Lecture 17 - The Somatic Nervous System Part 2 (2024) PDF

The Somatic Nervous System (Part 2) Monday’s Lecture: 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).  Efferen...

The Somatic Nervous System (Part 2) Monday’s Lecture: 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 Monday’s Lecture: 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  Special Senses - Vision (Sight) Accessory Structures of the Eye Anatomy of the Eye The retina, photoreceptors & photoreception Vision Perception of objects via emitted or reflected light “Light” refers to visible electromagnetic radiation Human vision is limited to wavelengths of light between 400-700nm Light stimuli is received through the eyes and transduced to the CNS Image Source: UCSB Science Line Accessory Structures of the Eye Eyes sit within the orbit (surrounded by cranial bones) Soft tissues of the eye attach to the orbit. Protect it and support its function Corner of the eye, where upper/lower eyelids meet White part of the eye Mucus membrane providing lubrication Image Credit: Jonathan Bubb Jr, Anatomy and Physiology for Health Professionals Accessory Structures of the Eye Palpebrae (eyelids) Blink reflexively to moisten eye Protect eye from abrasions & foreign particles Block light and prevent dehydration Eyelashes Sensory receptors initiate blinking Block debris Tarsal glands – secrete oil that reduces tear evaporation Eyebrows Shade eyes Block sweat  Accessory Structures of the Eye Bulbar conjunctiva Conjunctival fornix Palpebral conjunctiva Lacrimal Apparatus The lacrimal glands produce tears Palpebral Conjunctiva > Saline, mucus, antibodies, enzymes Mucus membrane providing These drain into the lacrimal sac lubrication Nasolacrimal duct connects lacrimal sac and nasal Palpebral and bulbar/ocular regions cavity Image Source: Jarvis, 1996 Image Source: https://doi.org/10.3390/bioengineering8100135 The Extraocular Muscles Origin: the body orbit Insertion: on the eyeball x Holds the eye inside, helps maintain its shape, allows it to move to follow objects Disorders of the Extraocular Muscles Diplopia (double-vision) Can result from paralysis, extrinsic muscle weakness or neurological disorders Movements of external eye muscles are not perfectly coordinated Person cannot properly focus images of same area of the visual field from each eye, so sees two images instead of one Strabismus (‘cross-eye’) Caused by congenital weakness of external eye muscles Eye rotates medially or laterally May alternate between which eye is focused, or only use the controllable eye Image Source: Chatswood Eye Specialists, All About Vision Anatomy of the Eye The sphere of the eye can be divided into anterior and posterior chambers. The wall of the eye is composed of three layers: > fibrous tunic > vascular tunic > neural tunic The Fibrous Tunic Outer layer Sclera White portion 5/6 of eye surface Mostly not visible Cornea Clear portion Covers anterior tip Allows light to enter The Vascular Tunic Middle layer Choroid Layer of highly vascularized connective tissue Provides blood supply to eyeball Ciliary Body Muscular structure Attached to the lens by suspensory ligaments Bends the lens, so that light is focused on the back of the eye. The Vascular Tunic Middle layer Iris The coloured part of the eye Smooth muscle that opens/closes pupil Pupil Hole at the center of the eye Allows light to enter The iris constricts the pupil x The iris dilates the pupil The Neural Tunic Inner layer Retina 3 layers of cells Two synaptic layers in between Small indentation at the centre called the fovea. Responsible for photoreception. Photoreceptors Cells that are specialised for the detection of light Inner Segment Contain nucleus & other organelles Outer Segment Contain membrane arrays Filled with photosensitive opsin molecules There are two types of photoreceptors, which differ in the shape of their outer segment: Rods – long, columnar, stack of opsin-containing discs Cones – short, tapered, folds of opsin-containing membranes Image Source: Medline Plus Organisation of the Retina Three Major Cell Types Ganglion cell layer Bipolar cell layer Photoreceptor Layer Two Synaptic Layers Inner plexiform layer Outer plexiform layer Image Source: Biorender Organisation of the Retina Light enters the eye, travels through the layers of the retina The light energy triggers change in membrane potential of the photoreceptors They alter the release of neurotransmitters at the OPL Signal is passed to bipolar cells Bipolar cells pass the signal to retinal ganglion cells (RGCs) at the IPL Axons of the RGCs collect at optic disc to form the optic nerve Image Source: Biorender Photoreception The outer segment of photoreceptors