BMS2-6 Introduction to Cranial Nerves and CN I, III, IV, VI PDF

Spinal Nerve Fibers • Contain sensory & motor fibers • Some of each kind are related to visceral structures & some to somatic structures • A given axon entering or exiting the spinal cord can be grouped into 4: - Somatic sensory fibers: Convey info from receptive endings for pain, temperature, mec...

Spinal Nerve Fibers • Contain sensory & motor fibers • Some of each kind are related to visceral structures & some to somatic structures • A given axon entering or exiting the spinal cord can be grouped into 4: - Somatic sensory fibers: Convey info from receptive endings for pain, temperature, mechanical stimuli - Visceral sensory fibers: Convey info from receptive endings in visceral structures, e.g. walls of blood vessels, GI tract - Somatic motor fibers: Innervate skeletal muscle. I.e. Axons of alpha & gamma motor neurons - Visceral motor fibers: Preganglionic autonomic axons • Axons from all 4 categories of spinal nerves are also found in various cranial nerves. Some cranial nerves contain axons from additional categories ➔ Special sensory ➔ Branchiomeric muscles 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 innervating 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) 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 the olfactory epithelium either through the 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 : • Both receptors & conductors • True neurons • 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 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 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 • 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 tract in 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