Nervous System: Neurons and Action Potentials PDF

9/7/2023 Nervous System: Neurons and Action Potentials Fox Chapter 7 © Stanbridge University, 2023 1  9/7/2023 Objectives:  Understand the difference between the central and per...

9/7/2023 Nervous System: Neurons and Action Potentials Fox Chapter 7 © Stanbridge University, 2023 1  9/7/2023 Objectives:  Understand the difference between the central and peripheral nervous system  List the glial cells of the central and peripheral nervous system and name their functions  List the differences between a graded potential and an action potential  Describe how an action potential occurs with an electrical and chemical trigger © Stanbridge University, 2023 2  9/7/2023 Central and Peripheral Nervous System (Marieb, 2019) © Stanbridge University, 2023 3  9/7/2023 Organization of the Nervous System (Marieb, 2019) © Stanbridge University, 2023 4  9/7/2023 Cells of the Nervous System  Neurons:  Basic structural and functional unit of the nervous system  Respond to physical and chemical stimuli  Conduct electrochemical impulses  Release chemical regulators  Types: Motor, sensory, and interneurons  Neuroglia:  Supporting cells  Aid the functions of neurons  Types: Astrocytes, ependymal cells, Schwann cells, oligodendrocytes, microglia, and satellite cells © Stanbridge University, 2023 5  9/7/2023 Structure of a Neuron a) Motor Neuron b) Sensory Neuron (Fox, 2021) © Stanbridge University, 2023 6  9/7/2023 Structure of a Neuron (Fox, 2021) Major Components: 1. Cell Body – information processing, nutritional center of the neuron 2. Dendrites – receives information and transmits it to the cell body 3. Axon – conducts impulses away from the cell body © Stanbridge University, 2023 7  9/7/2023 Myelination of a nerve fiber in the PNS (Marieb, 2019) © Stanbridge University, 2023 8  9/7/2023 Structure of a Neuron (Fox, 2021) © Stanbridge University, 2023 9  9/7/2023 Structure of a Neuron (Fox, 2021) © Stanbridge University, 2023 10  9/7/2023 © Stanbridge University, 2023 Marieb, 2019 11  9/7/2023 Neuroglia in the CNS Astrocytes: most abundant and versatile glia in CNS  Support and brace neurons  Anchor neurons to nutrient supply and determine capillary permeability  Guide young neuron migration and formation of synapses  Control chemical environment around neurons Marieb, 2019 © Stanbridge University, 2023 12  9/7/2023 Neuroglia in the CNS Microglial cells:  Monitor neuron health  Can change into type of macrophage for microorganisms and neural debris Marieb, 2019 © Stanbridge University, 2023 13  9/7/2023 Neuroglia in the CNS Ependymal cells:  Line central cavities of brain and spinal cord  Permeable barrier between cerebral spinal fluid (CSF) and fluid around cells in the central nervous system  Cilia (in most) help circulate cerebral spinal fluid Oligodendrocytes:  Form myelin sheath around larger axons Marieb, 2019 in the CNS © Stanbridge University, 2023 14  9/7/2023 Neuroglia in the PNS Satellite Cells: thought to be like astrocytes Schwann Cells:  Surround all nerve fibers in the PNS  Form myelin sheath around thicker axons (like oligodendrocytes)  Vital to regeneration of peripheral nerve fibers Marieb, 2019 © Stanbridge University, 2023 15  9/7/2023 Action Potential Animation (https://youtu.be/ZAmUjvgoO0A) © Stanbridge University, 2023 16  9/7/2023 Graded Potentials and Action Potentials: Gated Channels Marieb, 2019 © Stanbridge University, 2023 17  9/7/2023 Resting Membrane Potential: Concentration of Na+ and K+ inside and outside is different © Stanbridge University, 2023 Marieb, 2019 18  9/7/2023 © Stanbridge University, 2023 Marieb, 2019 19  9/7/2023 © Stanbridge University, 2023 Marieb, 2019 20  9/7/2023 © Stanbridge University, 2023 Marieb, 2019 21  9/7/2023 Depolarization and Hyperpolarization Depolarization: decrease in membrane potential Inside of cell becomes less negative than resting membrane potential Can reverse the membrane potential where in the inside of the cell becomes positive Hyperpolarization: increase in membrane