contain light-sensitive pigments Rods contain rhodopsin Work best in dim light React to variations in light to produce a black/white response Cones contain three different opsins Work best in bright light Each reacts to a specific wavelength of light –R/G/B Combination of stimulated photopigments produces colour vision Image Source: WebRN-MacularDegeneration.com Photoreception E.g. 450nm Excite the “blue” cones a lot Excite the “red” and “green” cones minimally Colour Blindness A hereditary x-linked condition, so more common in males Leads to lack of one or more cone pigments Red/Green blindness is most common (but this is a generic term!) protanopia (red-blindness) protanomaly (red-weakness) deuteranopia (green-blindness) deuteranomaly (green-weakness) Tritanopia (blue-yellow blindness) Image Source: AllAboutVision Colour Blindness & Traffic Lights Horizontal signals: Red is always at the LEFT. This is a standard to help the colour blind. Red signals are required to leak some yellow. Yellow signals are required to leak some red, and green signals are required to leak some blue, so colour blind people can tell green from the rest Image Sourcehttps://dribbble.com/shots/4518773-Color-Blindness# How does *light* trigger a change in membrane potential? The opsins are transmembrane proteins They contain a cofactor called retinal When a photon hits retinal, the molecule is biochemically altered (photoisomerization) This shape change activates a G-protein This G-protein activates a cascade of events inside the cell Ultimately triggers closure of Na+ channels Special Sensory Pathways to the CNS Auditory Pathway Gustatory Pathway Optic Pathway Travelling from the Sensory Receptors  CNS Sensed information needs to be transmitted back along sensory nerves to the CNS. Sensory pathways  the chain of neurons, from receptor organ to cerebral cortex, that are responsible for the perception of sensations. The various sensory modalities each follow specific pathways through the CNS Sensory information from the head & neck travels through cranial nerves Sensory information from below the neck travels in through the spinal nerves/cord Gustatory Pathway 1) Sensory information travels along the three cranial nerves shown. 2) They synapse with neurons of the solitary nucleus in the medulla oblongata. 3) Axons from here project to and synapse in the thalamus 4) Axons from here project to and synapse in the gustatory cortex of the cerebral cortex. Connections are ipsilateral (LH body > LH brain) Image Source: Nursing Times, 21st Nov 2022 Auditory Pathway 1) Sensory information travels along the auditory/vestibulocochlear (VIII) nerve 2) Synapse with neurons of the cochlear nuclei of the medulla oblongata. 3) Axons from here project to and synapse in the inferior Pons colliculus of the pons. 4) Axons from here project to and synapse in the medial Medulla geniculate nucleus of the thalamus oblongata 5) Axons from here project to and synapse in the primary auditory cortex. Connections are ipsilateral (LH body > LH brain) Image Source: DOI:10.3389/fnsys.2013.00092 Optic Pathway Connections of the optic nerve are more complex than other cranial nerves LH side processes the left side of each eye RH side processes the right side of each eye  Left side of each eye / right side of each eye Axons from lateral side of retina project straight back (ipsilateral) Axons from medial side of retina decussate at the optic chiasm (contralateral) Where is visual information delivered to? Optic Tract Thalamus (diencephalon) Suprachiasmatic Nucleus A small number of photosensitive RGC axons Lateral Geniculate Nucleus project here Projects to visual They simply respond to cortex of cerebrum presence/absence of light (occipital lobe) This info is passed to other regions of the brain to establish our circadian rhythm e.g. pineal gland, hypothalamus Topographic Mapping of the Retina  Visual Cortex L/R visual information is sorted at the optic chiasm x > R visual field is processed in the L visual cortex > L visual field is processed in the R visual cortex Superior/inferior visual information is maintained topographically x > Superior visual field is processed in inferior cortex > Inferior visual field is processed in superior cortex = image is inverted and reverse as it enters visual cortex. The foveal-processing area is disproportionately large! Defects in Image Formation Image Course: University Children’s Eye Center Somatosensory Pathways to the CNS Dorsal Column System Spinothalamic Tract Somatosensory Pathways Somatosensory stimuli are detected by receptors throughout the entire body (e.g. skin, muscles, joints, organs) Somatosensory stimuli above the neck travel