PHYSIO EXAM 1 NOTES PDF

PHYSIO Lectures Summary MEMBRANE POTENTIALS Different cells have different resting membrane potentials. Neurons have a resting membrane potential generally in the range of -40 to -90 mV. It is generally accepted in textbooks for the standard Resting...

PHYSIO Lectures Summary MEMBRANE POTENTIALS Different cells have different resting membrane potentials. Neurons have a resting membrane potential generally in the range of -40 to -90 mV. It is generally accepted in textbooks for the standard Resting Membrane Potential (RMP) is at -70 mV The changes in potential are due to the movement of ions in and out of the cell The semipermeable cell membrane allows for the selective passage of materials into and out of the cell. These transfers are allowed due to channels. Ligand-gated channels are opened by a chemical stimulus Acetylcholine (ACh) for instance (neuromuscular junction) Mechanically-gated channels are opened due to deformation/pressure Voltage-gated channels open and close depending on the membrane potential. These channels are important in the generation and relaying of the action potential. If ions have to pass through the membrane against the potential (for example a positive ion that wants to move to a more positive region), they require pumps. An example of a pump is the Na+/K+ ATPase pump. The Na/K ATPase pump pushes Na out of the cell (against the potential), and trades in K ions. 3 Na moves out in exchange for 2 K. This makes the net charge of the cell -1. PHYSIO Lectures Summary 1  The movement of the RMP can be describes as follows: Depolarization is the movement of the potential from the RMP to a less negative value. Repolarization is the movement of the potential from a less negative value toward the RMP. Hyperpolarization is the movement of the potential from the RMP to a more negative value. ACTION POTENTIALS Any change in the membrane potential due to a stimulus is known as a graded potential. This change can be either a depolarization or a hyperpolarization. There are techniques in medicine where you hyperpolarize the cell to make it more difficult for that cell to develop an action potential (for instance against pain). The generation of the AP is prohibited by local anesthetics such as procaine and lidocaine. These two anesthetics block the voltage-gated Na+ channels Some animals produce toxins that interfere with nerve conduction as well. An example is tetrodotoxin, which comes from pufferfish The magnitude of the potential change can vary (is “graded”) depending on the strength of the stimulus. When graded potentials reach a certain threshold potential, they generate action potentials. Action potentials are generally very rapid (as brief as 1-4 milliseconds). They are “all-or-none” responses, the deciding factor is whether or not the potential change reaches a certain threshold. The threshold potential is usually 15 mV above the RMP, which is about -55 mV. The ability of a neuron to generate an action potential is known as its excitability. PHYSIO Lectures Summary 2  Neuroreceptors in the dendrite of the neuron are activated by stimuli (neurotransmitters like acetylcholine for example). These then open the ligand-gated channels, which allow the Na+ ions outside the cell to enter, making the membrane potential more positive (depolarization). Once enough Na+ ions enter such that the membrane potential reaches -55 mV (the threshold potential), the Na+ voltage gated channels around the axon hillock all open. The action potential propagates through the axon as more gates open. Once the potential in the cell flips to positive, the Na+ voltage gated channels close and the K+ voltage gated channels open. This allows the K+ ions inside the cell to go back out, making the potential in the cell more negative (repolarization). There is an overshoot as the K+ channels close gradually, but after a while the membrane potential goes back to its rest state (-70mV) (polarization). Right after an AP is generated, the cell undergoes a refractory period, which is basically the “rest period” of the neuron that occurs before another AP can be generated. These are caused by the hyperpolarization overshoots the membrane potential undergoes. There are 2 types of refractory periods: The absolute refractory period is the period wherein a second stimulus, no matter how strong, will not produce a second action potential no matter