potential The inside of the neuron becomes more negative than the resting membrane potential © Stanbridge University, 2023 22  9/7/2023 Action Potentials (AP)  All or none event; occurs in one section at a time along axon  Depolarization of the cell must reach threshold to trigger an AP  Initiated by graded potentials that spread along the dendrites and cell body membranes → change in membrane potential → open or close voltage gated ion channels  Stronger stimuli → depolarize more → Na+ permeability rises → positive feedback cycle → brief reversal of membrane potential from -70mV to +30mV → AP  (Weaker stimuli have to go longer to provide enough depolarization)  Generated only in the axon  Only neurons and muscle cells have excitable membranes which can have an AP © Stanbridge University, 2023 23  9/7/2023 Action Potentials (AP) (Marieb, 2019) © Stanbridge University, 2023 24  9/7/2023 © Stanbridge University, 2023 Marieb, 2019 25  9/7/2023 Nonmyelinated vs. myelinated axons Myelin speeds up conduction of an action potential by about 30x! © Stanbridge University, 2023 (Marieb, 2019) 26  9/7/2023 Electrical Synapses  Less common  Has gap junctions  Has protein channels that connect adjacent neurons and allow ions and small molecules to pass  Neurons are electrically coupled → rapid transmission  Can be unidirectional or bidirectional transmission  Synchronized activity  Ex. eye movements © Stanbridge University, 2023 27  9/7/2023 Chemical Synapses  More common  Release and reception of neurotransmitters (in synaptic vesicles) between neurons across a synaptic cleft  Unidirectional communication  Converts electrical signal to chemical signal, then back to electrical signal for transmission © Stanbridge University, 2023 28  9/7/2023 Steps of an Action Potential from a Chemical Synapse  Depolarization:  Na+ enters the axon through voltage-gated channels, changing the polarity of the membrane (to become positive inside of the cell, from negative) © Stanbridge University, 2023 29  9/7/2023 Steps of an Action Potential from a Chemical Synapse  Repolarization  K+ leaves the axon to temporarily make the axon negative again  Hyperpolarization  Too much K+ leaves the axon for a brief period which makes the membrane potential more negative (inside of the cell)  Na+/K+ pumps quickly restore the resting membrane potential of charge (ion concentration takes longer to restore but action potentials can continue usually) © Stanbridge University, 2023 30  9/7/2023 Axon Terminals: AP reaches axon terminal, neurotransmitters are released and stimulate dendrites of the next neuron Step 1: AP causes calcium (Ca2+) channels in the terminal to open Step 2: Ca2+ causes vesicles filled with neurotransmitters to migrate to the end of the terminal There are many types of neurotransmitters Step 3: Exocytosis releases the neurotransmitters Serotonin into the synaptic cleft Acetylcholine Step 4: Neurotransmitters bind to receptors on Dopamine Epinephrine dendrites of the receiving neuron. This triggers Norepinephrine an action potential in the next neuron Step 5: Neurotransmitters are degraded by enzymes and “taken up” by endocytosis Step 6: Neurotransmitters are reassembled into vesicles again © Stanbridge University, 2023 31  9/7/2023 Central Nervous System CNS (Fox CHAPTER 8) © Stanbridge University, 2023 32  9/7/2023 Objectives:  Identify the five brain regions and the major structures they contain, including the ventricles  Describe the organization of the sensory and motor areas of the cerebral cortex  Describe the locations and functions of the diencephalon, midbrain, hindbrain, cerebellum, and spinal cord © Stanbridge University, 2023 33  9/7/2023 Components of the CNS Brain Cerebrum Diencephalon Cerebellum Brainstem Marieb, 2019 Spinal Cord (SC) © Stanbridge University, 2023 34  9/7/2023 Brain  100 billion neurons  Average 2.95# in males and 2.64# in females (Hartman, et al. 1994)  20% blood flow  High metabolic rate- constant during exercise  Requires constant O2, nutrients-glucose  10 seconds without O2 results in unconsciousness  Sensitive to toxins and drugs Hartman P., Romseier, A., Gudat F., Mihatsch MJ., Polasek W. Normal Weight of the Brain in Adult in Relation to Age, Sex, Body Height, and Weight. Pathologe. 