through cranial nerves back to the CNS (e.g. somatosensory information from the face travels through the trigeminal (V) nerve) Somatosensory stimuli below the neck travels up the spinal cord through one of two pathways: The Dorsal Column System  Primarily responsible for sensing touch and movement The Spinothalamic Tract  Primarily responsible for sensing pain and temperature Somatosensory pathways often pass information contralaterally (LH body > RH brain) Image Source: UNSW Sydney Embryology, 2018 Ascending Pathways/Tracts Two major pathways that bring sensory info to the brain: Dorsal Column System Axon of dorsal root ganglion Synapse with second neuron in the nuclei of medulla oblongata Synapse with a third neuron in the thalamus These axons project into the primary somatosensory cortex (post-central gyrus). Primarily responsible for sensing touch and movement *decussate – to cross sides Ascending Pathways/Tracts Two major pathways that bring sensory info to the brain: Spinothalamic Tract Axon of dorsal root ganglion Synapse with second neuron in the dorsal horn Synapse with a third neuron in the thalamus These axons project into the primary somatosensory cortex (post-central gyrus). Primarily responsible for pain & temp sensation *decussate – to cross sides Motor Responses Prefrontal Cortex Primary and Secondary Motor Cortices Descending Pathways  *cortical – relating to cerebral cortex Cortical Responses Somatic senses convey stimuli back to nervous system. Ascending pathways  signals are processed and spread throughout the cerebral cortex The response is voluntary skeletal muscle movement (with or without conscious control) ) Sensory cortical areas (occipital/temporal/parietal) Motor cortical areas (frontal lobe) Image Credit: Bioninja Cortical Anatomy – Planning Voluntary Movement The prefrontal cortex carries out executive functions The rest of the frontal lobe are the regions that produce movement Posterior Parietal Cortex Processes sensory info from visual and somatosensory pathways Integrates visual, tactile and movement information to create a representation of the world Sends connections to the frontal lobe    Image Credit: Casey Henley, Foundations of Neuroscience (CC BY-NC-SA) 4.0 Cortical Anatomy – Planning Voluntary Movement The prefrontal cortex carries out executive functions The rest of the frontal lobe are the regions that produce movement Prefrontal Cortex Deciding on the most appropriate behaviour for the situation Evaluating the consequences of that behaviour Image Credit: Casey Henley, Foundations of Neuroscience (CC BY-NC-SA) 4.0 Cortical Anatomy – Planning Voluntary Movement The prefrontal cortex carries out executive functions The rest of the frontal lobe are the regions that produce movement Secondary Motor Cortices Premotor cortex/area > Helps to plan and organise movements, decide which actions should be used for a situation (based on stimuli) Supplementary motor area > more medial/superior > Helps to plan & coordinate movement (based on remembered sequences of movements) Image Credit: Casey Henley, Foundations of Neuroscience (CC BY-NC-SA) 4.0 Cortical Anatomy – Planning Voluntary Movement The prefrontal cortex carries out executive functions The rest of the frontal lobe are the regions that produce movement Primary Motor Cortex Receives input from several areas to plan movements Controls the initiation and execution of voluntary movements Major output  stimulate spinal cord neurons to trigger contraction of skeletal muscles Image Credit: Casey Henley, Foundations of Neuroscience (CC BY-NC-SA) 4.0 Descending Pathways The Corticospinal Tract The major descending tract that controls muscle movement Upper motor neuron (contralateral) > cell body in primary motor cortex Synapses with a lower motor neuron in the ventral horn of the spinal cord The axon of this lower motor neuron projects to a skeletal muscle in the periphery Lateral tract  controls appendicular muscles Anterior tract  controls axial muscles What do I need to have learned from this session? Recognise the key accessory structures of the eye and explain their function/purpose Recognise the three layers of the eye, and explain key structures present in each Explain the structure and function of photoreceptors, how they are organised in the retina, and how they are stimulated by light. Outline the gustation, auditory and optic sensory pathways Outline the two routes by which somatosensory information travels back to the CNS: the dorsal column system and the spinothalamic pathway Outline how motor signals are generated in the frontal lobe, and how they travel down the corticospinal tract.  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