Nervous System and Nervous Tissue

Nervous System Master control and communication Functions (system level and cell level) Sensory input – monitoring stimuli Integration – interpretation of sensory input Motor output – response to stimuli

Cellular v. System level PNS Dendrites: input Cell body: integration Axon: output CNS PNS

Nervous System Organization Central nervous system (CNS) Form: Brain and spinal cord Function: Integration and command center Peripheral nervous system (PNS) Form: Paired spinal and cranial nerves Function: Carries messages to and from the spinal cord and brain

Central Nervous System Peripheral Nervous System Motor (efferent) Sensory (afferent) Somatic (voluntary) Autonomic (involuntary) Parasympathetic (Stop! ) Sympathetic (Action! Go!)

Peripheral Nervous System (PNS) INPUTS: Sensory (afferent) division Sensory afferent fibers – from skin, skeletal muscles, and joints to the brain Visceral afferent fibers – from visceral organs to the brain OUTPUTS: Motor (efferent) division Transmits impulses from the CNS to effector organs

Motor Division Organization Somatic nervous system (SNS) Conscious control of skeletal muscles Autonomic nervous system (ANS) Regulates involuntary muscle (smooth and cardiac) and glands Sympathetic (Stimulates = Go!) Parasympathetic (Conserves = Stop!)

The Autonomic Nervous System Is part of the CNS Is part of the PNS 25

The Central Nervous System Includes only the brain Includes the brain and spinal cord Includes the brain, spinal cord and all peripheral nerves 25

Nervous System Cell Types Neurons Transmit electrical signals Neuroglia (“nerve glue”) Supporting cells Neuroglia in the CNS Astrocytes Microglia Ependymal cells Oligodendrocytes Neuroglia in the PNS Satellite cells Schwann cells

Neurons Structural units of the nervous system Long-lived (100+ years) Amitotic (no centrioles = can’t divide) High metabolic rate (glucose gobblers!)

Neuron Classification (function) Sensory (afferent) transmit impulses toward the CNS Motor (efferent) transmit impulses away from the CNS Interneurons (association neurons) shuttle signals through CNS pathways

Neuron (nerve cell) Neuron cell body Cell body Dendrites Dendritic (a) Dendrites Cell body Nissl bodies Axon terminals (secretory component) Axon hillock Node of Ranvier Impulse direction Schwann cell Neuron cell body Dendritic spine

Nerve Cell Body (Soma) Contains nucleus and nucleolus Major biosynthetic center Focal point for the outgrowth of neuronal processes (dendrites and axons) Axon hillock – where axons arise

Neuronal processes (fibers) Dendrites Numerous Short and tapering Diffusely branched Contain “spines” where synapses form Axons One per cell Long (up to 4 ft. in length) Form synapses at terminals (release neurotransmitters) Anterograde and retrograde transport (out and back!)

Supporting Cells: Neuroglia Provide a supportive scaffolding for neurons Segregate and insulate neurons Guide young neurons to the proper connections Promote health and growth Help regulate neurotransmitter levels Phagocytosis

Astrocytes Most abundant and versatile Cling to neurons and synaptic endings Cover capillaries (blood-brain barrier) Support and brace neurons Guide migration of young neurons Control the chemical environment

Microglia Monitor health of neurons Transform into macrophages to remove cellular debris, microbes and dead neurons NOTE: Normal immune system cells can’t enter CNS

Ependymal Cells Shape: squamous to columnar (often ciliated) Location: Line the central cavities of the brain and spinal column Function: Circulate cerebrospinal fluid

Oligodendrocytes Wrap CNS axons like a jelly roll Form insulating myelin sheath

Schwann Cells and Satellite Cells Surround axons of the PNS Form insulating myelin sheath Satellite cells Surround neuron cell bodies Nodes of Ranvier

Myelin Sheath and Neurilemma White, fatty sheath protects long axons Electrically insulates fibers Increases the speed of nerve impulses Neurilemma remaining nucleus and cytoplasm of a Schwann cell

Axons of the CNS Both myelinated and unmyelinated fibers are present Oligodendrocytes insulate up to 60 axons each White matter: dense collections of myelinated fibers Gray matter: mostly soma and unmyelinated fibers

Which cell myelinates CNS neurons? Schwann cells Ependymal cells Oligodendrocytes 25

