Neurons_8a8ca9093a569d3b3d6bb477d66a9bc2.pdf

Topic 4 Neurons and Electrophysiology 1 © McGraw Hill, LLC  Functions of the Nervous System 1. Sensory input Information gathered by sensory recep...

Topic 4 Neurons and Electrophysiology 1 © McGraw Hill, LLC  Functions of the Nervous System 1. Sensory input Information gathered by sensory receptors about internal and external changes 2. Integration Interpretation of sensory input 3. Motor output Activation of effector organs (muscles and glands) produces a response © McGraw Hill, LLC  Sensory input Integratio n Motor output © McGraw Hill, LLC Figure 11.1  The Nervous System The nervous system has two major divisions: the central nervous system (CNS), composed of the brain and spinal cord the peripheral nervous system (PNS), consisting of the nerves that connect the brain or spinal cord with the body’s muscles, glands, sense organs, and other tissues The 2 major cell types found in the nervous system: Neuron: the functional unit of the nervous system , which generates electrical signals called action potentials or nerve impulses Glial cells: non neuronal cells that support neurons but do not generate nerve impulses © McGraw Hill, LLC 4  Glial Cells of the CNS Glial cells provide neurons with physical and metabolic support. Types of glial cells in the CNS: Astrocytes: help regulate the composition of the extracellular fluid in the CN S by removing potassium ions and neurotransmitters around synapses, stimulate the formation of tight junctions (for the blood-brain barrier), and sustain neurons metabolically. Microglia: specialized, macrophage-like cells that perform immune functions in the CNS and may also contribute to synapse remodeling and plasticity. Ependymal cells: line the fluid-filled cavities within the brain and spinal cord and regulate the production and flow of cerebrospinal fluid. Oligodendrocytes: form the myelin sheath of CNS axons. © McGraw Hill, LLC 5  Figure 6.6 Glial Cells of the Central Nervous System © McGraw Hill, LLC Access the text alternative for slide images. 6  Structure of a Neuron Parts of a Neuron: Cell body (Soma): Contains nucleus and ribosomes Dendrites: Branches that receive information, typically through neurotransmitters Axon: Carries outgoing signals to target cells Initial segment (Axon hillock): Portion of axon that arises from cell body; generates action potentials; may branch into collaterals Axon terminal (Synaptic knob): End of each branch; releases neurotransmitters © McGraw Hill, LLC 7  Figure 6.1 Diagrammatic Representation of One Type of Neuron (a) Illustration of a typical neuron (b) Micrograph of a neuron © McGraw Hill, LLC Access the text alternative for slide images. 8  Figure 6.2 (a) Myelin Formed by Schwann Cells and (b) Oligodendrocytes on Axons. (c) False color photomicrograph of a section through a myelinated axon in the PNS. (a) Myelin sheath in PNS (b) Myelin sheath in CNS (c) Myelinated axon in cross section (c) Don W. Fawcett/Science Source Access the text alternative for slide images. © McGraw Hill, LLC 9  Transport Through the Axon Axonal Transport: Movement of substances & organelles between the Soma and the Axon terminals (Synaptic knobs). Movement proceeds along microtubule frameworks, with help of Motor Proteins: Kenesins: Move materials forward, from cell body toward axon axon terminals (Anterograde transport). Nutrients, synaptic vesicles, mitochondria. Dyneins: Move materials backward, from axon terminals toward cell body (Retrograde transport). Recycled growth factors, vesicles. Also route of entry for pathogens: tetanus toxin, herpes simplex, rabies, polio virus. © McGraw Hill, LLC 10  Figure 6.3 Axonal Transport Along Microtubules by Dynein and Kinesin © McGraw Hill, LLC Access the text alternative for slide images. 11  Figure 6.4a Three Classes of Neurons (a) Information flow in the nervous system by three types of neurons © McGraw Hill, LLC Access the text alternative for slide images. 12  Figure 6.4b Transverse Section of a Nerve as Seen in a Light Micrograph (b) Section through a nerve © McGraw Hill, LLC 13  Table 6.1 Characteristics of Three Classes of Neurons Afferent neurons Transmit information into the NS from receptors at their peripheral endings C Single process from the cell body splits into a long peripheral process (axon) that is in the PNS and a short central process (axon) the enters the CNS Efferent neurons Transmit information out of the CNS to effector cells, particularly muscles, glands, neurons, and other cells Cell body with multiple dendrites and a small segment of the axon are in the CNS; most of the axon is in the PNS Interneurons Function as integrators and signal changers Integrate groups of afferent and efferent neurons into reflex circuits Lie entirely within the CNS Account for > 99% of all neurons © McGraw Hill, LLC 14  Figure 6.5 A Neuron Postsynaptic to One Cell Can be Presynaptic to Another Synapses can use both chemical and electrical stimuli to pass information. Synapses can also be inhibitory or excitatory depending on the signal/ neurotransmitter being transmitted. © McGraw Hill, LLC Access the text alternative for slide images. 