Transcript Structure of a Neuron

Neurons
• The two principal cell types of the nervous
system are:
– Neurons – excitable cells that transmit
electrical signals.
• There are 150 different types.
• Vary in function and size.
– Supporting cells (neural glia)– cells that
surround and wrap neurons.
• Oligodendrocytes, Schwann cell, Astrocytes
Supporting Cells: Neuroglia
• The supporting cells
– Provide a supportive
scaffolding for neurons
– Segregate and
insulate neurons
– Guide young neurons
to the proper
connections
– Promote health and
growth
Cells of the CNS
Fundamental Types of Neurons
• Sensory (afferent) neurons
– detect changes in body and external environment
– information transmitted to brain or spinal cord
• Interneurons (association neurons)
– lie between sensory and motor pathways in CNS
– 90% of our neurons are interneurons
– Integration and retrieving information
• Motor (efferent) neuron
– send signals out to muscles and gland cells
– organs that carry out responses called effectors
Structure of a Neuron
• Cell body (soma)
– single, central nucleus
– contains many multibranched dendrites
– Which receive signals from
other neurons.
• Axon
– (nerve fiber) arising
from axon hillock for
rapid conduction
– Axon terminals
release will
neurotransmitters that
communicate a
chemical message to
another nerve or
muscle
Neuron Physiology
• Living nerve cells are polarized
– The inside of the cell (Intracellular fluid) ICF is
negatively charged compared to the outside of the
cell extracellular fluid (ECF)
– The cell is able to maintain a resting membrane
potential of -70 mV (negative charge on the inside
of membrane by active transport and specific
voltage gated channels.
Resting Membrane Potential
Resting Membrane Potential
The cell membrane is considered a semi-permeable
membrane which selectively allows things in and out of
the cell.
– Large negatively charged molecules found in the ICF
such as proteins and phosphates are confined to the
inside of the cell. The membrane is impermeable to
these molecules which contributes to the ICF
maintaining RMP of -70mV
Ionic Basis of Resting Membrane
Potential
• Na+ concentrated outside of cell (ECF)
• K+ concentrated inside cell (ICF)
Basis of the Resting Membrane Potential
• Since Na+ ion are more concentrated in the ECF when
a specific voltage gated Na+ channel opens Na+ will
always rush into the cell by diffusion.
• Since K+ ion channels are more concentrated in the
ICF when a specific voltage gated K+ channel opens
K+ will always rush out of the cell by diffusion
• In order to keep the resting membrane potential at –70
mV the cell is constantly hydrolyzing ATP with the
Na+,K+-ATPase pump.
Resting Membrane Potential
Chemical Excitation
Depolarization
Repolarization
Action Potentials (APs)
• There are 3 phases to an AP:
– Depolarization
• a reduction in the polarity of the membrane
potential by allowing Na+ to enter the cell.
– Repolarization
• membrane potential returns towards the resting
value closing Na channels and opening K+
channels. K+ travels along its concentration
gradient out of the cell returning the inside of the
cell to a negative value.
– Hyperpolarization
• Slow closing K+ channels cause the inside of the
cell to be more negative than the resting value
• All APs have the same magnitude regardless of the
size of the stimulus
Action Potentials
The Refractory Period
• Absolute refractory period
– as long as Na+ gates are
open
– no stimulus will trigger AP
• Relative refractory period
– as long as K+ gates are
open
– only especially strong
stimulus will trigger new AP
• Refractory period is
occurring only to a small
patch of membrane at
one time (quickly
recovers)
Impulse Conduction in Unmyelinated Fibers
• The action potential in trigger zone begins
impulse
• Nerve signal (impulse) - a chain reaction
of sequential opening of voltage-gated
Na+ channels down entire length of axon
• This is a very slow process. (2 m/sec)
• We need something to speed the process
up.
Impulse Conduction - Unmyelinated Fibers
Saltatory Conduction - Myelinated Fibers
• The velocity of an action potential propagates along the length of the axon
depends on:
– axon diameter
• The larger the diameter of the axon the greater the velocity of the
action potential travels along the axon to the axon terminal.
– Myelin sheath increases the diameter of sections of the axon which
dramatically increases impulse speed. (120 m/sec)
Myelin Sheath
• Myelin is a white, fatty insulating covering around most of the long
axons. It plays an important role in both conduction velocity and
protection of the axon.
• In the CNS Oligodendrocytes can myelinate many different neurons.
• In the PNS Schwann cells are can only myelinate a portion of one
axon.
Diseases of the Nervous System
• What would be worse?
– A disease that attacks neurons of the CNS
– A disease that attacks neurons of the PNS
• What are the deficits one might expect to see if
the neurons loose their Myelination?
Saltatory Conduction
• Notice how the action potentials jump from
node of Ranvier to node of Ranvier.
Presynaptic Neurons
• Presynaptic neurons
– Nerve signal(AP) opens
voltage-gated calcium
channels allowing it to diffuse
into the synaptic knob.
– Calcium triggers the release
of a neurotransmitter such as
acetylcholine (Ach) from the
(synaptic vesicles).
– The neurotransmitters are
released into the synaptic
cleft.
Postsynaptic Neuron
• Neurotransmitters diffuse across the
synaptic cleft binding to ligand-gated
channels on the postsynaptic
neuron.
• Graded Potential
•
Specific neurotransmitters can influence
the permeability of the voltage gated
channels.
• This influences the post synaptic
neuron to become more likely to
generate an AP (depolarization) or
less likely (hyperpolarization).
Postsynaptic Potentials- EPSP
• Excitatory postsynaptic potentials (EPSP)
– a positive voltage change causing postsynaptic cell to be more
likely to fire
• result from Na+ flowing into the cell
– glutamate and aspartate are excitatory neurotransmitters
Postsynaptic Potentials- IPSP
• Inhibitory postsynaptic potentials (IPSP)
– a negative voltage change causing postsynaptic cell
to be less likely to fire (hyperpolarize)
• result of Cl- flowing into the cell or K+ leaving the cell
– glycine and GABA are inhibitory neurotransmitters
The membrane potential of a real neuron typically undergoes
many EPSPs (A) and IPSPs (B), since it constantly receives
excitatory and inhibitory input from
the axons terminals that reach it.
Summation - Postsynaptic Potentials
• Net postsynaptic potentials
in trigger zone
– firing depends on net input of
other cells
• The trigger zone takes in both
EPSP and IPSPs. If there are
more EPSPs threshold will be
reached.
– temporal summation
• single synapse receives
many EPSPs in short time
– spatial summation
• single synapse receives many
EPSPs from many cells
Summation of EPSP’s
• Does this represent spatial or temporal
summation?
Action Potentials vs. Graded Potentials
• Action Potentials (All-or-none phenomenon)
– action potentials either completely, or not at
all
• Graded Potentials ( sub threshold)
• EPSP and IPSPs are
– are graded (vary in magnitude with stimulus
strength)
– are decremental (get weaker the farther they
spread)
– are reversible as K+ diffuses out of cell
Clinical Applications
• Many drugs work by altering neuronal
functions.
– Block the receptor site
• Beta blockers: prevent sympathetic input to the
heart and various organs
– Block the reabsorption of the
neurotransmitter
• SSRI: Prevent the reuptake of serotonin so it stays
in the synaptic cleft longer and continue to
stimulate nerve
– Bind to the receptor site
• Curare blocks the (ACh) acetylcholine receptors
by binding to the same position on the receptor