what This is because all the Na+ gated channels are already open, and so you can’t open any more. The relative refractory period is the period following the absolute refractory period, and is the interval in which a second AP can be produced, but only if the stimulus is considerably strong The refractory period makes it difficult to generate an AP in the other direction. PHYSIO Lectures Summary 3  Action potentials only occur at the nodes of Ranvier. Hence, in myelinated axons, they jump from one node to the next. This process is known as saltatory conduction. SYNAPSES Synapses are junctions between two neurons, and can be either chemical or electrical in nature. For electrical synapses, the electrical activity of the presynaptic neuron affects the electrical activity of the postsynaptic neuron. Chemical synapses on the other hand utilize neurotransmitters Neurotransmitters are produced and stored in vesicles at the axon terminals. When the cell is stimulated, the intracellular Ca2+ levels increase, and stimulate the vesicles to translocate and bind to the plasma membrane through the SNARE proteins. PHYSIO Lectures Summary 4  Synaptic vesicles dump the neurotransmitters to the synaptic cleft, passing it onto the postsynaptic neuron via exocytosis. To then terminate the signal in a chemical synapse, the neurotransmitter must be removed. This is done by: Diffusion of the transmitter from the cleft Degradation of the transmitter by enzymes Reuptake into the presynaptic neurons for reuse For the activation of the postsynaptic cell, we have the excitatory chemical synapses and the inhibitor chemical synapses. The excitatory chemical synapses generate an excitatory postsynaptic potential (EPSP), which serve to bring the membrane potential closer to the threshold to more easily generate an AP (depolarization). The inhibitor chemical synapses generate an inhibitory postsynaptic potential (IPSP), which brings the membrane potential to more negative values to make it more difficult to generate an AP (hyperpolarization) PHYSIO Lectures Summary 5  Drugs can have an effect on synaptic transmission by the following: One example is the clostridium botulinum bacilli toxin (botulism, botox), which interferes with the actions of SNARE proteins at excitatory synapses that activate muscles; botulism is characterized by muscle paralysis NEUROTRANSMITTERS AND NEUROMODULATORS Neurotransmitters are chemical messengers that transmit signals across synapses from one neuron to another, and neuromodulators are substances that modulate the activity of these neurotransmitters Neurotransmitters are involved with more rapid communication Neuromodulators are associated with slower events such as learning, development, motivational states, and some types of sensory or motor activities. Now let’s delve into some of the known neurotransmitters or neuromodulators. ACETYLCHOLINE ACh is found in the neuromuscular junction (NMJ) Neurons that use ACh as the primary neurotransmitter are known as cholinergic neurons. It acts at muscarinic (G-protein coupled) or nicotinic (ion channels) receptors. PHYSIO Lectures Summary 6  Nicotinic receptors are found at the NMJ of skeletal muscles ACh is produced in the presynaptic axon by the enzyme choline acetyl transferase (CAT) as follows Acetyl COA + choline → acetylcholine + CoA The degradation of ACh occurs in the synaptic cleft, and is done by the enzyme acetylcholinesterase. BIOGENIC AMINE NEUROTRANSMITTERS CATECHOLAMINES Made from Tyrosine Dopamine, Norepinephrine (NE), Epinephrine (Epi) NE and Epi utilize adrenergic receptors Adrenergic receptors are G protein coupled that are generally linked to second messenger signal transduction pathways You have your Alpha (α1 , α2 ) and Beta (β1 , β2 , β3 ) adrenergic receptors ​ ​ ​ ​ ​ NE is found in both the CNS and PNS, but Epi is found mainly in the PNS. SEROTONIN Made from tryptophan Also known as 5-hydroxytryptamine or 5-HT It’s mainly located in the CNS, particularly in the brainstem Some of its functions include Regulating sleep Emotions Involved in the vomit reflex Regulates cell growth Vascular smooth muscle cell contraction HISTAMINE PHYSIO Lectures Summary 7  Made from histidine It’s mainly located in the hypothalamus It’s commonly known for paracrine actions Also found in the PNS, and is involved in allergic reactions, nerve sensitization, and acid production in the stomach The enzymes that