1994 June 15(3): 165-170. © Stanbridge University, 2023 35  9/7/2023 Brain Structure  Cerebrum and cerebellum have an outer layer of gray matter (called cortex)  Gray Matter = made up of cell bodies and dendrites, location of information processing  Pattern changes in brain stem, no cortex, but gray matter nuclei scattered throughout white matter  White Matter = made up of axons, location of information transport © Stanbridge University, 2023 36  9/7/2023 Gray Matter and White Matter in the Brain, including the brainstem, and the Spinal Cord Marieb, 2019 © Stanbridge University, 2023 37  9/7/2023 Brain Structure: Cerebral Hemispheres  Superior part of the brain  83% of brain mass between two sides  They cover diencephalon and top of brainstem  Largest portion of the brain  Located on the top and more of the front of the brain  Involved with speech, sensation, communication, memory, motor function, reasoning, will, and emotions © Stanbridge University, 2023 (Kessler, 2021) 38  9/7/2023 Brain Structure: Cerebrum  Cerebral cortex: neuron cell bodies, dendrites, glia and blood vessels (no fiber tracts)  Cerebral white matter: communication between cerebral areas and lower CNS centers; largely myelinated fibers bundled into tracts  Basal nuclei (basal ganglia): group of subcortical nuclei: caudate nucleus, putamen, and globus pallidus © Stanbridge University, 2023 39  9/7/2023 Brain Structure: Cerebral Cortex  Gyri: elevated ridges of tissue  Sulci: shallow grooves  Fissures: deeper grooves separate larger regions © Stanbridge University, 2023 (Fox, 2021) 40  9/7/2023 Functions of the Cerebral Lobes Frontal lobe  Higher mental process (intelligence, motivation, judgement)  Motor area (voluntary motor)  Motor Speech (Broca’s Area) Parietal lobe  Sensory area (touch, vibration, kinesthesia, proprioception, etc)  Estimation of distances, sizes, shapes Temporal lobe  Auditory area  Speech Comprehension (Wernicke’s Area)  Olfactory area © Stanbridge University, 2023 41  9/7/2023 Functions of the Cerebral Lobes Occipital lobe  Visual interpretation and association Insula  cortex: conscious perception of visceral sensation  posterior: vestibular sensory area  taste (gustatory cortex); perceiving taste © Stanbridge University, 2023 42  9/7/2023 Cerebral Cortex: Motor Areas Located in posterior part of frontal lobe Primary motor cortex Premotor areas Broca’s area Frontal eye field © Stanbridge University, 2023 (Fox, 2021) 43  9/7/2023 Cerebral Cortex: Sensory Areas  Located in parietal, insular, temporal, and occipital lobes  Primary somatosensory cortex  Somatosensory association area  Visual areas  Auditory areas  Vestibular areas  Olfactory cortex  Gustatory cortex  Visceral sensory area © Stanbridge University, 2023 (Fox, 2021) 44  9/7/2023 Homunculi  A form of cortical mapping  Mapping takes place in the precentral cortex (frontal lobe - motor) and the postcentral cortex (parietal lobe – sensory)  Areas of the cortex responsible for different body parts  Size of body parts on the map correlate with number of receptors in that body part  Ex. Hands are depicted as a large area in the homunculi due to high number of sensory and motor receptors in this area © Stanbridge University, 2023 45  9/7/2023 Homunculi © Stanbridge University, 2023 Marieb, 2019 46  9/7/2023 Cerebral White Matter  Largely myelinated fibers bundled into tracts  Association fibers: connect within a hemisphere  Commissural fibers: connect corresponding gray matter of 2 hemispheres: largest is corpus callosum, smaller anterior and posterior commissures  Projection fibers: enter or exit to and from lower brain areas or spinal cord (Ex. Spinal tracts) © Stanbridge University, 2023 47  9/7/2023 Cerebral Cortex: Multimodal Association Areas In most of frontal lobe cortex: sensations, thoughts, and emotions