Lecture 18 Somatic Nervous System PDF

The Somatic Nervous System Dr. Elita Partosoedarso Links to segment recordings:Part A, Part B, Part C  Overview of Somatic Nervous System Classification by structure Sensory...

The Somatic Nervous System Dr. Elita Partosoedarso Links to segment recordings:Part A, Part B, Part C  Overview of Somatic Nervous System Classification by structure Sensory Classification by function receptors Classification by location Somatic NS Vision, hearing, equilibrium, smell and Special senses taste Connections of cerebrum Cerebral cortex Sensory and motor homunculus processing Ascending and descending somatic tracts 2 Sensory Receptors 1.Sensory receptors are activated by stimuli caused by changes in our internal or external environment 2.Sensory information is acquired through depolarization of sensory nerve endings 3.Sensory capability become more acute with maturation and is affected by age, disease, structural defects, or lack of maturation Basic Definitions Receptors: structures that detect sensations and generate an action potential in neurons. Each receptor can only receive input from one type of stimulus (sensory modality) Sensation: activation of sensory receptor cells at level of stimulus Perception: central processing of sensory stimuli into a meaningful pattern 3 3  Receptor potential Stimulus acts on a receptor Develops potential (graded response) If graded response is large enough, threshold is reached to start an action potential (impulse) Impulses travel across the axon to CNS Sensation is ‘felt’ or reflex action or response is started 4 4  Principle of adaptation: a functional characteristic of receptors Definition Receptor potential decreases over time in response to a continuous stimuli Function Leads to a decreased rate of impulse conduction and a decreased intensity of sensation Types of adaptation Slowly adapting receptors Rapidly adapting receptors 5 5  Classification of Sensory Neurons/Receptors Based on Location of Sensory Endings Based on Structure of Sensory Endings 1.Visceroceptors (interoceptors) 44.Free nerve endings (most common) contain Location: internally, often within body dendrites embedded in tissue that receive organs sensations, eg visceroceptors, nociceptors, thermoreceptors, mechanoreceptors Type of information received: internal 5.Encapsulated nerve endings: encapsulation environment, eg pressure, stretch, 5 enhance nerve ending sensitivity for touch, chemical changes, hunger, thirst pressure, vibration, stretch 2.Exteroceptors 6 6.Specialized receptor cells contains distinct Location: on or near the body surface structural components that interpret a Type of information received: external specific type of stimulus, eg light, smell, environment, eg pressure, touch, pain, taste temperature 3.Proprioceptors (special type of visceroceptor) 4 Location: skeletal muscles, tendons, joint capsules Type of information received: body 5 movement, body position, orientation in space 6 6  Classification of Receptors by stimulus detected 1. Osmoreceptors ○ Location: hypothalamus ○ Activated by changes in concentration of electrolytes (osmolarity) in extracellular fluids 2. Chemoreceptors ○ Location: Throughout the body ○ Activated by changing concentration of certain chemical ex. eg, taste, smell, CO2, glucose 3. Mechanoreceptors ○ Location: Throughout the body ○ Activated by mechanical stimuli eg, stretch, pressure, hollow organ filling 4. Thermoreceptors ○ Location: Throughout the body ○ Activated by temperature; EITHER hot or cold 5. Photoreceptors ○ Location: only in the eye ○ Activated by light 6. Nociceptors ○ Location: Throughout the body ○ Activated by intense stimuli (pain sensation) that may 7 7 damage tissue  gustatory cells register presence of chemical Gustation (Taste) Releases neurotransmitter into synapse Neurotransmitter binds to receptor on dendrites of sensory neurons on cranial nerves: Facial, glossopharyngeal and vagus nerves Raised bumps on surface of tongue (papillae) contain taste buds with specialized gustatory receptor cells for transduction of taste stimuli. 1. Salty taste: due to perception of sodium ions (Na+) in the saliva. 2. Sour taste: due to perception of H+ concentration. 3. Sweet taste: due to perception of glucose dissolved in the saliva. 4. Bitter taste: respond to alkaloids (coffee, hops (in beer), tannins (in wine), tea, aspirin). Can cause gagging reflex via vagus nerve 5. Umami (savory) taste: respond to amino acid L-glutamate found in protein- 8 rich foods, eg meat.  