are responsible for the degradation of the biogenic amine neurotransmitters are: Monoamine oxidase (MAO) Catechol-o-methyltransferase AMINO ACID NEUROTRANSMITTERS Found at excitatory synapses: Aspartate Glutamate It’s the primary neurotransmitter in 50% of the excitatory synapses in the CNS There are 2 types of receptors: Metabotropic glutamate receptors G-protein coupled receptors Ionotropic glutamate receptors AMPA receptors NMDA receptors implicated in mediating excitotoxicity, a phenomenon in which the injury or death of some brain cells rapidly spreads to adjacent regions. Excitotoxicity may be involved in stroke, traumatic brain injury, and other neurodegenerative diseases. Activity of AMPA and NMDA receptors has been implicated in long-term potentiation (LTP) phenomenon, or muscle memory. Found at inhibitory synapses: Glycine Glycine is the major neurotransmitter released from inhibitory interneurons in the spinal cord and the brainstem. They bind to ionotropic receptors on postsynaptic cells that allow Cl- to enter Glycinergic neurons are essential for maintaining a balance of excitatory and inhibitory activity in the spinal cord integrating centers that regulate skeletal muscle contraction. Gamma-Aminobutyric Acid (GABA) PHYSIO Lectures Summary 8  GABA is the major inhibitory neurotransmitter in the brain Postsynaptically, GABA may bind to ionotropic or metabotropic receptors. The ionotropic receptor increases Cl- flux into the cell, which results in the hyperpolarization of the postsynaptic membrane. Ethanol stimulates GABA, which inhibits excitatory glutamate synapses. The overall effect is the global depression of the electrical activity of the brain. NEUROPEPTIDES These are short chains of amino acids with peptide bonds Their physiological roles are not all known Neurons that release one or more of the peptide neurotransmitters are called peptidergic neurons. In many cases, they are co-secreted with another type of neurotransmitter and act as meuromodulators You have your: ENDOGENOUS OPIOIDS Enkephalins Endorphins Morphine and Codeine They are synthetic opioids that are used as analgesics (pain reducers) SUBSTANCE P Released by afferent neurons that relay sensory information into the CNS Known to be involved in pain sensation GAS NEUROTRANSMITTERS They are produced by enzymes in axon terminals (in response to Ca2+ entry) They simply diffuse from their sites of origin in one cell into the intracellular fluid of other neurons or effector cells, where they bind to and activate proteins. You have your: NITRIC OXIDE (NO) Produced by nitric oxide synthetase (eNOS, nNOS, iNOS) and undergoes rapid degradation They activate cGMP signaling pathways PURINE NEUROTRANSMITTERS Non-traditional Include the purines ATP and adenosine, which act principally as neuromodulators PHYSIO Lectures Summary 9  ATP is present in all pre-synaptic vesicles and is co-released with other classical neurotransmitters in response to Ca2+ influx into the terminal. OVERVIEW OF THE NERVOUS SYSTEM The Central Nervous System (CNS) is composed of the brain and the spinal cord. The Peripheral Nervous System (PNS) is composed of the Afferent and Efferent divisions. The Afferent division travels from the PNS toward the CNS, and receives the stimuli. The Efferent division travels from the CNS toward the PNS and sends commands from the brain The brain consists of 3 major parts: The forebrain, cerebellum, and the brainstem In the forebrain, the cerebrum consists of the right and left cerebral hemispheres, and the diencephalon The cerebral hemispheres consist of: Cerebral Cortex Outer gray matter composed primarily of cell bodies, and an inner layer of white matter composed of primarily myelinated fiber tracts This is the integrating area of the nervous system Corpus Callosum The bundle of nerve fibers connecting the left and right cerebral hemispheres PHYSIO Lectures Summary 10  Each cortex is divided into four lobes: the frontal, parietal, occipital, and temporal. The cortex is highly folded, which leads to more surface area (more cell bodies) There are 2 basic types of cells in the human cerebral cortex: Pyramidal cells They are the major output cells of the cortex, sending their axons to other parts of the cortex and CNS. Nonpyramidal cells They are involved in receiving inputs into the cortex and in local processing of information The subcortical nuclei are groups of gray matter