Chapter 5 Neuron Structure and Function
I.
Overview
1.
Nervous (Nerve) system is the
collection of all neurons (nerve cells) in an animal's body.
2.
A nervous system contains 2
parts: neurons and neuroglia (glia; glial cells). Neuroglia are inexcitable
supporting cells which nourish neurons, insulate the axons of neurons, and help
maintain homeostasis.
3.
Neurons are specialized
excitable cells in the nervous system that communicate using electrical and
chemical signals to generate action potentials (APs).
4.
A neuron structurally contains
3 major parts:
(1)
Cell body (soma) consists of
the nucleus and organelles.
(2)
Dendrite is a neuron fiber
that receives signals from its tip inward, toward the rest of the neuron.
(3)
Axon is a neuron extension
that conducts signals to another neuron or to an effector cells.
5.
Resting membrane potential
(Vrest, usually –20 mV to –100 mV) is the voltage of normal and unstimulated membrane potential (Vm)
across the plasma membrane of a resting neuron.
6.
The physiological property of
a neuron contains:
(1)
passive electric properties
(regular conducted potential; graded potential)
(2)
active electric properties
(unusual action potential)
7.
Action potential (AP,
sometimes called spike or nerve impulse) is
a
transient transmembrane voltage (or membrane potential) across an excitable
membrane in a neuron generated by the activity of voltage-gated ion channels.
8.
Myelin sheath is a series of
Schwann cells surround the axon of a neuron. Each pair of cells in the sheath is
separated by a space called a node of Ranvior (regularly spaced interruption of
the myelin sheath (unmyelinated gap) along an axon).
9.
Synapse is a junction between
two neurons, where electrical and chemical signals are transmitted. A synapse
contains a presynaptic cell (terminus), the synaptic cleft, and a postsynaptic
cell.
II.
Signaling
in a Vertebrate Motor Neuron
1.
Electrical signals in neurons
(1)
Neurons can rapidly alter
their membrane potential in response to an incoming signal, and these changes in
membrane potential can act as electrical signals.
(2)
Most neurons have a
resting membrane potential of
approximately –70 mV, meaning that the inside of the cell membrane is
about 70 mV more negatively charged than the outside of the membrane.
2.
Two factors ionic
concentration gradients and membrane permeability establish mechanism potential.
3.
The
sodium-potassium (Na+/K+)
ATPase maintains the membrane potential. The Na+/K+ ATPase
is an electrogenic pump that pumps 3 Na+ ions out of the cell for
every 2 K+ ions that it pumps into the cell. Thus,
the membrane of a resting neuron is
polarized with the inside more negatively charged than the outside.
4.
The activity of the Na+/K+
pump can be inhibited by drugs, such as ouabain or digitalis.
5.
Changes in membrane
permeability cause electrical signals.
(1) Polarization represents the status of resting membrane potential, which is caused by ionic concentration gradients.
(2) Depolarization is a change in
the membrane potential of a neuron from its normally negative resting membrane
potential to a more positive value.
(3)
Repolarization is a return of
the membrane potential of a neuron toward the resting membrane potential
following a depolarization or hyperpolarization.
(4)
Hyperpolarization is a change
in the membrane potential of a neuron from its normally negative resting
membrane potential to a more negative value.
III.
Signals in the Dendrites and
Cell Body
1.
In the dendrites and cell
bodies of neurons, the electrical signals vary in magnitude with the stimulus
intensity, resulting from the opening and closing of ion channels, and are
graded potentials (passive electric properties).
2.
Graded potentials are
short-distance signals. Conduction with decrement shows graded potential can
travel through the cell, but they decrease in strength as they get further away
from the opened Na+ ion channel.
3.
Graded potentials can trigger
action potentials at the axon hillock. If a graded potential causes the membrane
potential at the axon hillock to depolarize beyond the threshold potential, the
axon will fire an action potential according to the all-or-none principle.
4.
Excitatory potential
(EP) is a
change in the membrane potential in a neuron that increases the probability of
action potential initiation in that cell. Inhibitory potential (IP) is a change in
the membrane potential that makes a neuron less likely to generate an action
potential.
5.
Graded potentials can be
integrated across time and space. Temporal summation is a process by which
graded potentials occurring at slightly different times combine to influence the
net graded potential of a cell. Spatial summation is a process by which graded
potentials occurring at the same time but at different points in the membrane
combine to influence the net graded potential of a cell.
IV.
Signals in the Axon
1.
Action potentials can be
transmitted across long distances without degrading. Action potentials differ
from graded potentials in many respects: magnitude, duration, distance,
position, and ion channels.
2.
An action potential typically
has 3 phases:
(1)
Depolarization phase: is the
initial part of an action potential during which the electrical difference
across the membrane. The action potential (about +30 mV) is triggered when the
membrane potential at the axon hillock reaches threshold.
