Lecture 16 - The Somatic Nervous System_Part 1 (2024) UPDATED Student Copy PDF

The Somatic Nervous System (Part 1) The Somatic Nervous System Conscious perception of the environment (through our senses)  Afferent nerves (sensory neurons) carry sensory information to the CNS Voluntary responses to that perception (through skeletal muscles)....

The Somatic Nervous System (Part 1) The Somatic Nervous System Conscious perception of the environment (through our senses)  Afferent nerves (sensory neurons) carry sensory information to the CNS Voluntary responses to that perception (through skeletal muscles).  Efferent nerves carry motor function information to the body from the CNS Image Source: Anatomy and Physiology for Health Professions General vs. Special Senses General senses Somatic (touch, tactile pressure, vibration, temperature, and pain perception) Detected by receptors found throughout the body Visceral (from visceral organs , e.g., balance, hunger, thirst) Special senses Specific organ devoted to it (eye, inner ear, tongue, or nose) Gustation (taste) Olfaction (smell) Audition (hearing) Equilibrium (balance) Vision (sight)  Sensory Receptor Cells Classification by structure Classification by location Classification by transduction method How do we Classify Sensory Receptor Cells? Based on their structure 1) Free nerve endings 2) Encapsulated ending 3) Specialised receptor How do we Classify Sensory Receptor Cells? Based on their structure Based on their location relative to the stimuli Exteroceptor  near stimuli in the external environment (e.g. receptors in the skin) Interoceptor  interprets stimuli from internal organs and tissues (e.g. receptors in the walls of blood vessels) Proprioceptor  near a moving part of the body (e.g. muscles and tissues) How do we Classify Sensory Receptor Cells? Based on their structure Based on their location relative to the stimuli Based on the type of stimuli they transduce** ** Transduction means ‘the process of converting energy or a message into another form’. In this case, a stimulus into an electrical signal. Chemoreceptors  chemical stimuli (basis for taste) Osmoreceptors  osmotic pressure (fluid balance) Nociceptors  harmful stimuli such as heat, cold, force or chemicals (pain) Mechanoreceptors  mechanical stimuli (hearing, balance and most somatic senses) Thermoreceptors  temperature changes Photoreceptors  light energy (vision)  General Senses (Somatosensation) x General vs. Special Senses General senses Somatic (touch, tactile pressure, vibration, temperature, and pain perception) Detected by receptors found throughout the body Visceral (from visceral organs , e.g., balance, hunger, thirst) Special senses Specific organ devoted to it (eye, inner ear, tongue, or nose) gustation (taste) olfaction (smell) audition (hearing) equilibrium (balance) vision General Senses Many somatosensory receptors are located in the skin… Includes free nerve endings that wrap (Light pressure and around the base of low-frequency hair follicles, detect vibration) bending. (Stretch) (Low-frequency vibration) (Deep pressure and high-frequency vibration) Image source: Lumen Learning General Senses Many somatosensory receptors are located in the skin… but elsewhere too! Sensory receptors in Muscles Proprioceptors (detect body position & movement) Muscle contraction causes tension in As the main muscle the tendon, which contracts, the deforms sensory intrafusal fibers follow nerve endings. and deform the sensory nerve endings. The degree of muscle tension can The shape change (how be detected and much & how fast) can sent to the CNS. be detected and sent to the CNS. Image Source: The Anatomy of Stretching, Second Edition General Senses Many somatosensory receptors are located in the skin… but elsewhere too! Sensory receptors in Joint Capsules Ruffini (bulbous) corpuscles Joint position and movement causes stretch Detects this, aids in proprioception Pacinian (lamellated) corpuscles Changes in joint position and acceleration cause pressure and vibration Detect this, aids in proprioception. General Senses Many somatosensory receptors are located in the skin… but elsewhere too! Sensory receptors in Walls of Visceral Organs Mechanoreceptors Monitor the filling of organs like the bladder, stomach etc. Chemoreceptors Detect changes in pH, oxygen and CO2 content E.g. the carotid body in the neck, medulla oblongata Nociceptors Detect harmful stimuli e.g. chemicals or excessive stretch Image Source: NCBI StatPearls  Mechanoreceptors of Somatosensation Name Historical (eponymous) name Location(s) Stimuli Dermis, cornea, tongue, joint Pain, temperature, Free nerve endings * capsules, visceral organs mechanical deformation Epidermal–dermal junction, Low frequency vibration (5– Mechanoreceptors Merkel’s discs mucosal membranes 15 Hz) Bulbous corpuscle Ruffini’s corpuscle Dermis, joint capsules Stretch Papillary dermis, especially in Light touch, vibrations below Tactile corpuscle Meissner’s corpuscle the fingertips and lips 50 Hz Deep pressure, high- Deep dermis, subcutaneous Lamellated corpuscle Pacinian