The inputs to a neuron are the… Cell body (soma) Dendrites Axons 25

Which cell forms the blood-brain barrier? Schwann cells Ependymal cells Astrocytes Microglia 25

Which is NOT found in the CNS? Schwann cells Ependymal cells Astrocytes Microglia 25

Action Potentials (nerve impulse) Electrical impulses carried along the length of axons Always the same regardless of stimulus Based on changes in ion concentrations across plasma membrane This is HOW the nervous system functions

Electricity Definitions Voltage (V) potential energy from separation of charges (+ and -) For neurons, measured in millivolts Current (I) the flow of electrical charge between two points Insulator substance with high electrical resistance Think myelin sheath!

Ion Channels Passive (leakage) channels: always open Voltage-gated channels: open and close in response to membrane potential Ligand-gated (chemically gated) channels: open when a specific neurotransmitter binds Mechanically gated channels: open and close in response to physical forces

Let’s review! The sodium-potassium pump

Voltage-Gated Channels

Chemically Gated Channels

Gated Channels When gated channels are open: Ions move along electro-chemical gradients Takes into account charge differences Takes into account concentration differences An electrical current is created Voltage changes across the membrane

Measuring Membrane Potential

Resting Membrane Potential Resting membrane potential (–70 mV) The inside of a cell membrane has more negative charges than outside the membrane Major differences are in Na+ and K+

Changes in Membrane Potential Depolarization the inside of the membrane becomes less negative Hyperpolarization the inside of the membrane becomes more negative than the resting potential Repolarization the membrane returns to its resting membrane potential

Action Potentials = nerve impulse Principal means of neural communication A brief reversal of membrane polarity All or nothing event Maintain their strength over distance Generated only by muscle cells and neurons

Phases of an Action Potential Resting state Depolarization Repolarization Hyperpolarization Return to resting potential

Action Potential: Resting State Na+ and K+ GATED channels are closed Each Na+ channel has two voltage-regulated gates Activation gates Inactivation gates

Action Potential: Depolarization Na+ permeability increases; membrane potential reverses Na+ gates are opened, but K+ gates are closed Threshold: critical level of depolarization (-55 to -50 mV) Once threshold is passed,action potential fires

Action Potential: Repolarization Sodium inactivation gates close Voltage-sensitive K+ gates open K+ rushes out Interior of the neuron is negative again

Action Potential: Hyperpolarization Potassium gates remain open Excess K+ leaves cell Membrane becomes hyperpolarized Neuron is insensitive to stimuli until resting potential is restored

Return to resting potential: Sodium-potassium pump Repolarization ONLY restores the electrical differences across the membrane DOES NOT restore the resting ionic conditions Sodium-potassium pump restores ionic conditions More sodium outside More potassium inside

Resting state Hyperpolarization Depolarization Repolarization Sodium channel Na+ Potassium channel Activation gates K+ Inactivation gate Na+ Na+ 1 Resting state K+ K+ 4 Hyperpolarization Na+ 2 Depolarization K+ 3 Repolarization

ACTION! http://outreach.mcb.harvard.edu/animations/actionpotential.swf http://www.youtube.com/watch?v=SCasruJT-DU http://bcs.whfreeman.com/thelifewire/content/chp44/4402s.swf http://www.blackwellpublishing.com/matthews/channel.html

Absolute and Relative Refractory Periods

Refractory Periods Absolute refractory period (NO WAY! NO HOW!) Neuron CANNOT generate an action potential Ensures that each action potential is separate event Enforces one-way transmission of nerve impulses Relative refractory period (Well, maybe…) Threshold is elevated Only strong stimuli can generate action potentials

Propagation of an Action Potential –70 +30 (a) Time = 0 ms (b) Time = 2 ms (c) Time = 4 ms Voltage at 2 ms at 4 ms at 0 ms Resting potential Peak of action potential Hyperpolarization Membrane potential (mV))

Action Potential Frequency Stronger stimuli generate more frequent action potentials

How fast does a signal travel? Velocity determined by Axon diameter the larger the diameter, the faster the impulse Presence of a myelin sheath Myleinated neurons have much faster impulses Why? Node-jumping! (Saltatory conduction)

Saltatory Conduction

Sodium rushes into the cell during… Depolarization Repolarization Hyperpolarization

Hyperpolarization means… The inside is more negative The cell has crossed threshold The inside is more positive