Olfaction (Smell) Olfactory epithelium within superior nasal cavity responsive to chemical stimuli: contains bipolar olfactory sensory neuron with dendrites that extend from apical surface of the epithelium into mucus lining the cavity Inhaled airborne (odorant) molecules pass over olfactory epithelial region and dissolve into the mucus odorant molecules bind to proteins to keep them dissolved in mucus odorant–protein complex is transported to olfactory dendrites where it binds to G protein–coupled receptor on the dendrite of an olfactory neuron Graded membrane potential is produced in olfactory neurons which projects to frontal lobe Signal splits to cerebrum (primary olfactory cortex), limbic system and hypothalamus (associated with long-term memory 9 and emotional responses)  Audition (Hearing) Sound enters the external ear directed towards auditory vibrate tympanic (ear) canal by C-shaped membrane (ear curves of auricle drum) Sound enters middle ear (space with 3 ossicles- malleus, incus, and stapes) connected to pharynx via Eustachian tube to equilibrate air pressure across the tympanic membrane Sound travels to inner ear Sound waves are transduced into a neural signal 10  Equilibrium (Sense of Balance) 1.Inner ear is responsible for encoding information about equilibrium 2.Sensory stimuli about head position, head movement, and body motion is detected by mechanoreceptor in vestibule of the inner ear: mechanoreceptors consist of hair cell with stereocilia and support cells (macula). 3.The stereocilia extend into the viscous gel of the otolithic membrane 4.Otoliths: layer of calcium carbonate crystals on top of otolithic membrane which make the otolithic membrane top-heavy. 5.The otolithic membrane can move independently from the macula. 6.The exact position of the head is interpreted by the brain based on the pattern of hair-cell depolarization. 11  Vision Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes 6 extraocular muscles that originate from orbital bones and insert into the surface of the eyeball superior rectus, medial rectus, inferior rectus, and lateral rectus: contraction of each muscle moves the eye towards it superior oblique and inferior oblique muscles is threaded through the trochlea (pulley-like piece of cartilage): contraction of these help to move eye levator palpebrae superioris elevates and retracts upper eyelidnerve (II): receives info from retinal sensory Optic neurons (sight) Oculomotor nerve (III): controls all other eye muscles Trochlear nerve (IV): controls superior oblique muscles Abducens nerve (VI): controls lateral rectus muscles 12  How does the cerebral cortex process so many different types of input? Cortical processing Broca's area: production of human language Wernicke's area: comprehension of human language 13 Cerebral tracts: cerebrum’s white matter Tracts are bundles of myelinated axons in the CNS which serve a similar function and/or travelling to and from the same locations. They promote communication between cerebral hemispheres and different parts of CNS Types of tracts 1.Association tracts (most numerous): axons that extend from one convolution to another in the same hemisphere 2.Commissural tracts (Corpus callosum): axons that extend from one convolution to the corresponding convolution in the other hemisphere 3.Projection tracts: axons that connect the cerebrum to other parts of the 14 nervous system 4.Connectome: entire network of neural connections in the brain  Ascending Somatic Sensory Pathways in CNS Somatosensory stimuli (touch, temp, pain, Tracts are collection of axons proprioception) activate receptors in skin, muscles, travelling to the same direction tendons, joints First order neurons: from periphery to spinal cord/brainstem cell bodies in Axon projects to brainstem (dorsal dorsal root column) or to spinal cord ganglion (spinothalamic tract) Second order neurons (spinothalamic tracts) axons decussates (crosses) at Cell bodies in brainstem (dorsal column) or spinal gray matter cord (spinothalamic tract) Third order sensory neurons (thalamocortical tracts) Projects to postcentral gyrus of conscious Two major pathways bring parietal lobe (somatosensory perception sensory information to the cortex) for initial processing occurs 15 brain  Somatic areas of cerebral cortex Somatic areas of the cerebral cortex control voluntary movements. For normal movements to occur, many parts of the nervous system must function Primary somatic motor Primary somatic sensory area area Location: Postcentral gyrus Location: Precentral gyrus Function: Receives inputs Function: sends info to from areas activated by stimulate individual heat, cold, touch muscles Somatic sensory association Premotor area area Location: area Location: Compares immediately anterior to incoming stimuli with known precentral gyrus stimuli 16 Descending Somatic Motor Pathways Upper motor neuron Descends via Decussat Cell bodies in corticobulbar and es at cerebral cortex corticospinal tracts pyramids Lower (anterior horn) motor neuron Cell bodies in spinal cord Final common (anterior horn) pathway specific motor unit within a skeletal muscle conscious or voluntary movements of skeletal muscles Axons of the lateral corticospinal tract which decussate in the medulla control appendicular muscles while those in anterior corticospinal tract which do not decussate in the medulla control axial muscles. Cell bodies of motor neurons controlling the upper and lower limbs are found in the ventral horn at C5-T1 (cervical enlargement) and 17 L2-L3 (lumbar enlargement) respectively. The cervical