(2)
Repolarization phase: is a
return of the membrane potential of a neuron toward the resting membrane
potential.
(3)
(After-)hyperpolarization phase:
varies greatly among neurons for the duration (2-15 msec) and size.
3.
Voltage-gated ion channels
generate the action potential.
(1)
Opening of voltage-gated Na+
ion channels (increased influx of Na+) at the threshold potential causes the depolarization phase of the
action potential, and slow opening of voltage-gated K+ ion channels
(increased outflux of K+)
initiates the repolarization and hyperpolarization phases in neurons.
(2)
The Hodgkin cycle is a
regenerative, or positive feedback, loop responsible for the upstroke of the
action potential; depolarization causes an increase in the Na+
permeability, permitting an increased influx of Na+, which further
depolarizes the membrane.
4.
Vertebrate motor neurons are
myelinated.
(1)
Schwann cells (glial cells)
form the myelin sheath by wrapping in a spiral pattern around the axon of the
neuron.
(2)
Areas of exposed axonal
membrane are nodes of Ranvier that contain high densities of voltage-gated
ion channels.
(3)
Saltatory conduction is the
mode of conduction of action potentials in myelinated axons in which action
potentials appear to jump from one node of Ranvier to the next.
V.
Signals across the Synapse
1. Synapses can be divided into two general classes: electrical synapses and chemical synapses.
(1) Electrical synapses are specialized connections between neurons that facilitate direct electrical, ionic, and small metabolite communication. They are composed of tens to thousands of gap junctions clustered together into plaques.
(2) Chemical synapses are neuron-to-neuron connections via which neurotransmitters transfer nerve impulses in one way direction.
2.
Intracellular Ca2+
regulates neurotransmitter release.
(1)
When an action potentail
reaches the membrane of the presynaptic axon terminus of a synapse or the neuromuscular
junction (NMJ), the resulting depolarization triggers the opening of
voltage-gated Ca2+ ion channels on the cell membrane of the axon
terminus.
(2)
The resulting increased Ca2+
concentration provides a signal for exocytosis of synaptic vesicles to release
neurotransmitter into the synaptic cleft.
3.
Neurotransmitter diffuses
across the synaptic cleft and bind to specific receptors (e.g., acetylcholine
receptor) on the membrane of postsynaptic cell.
4.
Binding of neurotransmitter to
receptor activates signal transduction pathways.
5.
Acetylcholine (ACh) is
the first identified and the
primary neurotransmitter at the vertebrate neuromuscular junction.
(1)
ACh synthesis occurs in
the axon terminus in a reaction catalyzed by the enzyme choline
acetyltransferase (ChAT): acetyl CoA from the mitochondria is
combined with choline to form ACh and coenzyme A (CoA).
(2)
Signaling is terminated by
acetylcholinesterase (AChE) to break the ACh down into choline and acetate.
VI.
Diversity of Neural Signaling
1.
Structural diversity of
neurons
(1)
Neurons can be divided into 3
classes based on their function:
1)
Sensory (or afferent) neurons
2)
Interneurons
3)
Motro (or efferent) neurons
(2)
Neurons can be associated with
glial cells in vertebrates, e.g., astrocytes, oligodendrocytes, microglial
cells, and
ependymal cells.
2.
Diversity of signal conduction
(1)
Conduction speed varies among
axons.
Animals use 2 main strategies for
increasing the speed of action potential conduction: myelination and increasing
the diameter of the axon.
(2)
The cable properties of the
axon influence current flow. The passive conduction properties are called cable,
because they are analogous to properties of long copper telecommunication
cables. For example, Ohm's law describes the relationship between current (I)
and voltage (V): V = IR, whereas R is the resistance of the circuit. Thus,
current is proportional to the voltage drop across a circuit, and inversely
proportional to the resistance.
3.
Diversity of synaptic
transmission
(1)
Electrical synapse is a
junction that the electrical signal in the presynaptic cell is directly
transferred to the postsynaptic cell through the gap junctions.
(2)
Chemical synapse is a junction
between a neuron and another cell in which the signal is transmitted across the
synapse in the form of a neurotransmitter. Most neurons, however, do not form
gap junctions with their target cells.
(3)
There are many types of
neurotransmitters.
(4)
Neurotransmitters can be
excitatory or inhibitory.
1)
Excitatory neurotransmitters
generally cause depolarization, making the postsynaptic cell more likely to
generate an action potential. These depolarizations are called excitatory
postsynaptic potentials (EPSPs).
2)
Inhibitory neurotransmitters
generally cause hyperpolarization, making the postsynaptic cell less likely to
generate an action potential. These hyperpolarizations are called inhibitory
postsynaptic potentials (IPSPs).