Ch 11 - Nervous Tissue PDF

BIOL243 – HUMAN ANATOMY & PHYSIOLOGY I Charles Smith, PhD CSCS HUMAN ANATOMY & PHYSIOLOGY Ch. 11 – Fundamentals of the Nervous System & Nervous Tissue FUNCTIONS OF THE NERVOUS SYSTEM Nervous system is master controller & communicator for body Cells send out chemical & electrical...

BIOL243 – HUMAN ANATOMY & PHYSIOLOGY I Charles Smith, PhD CSCS HUMAN ANATOMY & PHYSIOLOGY Ch. 11 – Fundamentals of the Nervous System & Nervous Tissue FUNCTIONS OF THE NERVOUS SYSTEM Nervous system is master controller & communicator for body Cells send out chemical & electrical signals Signals are rapid resulting in near immediate responses Signals are specific 3 Overlapping Functions: 1. Sensory Input (afferent) Sensory receptors gather information about internal and external changes i.e., touch, temperature, hormone levels, blood pressure, carbon dioxide concentration Information gets relayed to the central nervous system 2. Integration Brain/Spinal Cord process & interpret info provided by sensory receptors 3. Motor Output (efferent) Effector organs (i.e., muscles, glands) get activated Response produced Remember positive/negative feedback loops? DIVISIONS OF THE NERVOUS SYSTEM Central Nervous System (CNS) Brain & Spinal Cord Integration & control center Receives sensory input & determines appropriate output (response) Peripheral Nervous System (PNS) Anything outside the CNS Mainly cranial (to and from brain) and spinal (to and from spinal cord) nerves Sensory (Afferent) Division Convey impulses to CNS from Internal organs (visceral) Skin, skeletal muscles, and joints (somatic) Motor (Efferent) Division Carry response from CNS to Voluntary skeletal muscles (somatic) Involuntary smooth muscle, cardiac muscle, or glands (autonomic) Sympathetic nervous response = “Fight or Flight” Parasympathetic nervous response = “Rest & Digest” NEUROGLIA Aka glial cells surround and wrap delicate neurons Supporting cells Where most brain tumors originate Peripheral Nervous System (CNS) Glial Central Nervous System (CNS) Glial Cells Cells 1. Astrocytes (CNS) 1. Satellite Cells (PNS) Most abundant Function similarly to astrocytes for PNS Form Blood-Brain Barrier protecting CNS from substances in cardiovascular system Surround PNS neuron cell bodies 2. Microglia (CNS) 2. Schwann Cells (PNS) Have thorny processes that touch and monitor Function similarly to oligodendrocytes for PNS neurons Form myelin sheath surrounding peripheral nerve Tend to migrate towards injured neurons to fibers phagocytize microorganisms & debris 3. Oligodendrocytes (CNS) Form insulating myelin sheath surrounding thicker neurons’ axons 4. Ependymal Cells (CNS) Produce cerebrospinal fluid (CSF) which fill brain ventricles & spinal canal NEURONS Structural units of the nervous system Large, specialized cells that conduct impulses Neurons are special compared to other cells They last your entire lifespan They have a high metabolic rate Require constant supplies of oxygen & glucose Amitotic Once neuron has assumed its role, it loses its ability to divide Exceptions: olfactory epithelium & some hippocampal (memory) brain areas Classed based upon which direction it carries impulses relative to CNS Sensory: carry impulses from sensory receptors to CNS (afferent) Motor: carry impulses from CNS to effector cells (efferent) Interneurons: connect motor & sensory neurons Make up 99% of body’s neurons Almost entirely found in CNS NEURONAL STRUCTURE Neurons are made from a Cell Body (Soma) Contains nucleus & is where most proteins and chemicals are made Mostly located in CNS Often clustered Nuclei = cluster of cell bodies in CNS Ganglion = cluster of cell bodies in PNS 1+ Processes Arm-like extensions off the soma Dendrites (receivers): convey incoming information Axons (communicators): conduct information to axon terminal Terminal secretes excitatory or inhibitory neurotransmitters Often bundled together Tract = CNS bundle Nerve = PNS bundle MYELIN Myelin sheathing protects and electrically insulates the axon of myelinated fibers Allows for fast impulse transmission Whitish, protein-lipid substance Why brain & spinal white matter are white Gray matter = neuronal soma & non-myelinated fibers Myelinated fibers typically have larger diameter axons Multiple Sclerosis: