LECTURE STRUCTURE PART 1: Background / Introduction PART 2: Structure of the NS, how it operates PART 3: CNS PART 4: PNS Why did the action potential cross the synaptic junction? To get to the other side ASC171 NERVOUS SYSTEM Module 5 PART 1: BACKGROUND - Nervous system evolved from simple reflex arcs to centralised brains, with hierarchic regulation. - Nerve nets are the most simplest found in most animals - In vertebrates = nerve plexus -TED Talk Why do we have a brain? - Brain size varies with body size across different species, but number of neurons doesn t correspond to intellect - Vertebrate brains have a level of plasticity 1
LEARNING OBJECTIVES: PART 2 1. Describe the major anatomical and functional subdivisions of the nervous system 2. Describe the typical structure of a neuron 3. Describe the role of each anatomical element in normal neuronal function 4. Describe the ways that neurons may be classified 5. List the different types of glial cells and state their role 6. Describe the conditions during resting membrane potential and the processes involved in an action potential 7. Detail how action potentials are propagated between neurons VERTEBRATE NERVOUS SYSTEM: OVERVIEW AND PERIPHERAL SYSTEM Central nervous system (CNS) Brain Spinal cord Peripheral nervous system (PNS) Nerve fibres extending to other parts of the body Afferent division -- carries information from sensors to CNS Efferent division -- transmits instructions from CNS to effector organs DESCRIBE THE MAJOR ANATOMICAL AND FUNCTIONAL SUBDIVISIONS OF THE NERVOUS SYSTEM: LO1 DESCRIBE THE TYPICAL STRUCTURE OF A NEURON: LO2 Neurons are the basic functional unit Excitable cells that transmit electrical signals: Produce and conduct electrochemical impulses Respond to physical and chemical stimuli Release chemical regulators Cannot divide by mitosis 2
Dendrites Nucleolus Nucleus hillock (microtubules) Cell body Nissl body Schwann cell Node of Ranvier Schwann Cell nucleus Terminals Terminal branches DESCRIBE THE ROLE OF EACH ANATOMICAL ELEMENT IN NORMAL NEURONAL FUNCTION: LO3 STRUCTURE Cell body Dendrites Terminals Terminal branches Nucleus Nucleolus Nissl body hillock Schwann cell Schwann cell nucleus Nodes of ranvier FUNCTION Normal cell functioning Process that conducts electrical impulse towards cell body Conducts impulse away from cell body End of the axon Branches of the axon Rough endoplasmic reticulum Where the axon starts, start of the action potential Glial cell covering the axon (makes fibre myelinated) Myelin-free gaps between the Schwann cells DESCRIBE THE WAYS THAT NEURONS MAY BE CLASSIFIED: LO4 Structural classification: Based on the number of processes that extend from cell body Pseudounipolar Bipolar neurons Multipolar Functional classification: Sensory (afferent) transmit impulses toward the CNS Motor (efferent) carry impulses away from CNS Interneurons (association neurons) shuttle signals through CNS pathways TYPES OF NEURONS Afferent neurons Sensory receptor at peripheral end Peripheral axon extends from sensory receptor to cell body Cell body in dorsal root ganglion outside of CNS Central axon extends from cell body into spinal cord Efferent neurons Dendrites and cell body in CNS projects to effector organ Interneurons Lie entirely within the CNS Integration of peripheral responses to peripheral information 3
Central nervous system (spinal cor d) terminals Cell body Peripheral nervous system Afferent neuron LIST THE DIFFERENT TYPES OF GLIAL CELLS AND STATE THEIR ROLE: LO5 Central axon Peripheral axon (afferent fiber) Receptor Glial cells (neuroglia) serve as the connective tissue of the CNS. Occupy half the volume of the brain Interneuron Do not initiate or conduct nerve impulses Efferent neuron* Maintain composition of the ECF environment surrounding neurons Cell body (efferent fiber) terminals Effector organ (muscle or gland) Stepped Art Fig. 5-6, p.159 Modulate synaptic function Important in learning and memory GLIAL CELLS Space containing cerebrospinal fluid Types of glial cells Astrocytes Most abundant Support neurons Oligodendrocytes Myelin sheaths Ependymal cells Form cerebrospinal fluid Microglia Immune defense of CNS Brain interstitial fluid Neurons Oligodendrocyte Ependymal cell Capillary Astrocyte Microglia Figure 5-11 p167 4