15  Growth and Development of Neurons 1 Development of the nervous system in the embryo begins with stem cells that can develop into neurons or glia. After the last cell division, each neuronal daughter cell differentiates, migrates to its final location, and sends out processes that will become its axon and dendrites. A specialized enlargement, the growth cone, forms the tip of each extending axon and is involved in finding the correct route and final target for the process. As the axon grows, it is guided along the surfaces of other cells, most commonly glial cells. © McGraw Hill, LLC 16  Growth and Development of Neurons 2 Route followed by an axon depends on attracting, supporting, deflecting, or inhibiting influences exerted by cell adhesion molecules and soluble neurotrophic factors (growth factors for neural tissue) in the extracellular fluid round the growth cone or its distant target. Once the target of the advancing growth cone is reached, synapses form. During early stages of neural development, which occur during fetal life and infancy, alcohol and other drugs, radiation, malnutrition, and viruses can cause permanent damage to the developing nervous system. © McGraw Hill, LLC 17  Growth and Development of Neurons 3 Throughout the life span, our brain has an amazing ability to modify its structure and function in response to stimulation or injury, which is called plasticity. The degree of neural plasticity varies with age. The basic shapes and locations of major neuronal circuits in the mature central nervous system do not change once formed. The creation and removal of synaptic contacts begun during fetal development continue throughout life as part of normal growth, learning, and aging. © McGraw Hill, LLC 18  Figure 6.7 Types of Electrical Interactions and Effects of Quantity and Distance Between Charges on Electrical Forces (a) Interactions between positive and negative charges (b) Relationships between type and amount of charge and distance © McGraw Hill, LLC Access the text alternative for slide images. 19  Figure 6.8 Measurement of the Resting Membrane Potential (a) Experimental method for recording membrane potential (b) Membrane potential recorded using a voltmeter © McGraw Hill, LLC Access the text alternative for slide images. 20  The Resting Membrane Potential © McGraw Hill, LLC 21  Table 6.2 Distribution of Major Mobile Ions Across the Plasma Membrane of a Typical Neuron Ion Concentration Concentration (millimoles/liter) (millimoles/liter) Extracellular Intracellular Na+ 145 15 Cl− 100 7* K+ 5 150 A more accurate measure of electrical driving force can be obtained using a measurement called milliequivalents/liter (mEq/L), which factors in ion valence. Because all the ions in this table have a valence of 1, the milliequivalents/liter is the same as the millimole/liter concentration. *Intracellular Cl- concentration varies significantly between neurons due to differences in expression of membrane transporters and channels. © McGraw Hill, LLC 22  Figure 6.9 The Excess Positive Charges Outside the Cell and the Excess Negative Charges Inside Collect in a Tight Shell Against the Plasma Membrane © McGraw Hill, LLC Access the text alternative for slide images. 23  Figure 6.10 Generation of a Membrane Potential Due to Diffusion of K+ Through K+ Channels © McGraw Hill, LLC Access the text alternative for slide images. 24  Figure 6.11 Generation of a Membrane Potential Due to Diffusion of Na+ Through Na+ Channels © McGraw Hill, LLC Access the text alternative for slide images. 25  Resting Membrane Potential Negative ions along inside of cell membrane & positive ions along outside potential energy difference at rest is -70 mV cell is “polarized” Resting potential exists because concentration of ions different inside & outside extracellular fluid rich in Na+ and Cl- cytosol full of K+, organic phosphate & amino acids membrane permeability differs for Na+ and K+ 50-100 greater permeability for K+ inward flow of Na+ can’t keep up with outward flow of K+ Na+/K+ pump removes Na+ as fast as it leaks in © McGraw Hill, LLC  Figure 6.12 Forces Influencing Na+ and K+ Ions at the Resting Membrane Potential © McGraw Hill, LLC Access the text alternative for slide images. 27  Figure 6.13 Steps Explaining the Resting Membrane Potential (a) Establishment of a concentration gradient by ion pump (b) Greater efflux of creates charge difference. (c) Stable negative resting potential achieved © McGraw Hill, LLC Access the text alternative for slide images. 28  Terminology for Graded and Action Potentials When describing action potentials and graded potentials we use specific terms to describe the direction of the changes in membrane potential relative to the resting membrane potential (RMP) in an excitable cell. Depolarization refers to the potential moving from RMP to less negative (closer to zero) values. Overshoot refers to a reversal of membrane polarity, when the inside of the cell becomes more positive than the outside. Repolarization refers to the potential returning to the RMP from a depolarized state. Hyperpolarization refers to the potential becoming more negative than the RMP. © McGraw Hill, LLC 29  Figure 6.14 Depolarizing, Overshoot, Repolarizing, and Hyperpolarizing Changes in the Membrane Potential Relative to the Resting Potential © McGraw Hill, LLC Access the text alternative for slide images. 30  Table 6.3 A Miniglossary of Terms Describing the Membrane Potential Potential or potential difference The voltage difference between two points due to separated electrical charges of opposite sign Membrane potential The voltage difference between the inside and outside of a cell Equilibrium potential The voltage difference across a membrane that produces a flux of a given ion species that is equal but opposite to the flux due to the concentration gradient of that same ion Resting membrane potential The steady potential of an unstimulated cell Graded potential A potential change of variable amplitude and duration that is conducted decrementally; has no threshold or refractory period Action potential A brief all-or-none depolarization of the membrane, which reverses polarity in neurons; has a threshold and refractory period and is conducted without decrement Synaptic potential A graded potential change produced in the postsynaptic neuron in response to the release of a neurotransmitter by a presynaptic terminal; may be depolarizing (an excitatory postsynaptic potential or EPSP) or hyperpolarizing (an inhibitory postsynaptic potential or IPSP) Receptor potential A graded potential produced at the peripheral endings of afferent neurons (or in separate receptor cells) in response to a stimulus Pacemaker potential A spontaneously occurring graded potential change that occurs in certain specialized cells Threshold potential The membrane potential at which an action potential is initiated © McGraw Hill, LLC 31