that lie within the cerebral hemispheres, and they include the basal nuclei (basal ganglia) which is important in controlling movement and posture and in more complex aspects of behavior. Parkinson’s disease affects the dark parts of the basal ganglia. You lose the dopamine secreting cells located in the substantia nigra. The diencephalon contains the thalamus, hypothalamus, and epithalamus. The epithalamus is a small mass of tissue that includes the pineal gland which has a role in regulating biological rhythms. “Epi” means “around.” In the thalamus are relay stations and important integrating centers for most inputs to the cortex. It plays a role in general arousal and in focusing attention. The hypothalamus forms the the master command center for neural and endocrine coordination. It controls behaviors having to do with individual preservation (eating and drinking) and the preservation of the species (reproduction). It is connected by a stalk to the pituitary gland. The cerebellum is an important center for coordinating movements and for controlling posture and balance. It receives information from the muscles and joints, skin, eyes and ears, viscera, and all the parts of the brain involved in the control of movement. PHYSIO Lectures Summary 11  The brainstem is where the nerve fibers that relay signals between the forebrain, cerebellum, and spinal cord pass through. It receives and integrates input from all regions of the CNS and processes a great deal of neural information. The reticular formation covers an expansive portion of the brainstem. It is comprised of neuron cell bodies that run through the core of the brainstem. It is involved in motor functions, cardiovascular and respiratory control, regulating sleep and wakefulness, and focus of attention. The brainstem contains nuclei involved in processing information for 10 out of the 12 pairs of cranial nerves. In order, you have: I. Olfactory (Sensory) VII. Facial (Both) Carries input from olfactory neuroepithelium Innervates skeletal muscles for facial expression and swallowing II. Optic (Sensory) VIII. Vestibulocochlear (Sensory) Carries input from receptors in the eye Transmits information from receptors in the inner ear III. Oculomotor (Motor) IX. Glossopharyngeal (Both) Innervates skeletal muscles involving the eye (medial movement), adjusting pupil width Innervates skeletal muscles involved in swallowing and transmits information from taste buds IV. Troclear (Motor) X. Vagus (Both) Innervates skeletal muscles involving the eye (lateral movement) Innervates skeletal muscles in the throat V. Trigenimal (Both) XI. Accessory (Motor) Innervates skeletal chewing muscles, transmits Innervates neck skeletal muscles information from skin, skeletal muscles of the face XII. Hypoglossal (Motor) VI. Abducens (Motor) Innervates skeletal muscles of the tongue Innervates skeletal muscles involving the eye (lateral movement) The mnemonic for the names of the nerves is : Oh Oh Oh To Touch And Feel Very Good Velvet, AH The mnemonic for the functions of the nerves is : Some Say Marry Money, But My Brother Says Big Brains Matter Most. The spinal cord lies within the bony vertebral column and consists of the following parts: PHYSIO Lectures Summary 12  Gray Matter A butterfly-shaped area composed of interneurons, the cell bodies and dendrites of efferent neurons, the entering axons of afferent neurons, and glial cells Dorsal Horns Is the gray matter projecting toward the back of the body Ventral Horns Is the gray matter oriented toward the front White Matter Areas surrounding the gray matter which consists of groups of myelinated axons There are fiber tracts that run longitudinally through the cord: Descending pathway → Relay information from the brain to the spinal cord Ascending pathway → Transmits information toward the brain A short distance from the cord, the dorsal and ventral roots combine to form a spinal nerve Now the PNS transmits signals between the CNS and the receptors or effector organs. It contains a total of 43 pairs of nerves: 12 pairs of cranial nerves (see earlier), and 31 pairs of spinal nerves. The spinal nerves are divided into 5 segments: PHYSIO Lectures Summary 13  1. Cervical Nerves (8) Control the muscles and glands and receive sensory input from the neck , shoulders, arms, and hands 2. Thoracic Nerves (12) Associated with the chest and the upper abdomen 3. Lumbar Nerves (5) Associated