BISC 101 Lecture Notes - Lecture 19: Resting Potential, Axon Hillock, Peripheral Nervous System

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BISC 101 – Lecture 18 – Neurons, Synapses and Signaling
Lines of Communication
The human brain contains an estimated 100 billion neurons
oEach neuron may communicate with thousands of other neurons in
complex information-processing circuits
Neurons: Nerve cells that transfer information within the body
Neurons use two types of signals to communicate
1. Electrical Signals: Long-distance
2. Chemical Signals: Short-distance
Neuron Organization
Neuron organization and structure reflect function in information transfer
Nervous systems process information in three stages:
1. Sensory input
2. Integration
3. Motor output
Sensors: Detect external stimuli and internal conditions and transmit information
along sensory neurons
Sensory information is sent to the brain or ganglia, where interneurons integrate
and process the information
oGanglia: Nerve cell cluster or a group of nerve cell bodies located in
peripheral nervous system
Motor output leaves the brain or ganglia via motor neurons, which trigger muscle
or gland activity
Many animals have a complex nervous system which consists of:
oThe Central Nervous System or CNS: Where integration takes place; this
includes the brain and a nerve cord (or spinal cord for humans)
oThe Peripheral Nervous System or PNS: Leaves the brain and spinal cord
and travel to certain areas of the body
The PNS brings information into and out of the CNS
Signal Transmission
Neuron is the structural and functional unit of the nervous system
Most of a neuron’s organelles (such as the nucleus) are in the cell body
Most neurons have dendrites
oDendrites: Highly branched extensions of the cell body that receive signals
from other neurons
Axon: A much longer extension that transmits signals from its terminal branches
to other cells at synapses
An axon joins the cell body at the Axon hillock
oAxon Hillock: Region responsible for summating signals from dendrites
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oIf total strength of overall signal exceeds threshold limit, then an action
potential will fire and move down the axon
Most neurons are nourished or insulated by cells called glia
oThese supporting cells that are essential for the structural integrity of the
nervous system and for the normal functioning of neurons
There are several types of glia:
o Astrocytes: Found within the CNS provide structural support for neurons
and regulate extracellular concentrations of ions and neurotransmitters
Some astrocytes respond to activity in neighboring neurons by
facilitating information transfer at those neuron’s synapses
By inducing the formation of tight junctions between capillary cells,
astrocytes help form the blood – brain barrier
Blood – Brain Barrier: Restricts the passage of substances into the
CNS
o Oligodendrocytes (in the CNS) and Schwann Cell (in the PNS): Glia that
form myelin sheaths around the axons of vertebrate neurons
These sheaths provide electrical insulation of the axon
Multiple Scierosis: Myelin sheaths gradually deteriorate, resulting
in a progressive loss of body function due to the disruption of nerve
signal transmission
Resting Potential
Membrane Potential: Refers to the voltage (difference in electrical charge) across
a cell’s plasma membrane
oDifferences in concentration of ions on opposite sides of a cellular
membrane lead to a voltage called the membrane potential
Messages are transmitted as changes in membrane potential
The Resting Potential: The membrane potential of a neuron not sending signals
The resting membrane potential is between -60 mV and -80 mV when the cell is
not transmitting signals (so average value is -70 mV)
Formation of the Resting Potential
At resting potential, concentration of K+ is greater inside the cell, Na+ is greater
outside the cell
K + ions slowly move through potassium leak channels from inside neuron to
outside while Na+ ions slowly move through sodium leak channels from outside
of neuron to inside
oThe ions move along their concentration gradient
oThe leak channels open and close randomly
There are significantly more potassium leak channels compared to sodium leak
channels
More positive charges leak out to the outside of the neuron
oThis means the resting neuron becomes more negatively charged
oThe resting potential is 25 to 30 times more permeable to K+ than Na +
ions
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