become conscious  Anterior association: in prefrontal cortex  Recall, intellect, complex learning activities, personality, “working memory”  Posterior association: parts of temporal, parietal, and occipital lobes  Recognize patterns and faces, self in space, Wernicke’s area  Limbic association: cingulate gyrus, parahippocampal gyrus, and hippocampus  Emotional impact of a situation  Hippocampus: memories © Stanbridge University, 2023 48  9/7/2023 Cerebral white matter  Projection fibers: at top of brain stem, form a band called internal capsule, passes between thalamus and some of basal nuclei, then fibers radiate called corona radiata © Stanbridge University, 2023 49  9/7/2023 White Matter Tracts (Frontal Section) © Stanbridge University, 2023 Marieb, 2019 50  9/7/2023 White Matter Tracts (parasagittal section) Marieb, 2019 © Stanbridge University, 2023 51  9/7/2023 Cerebral Basal Nuclei  Deep within cerebral white matter  Not completely defined roles  Motor functions overlap with cerebellum  Cognition and emotion  Filter out incorrect or inappropriate responses  Start, stop, monitor intensity of movements from cortex ex. arm swing with walking  Inhibit antagonistic or unnecessary movements © Stanbridge University, 2023 52  9/7/2023 Basal Nuclei (3D view) Marieb, 2019 © Stanbridge University, 2023 53  9/7/2023 Basal Nuclei (transverse section) Marieb, 2019 © Stanbridge University, 2023 54  9/7/2023 Diencephalon – 3 parts  Thalamus  Hypothalamus  Epithalamus © Stanbridge University, 2023 55  9/7/2023 Diencephalon – 3 parts Thalamus (many nuclei) Hypothalamus (many nuclei)  Integrate sensory and motor info  Main visceral control center  Sensory relay to cortex  Homeostasis: ANS Epithalamus Physical response to emotion  Pineal gland: sleep wake cycle Regulate body temp Regulate food intake Regulate water balance and thirst Sleep wake cycle Control endocrine function © Stanbridge University, 2023 56  9/7/2023 Brain stem: relays and transmits impulses  Midbrain: connects cerebrum and the lower nervous system. Reflexes of the eyes and ears  Pons: between midbrain and medulla oblongata. Connects cerebellum to brain and some reflexes of respiration  Medulla Oblongata: continuation of the spinal cord. Respiratory Center, Cardiac Center, Vasomotor Center, swallowing, coughing, sneezing reflexes  Site where motor signals cross to the opposite side of the body © Stanbridge University, 2023 57  9/7/2023 Cerebellum  Sits interior and posterior to cerebrum  Maintains: Body balance, coordinates voluntary muscles, makes muscular movements graceful and smooth Marieb, 2019 © Stanbridge University, 2023 58  9/7/2023 Cerebellum  Cerebellar hemispheres  Cerebellar peduncles: connect cerebellum to brain stem  Input from cerebral motor cortex, brain stem nuclei, sensory receptors → precise timing and appropriate patterns of movement and agility  Involved with equilibrium and balance © Stanbridge University, 2023 59  9/7/2023 Functional Brain System  Networks of neurons working together and spanning wide areas of the brain  Two systems:  Limbic system  Reticular formation © Stanbridge University, 2023 60  9/7/2023 Limbic System  Located on the medial aspects of cerebral hemispheres and diencephalon  Limbic System: Emotion and cognition/memory  Amygdala – responding to perceived threats; memory  Rhinencephalon – olfactory (identifies smells)  Cingulate gyrus (anterior part)- express emotion through gestures  Hypothalamus – most limbic system output through here  Anterior thalamic nuclei – relays info to and from spinal cord and cerebrum  Hippocampus: memory © Stanbridge University, 2023 61  9/7/2023 Reticular Formation  Maintains cerebral cortical alertness (RAS)  Filters out repetitive stimuli  Helps regulate skeletal and visceral muscle activity (vasomotor, cardiac, respiratory centers of medulla)  Loosely clustered neurons in white matter  Central core through medulla oblongata, pons, and midbrain send messages to hypothalamus, thalamus, cerebral cortex, cerebellum, and s.c. © Stanbridge