with the lower abdomen, hips, and legs 4. Sacral Nerves (5) Associated with the genitals and lower digestive tract 5. Coccygeal nerve (1) Associated with the tailbone The PNS is divided into the Somatic and Autonomic Nervous Systems. PHYSIO Lectures Summary 14  The autonomic nervous system can be further divided into the enteric, sympathetic and parasympathetic nervous systems. Enteric Nervous system Specific for the gastrointestinal tract Sympathetic Nervous system “Fight or flight” Another name is the thoracolumbar division Has short pre-ganglionic and long post-ganglionic synapses PHYSIO Lectures Summary 15  The major neurotransmitters are ACh at the pre-ganglionic synapses, and NE and Epi at the post-ganglionic synapses Parasympathetic Nervous system “Rest and digest” Another name is the craniosacral division Has long pre-ganglionic and short post-ganglionic synapses The major neurotransmitter is ACh. Muscarinic and nicotinic receptors are both types of cholinergic receptors (activated by acetylcholine). Muscarinic receptors are located on smooth muscle, cardiac muscle, and gland cells Nicotinic receptors are located on skeletal muscle fibers, innervated by the somatic motor neurons PHYSIO Lectures Summary 16  For the physical support of the CNS, you have the bones, meninges, and cerebrospinal fluid. The cranium and vertebrae are the bones that serve to support and protect the structures of the CNS and PNS Meninges are the membranes that line the structures and add additional support and protection. CSF protects and cushions the structures The meninges consist of three layers, the dura mater, arachnoid mater, and pia mater. The job of the meninges is to: Cover and protect the CNS Protect blood vessels and enclose the venous sinuses Contain CSF Form partitions in the skull PHYSIO Lectures Summary 17  When there’s an inflammation of the meninges, it’s called meningitis. If the brain itself is inflamed, it’s called encephalitis. CSF is secreted by the ependymal cells of the choroid plexus, and circulates through the subarachnoid space and ventricles. They are reabsorbed by arachnoid villi. The blood-brain barrier is a protective mechanism that helps maintain a stable environment in the brain. SENSORY PHYSIOLOGY Sensory receptors are specialized cells that generate graded potentials called receptor potentials in response to a stimulus. They can be specialized endings of the primary afferent neurons or separate receptor cells that signal the primary afferent neurons using neurotransmitters. There are five major divisions of these sensory receptors based on the stimuli that they respond to: 1. Mechanoreceptors - physical stimuli 2. Thermoreceptors - temperature 3. Photoreceptors - light 4. Chemoreceptors → chemicals (like for taste) 5. Nociceptors → pain Coding is the conversion of a stimulus into a signal that is conveyed to the CNS. Information is conveyed by both the frequency and amplitude of the resulting signals. The number of action potentials determine the strength of the sense, and NOT the amplitude of the action potential. All action potentials depolarize equally, so stacking them is what makes the sense stronger and not how strong one individual AP is. Adaptation refers to a decrease in receptor sensitivity and results in a decrease in AP frequency in an afferent neuron despite a stimulus of constant strength. Think of the rubber band exercise, where if you place one around your head eventually you won’t feel it anymore. As mentioned earlier, the intensity of the stimulus is increased by the frequency of AP in a single afferent neuron. As the strength of a local stimulus increases, receptors on adjacent branches of an afferent neuron are activated PHYSIO Lectures Summary 18  The stimulus location is coded by the site of a stimulated receptor. APs from from each receptor travel along unique pathways to a specific region in the CNS. Acuity refers to the clearness or the sharpness of a sensory function. One stimulus can be discerned from an adjacent one depending on the amount of convergence of neuronal input in the specific ascending pathway. Think about the 2 point determination experiment. A high AP frequency in one neuron but low in the others provides more accurate localization of the stimulus. Lateral inhibition enables the localization of a stimulus site. It happens by inhibiting information from adjacent afferent neurons. This enhances