medial olfactory stria carries axons that cross median plane in ant. commissure to pass to olfactory bulb of opposite side Olfactory Cortex • Not a single structure - combined areas of cerebral cortex, generally within temporal lobe • Receive input directly from the olfactory bulb Including: • Piriform cortex: Located below the lateral olfactory stria • Amygdala: Located ant. to inferior horn of lateral ventricle Associated w/ emotion of fear • Entorhinal cortex: Ant. part of parahippocampal gyrus Involved in the formation of memory Olfactory Cortex Primary Olfactory Cortex • Periamygdaloid & prepiriform areas • Sends fibers to many other centers within brain to establish connections for emotional & autonomic responses to olfactory sensations Secondary Olfactory Cortex • Entorhinal area (area 28) of parahippocampal gyrus, – medial aspect of temporal lobe : UNCUS (Lat: Hook) • Appreciation of olfactory sensations • Olfactory afferent pathway reaches cerebral cortex without synapsing in one of the thalamic nuclei – EXCEPTION • Olfactory info bypasses thalamus on its way to cortex! • Thalamus part of subsequent stages of olphactory circuitry Output of Olfactory Cortex • Olfactory cortexes project to orbital surface of frontal lobe via the dorsal medial nucleus of the thalamus • Gustation also projects here • Parts of orbital surface of frontal lobe serves as an association area (cortex) for flavour • Same areas receive converging inputs conveying somatosenory & visual info • Some neurons respond to smell, taste, texture or sight of a particular kind of food • Orbital neurons have role in signaling reward value of food & beverage • An individual neuron may respond differently when hungry or satiated Major Projections of Olfactory Areas • All olfactory areas interface w/ various autonomic centers via a complex communications network For example: • Reticular formation & various cranial nerve nuclei involved in visceral responses • E.g. dorsal motor nucleus of vagus - mediates nausea, gastric motility and secretion, and intestinal peristalsis • Superior & inferior salivatory nuclei - mediate salivation ➔ Information about odors thus reaches a variety of forebrain regions, allowing olfactory cues to influence cognitive, visceral, emotional, and homeostatic behaviors Flavour Perception & Olfaction • Flavor perception is aggregation of auditory, taste, touch & smell sensory info • During mastication, tongue manipulates food, odorants are released • Odorants enter nasal cavity during exhalation • Co-activation of motor cortex & olfactory epithelium during mastication results in smell being felt in mouth • During exhalation, olfaction contribution to flavor occurs • In contrast to that of proper smell, which occurs during inhalation phase of breathing • Olfaction, taste, and trigeminal receptors (also called chemesthesis) together contribute to flavor Clinical Correlate: Anosmia • Deprived of sense of smell • Injury or disease • Tumours growing at base of skull can become large before causing any symptoms other than unilateral anosmia • 2 types of processes can disrupt sense of smell • Processes preventing odorants from reaching olfactory epithelia – conductive olfactory deficit • E.g. Nasal polyps, septal deviation, inflammation • Processes that damage olfactory receptor neurons or parts of olfactory CNS – sensorineural olfactory deficit • • • • E.g. Consequence of head injuries or neurodejenerative diseases such as Parkinson’s, Alzheimer’s, Schizophrenia Head trauma can tear away olfactory fibers from olfactory bulb Cribriform plate may injure and fibers can not regrow through Some patients suffer permanenet damage to receptor neurons after severe URTI – unknown reason • Sensorineural deficit patients likely to have intact common chemical sense – can perceive range of volatile substances such as ammonia, menthol – most of which (not all) are irritating • Dysosmia: Distortons in smell References • Standring S (2020) Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd Edition • Dalley A. F. (2022) Moore's Clinically Oriented Anatomy. 9th Edition • Drake R.L. (2019) Gray's Anatomy for Students: 4th Edition • Paulsen F. (2109) Sobotta Clinical Atlas of Human Anatomy, one volume, English 1st Edition • Scheunke M. (2020) Atlas of Anatomy, Head, Neck, and Neuroanatomy 3rd Edition. Thieme • Gilroy M., (2020) Atlas of Anatomy 4th Edition, Thieme • Snell R. Clinical Neuroanatomy, 7th Edition