Chapter 8: Neurons
● Neurons carry electrical signals and release neurotransmitters (NT) into extracellular
fluid to communicate with neighboring cells
○ Cells are linked by gap junctions which allow electrical signals to pass directly
from cell to cell
● Nervous system is divided into
○ Central nervous system (CNS)
■ Brain and spinal cord
■ Interneurons only
○ Peripheral nervous system (PNS)
■ Sensory (afferent) neurons- that bring info into the CNS from outside
signal (receptor)
■ Motor (efferent) neurons- that carry info away from the CNS back to other
parts of body (effector)
● Somatic motor neurons- control skeletal muscles
● Autonomic neurons- control smooth and cardiac muscles, glands,
and some adipose tissue
○ Sympathetic- fight or flight
○ Parasympathetic- rest and digest
● Info flows through the nervous system as simple reflex arc:
○ Stimulus
○ Sensor
○ Input signal
○ Integrating center
○ Output signal
○ Target
○ Response
● Sensory neurons: carry info about temp, pressure, light, other stimuli from sensory
receptors to the CNS
● Interneurons: lie entirely within the CNS, have branching processes to communicate with
many other neurons by connecting them
● Motor neurons: have axons that divide several times into branches (collaterals), enlarged
endings (axon terminals) that store/releases NTs
● Neurons have a cell body with a nucleus and organelles to direct cellular activity,
dendrites to received incoming signals, and an axon to transmit electrical signals from
the cell body to the axon terminal
● The region where an axon terminal meets its target cell is called a synapse, the target
cell is called the postsynaptic cell, and the neuron that releases the chemical signal is
known as the presynaptic cell, the region between these two cells is the synaptic cleft
● Glial cells: provide physical support and communicate with neurons
○ Schwann cells- PNS
○ Oligodendrocytes- CNS
, ○ Both form insulating myelin sheaths around neurons, the section that are
uninsulated are called the nodes of Ranvier
● Nernst equation: describes the membrane potential of a cell that is permeable to only
ONE ion
● There is a higher concentration of Na+, Cl-, and Ca2+ OUTSIDE the cell, whereas there
is a higher concentration of K+ INSIDE the cell
○ The resting cell membrane is MUCH MORE permeable to K+ than Na+, which is
why K+ is the major ion contributing to the resting membrane potential
○ Na+/K+ exchanger pump brings 2 K+ into the cell and 3 Na+ out of the cell,
which produces the driving force and maintains the Na+/K+ gradients
● Nernst potential of K+ is -90 mV, of Na+ is +60 mV
○ Leaky K+ channels and cell’s slight permeability to Na+ creates a resting
membrane potential of -70 mV
● Permeability of a cell to ions change when ion channels in the membrane open/close
○ Movement of only a few ions can significantly change the membrane potential
● I=V/R: as resistance increases, the current flow decreases
○ Resistance to current flow is from cell membrane (think of a river and the walls of
the shore) and the internal resistance of the cytoplasm (proteins and vesicles
getting in the way)
○ Internal resistance decreases as the cell diameter increases, allowing more ions
to flow leading to faster conduction velocity
■ Axons with large diameters have higher conduction velocity which
means they send signals faster
○ Likewise, opening ion channels decreases membrane resistance
● Graded potentials: depolarizations or hyperpolarizations whose strength is directly
proportional to the strength of the triggering event, they lose strength as they move
throughout the cell
● Action potential: rapid electrical signals (all-or-none depolarizations) that travel
undiminished in strength down the axon from the cell body to the axon terminals
○ Begin in the trigger zone (axon hillock) of a single graded potential or the sum
of multiple exceeds the threshold voltage (-55 mV)
○ Additional Na+ entering the cell from channels being opened by the AP reinforce
the depolarization, which is why strength is not loss over distance
○ Steps of an action potential:
■ Resting membrane potential at -70 mV
■ Depolarizing stimulus (sudden temporary increase in Na+ permeability)
■ Graded potential reaches the trigger zone and depolarizes to threshold
(-55 mV) which makes VG Na+ channels open, Na+ starts entering the
cell down its electrochemical gradient (driving force)
■ The influx of Na+ further depolarizes the cell, as soon as the membrane
potential becomes positive the electrical driving force moving Na+ in
, disappears; however, the chemical gradient remains so Na+ keeps
moving into the cell
■ The AP peaks at +30 mV when Na+ channels in the axon close and K+
channels start to open, increasing permeability to K+