the contrast between stimulated regions. Sensory signals are subject to extensive modification before they reach higher levels of the CNS. Modification can come from: Lateral inhibition Pathways descending from the brain Synapses on the axon terminals of the afferent neurons (presynaptic inhibition) Interneurons that affect other neurons in the sensory pathways. PHYSIO Lectures Summary 19  When the afferent neurons enter the CNS and synapse with the interneurons, the process may diverge to terminate on several interneurons or many afferent neurons terminate upon a single interneuron. The processing of afferent information happens in either primary cortical receiving areas, or association areas in the cerebral cortex where complex integration occurs. Some factors that affect perception are: Receptor adaptation and afferent processing Emotions and experiences Not all stimuli give rise to a conscious sensation (like stretch receptors monitor blood pressure) Lack of receptors for certain stimuli (like radio waves) Damaged neural pathways Drugs Somatic sensation refers to the perception of physical sensations that arise from the body’s surface or musculoskeletal system including touch, pain, temperature, vibration, joint sense, and muscle sense. We have different types of sensory receptors in the skin for somatic sensation. These are: Meissner’s corpuscle Rapidly adapting mechanoreceptor, touch and pressure Merkle’s corpuscle Slowly adapting mechanoreceptor, touch and pressure Free nerve endings Slowly adapting, some are nociceptors, thermoreceptors, and mechanoreceptors Pacinian corpuscles Rapidly adapting mechanoreceptor, vibration and deep pressure Ruffini corpuscle Slowly adapting mechanoreceptor, skin stretch Hyperalgesia refers to an increased sensitivity to painful stimuli. Analgesia is the selective suppression of pain without effects on consciousness or other sensations. Some mechanisms to achieve pain relief include: Electrical stimulation of the CNS PHYSIO Lectures Summary 20  Drugs → NSAIDs (non-steroidal anti-inflammatory drugs) and Morphine (opioids) Neurons in the inhibitory pathways that release morphine-like endogenous opioids Acupuncture Transcutaneous Electrical Nerve Stimulation (TENS) Massage Referred pain is when the brain gets confused and you feel pain from an internal organ as another area of the body. It’s when you feel pain in a place where it’s not happening due to the development of the nerves from the same embryonic location. Homunculus is the name of the small, exaggerated human figure. VISION The eyes are composed of both an optical and a neural component. The optical component focuses the visual image on the receptor cells while the neural component transforms the visual image into a pattern of graded and action potentials. The lens of they eye is convex (converging lens). The eyes focus and unfocus due to the interaction of the ciliary muscles and the zonular fibers. When you relax the ciliary muscles, the zonular fibers tense and the lens is flattened and vice versa. Nearsightedness (Myopia) happens when the focal point occurs before the fovea centralis due to an elongated eyeball. Farsightedness (Hyperopia) happens when the focal point occurs beyond the fovea centralis due to a shorter eyeball. PHYSIO Lectures Summary 21  The blind spot occurs when the focus hits below the fovea centralis, into the area where the optic nerves are. That area of your vision is actually just filled up by the brain. Light signals are converted into AP by the interaction of photoreceptors, bipolar cells, and ganglion cells. The photoreceptors and bipolar cells only undergo graded responses due to the lack of voltage-gated channels. The ganglion cells are the first cells in the pathway where AP are initiated. Photoreceptors interact with bipolar and ganglion cells in two distinct ways — ON and OFF pathways. Photoreceptors are depolarized in the absence of light, releasing glutamate onto the bipolar cells. Light striking the photoreceptors hyperpolarizes them, decreasing the glutamate output. PHYSIO Lectures Summary 22  The bipolar cells for the ON and OFF pathways respond differently to glutamate due to the type of glutamate receptors present on each. For the ON bipolar cells they have metabotropic glutamate receptors, and for the OFF bipolar cells they have ionotropic glutamate receptors. The metabotropic receptors (ON) hyperpolarize