University, 2023 62  9/7/2023 Reticular Formation © Stanbridge University, 2023 Marieb, 2019 63  9/7/2023 Reticular Formation: RAS and Motor Function  RAS – Reticular Activating System  Sends impulses to the cerebral cortex to keep it conscious and alert  Filters out repetitive and weak stimuli  Motor function  Helps control coarse motor movements  Autonomic centers regulate visceral motor functions – e.g., vasomotor, cardiac, and respiratory centers © Stanbridge University, 2023 64  9/7/2023 Function of Meninges  3 connective tissue membranes just external to CNS organs  Mening = “membranes”  Cover and protect CNS  Protect blood vessels and enclose venous sinuses  Contain CSF  Form partitions in the skull © Stanbridge University, 2023 65  9/7/2023 Meninges  connective tissue pad that surrounds and protects the CNS  Pia mater: deep layer  Arachnoid mater : middle layer  Dura mater: superficial layer  Meningitis- life threatening infection © Stanbridge University, 2023 66  9/7/2023 Meninges © Stanbridge University, 2023 Marieb, 2019 67  9/7/2023 More about the meninges (PAD)  The word Mater means “Mother”  Pia Mater = Tender (soft) Mother  The gentle, soft, inner layer that rests against the tender brain  Arachnoid Mater = Spider-Like Mother  Very thin middle layer that resembles a spider web  Dura Mater = Tough Mother (folds in to secure the brain)  The tough, thick, outer protective layer. © Stanbridge University, 2023 68  9/7/2023 Pia Mater  Follows all brain convolutions  Located closest to the brain tissue  Many tiny blood vessels (Fox, 2021) © Stanbridge University, 2023 69  9/7/2023 Arachnoid Mater  Loose brain covering  Separated from dura mater by narrow serous cavity (subdural space) contains film of fluid  Subarachnoid space:  below arachnoid membrane  filled with CSF  contains largest blood vessel serving brain (poorly protected) © Stanbridge University, 2023 70  9/7/2023 Dura Mater  In brain: 2 layers  Outer periosteal layer attaches to skull periosteum (at dural septa)  Inner meningeal layer: true external covering of brain *When the two layers split, they create the dural venous sinuses © Stanbridge University, 2023 71  9/7/2023 Dural septa  In some places, meningeal layer of dura mater extend inward to partition cranial cavity  Limit excessive movement of the brain within cranium © Stanbridge University, 2023 72  9/7/2023 Dural Venous Sinuses  Dura mater in brain: 2 layers fused except when separate to enclose dural venous sinuses  Collect venous blood from brain and direct it to internal jugular veins of neck (Fox, 2021) © Stanbridge University, 2023 73  9/7/2023 Cerebrospinal Fluid (CSF) CSF mixes with extracellular fluid around neurons Moves through ventricles, enters subarachnoid space via walls of 4th ventricle Long cilia keep it in motion Bathes brain and spinal cord, then returns to blood via dural sinuses through arachnoid villi © Stanbridge University, 2023 74  9/7/2023 Cerebrospinal Fluid (CSF)  Supports nervous tissue, cushions from shock  Carries nutrients to cells and carries chemical signals  Transports waste products from cells  Located in and around brain and spinal cord  Produced by the Choroid Plexus in ventricles of brain  Regulates CSF composition and cleanses CSF Marieb, 2019 © Stanbridge University, 2023 75  9/7/2023 Ventricles of the Brain 2 lateral ventricles Third Ventricle Fourth Ventricle © Stanbridge University, 2023 76  9/7/2023 Ventricles of the Brain Marieb, 2019 © Stanbridge University, 2023 77  9/7/2023 © Stanbridge University, 2023 Marieb, 2019 78  9/7/2023 Blood Brain Barrier  Endothelial cells triggered by astrocytes to form tight junctions → least permeable capillaries in the body  Blood bourne substances in the brain’s capillaries must pass through 3 layers before reaching neurons:  Endothelium of capillary wall  Thick basal lamina surrounding external face of each capillary  “feet” of astrocytes clinging to capillaries © Stanbridge University, 2023 79  9/7/2023 Blood Brain Barrier  Not inform throughout (some areas more