■ VG K+ open in response to the depolarization (like Na+ channels) but are
just much slower, electrochemical driving force makes K+ leave the cell
so the membrane potential becomes more negative
■ By the time the membrane potential gets back to -70 mV, the K+
permeability still hasn’t returned to resting state so the cell is further
hyperpolarized
■ The slow VG K+ channels finally close and less K+ leaves the cell now
■ Retention of K+ in the cell and the leak of Na+ into the axon bring the
membrane potential back to -70 mV
○ Na+ channels open faster than K+ channels because they open at a lower
voltage than K+ channels, this is what causes the steep depolarization of an AP
● Once an AP has begun, there is an absolute refractory period during which a second
AP cannot be triggered no matter how large the stimulus, this is why an AP cannot be
summed
○ Na+ channels opening for AP, then closing as K+ channels start opening
○ Second AP cannot occur before the first one is finished
○ AP moving from hillock to terminal cannot overlap/go backwards
● During a relative refractory period, a higher-than-normal graded potential is required to
trigger an action potential
○ Follows the absolute refractory period
○ Time required for Na+ channel gates to reset to their resting position
○ Some Na+ channels gates are reset to resting position but K+ channels still open
● The myelin sheath around an axon speeds up conduction by increasing membrane
resistance and decreasing current leakage
○ AP are faster in high-resistance membranes so that current leakage is
minimized
■ The larger the diameter of the axon is, the more leak-resistant the
membrane is and thus the faster the AP will move
○ Myelinated axons limit the amount of membrane in contact with the extracellular
fluid and thus prevents ion flow out of cytoplasm
○ Also moves charge on inside the cell further away from charge on outside of cell
(decreasing charge interactions--capacitance)
● Saltatory conduction is when APs “jump” from node to node in myelinated axons
○ Depolarization occurs at nodes where Na+ enters, the + charge will diffuse
through the myelinated regions without needing to regenerate the AP because
there is very little charge interaction and leakage
○ Once the charge reaches the next Node of Ranvier, another AP can be initiated
○ Allows much smaller axons to be used and able to rapidly convey info
● Neurons carry electrical signals and release neurotransmitters (NT) into extracellular
fluid to communicate with neighboring cells
○ Cells are linked by gap junctions which allow electrical signals to pass directly
from cell to cell
● Nervous system is divided into
○ Central nervous system (CNS)
■ Brain and spinal cord
■ Interneurons only
○ Peripheral nervous system (PNS)
■ Sensory (afferent) neurons- that bring info into the CNS from outside
signal (receptor)
■ Motor (efferent) neurons- that carry info away from the CNS back to other
parts of body (effector)
● Somatic motor neurons- control skeletal muscles
● Autonomic neurons- control smooth and cardiac muscles, glands,
and some adipose tissue
○ Sympathetic- fight or flight
○ Parasympathetic- rest and digest
● Info flows through the nervous system as simple reflex arc:
○ Stimulus
○ Sensor
○ Input signal
○ Integrating center
○ Output signal
○ Target
○ Response
● Sensory neurons: carry info about temp, pressure, light, other stimuli from sensory
receptors to the CNS
● Interneurons: lie entirely within the CNS, have branching processes to communicate with
many other neurons by connecting them
● Motor neurons: have axons that divide several times into branches (collaterals), enlarged
endings (axon terminals) that store/releases NTs
● Neurons have a cell body with a nucleus and organelles to direct cellular activity,
dendrites to received incoming signals, and an axon to transmit electrical signals from
the cell body to the axon terminal
● The region where an axon terminal meets its target cell is called a synapse, the target
cell is called the postsynaptic cell, and the neuron that releases the chemical signal is
known as the presynaptic cell, the region between these two cells is the synaptic cleft
● Glial cells: provide physical support and communicate with neurons
○ Schwann cells- PNS
○ Oligodendrocytes- CNS
, ○ Both form insulating myelin sheaths around neurons, the section that are
uninsulated are called the nodes of Ranvier
● Nernst equation: describes the membrane potential of a cell that is permeable to only
ONE ion
● There is a higher concentration of Na+, Cl-, and Ca2+ OUTSIDE the cell, whereas there
is a higher concentration of K+ INSIDE the cell
○ The resting cell membrane is MUCH MORE permeable to K+ than Na+, which is
why K+ is the major ion contributing to the resting membrane potential
