PSL201Y1 Chapter Notes -Sodium-Potassium Alloy, Electrochemical Gradient, Active Transport
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7 NERVE CELLS AND ELECTRICAL SIGNALING
Overview of the Nervous System
Central Nervous System (CNS): consists of the brain and spinal cord. Receives and processes
information from sensory organs and the viscera to determine the state of the external
environment (sensory information) and internal environment (visceral information).
Peripheral nervous system (PNS): neurons that provide communication between the central
nervous system and organs throughout the body.
o Afferent: neurons transmit sensory and visceral information from the organs to the CNS.
Information transmitted includes somatic senses, special senses, and visceral info.
o Efferent: neurons transmit information from the central nervous system to organs in the
periphery called effector organs (muscles and glands).
o A neuron capable of transmitting messages or receiving information to an effector organ
is said to innervate that organ.
o Efferent is subdivided into somatic nervous system (motor neurons for skeletal muscle
contractions) and autonomic nervous system (regulate the function of internal organs
and other structures; have parasympathetic and sympathetic nervous system).
Cells of the Nervous System
Neurons are the functional unit of the tissue. They are the excitable cells that communicate by
transmitting electrical impulses.
Excitable cells: cells capable of producing large rapid electrical signals called action potentials.
Glial cells constitute 90% of the cells in the nervous system and provide structural and metabolic
Most neurons contain three main components: a cell body and two types of neural processes that
extend from the cell body – the dendrite(s) and an axon.
Cell body (soma): contains the cell nucleus and most of the cell’s organelles; carries out most of
the functions (protein synthesis, cellular metabolism).
In most areas of the nervous system, adults have all the neurons they will ever have. In few areas
in the brain, new neurons can develop from undifferentiated cells.
Dendrites: branch from the soma and receive input from other neurons at specialized junctions
Axon (nerve fiber): sends information. Neuron usually has one axon, but they can branch and
send signals to more than one destination. The branches of an axon are called collaterals.
Action potentials: brief, large changes in membrane potential during which the inside of the cell
becomes positively charged relative to the outside.
Axon hillock: the site where the axon originates from the cell body, is specialized in most
neurons for the initiation of action potentials.
Axon terminal: specialized to release neurotransmitter on arrival of an action potential.
LOCALIZATION OF ION CHANNELS IN NEURONS
o Leak channels: found in the plasma membrane throughout a neuron, are always open
and are responsible for the resting membrane potential.
o Ligand-gated channels: open or close in response to the binding of a chemical messenger
(neurotransmitter) to a specific receptor in the plasma membrane; densely located in
the dendrites and cell body.
o Voltage-gated channels: open or close in response to changes in membrane potential.
Have voltage-gated sodium and potassium channels in the axon and axon hillock.
Calcium channels are in the axon terminals (triggers the release of neurotransmitter).
FUNCTIONCAL CLASSIFICATION OF NEURONS
o Efferent neurons transmit information from the CNS to the effector organs. Are located
in the CNS, however the axon leaves the CNS and becomes part of the PNS as it travels to
the effector organ it innervates.
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7 NERVE CELLS AND ELECTRICAL SIGNALING
o Afferent neurons transmit either sensory info from sensory receptors (from outside
environment) or visceral receptors (from inside environment). Most are pseudo-
unipolar neurons, with the soma located outside the CNS. The peripheral axon is located
in the peripheral organ. The central axon terminates releases neurotransmitters in CNS.
o Interneurons account for 99% of all neurons in the body. Located in the CNS. They
perform all the sensory information from afferent neurons, sending out commands o
effector organs through efferent neurons.
STRUCTURAL ORGANIZATION OF NEURONS IN THE NERVOUS SYSTEM
o In the CNS, cell bodies of neurons are often grouped into nuclei and the axons travel
together in bundles called pathways, tracts, or commissures.
o In PNS, cell bodies of neurons are clustered together in ganglia, and the axons travel
together in bundles.
Their functions include providing structural integrity to the nervous system and chemical and
anatomical support that permits neurons to carry out their functions.
There are astrocytes, ependymal cells, microglia, oligodendrocytes, and Schwann cells. Only
Schewann cells are in the PNS.
Oligodendrocytes and Schwann cells form an insulating wrap of myelin around the axons of
neurons. Enables neurons to transmit action potentials more efficiently and rapidly.
Oligodendrocytes form myelin around axons in the CNS; one oligodendrocyte sends out
projections providing the myelin segments for many axons.
Schwann cells form myelin around axons in the PNS, but each Schwann cell provides myelin for
only one axon.
Because the lipid bilayer of a plasma membrane has low permeability to ions, the several layers
of membrane of the myelin sheath substantially reduce leakage of ions across the cell membrane.
Nodes of Ranvier: gaps in the myelin; the axonal membrane contains voltage-gated sodium and
potassium channels that function in the transmission of action of potential by allowing ion
movement across the membrane.
Establishment of the Resting Membrane Potential
Resting membrane potential: a cell at rest (not receiving or transmitting signals) has a potential
difference across its membrane (inside of the cell is negatively charged relative to the outside).
Neurons have a resting Vm of -70mV (inside of a neuron is 70mV more negative than the outside).
Neurons communicate by generating electrical signals in the form of changes in membrane
potential. Release neurotransmitter which then carries a signal to another cell.
DETERMINING THE EQUILIBRIUM POTENTIALS FOR POTASSIUM AND SODIUM IONS
The resting membrane potential depends on two critical factors: the concentration gradient of
ions (i.e. Na ions and K ions) across the plasma membrane, and the presence of ion channels in
the plasma membrane.
Na+/K+ pump creates concentration gradients by transporting 3 Na ions out and 2 K ions in per
ATP hydrolyzed. Na ions are highly concentrated outside the cell, so there is a chemical driving
force tending to push sodium ions into the cell. K ions are more highly concentrated inside the
cell, so there is a chemical driving force tending to push K ions out of the cell.
MEMBRANE POTENTIAL OF A CELL PERMEABLE ONLY TO POTASSIUM
o Na+ are at a higher concentration outside the cell and are balanced electrically by the
presence of Cl- outside the cell.
o K+ are at a higher concentration inside the cell and are balanced electrically by the
presence of organic anions (i.e. proteins) inside the cell.
o K will diffuse down its concentration gradient, or out of the cell, which leaves the
negative membrane potential.
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