corpuscle frequency vibration (around tissue 250 Hz) Wrapped around hair Hair follicle plexus * Movement of hair follicles in the dermis In line with skeletal muscle Muscle contraction and Muscle spindle * fibers stretch Tendon stretch organ Golgi tendon organ In line with tendons Stretch of tendons  Special Senses - Gustation (Taste) Organisation of the tongue & papillae Basic steps of signal transduction Different types of taste stimuli General vs. Special Senses General senses Somatic (touch, tactile pressure, vibration, temperature, and pain perception) Detected by receptors found throughout the body Visceral (from visceral organs , e.g., balance, hunger, thirst) Special senses Specific organ devoted to it (eye, inner ear, tongue, or nose) gustation (taste) olfaction (smell) audition (hearing) equilibrium (balance) vision Gustation (taste) The transduction of chemical taste stimuli into a neural signal Made possible by the tongue The tongue is covered with small bumps called papillae (contain taste buds) Different types of papillae are found in different regions of the tongue. Gustation (taste) Fungiform Papillae Mushroom-shaped, scattered Detect taste, pressure and temperature Red dots visible on anterior two thirds Filiform Papillae Club-shaped, numerous No tastebuds, but detect touch. Present on anterior two thirds Gustation (taste) Foliate Papillae Short, vertical folds. Leaf-like. Back & sides of the tongue Less common Circumvallate Papillae Dome-shaped Organised in a row in front of the posterior third 8-12 on your tongue Gustation (taste) ‘Taste hairs’ Each taste bud is a cluster of specialised gustatory (microvilli) receptor cells. These elongated cells have microvilli at the tip, which project outwards through the taste pore 1) Food molecules (tastants) dissolve in saliva 2) They bind to receptors in the microvilli 3) This triggers an action potential 4) The signal is passed to taste sensory nerves They carry the signal through the nearby cranial nerves Gustatory (VII/X), through the medulla oblongata, and up to the receptor cell gustatory cortex. The Different Tastes Salty Sour Sweet Bitter Umami Salt crystals Acidic foods Specific molecules bind to a specialised receptors. Triggers dissociate into increase the H+ internal cellular signaling that ultimately leads to Na+ and Cl- ions concentration in depolarization of the gustatory cell. in your saliva. your saliva. Sweet  binding of glucose, other monosaccharides or The ions diffuse This triggers artificial sweeteners. into cells, increasing causing graded Bitter  triggered by alkaloids (contain nitrogen). Can depolarisation of potentials in trigger gag reflex to stop us ingesting poisons! the membrane. gustatory receptor cells. Umami  the amino acid L-glutamate (found in protein-rich foods)  Special Senses - Olfaction (Smell) Organisation of the nose & olfactory epithelium Basic steps of signal transduction  Olfaction (Smell) The transduction of odor molecules into a neural signal Made possible by the structures of the nose The olfactory epithelium lines the superior nasal cavity Neurons in this epithelium pass upwards through the ethmoid bone to connect to neurons in the olfactory bulb Image Source: https://doi.org/10.1038/sj.embor.7401029  Olfaction (Smell) STEP 1: Inhaled molecules pass over the olfactory epithelial region and dissolve into the mucus (aided by odorant molecules). They bind to receptors in olfactory cilia Olfactory Receptor Cells Have dendrites extending into the mucus Each dendritic end broadens into a knob from which 5-20 cilia emerge Image Source: https://doi.org/10.1038/sj.embor.7401029  Olfaction (Smell) STEP 2: Odorant binding will trigger a graded potential in the olfactory receptor. If inputs are sufficient, an action potential will be triggered STEP 3: The signal will travel along the axon of the olfactory receptor, up through holes in the ethmoid bone, into the olfactory bulb. Receptors of the same type will converge in the same glomerulus (region of the bulb) Image Source: https://doi.org/10.1038/sj.embor.7401029  Olfaction (Smell) STEP 4: Here, they will excite mitral cells Transmit the signal up through the olfactory tract, into the brain  Primary olfactory cortex Conscious perception of smell  Limbic system Associate with memory and emotion Remember: smell is the one sensory modality that does not synapse in the thalamus before connecting to regions of the cerebral cortex. Image Source: https://doi.org/10.1038/sj.embor.7401029  Special Senses - Audition (Hearing) Organisation of the ear Basic steps of signal transduction Audition (Hearing) The transduction of sound waves into a neural signal Made possible by the structures of the ear Audition (Hearing) 1) Auricle The large, fleshy structure on the lateral aspect of the head. Curved like a “C” to direct sound waves toward the auditory canal 2) Ear canal Enters the skull through external auditory meatus of the temporal bone. Directs the sound waves towards… Audition (Hearing) 3) Tympanic Membrane Also known as the ‘ear drum’ Thin, semi-transparent Vibrates in response to