Chapter 2: Structures and Function of Cells of the Nervous System
Lecture Overview
Cell Specializations
Neurons
Glia
Membrane and action potentials
Neurotransmission
Neuron Components
Soma (Cell Body)
Neurites (any process that extends from cell body)
Axon
Dendrites
Terminal Buttons
Neuron Classification Schemes
Number of axon processes (unipolar, bipolar, multipolar)
Number of dendritic processes
Connections: Sensory, motor, interneurons
Axon length:
Long: Golgi type I
Short: Golgi type II
Neurotransmitter (NT) used by neuron
Effects of NT (excitatory vs.. inhibitory)
Cell Structure
Cell Specializations: Support, contraction, conduction, secretion
Nerve cells are specialized for communication (nerves conduct ELECTROCHEMICAL signals)
Cell components
Membrane: bilipid layer
Cytoplasm
- Mitochondria and Golgi apparatus
- Nucleus
- Microfilaments
- ER: smooth and rough
CNS Support Cells
Neuroglia ("glue") provide:
Physical support
Nutrient flow
Nerve "housekeeping"
Astrocytes and microglia
Oligodendroglia and Schwann Cells
"Blood-brain barrier": Barrier to entry of certain substances into brain
Measuring Nerve Cell Resting Membrane Potential
Giant squid axon is placed in sea water in recording chamber
Glass microelectrode is inserted into axon
Voltage measures -70 mV inside with respect to outside
Resting Membrane Potential
RMP is a balance point between
Concentration gradients
Electrical gradients
At rest, some K+ can leave cell, causing the exterior of the nerve cell membrane to be slightly positive relative to the inside of the axon
Local Potentials Degrade
Disturbances of membrane potential can be carried along membrane:
Degrade with time and distance
Above Threshold Changes in RMP Result in Action Potential
The Action Potential
AP is a stereotyped change in membrane potential
If RMP moves past threshold, membrane quickly moves to +40 mV and then returns to resting
Ionic basis of AP:
NA+ in: upswing of spike
K+ out: downswing of spike
Properties of the Action Potential
The action potential:
Is an "all or none" event
Insert favorite analogy here:
Firing a gun...
Is actively propagated down the axon
Notion of successive patches of membrane
Has a fixed velocity and amplitude
Is a property of the membrane
Membrane Refractory Periods
Absolute: ~1 msec (during impulse)
Relative: following repolarization
RP’s limit the firing rate of nerve cells
1 msec RP would = 1000 pulses per second
Absolute RP explains why AP typically cannot travel in 2 directions simultaneously
Saltatory Conduction
AP’s are propagated down axon
AP depolarizes each successive patch of membrane
Slows down transmission in nonmyelinated axons
Myelinated axons: AP jumps from node to node: only depolarizes membrane at node
Saltatory conduction speeds up velocity
and allows for smaller diameter axons
Synapse
AP is conducted along axon membrane to axon terminal
Eventually reaches the "synapse"
Physical gap between pre- and post-synaptic membranes (20-30 nmeters)
Synaptic web-intra synaptic threads
Postsynaptic thickening
Presence of vesicles (contain transmitter substances)
Exocytosis
AP sweeps along axon to arrive at terminal
Voltage change allows for entry of CA++ ions- triggers exocytosis of vesicles
Exocytosis: vesicles fuse with membrane
Heuser’s study with frog membrane
NT diffuses across cleft to interact with postsynaptic membrane (time consuming process)
NT release is a ‘quantal’ event
Postsynaptic Elements
Postsynaptic membrane shows thickening and small indentations
NT interacts with receptors to produce a change in membrane
Postsynaptic potentials (PSP)
Excitatory (EPSP)-depolarization of Em
Inhibitory (IPSP)-hyperpolarization of Em
PSP Characteristics
PSP’s are
Graded in size (--> # quanta released)
Are not propagated as are AP’s (are governed by time and space constants)
Not subject to a refractory period
Subject to a slow time course
PSP’s involve a change in P to one or more ions
Increased PK+ or PNA+
NT-gated receptors (ionotropic)
Second messenger systems (metabotropic)