NRSC_2125_Spring_ExamReview

Direction of Communication

• Afferent

  • To the brain

  • Example: Sensory input

• Efferent

  • Away from the brain

  • Example: Motor output

Cells of the Nervous System

• Neurons:

  • Receive, process, and send information

  • Found in the CNS (brain, spinal cord, retina) and PNS (sensory, motor, autonomic)

• Glia:

  • Support, nourish, and myelinate neighboring neurons

  • Modulate neurons locally; act as immune cells of the brain

Types of Neurons

• Sensory Neurons:

  • Receive specific information from the external world via specialized receptors

  • Send that information to the CNS; Afferent signals

• Motor Neurons:

  • Control movement; directions from brain and body (reflexes)

  • Efferent signals

• Interneurons:

  • Act locally between neurons in brain and spinal cord

• Projection Neurons:

  • Long-distance communication in brain and spinal cord

Axons and Dendrites

• Axons:

  • Send information to downstream neurons

  • One per neuron, longer, can branch (axon collaterals)

• Dendrites:

  • Receivers of information

  • One or more per neuron, shorter, heavily branched

Golgi vs. Cajal

• Golgi:

  • Reticular theory; neurites fuse to form a network

  • Brain does not follow Cell Theory

• Cajal:

  • Neuron doctrine; neurons communicate by direct contact

  • Cell Theory applies to neurons

Types of Glia

• Astrocytes:

  • Influence neurite growth; regulate chemical content of extracellular space

  • Move nutrients from blood to neurons

• Microglia:

  • Phagocytes; immune function

  • Respond to damage/neuroinflammation; increase cell excitability

• Myelinating Glia:

  • Oligodendrocytes (CNS) & Schwann cells (PNS); provide electrical insulation on axons

  • Disease: Multiple Sclerosis = demyelination

• Ependymal Cells:

  • Line the ventricular system; keep CSF flowing

Neuron Parts

• Soma

• Dendrites

• Axon

• Myelin Sheath

• Nodes of Ranvier

• Presynaptic Terminal

• Axon Hillock

• Neuronal Membrane

• Cytoplasm

• Nucleus

• Rough Endoplasmic Reticulum (ER)

• Smooth ER

• Free Ribosomes

• Golgi Apparatus

• Mitochondria

Synapse and Neurotransmission

• Synapse: The space/location where two neurons meet

• Presynaptic Terminal: Lined with vesicles ready to release neurotransmitters

• Synaptic Vesicles: Store neurotransmitters

• Synaptic Cleft: Space between pre and post synaptic parts

• Postsynaptic Membrane: Receiving side of the synapse; location of neurotransmitter receptors

Equilibrium and Resting Potentials

• Equilibrium Potential:

  • Na+: +62 mV

  • K+: -80 mV

  • Cl-: -65 mV

  • Ca2+: +123 mV

• Resting Membrane Potential:

  • Non-active state of the neuron; typically around -65mV

  • Vm at rest; no net movement of ions

Active and Passive Forces

• Active:

  • Sodium-Potassium Pump: Uses ATP, pumps 3 Na+ out and 2 K+ in

• Passive:

  • Diffusion

• Electrostatic Attraction/Repulsion

Ionic Driving Force

• Calculation: Vm - Eion

• No conductance when there are no open channels

EPSP and Action Potentials

• EPSP: Depolarizes spike-initiation zone; Na+ enters near axon hillock

• Threshold: If EPSPs and IPSPs sum enough to reach threshold

Voltage-gated Channels

• Na+ channels inactivate after 1 msec; Na+ influx halts

• K+ flows out rapidly, initiating peak period

After-Hyperpolarization

• Voltage-gated Na+ channels: Closed + De-inactivated

• Membrane potential returns to resting potential with the help of Na+/K+ pump

Sodium and Potassium Dynamics

• Rising Phase: Voltage-gated Na+ channels open; Na+ flows in rapidly

• Falling Phase: Membrane hyperpolarizes with K+ influx

Refractory Periods

• Absolute: Cannot fire another AP; VG Na+ channels inactive

• Relative: Could fire with enough stimuli to reach threshold; VG Na+ channels closed but can reopen

Action Potentials Conductance

• Factors affecting conduction:

  • Membrane Resistance: Fewer channels

  • Internal Resistance: Bigger diameter

  • Saltatory Conduction: Potential fades during myelin sheath, but reinvigorates at Nodes of Ranvier

Anesthetics and Pain

• Cocaine, Lidocaine, Novacaine:

  • Block voltage-gated Na+ channels; prevent Action Potential

  • Unmyelinated pain axons hardest hit

Synthesis and Storage of Neurotransmitters

• Large Molecule Neurotransmitters:

  • Synthesizing & packaging occurs at soma (peptides)

• Small Molecule Neurotransmitters:

  • Synthesizing & packaging at terminal (e.g., GABA, glutamate)

Neurotransmitter Signaling

• End of Signaling:

  • 1. Reuptake

  • 2. Degradation (enzymatic)

  • 3. Diffusion (out of the cleft)

Synaptic Process

• Ca2+ causes vesicles to move to presynaptic membrane and release neurotransmitter into cleft

• Neurotransmitter binds to receptors; triggers signal to postsynaptic neurons (EPSP or IPSP)

Synaptic Fusion Mechanism

• Synaptotagmin: Binds Ca2+ for vesicle fusion

• SNARE Proteins:

  • V-SNARE: Synaptobrevin

  • T-SNARE: Syntaxin and SNAP25

Non-Classical Transmission of Neurotransmitters

• Volume Transfer: Diffusion of lipid-permeable gases like nitric oxide; does not use synaptic vesicles

  • Useful for affecting other synapses

Electrical vs Chemical Synapses

• Electrical Synapses:

  • Gap Junctions; faster, smaller signals, local effects

• Chemical Synapses:

  • Presynaptic terminal, highly regulated; slower, larger signals, long-range effects

Toxins Affecting Neurotransmission

• Botulinum Toxin: Blocks ACh release, causes paralysis

• Tetanus Toxin: Blocks GABA/Glycine release, causes extreme muscle spasms