sound waves Audition (Hearing) The middle ear consists of the ossicles (3 small bones) and the Eustachian tube. The ossicles amplify and pass sound waves from the eardrum through to the inner ear. 4) Malleus (hammer shaped) 5) Incus (anvil shaped) 6) Stapes (stirrup shaped) Audition (Hearing) The middle ear consists of the ossicles (3 small bones) and the Eustachian tube. The ossicles amplify and pass sound waves from the eardrum through to the inner ear. The Eustachian tube connects the middle ear to the throat and nasal cavities. They help to drain fluid and balance air pressure. Audition (Hearing) The inner ear consists of the cochlea and the vestibule. The cochlea is responsible for transmitting sound waves. The Cochlea Sits within the bony labyrinth, a cavity of the temporal bone Contains three membrane-lined fluid-filled chambers. Divided in half by the basilar membrane On top of the basilar membrane sits the organ of Corti, covered by the tectorial membrane above The Cochlea The organ of Corti is a cellular layer, where sensory hair cells are found On their surface they have stereocilia, an array of microvilli-like structures that are arranged from tallest to shortest 7) Sound waves trigger ripples in the cochlear fluid, which in turn move the basilar membrane. Stereocilia are tethered together by proteins that open ion channels when the array is bent toward the tallest member of their array, and closed when the array is bent toward the shortest member of their array. The Cochlea The organ of Corti is a cellular layer, where sensory hair cells are found On their surface they have stereo-cilia, an array of microvilli-like structures that are arranged from tallest to shortest 9) Movement of the basilar membrane triggers stereocilia bending opening of ion channels  action potentials. 10) This signal is passed to auditory nerve fibers. They transport the signal through the cochlear nerve, through the ascending auditory pathway, to the primary auditory cortex in the temporal lobe. Detecting Different Frequencies of Sound Sound waves have both amplitude (height/strength) and frequency Different parts of the basilar membrane will only move in response to waves of a specific frequency. Higher frequency = moves near the base of cochlea Lower frequency = moves near the tip of the cochlea Humans can hear in the range of 20 – 20,000 Hz Audition (Hearing) The inner ear consists of the cochlea and the vestibule. The cochlea is responsible for transmitting sound waves. The vestibule is responsible for balance. Equilibrium (Balance) Vestibule senses head position, movement and motion Similar way – using a hair cell with stereocilia Sensing Head Position Sensed by the utricle and saccule Contain macula tissue Hair cells - have stereocilia that extend upwards Otolithic membrane – viscous gel Otoliths – calcium carbonate crystals Equilibrium (Balance) Vestibule senses head position, movement and motion Similar way – using a hair cell with stereocilia Sensing Head Position Sensed by the utricle and saccule Contain macula tissue 1) Tilting the head causes otolithic membrane to slide 2) This bends the stereocilia, opening ion channels 3) Hair cells depolarise and hyperpolarise in response 4) Signals are passed to the underlying neurons of the vestibular nerve Equilibrium (Balance) Vestibule senses head position, movement and motion Similar way – using a hair cell with stereocilia Sending Head Movement Sensed by the semicircular canals Filled with fluid At the base where they meet the vestibule, contain ampullae ○ Contains hair cells with stereocilia ○ These project into the cupula membrane Equilibrium (Balance) Vestibule senses head position, movement and motion Similar way – using a hair cell with stereocilia Sending Head Movement Sensed by the semicircular canals 1) You rotate your head. 2) The fluid in the semicircular canal lags slightly, pushing the cupula in the opposite direction 3) This pushing causes the stereocilia to bend, opening ion channels and triggering depolarisation. 4) The signal is passed to neurons in the vestibular nerve This information is then passed to many areas of the brain for processing. Equilibrium (Balance) Vestibule senses head position, movement and motion Similar way – using a hair cell with stereocilia Sending Head Movement Sensed by the semicircular canals Comparing the activation of different ampullae in different planes helps us to obtain a precise 3D position What do I need to have learned from this session? Understand the key classifications of sensory receptor Explain how somatosensory stimuli are detected throughout the body (with reference to specific sensory receptor types) For the special senses (gustation, olfaction, audition & equilibrium): Explain the macro- and microscopic anatomy of their sensory organ Explain how stimuli are transduced into an electrical signal Recall the route by which this signal travels back to the CNS  NortheasternLDN Tel: +44 (0)20 76374550 Devon House [email protected] 58 St Katharine’s Way [email protected] www.nulondon.ac.uk London E1W 1LP United Kingdom