permeable)  Glucose, essential a.a., and some electrolytes pass by facilitated passive diffusion  Some substances actively pumped out of brain  Fats, fatty acids, oxygen and carbon dioxide pass by simple diffusion © Stanbridge University, 2023 80  9/7/2023 Blood Brain Barrier Capillaries © Stanbridge University, 2023 Marieb, 2019 81  9/7/2023 Spinal Cord  CNS tissue is enclosed within the vertebral column from the foramen magnum to L1-L2  Enlargements in cervical and lumbar regions for limbs  Protected by bone, meninges, and CSF (between arachnoid and pia mater) – same meninges as in brain  Epidural space – space between the vertebrae and the dura mater filled with fat and a network of veins © Stanbridge University, 2023 Marieb, 2019 82  9/7/2023 Spinal Cord  Terminates into conus medullaris at L1 vertebrae (cone shaped structure)  Cauda equine (horse’s tail): lumbar and sacral nerves angle downward through canal to reach their intervertebral foramina  Dural and arachnoid membranes continue down to level of S2 vertebra © Stanbridge University, 2023 Marieb, 2019 83  9/7/2023 Spinal cord with vertebrae cut away Cervical, Thoracic, and Lumbar regions Marieb, 2019 © Stanbridge University, 2023 84  9/7/2023 Spinal Cord  Typically, spinal cord ends between L1 and L2 vertebrae  End of the spinal cord is called the Conus Medullaris  Bunch of fibers resembles the strands of a horse tail: The Cauda Equina © Stanbridge University, 2023 85  9/7/2023 Lateral View of spinal cord (Marieb, 2019) © Stanbridge University, 2023 86  9/7/2023 Spinal Cord  During fetal development, vertebral column (bone) grows faster than the spinal cord  Spinal nerve roots have to extend further to exit at the corresponding level © Stanbridge University, 2023 87  9/7/2023 Spinal Nerve Review 31 pairs of spinal nerves in humans:  8 cervical pairs (1-7 exit above corresponding vertebra and 8 exits below C7)  12 thoracic pairs  5 lumbar pairs  5 sacral pairs  1 pair of coccygeal nerves (C01)  More during PNS lecture © Stanbridge University, 2023 88  9/7/2023 Spinal Cord in vertebral Column © Stanbridge University, 2023 Marieb, 2019 89  9/7/2023 Spinal Cord and Meningeal Coverings Marieb, 2019 © Stanbridge University, 2023 90  9/7/2023 Gray Matter in the Spinal Cord:  Gray matter: cell bodies and interneurons  Dorsal horn: only interneurons receiving input from sensory neurons  Ventral horn: mainly cell bodies from somatic motor neurons and some interneurons  Lateral horn: only in thoracic and upper lumbar segments of s.c.; mostly cell bodies of autonomic NS (sympathetic division) motor neurons that serve visceral organs, exit via ventral root © Stanbridge University, 2023 91  9/7/2023 Organization of Gray Matter in Spinal Cord © Stanbridge University, 2023 Marieb, 2019 92  9/7/2023 White Matter in the Spinal Cord: White matter:  Nerve fibers (myelinated and nonmyelinated)  forms ascending and descending tracts © Stanbridge University, 2023 93  9/7/2023 Ascending and descending tracts in the Spinal Cord © Stanbridge University, 2023 Marieb, 2019 94  9/7/2023 Pathways of Ascending Spinal Cord Tracts (Marieb, 2019) Carries sensory information from the periphery of the body to the brain Dorsal Column = spinal tract that carries touch, vibration, 2-pt discrimination, proprioception Spinothalamic Tract = spinal tract that carries pain and temperature sensations © Stanbridge University, 2023 95  9/7/2023 Pathways of Descending Spinal Cord Tracts (Marieb, 2019) Carries sensory information from the brain to the periphery of the body Corticospinal Tract = spinal tract that carries voluntary motor © Stanbridge University, 2023 96  9/7/2023 Assess Your Learning! 1. Which of these statements about the precentral gyrus is true? A) It is involved in motor control B) It is involved in sensory perception C) It is located in the frontal lobe D) Both a and c are true E) Both b and c are true 2. Which portion of the diencephalon controls homeostasis in the body? A) Thalamus B) Hypothalamus C) Epithalamus © Stanbridge University, 2023 97