ANP1111 Lecture 17 Neuroanatomy and Neurophysiology Part 4 PDF

ANP1111 –Lecture 17 Neuroanatomy & Neurophysiology Part 4 Ch. 13, section 13.2 3 Levels of Processing of Sensory Information: 1. Receptor level 2. Circuit level 3. Perceptual level 1. To generate a signal at the receptor level: stimulus energy must match receptor specificity stim...

ANP1111 –Lecture 17 Neuroanatomy & Neurophysiology Part 4 Ch. 13, section 13.2 3 Levels of Processing of Sensory Information: 1. Receptor level 2. Circuit level 3. Perceptual level 1. To generate a signal at the receptor level: stimulus energy must match receptor specificity stimulus must be applied within the receptive field of the receptor (smaller receptive fields allow more (Fig. 13.2) precise localization of the stimulus site) transduction needs to occur [conversion of stimulus energy into a graded potential (EPSP or IPSP) graded potentials must reach threshold in first-order sensory neuron Reminder: Generator Potentials for free dendrites or encapsulated receptors; Receptor Potentials for special senses What is adaptation and how does that related to phasic receptors versus tonic receptors? Adaptation: reduction in sensitivity (electrical activity) in the presence of a constant stimulus and can occur quickly for stimuli that are not painful Peripheral adaptation is at the level of the receptor and reduces how much information is sent to the CNS; Central adaptation is at the level of the neural pathway to the brain and involves brain nuclei 1.Phasic receptors: fast adapting; e.g. lamellar & tactile corpuscles and some special senses (e.g. going from darkness to bright light – you adjust); provide information on rate of change of the stimulus 2.Tonic receptors: sustained response with little to no adaptation; e.g. nociceptors (pain) and proprioceptors; their information is important and informs about the presence and strength of a stimulus https://psychologenie.com/understanding-se nsory-adaptation-with-examples  Fig. 12.34a – sample pathway for discriminative touch and conscious proprioception 2. Processing at the Circuit Level goal is to get information to the correct area of the cortex so that one is aware (perception) and can localize source of stimulus chain of 3 neurons 1st order neuron: (cell bodies are in dorsal root or centra ganglia; they bring the information to the CNS (spinal cord) branching now occurs: some branches lead to motor reflexes while other branches synapse with 2nd order neurons to ascend toward the brain (thalamus or cerebellum) 3rd order neurons have cell bodies in the thalamus and take the information to the correct sensory area of the cerebral cortex Three main pathways for somatosensory information to ascend the spinal cord 1.Dorsal column-medial lemniscal pathways: target is the thalamus; usually a single type of receptor (or a few related types) that can be localized precisely on the body surface (discriminative touch, vibrations; also proprioceptors); decussation at the level of the medulla 2.Spinothalamic pathways: target is also the thalamus; input from several types of receptors with information pertaining to pain, temperature, coarse touch and pressure; don’t localize source as precisely; decussation at level of spinal cord 3.Spinocerebellar pathways: target is the cerebellum; information about muscle or tendon stretch so cerebellum can coordinate skeletal muscle activity; ipsilateral, so do not decussate; we are not consciously aware Fig. 12.34 complete – adding in sensory transmission of pain and temperature information 2. Processing at the Perceptual Level Information has to get to the right place to be understood and localized; using the correct neural pathway to permit arrival at the correct destination is key Notion of sensation (aware of changes in internal/external environment) versus perception (conscious interpretation of those changes); perception determines how you will respond Properties of Sensory Perception 1. Perceptual detection – simply put, one is aware; need to sum inputs from several receptors to achieve detection 2. Magnitude estimation – intensity of the stimulus; encoded by action potential frequency 3. Spatial discrimination – localize the stimulus; notion of “two-point discrimination test” as a measure of how precise that localization can be; can vary between 5 and 50 mm, depending on body area 4. Feature abstraction – each neuron tuned to one feature or property of a stimulus – often several features come together for the sensory experience (temperature, texture, firmness, & dimensions of something you are touching) 5. Quality discrimination – distinguish submodalities of a particular sensation (e.g. submodalities of taste) 6. Pattern recognition – e.g. recognition of a familiar face, a letter, a piece of music  PAIN Can be helpful - warns of tissue damage and motivates us to take action Pain is very personal – can’t really be measured Extreme temperature or pressure Pain chemicals include histamine, K+, ATP, acids, bradykinin Sharp pain followed by burning or aching 1. Sharp pain is carried by smallest of myelinated sensory fibers – A delta fibers 2. Burning pain carried more slowly by small nonmyelinated C fibers – inflammatory reaction now occurring 3. Neurotransmitters for both are glutamate & substance P; 2nd order axons from both ascend usually via spinothalamic tract Pain suppression: Endogenous opioids are endorphins and enkephalins; their release can be triggered by activation of the SNS. Remember the periaqueductal gray matter of the midbrain? Descending pathways relay cortical & hypothalamic pain signals – these activate spinal cord interneurons that then release enkephalins to block the pain signals generated by nociceptors Pain Tolerance Pain threshold (amount of stimulus needed to generate sensation of pain) is the same for everyone, but tolerance (ability to withstand high levels of pain) varies and can be influenced by genetics as well as mental state, even gender, social isolation, stress. Some Pain Terminology 1. Somatic pain – musculoskeletal pain; often described as aching, gnawing, throbbing or cramping; usually easy to localize because the bones and muscles are well innervated 2. Visceral pain – pain associated with organs of the thorax and abdominal cavity – aching, burning, gnawing – can be the result of extreme stretching of tissues, ischemia, irritating chemicals, muscle spasms – uses same pathways as somatic pain making