DESCRIBE THE CONDITIONS DURING RESTING MEMBRANE POTENTIAL & PROCESSES INVOLVED IN ACTION POTENTIAL: LO6 TERMINOLOGY MEMBRANE POTENTIAL Recall Week 1 lectures on cell membrane Resting membrane potential: Neurons have a resting membrane potential of -70mV Lot of negatively charged proteins inside cell Na+/K+ pump: 3 Na+ out, 2 K+ in, more K+ in ICF, more Na+ in ECF Polarization -- Charges are separated across the plasma membrane Depolarization -- membrane is less negative Repolarization -- Membrane returns to resting potential after being depolarized Hyperpolarization -- membrane is more negative than the resting potential 4.1 INTRODUCTION 5
CHANGING MEMBRANE POTENTIAL: LO6 DESCRIBE CONDITIONS DURING RESTING MEMBRANE POTENTIAL & PROCESSES INVOLVED IN ACTION POTENTIAL: LO6 When a cell is at rest the membrane is relatively impermeable to these ions. Following stimulation, ion channels open, causing these ions to move. This causes an action potential. APs are stimulated from chemical messages arriving at dendrites Na+ is highest in concentration outside the cell (ECF). When these channels open, what direction will it move in? Action potentials occur because of ion movement. The name of the steps in an AP refer to the overall charge of the cell. It is the movement of Na+ and K+ at different times that cause this change in charge. Action potentials are all or nothing SEQUENCE OF EVENTS IN AN ACTION POTENTIAL DEPOLARIZATION Sequence of electrical events during an action potential Depolarization proceeds slowly until threshold potential is reached (-50 to -55 mv) At threshold potential an explosive depolarization takes place (+30 to +40 mv) Membrane repolarizes back to resting membrane potential (-70 mv) Transient after hyperpolarization (-80 mv) 6
Membrane potential (mv) Membrane potential (mv) 26/07/2015 REPOLARIZATION HYPERPOLARIZATION +70 +60 +50 +40 +30 +20 +10 0 10 20 30 40 50 60 70 80 90 Action potential After hyperpolarization Time (msec) 1 msec Slow depolarization to threshold Threshold potential Resting potential Figure 4-5 p116 4 Na+ channel closes and is inactivated (activation gate still open; inactivation gate closes) Na + channel opens and is activated (activation gate opens; inactivation gate already open) 3 K + voltage-gated channel closed (activation gate closed) 2 ECF 1 6 Depolarizing ICF triggering event Na+ voltage-gated channel closed Time (msec) (activation gate closed; inactivation gate open) 7 5 K + channel opens (activation gate opens) Na + channel reset to closed but capable of opening K + channel (activation closes gate closes; (activation inactivation gate closes) gate opens) Threshold potential 8 Resting potential Figure 4-8 p119 7
DETAIL HOW ACTION POTENTIALS ARE PROPAGATED BETWEEN NEURONS: LO7 PROPAGATION OF AN ACTION POTENTIAL First we need to understand how they are propagated within neurons Similarly to a mexican wave, APs are unidirectional When one region of axonal membrane depolarises, the influx of positively charged ions impacts in an adjacent region of the axon, helping to depolarise that portion of the membrane, opening the ion gates in that region. Thus the action potential propagates down the axon The previously stimulated section of the cell cannot start an AP because still recovering from the last Not one continuous action potential but series of adjacent action potentials CONTIGUOUS CONDUCTION OF ACTION POTENTIALS (UNMYELINATED AXONS) CONTIGUOUS CONDUCTION Contiguous conduction of action potentials (unmyelinated axons) Depolarization current spreads from site of action potential to inactive area of membrane adjacent to it Threshold is reached in inactive area and a new action potential is generated Action potentials are propagated along the length of the axon with no loss of magnitude Provides long-distance signals without attenuation or distortion 8
Previous active area returned to resting potential New active area at peak of action potential New adjacent inactive area into which depolarization is spreading; will soon reach threshold Backward current flow does not reexcite previously active area because this area is in its refractory period Forward current flow excites new inactive area Direction of propagation of action potential Figure 4-11 p124 CONDUCTION VELOCITIES Rate of impulse propagation is determined by: diameter the larger the diameter, the faster the impulse Presence of a myelin sheath myelination dramatically increases impulse speed MYELINATION OF AXONS s of myelinated fibers are wrapped by myelinforming cells at regular intervals along the length of the axon Oligodendrocytes in CNS Schwann cells in PNS Myelin acts as an insulator Nodes of Ranvier Exposed regions between myelin-forming cells About 1 mm apart Contain high concentration of voltage-gated Na + channels 9
