Download Feb. 11

Neurophysiology
Study of the cellular properties of nerve cells and how these properties give rise to behaviors from simple to complex
Levels of organization of the nervous system
From Shepherd “Neurobiology”, 3rd Ed.
Comparative Approach
• Most of our knowledge of cellular physiology is based on organisms low on the phylogenetic scale
– Squid: Action potential
– Crab: Sensory transduction
– Frog: Synaptic transmission
– Slug: Synaptic plasticity
Foundations of the Neuron Doctrine
Jan Purkinje
Neurons of the cerebellum observed by Purkinje in 1837
Foundations of the Neuron Doctrine
Two kinds of fibers arising from the cell body
• “Protoplasmic prolongations”, later known as dendrites
• “Axis cylinder”, later known as the axon
Motoneuron of the spinal cord
observed by Otto Dieters in 1865
Foundations of the Neuron Doctrine
• The discovery of neural processes arising from the cell body led to a controversy
– Reticular theory: Stated that nerve cells were connected like a web
– Neuron doctrine: Cell theory* applied to nerve cells; Each nerve cell is a separate entity
*All organisms are composed of individual units, called cells (Schleiden and Schwann, 1839)
Foundations of the Neuron Doctrine
Camillo Golgi
Supporter of the
reticular theory
Neurons of the visual cortex
stained in their entirety using
the Golgi method developed
in 1873, published in 1885.
Foundations of the Neuron Doctrine
Santiago Ramόn y Cajal
• 1885: Cajal began developing the Golgi method
• Published many papers between 1888 and 1891 in which he applied the Golgi method to various parts of the nervous system
• Interpreted the results as proof that the cell theory applied to neurons
Foundations of the Neuron Doctrine
• Wilhelm His (1886) – embryological development of nerve cells supported Cajal’s interpretation
• August Forel (1887) – nerve cells respond singly to injury
• Wilhelm Waldeyer (1891) – comprehensive review concluding that the cell theory applied to nerve cells
– Called nerve cells “neurons”
– Cell theory applied to neurons: “neuron doctrine”
Foundations of the Neuron Doctrine
• Although Golgi and Cajal shared the Nobel prize in Medicine in 1906, Golgi held onto the reticular theory
• The neuron doctrine was finally fully proven when the electron microscope was invented
Electron micrograph of a neuron in the rat visual cortex showing that neurons are similar to other cells of the body
Neuronal Structure
• Two properties of neural structure posited by Cajal:
– Principle of dynamic polarization: Information flows in a predictable and consistent direction in each neuron.
– Principle of connectional specificity: neurons make specific connections at precise points of synaptic contact.
Four morphologically/functionally distinct regions of the neuron
• Dendrites – input
• Soma – metabolic activities and protein synthesis
• Axon ‐ conductive
• Axon (presynaptic) terminals ‐ output
Overview of the sequence of neural
signaling
• Input: graded signals that are summed and integrated
• Trigger component: due to voltage‐sensitive Na channels, sudden influx of Na ions. High density of channels at the axon initial segment (hillock) • Conduction: All‐or‐none signal travels down the axon, being regenerated at each node of Ranvier.
• Output: Release of neurotransmitter through exocytosis.
Techniques for monitoring nerve cell activity
• Intracellular
• Patch clamp
• Extracellular (single units and field potentials)
• Brain imaging (noninvasive)
Cytology of the Neuron
• Two classes of nerve cells
• What is so special about neurons?
• Structure of the neuron
Two classes of nerve cells
• Glial cells
– Astrocytes
– Oligodendrocytes
– Schwann Cells – Microglia
• Neurons Types of Glial cells
Astrocytes
Blood brain barrier
Removal of excess potassium from synapse
Myelin aids in signal conduction down the axon
PNS: Schwann cells
CNS: Oligodendrocytes
Types of neurons
•
•
•
•
Unipolar
Bipolar
Mutipolar
Other types
A neuron shares features with other eukaryotic cells Organelles
• Nucleus
• Endoplasmic Reticulum
• Golgi Apparatus
• Mitochondria
• Endosomes and lysosomes
The plasma membrane
The plasma membrane
• Composed of a double layer of phospholipid
molecules
• Keeps cytoplasm in and water soluble molecules out
• Need for membrane proteins: receptors; surface proteins; cytoplasmic proteins
Hydrophillic
Hydrophobic
What differentiates neurons from other animal cells?
• Neurons specialized to process and transmit information – Electrical signals can be in the form…
• Action Potentials (“all or none”)
• Local Potentials (graded, variable amplitude)
• The plasma membrane is electrically excitable
– Membrane potential: inside of cell negative relative to the inside
Functions of the cytoskeleton
Determines shape of the neuron and the arrangement of organelles within the cytoplasm
Composition of the cytoskeleton
• Microtubules
• Actin filaments, thin flexible structures • Neurofilaments: located in the axon (neurofibrillary
tangles in AD)
Questions
• Types of glial cells and their functions
• Organelles and their functions
• What differentiates neurons from other eukaryotic cells? What is similar?