Neurons: Exam 1 Study Guide

• Exam 1 will be next Thursday during the lab period
• The exam will consist of multiple choice, fill in the blank, free response, drawing/interpreting figures
• Students have the whole lab period to complete the exam
• A study outline is posted on Canvas
• The lab write up from week 4 is due the following week
• A reading for the discussion on Monday 24th February is posted in required readings on Canvas (Carlberg & Mycko 2023)

Course Outline

• The course includes the study of neurons
• The course includes the study of membrane potentials
• The course includes the study of action potentials
• The course includes the study of Multiple Sclerosis (MS) and Vitamin D

Neurons Overview

• Whole-animal integration is essential for proper functioning
• Neurons are key in processing information and coordinating responses
• The nervous system is a control system
• The nervous system controls the functioning of other cells
• The nervous system conveys information
• Neurons transmit electrical and chemical signals to specific target cells
• Neurons generate action potentials which are electrical signals that are self-propagating impulses
• Neurons have 4 functional Parts

Functional Parts of Neurons

• Dendrites are an input
• Cell body provides integration
• Axon provides conduction of electrical signals
• Presynaptic terminals are the output, creating a chemical signal across synapses
• The nervous system is composed of a network of neurons in the body.
• The central nervous system (CNS) differs from the peripheral nervous system (PNS) based on location and function

Types of Neurons

• Afferent neurons relay signals to the CNS
• Efferent neurons relay signals from the CNS
• Interneurons are entirely within the CNS

Neural Circuits and Reflexes

• Neural circuits guide all animal responses
• Circuits are a group of neurons involved in a response
• Reflexes are rapid, predictable, involuntary motor responses to stimuli
• A stimulus goes to receptors, to sensory neurons, to the integration center, to motor neurons, and finally to muscle cells
• Reflexes are stereotypical behavior in response to a stimulus
• Responses are not instantaneous
• Body size matters regarding neural circuits and reflexes

Neuron Structure

• Neurons are structurally adapted to transmit signals
• The neuron doctrine states that neurons are anatomically distinct cells
• Cajal won the Nobel Prize in 1906
• Neuron function and structure are linked
• The soma is similar to "normal" cell types
• Dendrites contain dendritic spines
• Axons contain mitochondria and myelin sheath

Glial Cells

• Surround neurons and provide physical and metabolic support
• In vertebrates, the number of glial cells is similar to the number of neurons, but glial cells are less than neurons in other species
• Schwann cells form the sheath in the PNS
• Oligodendrocytes form the sheath in the CNS
• Astrocytes have interactions with other cells
• Microglial cells provide immunity

Membrane Potentials Overview

• Membrane potentials relate to how electrical signals of neurons work
• Net movement of charges (ions) results in electrical currents
• Voltage or electrical potential difference (V) refers to the separation of positive and negative charges
• The separation of charges can do work if charges are able to flow
• Cell qualities affect electrical charge
• Electrical activity of a nerve cell is a property of the cell membrane
• The membrane stores charges by separating ions
• Neurons maintain inside-negative membrane potentials at rest
• The output of the measurement of the charge difference across a membrane is Vm, or the resting membrane potential

Ions and Membrane Potential

• Sodium-potassium ATPase pump maintains sodium and potassium concentrations
• Membrane potential is dependent on ion difference across the membrane
• Use the Nernst equation to calculate membrane potential for a single ion
• The Nernst equation relates the concentration difference of a permeating ion across a membrane and the membrane potential at equilibrium
• The Nernst equation variables include:
  • E, membrane potential
  • R, gas constant
  • T, absolute temperature in K
  • z, valence of ion species or charge
  • F, Faraday’s constant
  • Cout and Cin which are the ion concentrations on each side of the membrane
• Greater concentration difference results in a higher membrane potential
• Greater concentration out leads to positive potential
• Greater concentration in leads to negative potential
• Membrane potential is largely dependent on potassium, sodium, and chloride ions
• Contributions of each ion to membrane potential can be calculated using the Goldman equation

Action Potentials

• Action potentials are voltage dependent, all-or-none electrical signals
• Action potential is a momentary reversal of membrane potential, from -65 mV to about +40 mV
• Action potentials are triggered by membrane depolarization to voltage threshold
• Threshold is generally ~15 mV above resting
• Voltage dependent changes in membrane permeability
• Absolute and relative refractory periods exists

Action Potential Stages

• Resting membrane potential exists
• Rising phase: threshold depolarization occurs and sodium channels open
• Hodgkin cycle indicates that permeability to sodium is voltage dependent
• Falling phase happens with repolarization where sodium channels close and potassium channels open
• Recovery: undershoot leads to channels closing

Factors Affecting Conduction Velocity

• Axon diameter influences conduction velocity. As axon diameter increases, conduction velocity increases as well.
• Myelination affects conduction velocity
  • 200+ layers of glial membrane
  • Absent from nodes of Ranvier
  • Saltatory conduction occurs
• Temperature (high) affects conduction velocity

Multiple Sclerosis (MS)

• An autoimmune disease
• Characterized by the cells attacking myelin of neurons in the CNS
• Sclerosis refers to the scar tissue stemming from loss of myelin
• It disrupts transmission of action potentials along axons
• It features high variability in disease progression
• It is caused by a combination of genetic and environmental factors

MS and Vitamin D - Points for discussion

• Describe how vitamin D3 acts at the cellular level
• Why are the vitamin D intervention studies (VitDbol and VitDmet) relevant?
• How does vitamin D supplementation affect the progression of the disease, and how effective have treatments been?