○ Na+/K+ exchanger pump brings 2 K+ into the cell and 3 Na+ out of the cell,
which produces the driving force and maintains the Na+/K+ gradients
● Nernst potential of K+ is -90 mV, of Na+ is +60 mV
○ Leaky K+ channels and cell’s slight permeability to Na+ creates a resting
membrane potential of -70 mV
● Permeability of a cell to ions change when ion channels in the membrane open/close
○ Movement of only a few ions can significantly change the membrane potential
● I=V/R: as resistance increases, the current flow decreases
○ Resistance to current flow is from cell membrane (think of a river and the walls of
the shore) and the internal resistance of the cytoplasm (proteins and vesicles
getting in the way)
○ Internal resistance decreases as the cell diameter increases, allowing more ions
to flow leading to faster conduction velocity
■ Axons with large diameters have higher conduction velocity which
means they send signals faster
○ Likewise, opening ion channels decreases membrane resistance
● Graded potentials: depolarizations or hyperpolarizations whose strength is directly
proportional to the strength of the triggering event, they lose strength as they move
throughout the cell
● Action potential: rapid electrical signals (all-or-none depolarizations) that travel
undiminished in strength down the axon from the cell body to the axon terminals
○ Begin in the trigger zone (axon hillock) of a single graded potential or the sum
of multiple exceeds the threshold voltage (-55 mV)
○ Additional Na+ entering the cell from channels being opened by the AP reinforce
the depolarization, which is why strength is not loss over distance
○ Steps of an action potential:
■ Resting membrane potential at -70 mV
■ Depolarizing stimulus (sudden temporary increase in Na+ permeability)
■ Graded potential reaches the trigger zone and depolarizes to threshold
(-55 mV) which makes VG Na+ channels open, Na+ starts entering the
cell down its electrochemical gradient (driving force)
■ The influx of Na+ further depolarizes the cell, as soon as the membrane
potential becomes positive the electrical driving force moving Na+ in
, disappears; however, the chemical gradient remains so Na+ keeps
moving into the cell
■ The AP peaks at +30 mV when Na+ channels in the axon close and K+
channels start to open, increasing permeability to K+
■ VG K+ open in response to the depolarization (like Na+ channels) but are
just much slower, electrochemical driving force makes K+ leave the cell
so the membrane potential becomes more negative
■ By the time the membrane potential gets back to -70 mV, the K+
permeability still hasn’t returned to resting state so the cell is further
hyperpolarized
■ The slow VG K+ channels finally close and less K+ leaves the cell now
■ Retention of K+ in the cell and the leak of Na+ into the axon bring the
membrane potential back to -70 mV
○ Na+ channels open faster than K+ channels because they open at a lower
voltage than K+ channels, this is what causes the steep depolarization of an AP
● Once an AP has begun, there is an absolute refractory period during which a second
AP cannot be triggered no matter how large the stimulus, this is why an AP cannot be
summed
○ Na+ channels opening for AP, then closing as K+ channels start opening
○ Second AP cannot occur before the first one is finished
○ AP moving from hillock to terminal cannot overlap/go backwards
● During a relative refractory period, a higher-than-normal graded potential is required to
trigger an action potential
○ Follows the absolute refractory period
○ Time required for Na+ channel gates to reset to their resting position
○ Some Na+ channels gates are reset to resting position but K+ channels still open
● The myelin sheath around an axon speeds up conduction by increasing membrane
resistance and decreasing current leakage
○ AP are faster in high-resistance membranes so that current leakage is
minimized
■ The larger the diameter of the axon is, the more leak-resistant the
membrane is and thus the faster the AP will move
○ Myelinated axons limit the amount of membrane in contact with the extracellular
fluid and thus prevents ion flow out of cytoplasm
○ Also moves charge on inside the cell further away from charge on outside of cell
(decreasing charge interactions--capacitance)
● Saltatory conduction is when APs “jump” from node to node in myelinated axons
○ Depolarization occurs at nodes where Na+ enters, the + charge will diffuse
through the myelinated regions without needing to regenerate the AP because
there is very little charge interaction and leakage
○ Once the charge reaches the next Node of Ranvier, another AP can be initiated
○ Allows much smaller axons to be used and able to rapidly convey info