degenerative, autoimmune condition where myelin hardens Neuron conduction becomes slow, less efficient Neuronal function declines MEMBRANE POTENTIALS Neurons highly excitable Can change potential rapidly, compared to other cells Potential maintained by ion channels Leakage Channels: non-gated, always open Gated Channels: proteins change shape opening & closing channel; create a selective- permeability in membrane Chemically-gated: or ligand-gated; bind to specific chemical (i.e., neurotransmitter) Voltage-gated: open/close in response to change in membrane potential Mechanically-gated: open/close in response to physical deformation of receptors MEMBRANE POTENTIALS Resting Membrane Potential created by differences in ion concentrations between ECF & ICF ECF = more sodium (Na+) ICF = more potassium (K+) Ions leak following concentration gradient Na+ = low permeability; K+ = high permeability Some sodium IN; LOTS of potassium OUT Result: membrane polarized with a net negative charge (-70 mV) Sodium-Potassium pump: maintains this gradient When gated channels open: Ions diffuse fast along electrochemical gradient Sum of electrical & chemical gradients High to Low concentration & charge Flow creates an electrical current changing membrane’s voltage (charge) ACTION POTENTIALS Brief reversal of membrane potential by changing voltage Rest = -70 mV; Depolarized = +30 mV How neurons communicate Aka nerve impulse Involves opening of specific voltage-gated channels Only occur in muscle cells & neuronal axons 4 Main Stages: 1. Rest 2. Depolarization 3. Repolarization 4. Hyperpolarization ACTION POTENTIALS 1. Rest All gated Na+ & K+ channels closed Ion flow through leakage only Potential maintained at -70 mV 2. Depolarization Current opens voltage-gated Na+ channels ICF floods with sodium (now less negative) Potential rapidly rises to +30 mV ACTION POTENTIALS 3. Repolarization Na+ channel gates close Potential spike stops rising (peaks) Voltage-gated K+ channels open K+ follows concentration gradient; exits ICF Membrane returns to resting negative charge (repolarizes) 4. Hyperpolarization Some K+ channels stay open Too much K+ exits Membrane now too negative (more than resting; hyperpolarized) Sodium-potassium pumps restore ion balance Resting membrane potential restored REFRACTORY PERIODS Neurons can’t just depolarize again and again and again and again… Refractory Period: time in which neuron can’t trigger another action potential Absolute Refractory Period: FIXED amount of time from when sodium channels open until channels reset Ensures impulses transmit one-way only Relative Refractory Period: follows the absolute refractory period Only a VERY strong stimulus could override this CONDUCTION VELOCITY Saltatory Conduction Myelin insulate and prevent charge leakage Gaps in myelin sheath (Nodes of Ranvier) have sodium channels Action potentials in axons only generated at gaps Electrical signal, effectively, “jumps” from node to node 30x faster than continuous conduction in unmyelinated neurons Some things can interrupt this Local anesthetics (i.e., lidocaine) block voltage-gated sodium channels on sensory receptors Cold temperatures or pressure interrupt blood circulation to neuron Tetrodotoxin (i.e., pufferfish or fugu) block muscular impulses = paralysis THE SYNAPSE Neurons are functionally connected by synapses Neuron-to-neuron OR neuron-effector connections Facilitate flow of information Chemical Synapse (most common) Release and reception of neurotransmitters Presynaptic axon terminal releases neurotransmitter from synaptic vesicles Postsynaptic neuron’s dendrites or cell body receive neurotransmitter Electrical Synapse (less common) Gap junctions Channel proteins (connexons) connect adjacent neuron cytoplasms Ions & small molecules freely flow from one to the other Neurons are “coupled” Helps synchronize activity of all interconnected neurons  Chemical synapses transmit signals from one neuron to another Presynaptic using neurotransmitters. neuron CHEMICAL SYNAPSE Postsynaptic neuron Presynaptic neuron 6 Steps involved: 1 Action potential arrives at axon terminal. 