the cell in response to glutamate, while the ionotropic receptors (OFF) depolarize the cell. The co-existence of ON and OFF pathways in the retina greatly improves image resolution by increasing the brain’s ability to perceive contrast The right and left eye collaborate to form your visual field. The general term for visual field defects is anopsia, and for half of the field the term is hemianopsia. PHYSIO Lectures Summary 23  We are able to perceive color due to the cones in our eyes. These come in 3 types: L cones → respond optimally at long wavelengths M cones → respond at medium wavelengths S cones → best stimulated at short wavelengths Color blindness results from a recessive mutation in the genes that encode the cone pigments. The most common form of color blindness is red-green color blindness and is present predominantly in men (about 1 of 12). PHYSIO Lectures Summary 24  The macula lutea region of the retina provide the highest visual acuity. This region becomes impaired in macular degeneration, which is a defect characterized by loss of vision in the center of the visual field. Approximately 30% of individuals over the age of 75 have increased incidence of Age-related Macular Degeneration (AMD). Eye movement can be characterized as fast (saccades) or slow. For fast movements, these are small, jerking movements that rapidly bring the eye from one fixation point to another to allow a search of the visual field. These also occur during certain periods of sleep when dreaming occurs (REM). Slow eye movements are involved in tracking objects as they move and during compensation for movements of the head. This requires continuous feedback of visual information about the moving object. HEARING AND VESTIBULAR SYSTEM The cochlea is in charge of the hearing, while the semicircular canals are in charge of your sense of balance. PHYSIO Lectures Summary 25  The tympanic membrane (eardrum) seals of the inner ear from the outside. When sound waves reach the eardrum, the displacement caused by the vibration move the middle ear bones (ear ossicles), which moves the membrane in the oval window, which affects the basilar membrane, which then moves the membrane in the round window. It’s the 3-bone configuration that allows for the tempering of the sound. Cochlear nerve fibers synapse with interneurons in the brainstem. From the brainstem, a multineuron pathway transmits information through the thalamus to the auditory complex in the temporal lobe. The semicircular canals act like a gyroscope that allows us to perceive our orientation. Our vestibular system is used to gain information to control eye movement, maintain posture and balance, as well as to provide awareness of body position and acceleration. Vestibular nerve fibers transmit information through the brainstem and thalamus to the vestibular centers in the parietal lobe. TASTE AND SMELL Chemicals that bind to specific chemoreceptors are responsible for the detection of taste and smell. Taste buds are groups of cells arranged around a hollow pore. Microvilli increase the surface area and contain membrane proteins that transduce a chemical into a receptor potential. The basal cells of the taste buds divide and differentiate to continually replace taste receptor cells damaged in the harsh environment of the mouth. To enter the pores of the taste buds, food molecules must be dissolved first in liquid. We have the following types of taste receptors and their corresponding chemical trigger: Salt taste → Sodium ions Sour taste → Hydrogen ions Sweet taste → Glucose Bitter taste → Associated with poisonous substances, especially plant alkaloids Umami taste → Associated with the taste of glutamate Proteins in the mucus interact with the odorant molecules, transport them to the receptors, and facilitate their binding to the receptors. Stimulated odorant receptors activate a G protein-mediated pathway that increases cAMP (cyclic adenosine monophosphate), which then opens nonselective cation channels and depolarizes the cell. PHYSIO Lectures Summary 26  The lack of the ability to smell is called anosmia Some factors that affect the sense of smell are: Attentiveness Hunger (greater in hungry subjects) Gender (women have keener olfactory sensitivities) Smoking (decreases sensitivity) Age (decreases with age) State of olfactory mucosa (decreases when the mucosa is congested, as in a head cold) PHYSIO Lectures Summary 27