it possible to have referred pain 3. Referred pain – pain arising from one part of the body appears to come from somewhere else; (e.g. heart attack and pain along medial aspect of left arm – both are spinal nerves T1 to T5) 4. Phantom pain – a type of hyperalgesia (pain amplification) which involves NMDA receptors making it a “learned” pain – if limb amputated under general anesthesia, then spinal cord still experienced the pain of amputation; better to use epidural anaesthetics to block spinal cord during the surgery Fig. 13.3 Map of Referred Pain Special Senses: Chapter 15 of your Textbook 7.4.1 Taste: Describe the gustatory receptors & the neural pathway for taste Almost all 10,000 taste buds are located on the tongue – in papillae that give a bumpy surface to the tongue Three types of papillae: Fungiform: mushroom-shaped, over entire surface of tongue, 1-5 taste buds each Vallate: largest and have many taste buds each; 8-12 vallate papillae total that make a V at the back of the tongue Foliate: laterally on tongue – many taste buds in each during childhood but their numbers decrease with age A small number of taste buds scattered over soft palate, cheeks, Fig 15.22 pharynx, even the epiglottis What does a taste bud look like and how does it work? gustatory epithelial cells have long microvilli called gustatory hairs that extend through a taste pore to the surface of the tongue where they are bathed by saliva containing dissolved food chemicals gustatory hairs have receptors for food chemicals (tastants) and once they are activated by tastant binding, they depolarize, release transmitter (serotonin, ATP), and activate the cranial nerve responsible for that taste information (dendritic processes wrapped around the gustatory cells) turnover of taste cells is 7-10 days – stem cell populations called basal epithelial cells Fig. 15.22 While overall taste sensation is complex and results from a mixture of food chemicals, there are five basic taste modalities (taste buds can respond to all five, but a single taste cell has receptors for only one modality): 1. Sweet—sugars, saccharin, alcohol, some amino acids 2. Sour— acids (hydrogen ions in solution) 3. Salty—metal ions (inorganic salts); sodium chloride tastes saltiest 4. Bitter—alkaloids such as quinine and nicotine, caffeine, morphine, strychnine and nonalkaloids such as aspirin 5. Umami—amino acids glutamate and aspartate; example: beef (meat) or cheese taste, and monosodium glutamate (food additive)  taste receptors have different thresholds (most sensitive for bitter to protect against ingestion of toxic substances) taste receptors adapt rapidly (3-5 sec for partial adaptation and complete adaptation in 1-5 min) What we know about the mechanisms 1. Salt: influx of Na+ through channels – depolarizes gustatory epithelial cells 2. Sour: H+ can directly go in and/or block leaky K+ channels leading to depolarization 3. Sweet, Bitter, Umami: All bind to appropriate cell surface receptors (coupled to the G protein gustducin and use second messengers to open channels leading to depolarization and release of the neurotransmitter ATP Mouth also contains thermoreceptors, mechanoreceptors and nociceptors – temperature and texture can influence our perception of food taste Actual taste is a small component of the total experience – 80% of what we attribute to taste is actually smell (think of what happens to taste when you have significant nasal congestion) Two main cranial nerve pairs carry taste impulses from the tongue to the brain via the medulla (solitary nucleus) and the thalamus: Facial nerve (VII) carries impulses from anterior two-thirds of tongue Glossopharyngeal (IX) carries impulses from posterior one-third of tongue and from pharynx Vagus nerve (X) transmits from epiglottis and lower pharynx (very minor) Fig. 15.23 – Gustatory Pathway 7.4.2 Smell: describe the olfactory receptors and the neural pathway for smell like taste, these receptors are also chemoreceptors olfactory epithelium (pseudostratified columnar cells) located in roof of nasal cavity – not the best location to catch all smells covers superior nasal conchae contains olfactory sensory neurons bipolar neurons with radiating olfactory cilia surrounded and cushioned by columnar supporting cells stem cells at base of epithelium Fig. 15.20a  olfactory neurons have several long cilia (increase SA) projecting from single apical dendrite cilia are not motile; they are covered by mucus and odorant chemicals dissolve in this mucus axons gather into small fascicles to form filaments of the olfactory nerve (cranial nerve I) project superiorly through cribriform plate to synapse in olfactory bulbs axons of mitral cells form the olfactory tract Fig. 15.20 b neurons with a short (30- Specificity of Olfactory Receptors each smell may contain hundreds of different odorants – think of it as a puzzle with many pieces that come together to give the full experience humans have ~350 different odorant receptors – sensation of smell is not as tidy as for taste each receptor responds to one or more odorants and each odorant binds to several different receptor types but each receptor cell expresses only one type of receptor very sensitive – often only a few odorant molecules required to activate the receptor cell pain and temperature receptors are also in nasal cavities respond to irritants, such as ammonia, or can “smell” hot or cold (chili peppers, menthol) in order to smell substance, it must be volatile as it enters the nasal cavity and then it must be able to dissolve in the mucus-rich fluid that coats the olfactory epithelium in order to have access to the receptors of the olfactory cilia Fig. 15.21: Olfactory Transduction (portion of the membrane of an olfactory cilium) 1. Na+ influx leads to depolarization and impulse transmission 2. Ca2+ influx causes transduction process to adapt, decreasing its response to a sustained stimulus (suggested to open Cl- channels) Olfactory tracts (axons of mitral cells) have 2 destinations: 1. the olfactory cortex where smells are identified and interpreted (surprisingly, only some of this travels via the thalamus; most of it does not) 2. The limbic system (hypothalamus, amygdala and other areas) – link scent with memories and emotions 3. For example: smell smoke → fight or flight reaction; smell delicious food cooking → activate salivation; smell something unpleasant → sneezing, choking, even vomiting Olfactory Pathways