During an absolute refractory period… Action potentials absolutely fire Action potential can never fire The cell is hyperpolarized

Multiple Sclerosis (MS) Cause: Autoimmune disease with symptoms appearing in young adults (women at highest risk) UNKNOWN environmental and genetic factors Symptoms: visual disturbances, weakness, loss of muscular control, incontinence Physiology Myelin sheaths in the CNS are destroyed, producing a hardened lesion (scleroses) Shunting and short-circuiting of nerve impulses occurs Alternating periods of relapse and remission

Multiple Sclerosis Treatment Prognosis Drugs that modify immune response Prognosis Medications can prevent symptoms from worsening Reduce complications Reduce disability HOWEVER, not all drugs work long-term in all patients

Synapses Junction for cell  cell communication Presynaptic neuron Neuron neuron Neuron  effector cell Presynaptic neuron Conducts impulses toward the synapse Postsynaptic neuron/cell Receives signal May/may not act on signal

Synapses

Electrical Synapses (fast) Less common Resemble gap junctions Allow direct ion flow cell  cell Important in the CNS Neural development Synchronization of activity Emotions and memory

Electrical Synapses

Chemical Synapses (slower) Most common Excitatory or inhibitory Communication by neurotransmitters Presynaptic neuron releases neurotransmitter Postsynaptic neuron has membrane-bound receptors Neurotransmitters must be recycled, removed or degraded after release

Chemical Synapses vesicles containing Neurotransmitter Synaptic cleft Ion channel (closed) Ion channel (open) Axon terminal of presynaptic neuron Postsynaptic membrane Ion channel closed Ion channel open Receptor Degraded neurotransmitter Na+ Ca2+ 1 2 3 4 5 Action potential NOTE: Ion channels are chemically gated, not voltage-gated

Neurotransmitter Diversity Acetylcholine (ACh) Biogenic amines (dopamine, serotonin) Amino acids (glutamate, GABA) Peptides (endorphins, enkephalins) Novel messengers ATP Nitric oxide (why Viagra works!) Carbon monoxide

Neurotransmitter Actions Direct Alter ion channels Rapid response Important in sensory-motor coordination Ex.) ACh, GABA, glutamate Indirect Work via second messengers and G-proteins Slower action Important in memory, learning, and autonomic nervous system Ex.) dopamine, serotonin, norepinephrine

Postsynaptic Potentials EPSP: excitatory postsynaptic potentials Cell is depolarized Ex.) glutatmate IPSP: inhibitory postsynaptic potentials Cell is hyperpolarized Ex.) GABA

Postsynaptic Potentials Will the postsynaptic cell fire? It depends on… Which neurotransmitter is released The amount of neurotransmitter released The length of time the neurotransmitter is bound to receptors If threshold isn’t reached, no action potential

Summation Spatial summation Temporal summation Multiple potentials arrive at the same time Number of IPSPs v. EPSPs determine if action potential is generated Temporal summation Multiple potentials arrive at different times Time intervals determine if action potential is generated

Summation NOTE: This is oversimplified. One neuron can receive inputs from thousands of other neurons.

Neuronal signaling and health Depression Often linked to altered levels of serotonin Treated with SSRIs (selective serotonin reuptake inhibitors) Provides greater signal from less neurotransmitter WARNING: Suicide risk can actually increase in some patients, particularly adolescents and young adults.

Neuronal signaling and health Addiction Dopamine is essential in “reward” pathways Triggers pleasurable sensations Involved in both drug and alcohol addiction Glutamate is essential in memory pathways May trigger relapses

Neuronal signaling and health BoTox = botulinum toxin Works by blocking acetylcholine release at neuromuscular junction Facial muscles can’t contract, wrinkles disappear Also used for many spastic disorders Local anaesthesia Most block sodium channels, so action potentials aren’t generated

Chemical synapses are faster than electrical synapses True False 25

An inhibitory neurotransmitter will hyperpolarize the postsynaptic cell True False Too confused to even guess! 25

Spatial summation involves… Multiple stimuli from one neuron Multiple stimuli from multiple neurons Stimuli separated by time 25

Which neurotransmitter is associated with depression? Dopamine GABA Glutatmate Serotonin 25

Which neurotransmitter is associated with addiction? Dopamine GABA Nitric oxide Acetylcholine 25