Myelin sheath Nodes of Ranvier Myelin sheath Myelin sheath Plasma membrane of neuron Nodes of Ranvier Oligodendrocyte (c) Oligodendrocytes in central nervous system (a) Myelinated fiber Figure 4-13a p127 Figure 4-13c p127 Saltatory conduction VELOCITY OF AP PROPAGATION Myelinated fibers propagate action potentials more rapidly than unmyelinated fibers Action potential jumps from node to node (saltatory conduction) Conserves energy Large diameter fibers propagate action potentials more rapidly Lower resistance to current flow Giant axons of some invertebrates have been useful for studying electrical properties of neurons Myelin sheath Voltage-gated Na + and K + channels Node of Ranvier Adjacent inactive node into which Active node at depolarization is peak of action spreading; will soon Remainder of nodes still potential reach threshold at resting potential Local current flow that depolarizes adjacent inactive node from resting to threshold Direction of propagation of action potential Previous active node Adjacent node that was New adjacent inactive node returned to resting brought to threshold by into which depolarization is potential; no longer local current flow now active spreading; will soon reach active at peak of action potential threshold Figure 4-14 p128 10
DETAIL HOW ACTION POTENTIALS ARE PROPAGATED BETWEEN NEURONS (LO7) Neurons aren t physically connected Synapse: A junction that mediates information transfer from one neuron to another neuron OR an effector cell Presynaptic neuron conducts impulses toward the synapse Postsynaptic neuron transmits impulses away from the synapse 2 types of transmission signals Electrical (direct) Chemical (indirect) = most common CHEMICAL SYNAPSES Specialized for the release and reception of neurotransmitters Typically composed of two parts: al terminal of the presynaptic neuron, which contains synaptic vesicles Receptor region on the dendrite(s) or cell body of the postsynaptic neuron CHEMICAL SYNAPSES: NEURON TO NEURON Voltage-gated Ca2+ channel Ca 2+ Neurotransmitter molecule Chemically gated receptor-channel for Na +, K +, or Cl 3 1 2 4 Receptor for neurotransmitter 5 of presynaptic neuron Synaptic knob (presynaptic axon terminal) Synaptic vesicle Synaptic cleft Subsynaptic membrane Postsynaptic neuron 3 4 5 Figure 4-16 p131 11
NEUROTRANSMITTERS AMINO ACIDS Chemicals used for neuronal communication with the body and the brain About 50 different neurotransmitters have been identified Classified chemically and functionally Excitatory or inhibitory Amino acids; Acetylcholine (Ach); Biogenic amines; Peptides GABA Gamma ( )-aminobutyric acid Most prevalent neurotransmitter in the brain. Inhibitory. Glutamate Major excitatory neurotransmitter in the brain Found only in the CNS ACETYLCHOLINE BIOGENIC AMINES First neurotransmitter identified, and best understood Released at the neuromuscular junction Released by: All neurons that stimulate skeletal muscle (motor neurons) Some neurons in the autonomic nervous system Fast excitatory neurotransmitter Switched off by the enzyme, acetylcholinesterase Catecholamines dopamine, noradrenaline and adrenaline Indolamines serotonin and histamine Broadly distributed in the brain Play roles in emotional behaviours and are mimicked by many antidepressants and drugs of addiction Agonists or re-uptake inhibitors 12
PEPTIDE NEUROTRANSMITTERS Substance P mediator of pain signals Opiates- Beta endorphin, dynorphin, and enkephalins Analgesics. Mimicked by mind altering drugs (heroin, opium, LSD) Neuropeptide Y- cardiovascular & appetite Gut-brain peptides Ghrelin, and cholecystokinin SO WHY IS THIS IMPORTANT? -Cells need to communicate to maintain homeostasis - Convey information about changes that need a response eg. BP drop - Convey messages to muscles & glands to stimulate a response -Specialisation of muscle cells depends on cells ability to undergo AP 13