Lecture 1 The Nervous System F24 PDF

The Nervous System What is your nervous system doing for you right this very second? Learning Objectives – 2 lectures Differentiate between white matter and gray matter. Describe the functions of afferent and efferent nerves. List the components of the central nervous system and the periph...

The Nervous System What is your nervous system doing for you right this very second? Learning Objectives – 2 lectures Differentiate between white matter and gray matter. Describe the functions of afferent and efferent nerves. List the components of the central nervous system and the peripheral nervous system. Differentiate between the autonomic and somatic nervous systems. Describe the process of depolarization and repolarization of neurons. List the excitatory and inhibitory neurotransmitters and describe their role in conduction of nerve impulses. Describe the structures and functions of the neurons and neuroglia of the nervous system Describe the structures and functions of the cerebrum, the cerebellum, the midbrain, and the brain stem. Describe the connective tissue layers surrounding the brain and spinal cord. Explain the function of the cerebrospinal fluid. Learn the cranial nerves (names and numbers will be provided) and their functions. Differentiate between the sympathetic and parasympathetic nervous systems and between autonomic and somatic reflexes. Describe the components of a reflex arc and explain its role. Describe the stretch reflex, withdrawal reflex, palpebral reflex, and pupillary light reflex. A highly advanced communication system Nervous system Endocrine system communicates communicates using what using what molecules? molecules? Distance? Distance? Speed? Speed? Neurons and their support staff (glial cells) Neurons Main functional unit of the nervous system Neuron Structure Central cell body (soma or perikaryon) Nucleoli often visible Lots of mitochondria Cell processes Dendrites – receive stimuli Axons - conducts nerve impulses away from cell body (no Nissl) RER – grouped into Nissl bodies Types of Neurons Sensory Motor  Axons Conduct nerve impulse away from cell toward another neuron or an effector cell Ends of axons – axon terminals, telodendrons, synaptic terminals Single, sometimes long process; may be covered with myelin (made mostly of lipids) White matter in the central nervous Myelin sheath around the axon – makes nerve transmission faster. Eat more fat (omega 3) get smarter? Axons Myelinated axons conduct impulses faster than unmyelinated ones Myelin sheath: cell membrane of glial cells tightly wrapped around the axon oligodendrocytes in the brain and spinal cord Schwann cells in the nerves outside the brain and spinal cord Myelinat ed axon – electron microgra ph  Axons Multiple Schwann cells or oligodendrocytes (CNS) cover the entire length of the axon Nodes of Ranvier: gaps between sheaths (#1 below)  Nerve = bundle of axons (lateral hind limb of a dog with biceps femoris removed) Sciatic nerve The support staff – neurogl ia or glial cells Support staff for neurons singular = microglial cell CSF = cerebrospinal fluid Nervous system organization Nervous system organization: afferent and efferent nerves Afferent and efferent nerves  Organization of Nervous System Direction of Impulses Afferent nerves - conduct impulses toward CNS Also called sensory nerves - conduct sensations from sensory receptors in the skin and other locations in the body to the CNS Efferent nerves - conduct impulses away from CNS subset called motor nerves - cause skeletal muscle contraction and movement Cranial and spinal nerves in the PNS and nerve tracts (bundles of axons) in the CNS may carry nerve fibers that are sensory, motor, or both (mixed) How does the neuron function? Resting state – charged and ready to go How does the neuron function? Resting membrane How is this potential - difference in difference electrical charge across neuronal membrane maintained? Results from differences in distribution of positive and negative charges from sodium, potassium, proteins, and other charged ions on either side of the neuronal membrane  Sodium-Potassium (Na+-K+) Pump OUTSIDE CELL INSIDE CELL 2-Minute Neuroscience: Sodium-Potas sium Pump - YouTube Resting membrane potential Then what? Depolarization causing an action potential! Neuron receives a stimulus (either from another neuron or from sensory receptors – tack) Causes sodium channels to open (facilitated diffusion) – blocked by lidocaine Remember that at resting state, LOTS of sodium outside the neuron Depolarization Inside the cells rapidly becomes positively charged Then what? Repolarization Potassium channels open and potassium rushes out of the neuron Plus the sodium potassium pump keeps working moving 3 Na+ out for every 2 K+ in Depolarization and repolarization Threshold – initially after stimulus received only a few Na channels open – but if threshold reached LOTS of Na channels open All-or-nothing principle Each neuron, if threshold is met, depolarizes the same amount no matter the stimulus Similar to how muscle fibres are stimulated Propagation of signal along neuron Refractory periods Absolute Refractory Action Potential Vid eo to show to 3:11 How does this action potential move along the neuron? Depolarization of one part of the membrane opens Na+ channels on nearby parts of the membrane – chain reaction Myelinated axons – super high speed propagation of impulse Saltatory conduction – of AP along myelinated axons saltare = to jump APs only created at nodes of Ranvier APs appear to jump from node to node (not really) Less energy for sodium/potassium pumps needed and faster! The signal has gone from one end of the neuron to the other – now what?  The synapse Very similar to the neuromuscular junction Synapse - junction between two neurons or a neuron and a target cell Synaptic cleft - gap between adjacent neurons Presynaptic neuron - neuron bringing the depolarization wave to the synapse Releases neurotransmitter Postsynaptic neuron - contains receptors for the neurotransmitter AP video with synapses at the end 3:10 The synapse Neurotransmitters – general types Excitatory Inhibitory neurotransmitters neurotransmitters Usually cause an Move the charge influx of sodium so within the that the postsynaptic cell postsynaptic farther away from membrane moves threshold toward threshold for an action potential Neurotransmitters – specific types (know the first 3) Acetylcholine (Ach) Excitatory in neuromuscular junctions Inhibitory in parasympathetic nervous system when messaging the heart Skeletal muscle cells with neuromuscular junctions Tick neurotoxin inhibits Ach release from nerve in neuromuscular junction Tick paralysis #2 http://vethq.com.au/tick-paralysis/ Neurotransmitters – specific types catecholamines Norepinephrine (noradrenaline) arousal, fight or flight reactions of sympathetic NS Neurotransmitters – specific types catecholamines Epinephrine (norepinephrine) actually a hormone – released by adrenal gland fight or flight Neurotransmitters – specific types catecholamines Dopamine For interest only Brain Autonomic functions Muscle control Behaviour/learning Mood https://www.everydayhealth.com/dopamine/ Increased by cocaine Lacking in Neurotransmitters – specific types GABA (gamma-aminobutyric Glycine acid) Major inhibitory NT Inhibitory Brain Spinal cord Diazepam (Valium) Strychnine is an increases GABA in the brain, so does alcohol antagonist of Caffeine inhibits release of glycine GABA Gabapentin is a drug used For interest only for nerve pain that pretends to be GABA (crosses BBB easily) What happens to the neurotransmitters? Broken down by Broken down bits enzymes/reabsorbed are reabsorbed by Acetylcholinesterase pre-synaptic axon MAO (monoamine knob and reused oxidase) – breaks down norepinephrine after it is taken back into knob COMT (catechol-O- methyl transferase) – breaks down norepinephrine left in cleft Synapse - neurotransmitters start at 2 :30 Drugs and poisons Many act by pretending to be neurotransmitters or by acting on the enzymes that remove the neurotransmitters from the synaptic cleft Organophosphate insecticides – inhibit Ach- esterase Snake venom – Ach receptor antagonist Permethrins – act on sodium channels Permethrin toxicity (Advantix ) How organophosphates work as toxins Organophosphate toxicity – continuous signals to muscles from motor nerves https://www.youtube.com/watch?v=OrmJF0E 5D_s