Download Neuron

Neuron
 neurons have four distinct anatomical regions
 Nerve cell membranes contain a resting electrical membrane potential.
 The resulting membrane potential is the result of three major determinants
 The resting membrane potential can be changed by synaptic signals from a
presynaptic cell
 Action potentials begin at the axon’s initial segment and spread down the entire
length of the axon.
There are two classes of cells in the nervous system: the neuron (or nerve cell) and the
neuroglial cell (or glia-the network of supporting tissue and fibers that nourishes nerve
cells within the brain and spinal cord). The neuron is the basic functional unit of the
nervous system. The large number of neurons and their interconnections give the nervous
system its complexity. There are approximately 10 billion neurons in the average
vertebrate nervous system.
Neurons have four distinct anatomical regions
a typical neuron has four morphologically defined regions: the dendrites, the cell body
(also called the soma or perikaryon), the axon, and the presynaptic terminals to the axon.
Theses four anatomical regions are important to the four major electrical and chemical
responsibilities of neurons: receiving signals from neighbouring neurons, integrating
these often-opposing signals, transmitting electrical impulses some distance along the
axon, and signaling as adjacent cell at the presynaptic terminal.
 Three organelles are common in neuron bodies: the nucleus, the ER (membrane
proteins are synthesized), and Golgi apparatus (which carries out processing of
secretory and membrane components).
 The cell body usually gives rise to several branch-like extensions, called
dendrites whose surface area and extent far exceed those of the cell body. The
dendrites serve as the major receptive apparatus of the neuron to receive signals
from neighbouring neurons.
 The cell body also gives rise to the axon, a tubular process that is often long. The
axon is the conducting unit of the neuron, transmitting an electrical impulse (the
action potential) from its initial segment at the cell body to the other end of the
axon at the presynaptic terminal. Axon lacks ribosomes and therefore, cannot
synthesize proteins.
 Large axons are surrounded by a fatty insulating coating called myelin. In the
peripheral nervous system, myelin is formed by Schwan’s cells, specialized glial
cells that wrap around a broomstick.
 The myelin sheath is interrupted at regular intervals by sites called nodes of
Ranvier.
 Axons branch near their ends into several specialized endings called presynaptic
terminals. These presynaptic terminals transmit a chemical signal to an adjacent
cell, usually another nerve or muscle cell. The site of contact of the presynaptic
terminal with the adjacent cell is called the synapse. It is formed by the
presynaptic terminal of one cell (presynaptic cell), the receptive surface of the
1
adjacent cell (post synaptic cell), and the space between these two cells is called
synaptic cleft. Presynaptic terminals contain synaptic vesicles that contain a
chemical transmitter prior to its release into the synaptic cleft.
Nerve cell membranes contain a resting electrical membrane potential.
Nerve cells, like other cells of the body, have an electric charge that can be measured
across their outer cell membrane (resting potential). The resting membrane potential is
the result of the differential separation of charged ions, especially Na+ and K+, across the
membrane and the resting membrane’s differential permeability to these ions diffusing
back down their concentration gradients. Even though the net concentration of positively
and negatively charged ions are similar in the intercellular and extracellular fluids, an
excess of positively charged cations accumulates immediately outside the cell membrane,
and as excess of negatively charged anions accumulates immediately inside the cell
membrane. This makes the inside of the cell negative with respect to outside of the cell.
The electrical membrane potential in nerve and muscle cells is unique in that its
magnitude can be changed as the result of synaptic signaling from neighbouring cells or
within a receptor as a response to environmental changes. When a nerve or muscle’s
membrane potential is reduced (-75mV) sufficiently, a further and dramatic change in the
membrane potential occurs, called an Action Potential which spreads across along the
entire length of the nerve axon.
The resulting membrane potential is the result of three major determinants
Three major factors cause the resting membrane potential.
1) The Na+, K+ pump. Cell membranes have an energy dependant pump that pumps
Na+ ions out of the cell and K+ ions into the cell against their concentration
gradients. The pump itself generates some of the resting membrane potential,
because it pumps three molecules of Na+ out for every two molecules of K+ into
the cell, thus concentrating positively charged cations outside the cell.
2) Differential permeability of the membrane of the membrane to diffusion of ions.
The resting membrane is much more permeable to K+ ions than to Na+ ions.
Therefore, positively charged K+ cations are allowed to diffuse out of the cell
2
through nongated leak channels back down their concentration gradient until the
resulting electrical membrane potential reaches equilibrium with the driving force
of the K+ concentration gradient. This further contributes to the building of
positive charges immediately outside the membrane. Because the resting
membrane is almost completely impermeable to Na+ ions, once pumped out of
the cell, Na+ cannot diffuse back into the cell, even though both the electrical and
concentration gradients for Na+ would drive Na+ ions back into the cell if sodium
channels in the resting membrane were open.
3) Negatively charged anions trapped in the cell. Many intracellular anions are
macromolecules synthesized within the neuron and are too big to get back out
through the cell’s plasma membrane. Therefore, they are trapped within the cell
and attracted to the inner surface of the membrane by the accumulated positive
charges just outside the cell.
These three determinants are the primary source of the resting membrane potential.
3