Neurology Foundations Quiz OCCTH 583

Course Information

• Course name: OCCTH 583
• Course title: Neurology Foundations
• Instructor: Jennifer Krysa, MSc, Registered Occupational Therapist (AB)
• University: University of Alberta

Acknowledgements

• Images are obtained from SciDraw (https://scidraw.io/) and Wikimedia Commons.

Objectives

• Describe neurons and neuroglia functions
• Understand synaptic transmission (electrical and chemical)

Nervous System Review

• CNS (central nervous system): brain and spinal cord, the control center
• PNS (peripheral nervous system): nerves branching from CNS, carrying information to and from CNS
  • Sensory (afferent): sends sensory stimuli to CNS
  • Motor (efferent): sends info from CNS to muscles and glands
    • Somatic NS: voluntary muscle control
    • Autonomic NS: involuntary bodily functions (e.g., heart rate, respiration)
      • Sympathetic division: fight or flight response
      • Parasympathetic division: rest and digest response

Cells

• Neurons: the actual nerve cells; conductive
• Neuroglia (glial cells): support neurons; non-conductive

Glial Cell Types

• CNS: Astrocytes, Microglial cells, Ependymal cells, Oligodendrocytes
• PNS: Satellite cells, Schwann cells

Astrocytes

• Most abundant glial cell type in CNS
• Anchors neurons to blood supply
• Fills space between neurons
• Metabolic and homeostatic support
• Regulates chemical content in extracellular space
• Neuroprotective (restricts spread of released neurotransmitters)
• Stabilizes and regulates blood-brain barrier

Microglial Cells

• CNS macrophages
• Removes damaged neurons and infectious microorganisms in the brain and spinal cord

Ependymal Cells

• CNS
• Forms epithelial layer lining ventricles in the brain and central canal of spinal cord
• Produces cerebrospinal fluid (CSF) in the choroid plexus and controls CSF flow

Oligodendrocytes

• CNS
• Produces myelin sheath
• Electrical insulation for axons
• Contributes myelin to multiple axons

Satellite Cells

• PNS
• Equivalent of astrocytes
• Surrounds and supports neuron cell bodies

Schwann Cells

• PNS
• Equivalent of oligodendrocytes
• Produces myelin sheath
• Contributes myelin to a single axon

Neurons

• High longevity, the same as your lifespan.
• Non-replaceable; they don't divide (amitotic)
• High metabolic rate
• Require glucose and oxygen

Neuron Structure

• Soma (cell body): contains cytosol and cell organelles
• Dendrites: receive impulses to the cell body
• Axon: transmits impulses away from the cell body to other cells
• Neuronal membrane: separates the neuron's interior from the exterior
• Associated proteins: function as pumps

Neuron Shape

• Unipolar: rare, mostly sensory
• Bipolar: rare, specialized sensory (e.g., retina)
• Multipolar: most common

Interneurons (Association Neurons)

• Exclusively in CNS
• 99% of all neurons
• Multipolar
• Stimulate tissues through neurotransmitters
• Assemble to form circuits
• Involved in higher brain functions (e.g., cognition, perception)

Neuronal Transmission - Electrochemical

• Neurons transmit one signal at steady strength and speed
• Nerve impulse frequency can vary
• Body has equal +ve and -ve charges, but localized areas can be more charged one way (opposing charges attract; same charges repel)

Membrane Potential

• Electrical charges separated by the membrane create electrical potential (voltage).
• Difference in electrical charges is membrane potential.
• Larger difference = higher voltage
• Membrane is a phospholipid bilayer
• Membranes resist current flow
  • Conductors have low resistance
  • Insulators have high resistance
• Measured in millivolts in the body

Resting Membrane Potential

• Resting neuron has more negative charge inside than outside
• Difference in electrical charge across a membrane when the neuron is at rest = resting membrane potential
• -70mV

Sodium Potassium Pump

• Ion pump formed by proteins
• Na+ and Ca2+ more concentrated outside the membrane
• K+ more concentrated inside the membrane along with negatively charged proteins
• Polarized due to negative charge inside the cell

Types of Ion Channels

• Ions diffuse across the membrane through ion channels (protein spans the membrane)
  • Voltage-gated: open and close at specific membrane potentials
  • Ligand-gated: open when a specific neurotransmitter, drug, or hormone attaches to it
  • Mechanically-gated: open when the membrane is physically stretched

Action Potential (AP)

• Nerve impulse
• Brief reversal of resting membrane potential
• Stimulus opens ion channels
  • Na channels open
• Significant stimulus opens voltage-gated channels
  • At -55mV, voltage-gated Na channels open
  • All-or-none phenomenon
• Na rushes into the cell, briefly depolarizing it (+40mV)

AP - Propagation

• When a few voltage-gated channels open it causes a cascading effect down the axon
• Local current is strong enough to change neighboring gates

AP - Repolarization

• Voltage-gated K+ channels open, releasing K+ into extracellular space to rebalance charges
• Briefly hyperpolarizes to ~-75mV then Na+/K+ restores balance
• Followed by a refractory period

AP - Signals

• Strength of AP is always the same
• Frequency changes with stimulus intensity
  • Weak stimulus = lower frequency
  • Strong stimulus = higher frequency
• Conduction velocity is variable
  • Fastest in myelinated axons (Saltatory conduction via Nodes of Ranvier)

Synaptic Transmission

• Electrical: Immediate, via gap junctions; electrical impulses are never converted
• Chemical: Slower
  • More common
  • More selective and precise
  • Uses neurotransmitters
  • Signal converted: electrical -> chemical -> electrical

Synapses - continued

• Presynaptic neuron interfaces with dendrites, axon soma, or another axon at synaptic cleft
• AP travels down neuron, activating Na+/K+ channels in presynaptic terminal
• Activates voltage-gated Ca2+ channels; Ca2+ enters cytoplasm
• Causes synaptic vesicles to fuse with presynaptic membrane; neurotransmitters released into synaptic cleft

Neurotransmitters

• Diffuse across synaptic gap and bind to receptor sites on postsynaptic neuron
• Binding causes ion channels to open
• Neuron excited or inhibited
• Neurotransmitters bind to receptors

Excitatory Neurotransmitters

• Excite the neuron, transmitting the chemical message to the next cell
• Depolarizes the postsynaptic neuron (makes inside more positive)
• Closer to threshold for AP

Inhibitory Neurotransmitters

• Block or slow further chemical message transmission
• Hyperpolarizes the postsynaptic membrane (makes inside more negative)
• Pushes further from threshold

Summary/Conclusion about AP

• Likelihood of postsynaptic neuron developing an action potential depends on the sum of excitatory and inhibitory influences in that area
• Signal is transmitted in the form of neurotransmitters which are released, unbind, reabsorbed, and broken down
• Neurotransmitter reuptake at presynaptic membrane

Types of Neurotransmitters

• Amino acids (e.g., glutamate, GABA, glycine)
• Amines (e.g., acetylcholine, serotonin, dopamine, epinephrine, norepinephrine, histamine)
• Neuropeptides (e.g., enkephalins, cholecystokinin)

Drugs & Toxins

• Mimic neurotransmitters or block postsynaptic receptors
• Promote or inhibit neurotransmitter production/release
• Affect neurotransmitter reuptake (accumulate in cleft)
• Can lead to loss of postsynaptic receptors (need more of the substance to feel effect).