1. Action Potential arrives at axon terminal of 2 Voltage-gated Ca2+ channels Mitochondrion presynaptic neuron open and Ca2+ enters the axon terminal. Ca2+ Ca2+ Ca2+ 2. Voltage-gated calcium (Ca2+) channels open Ca2+ Ca2+ enters axon terminal Synaptic 3 Ca2+ entry causes cleft synaptic vesicles to Axon release neurotransmitter terminal 3. Ca entry stimulates synaptic vesicles to Synaptic 2+ by exocytosis. vesicles release neurotransmitter 4 Neurotransmitter diffuses across the synaptic 4. Neurotransmitter diffuses across synaptic cleft and binds to specific Postsynaptic receptors on the neuron postsynaptic membrane. cleft Binds to receptors on postsynaptic membrane Ion movement Enzymatic 5. Ligand-gated ion channels on postsynaptic Graded potential Reuptake degradation membrane open Diffusion away from synapse Result: short-lived, localized graded potentials 5 Binding of neurotransmitter opens ion channels, resulting in graded potentials. Stimulus can be excitatory or inhibitory 6. Neurotransmitter effects terminated 6 Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse. Presynaptic action potential ceases Reuptake, degradation, or diffusion of remaining neurotransmitters away from cleft GRADED TO ACTION All-or-Nothing Principle: membrane charge (potential) must attain or exceed a threshold potential for signal/stimulus to be transmitted Doesn’t need to happen off a single stimulus… Summative Property: multiple graded potentials (stimuli) can “add” together to generate a large stimulus Temporal Summation: single presynaptic neuron stimulates multiple graded potentials at a high frequency Larger depolarization than single stimulus Spatial Summation: multiple presynaptic neurons stimulate graded potentials simultaneously Larger depolarization than stimulus from single presynaptic neuron KEY NEUROTRANSMITTERS Acetylcholine Neuromuscular junctions Excitatory for skeletal muscle contraction Inhibitory for parasympathetic nervous system (i.e., slows heart rate) Blocked by anticholinergic drugs Catecholamines Epinephrine & norepihephrine Sympathetic (Fight or Flight) response Dopamine “Feel-good” neurotransmitter Key for motor control Parkinson’s Disease: brought on by lack of dopamine production in brain Increased secretion seen in schizophrenia KEY NEUROTRANSMITTERS Serotonin Mainly inhibitory neurotransmitter Plays role in sleep, appetite, nausea, migraines, mood Blocked transmission → anxiety, depression SSRIs (selective serotonin reuptake inhibitors): antidepressants; block proteins that reuptake serotonin Drugs like ecstasy (aka X, “Molly”, MDMA) enhance transmission → euphoria Histamine Wakefulness, appetite control, learning & memory Glutamate Mainly excitatory neurotransmitter Key for learning & memory KEY NEUROTRANSMITTERS GABA (γ-aminobutyric acid) MAJOR inhibitory brain neurotransmitter Important for presynaptic inhibition Blocking its synthesis, release, or action can results in convulsions Tachykinins/Substance P Excitatory Substance P mediates pain transmission in PNS Tachykinins help regulate respiratory and cardiovascular control in CNS Endorphins Mainly inhibitory “Natural opiates” Inhibits Substance P blocking pain Effects mimicked by morphine, heroine, & methadone NERVOUS TISSUE SUMMARY The nervous system is the great communicator of the body Carries afferent (sensory) information to the CNS & puts out efferent (response) stimuli to PNS These stimuli are carried by neurons Action potentials at the neurons are the impulses communicating to either other neurons or the effector cells Action potentials are generated by the opening & closing of ion channels in the neuronal membrane Change the potential of the membrane Action potential travels down the axon Often insulated and sped up by the presence of myelin sheathing Once action potential reaches axon terminal, most neurons will release a chemical signal (neurotransmitter) across the synapse Each neurotransmitter has specific roles & functions SAMPLE QUESTIONS 1. What two glial cells are responsible for a neuron’s myelin sheathing? 2. What is it called when a neuron’s membrane charge changes from negative to positive? 3. What ions flow across a neuron’s membrane to generate an action potential? 4. The secretion of neurotransmitters which bind to post-synaptic receptors defines what kind of synapse? 5. Parkinson’s disease is characterized by the lack of secretion of what neurotransmitter?  COPYRIGHT © Pearson Edited by Charles Smith, PhD CSCS 2024