Chapter 44 Flashcards
Nervous System Organization
All animals must be able to respond to environmental stimuli Sensory receptors – detect stimulus Motor effectors – respond to it Nervous system links the two Consists of neurons and supporting cells
Vertebrates have three types of neurons Sensory neurons (afferent neurons) carry impulses to central nervous system (CNS) Motor neurons (efferent neurons) carry impulses from CNS to effectors (muscles and glands) Interneurons (association neurons) provide more complex reflexes and associative functions (learning and memory)
Central nervous system (CNS ) Brain and spinal cord Peripheral nervous system (PNS) Sensory and motor neurons Motor pathways are divided into: Somatic NS stimulates skeletal muscles Autonomic NS stimulates smooth and cardiac muscles, as well as glands Sympathetic and parasympathetic NS Counterbalance each other
Cell body Enlarged part containing nucleus Dendrites Short, cytoplasmic extensions that receive stimuli Axon Single, long extension that conducts impulses away from cell body
Neuroglia
Support neurons both structurally and functionally
Schwann cells (PNS) and oligodendrocytes (CNS) produce myelin sheaths surrounding axons
In the CNS, myelinated axons form white matter
Dendrites/cell bodies form gray matter
In the PNS, myelinated axons are bundled to form nerves
Nerve Impulse Transmission
In all cells, there is a difference in ion distribution across the plasma membrane—membrane potential
Negative pole – cytoplasmic side (fewer + ions inside cell and/or more – ions inside
Positive pole –extracellular fluid side (more + ions outside the cell)
What makes neurons different from other cells is that
in the presence of a stimulus, the permeability of the
membrane to these ions changes
and
this allow ions to flow in or out of the neuron changing
the membrane potential.
When a neuron is not being stimulated, it maintains a resting potential
Ranges from –40 to –90 millivolts (mV)
Average about –70 mV
If no stimulus is present:
How is this resting potential maintained?
Sodium–potassium pump
Brings two K+ into cell for every three Na+ it pumps out—maintains high K+ and low Na+ inside neuron and high Na+ and low K+ outside the neuron
Ion leakage channels
Channel proteins allow specific ions to diffuse
across membrane
More K+ ion channels than Na+ ion channels
so more K+ to diffuse out than Na+ to diffuse in
Two other major forces act on ions in establishing the resting membrane potential
Diffusional force—concentration gradient produced by unequal concentrations of molecules from one side of the membrane to the other—due to Na+/K+ pump, there are more K+ inside the cell and more Na+ outside the cell. Thus K+ tend to diffuse out of the cell through K+ ion channels.
Electrical force—due to unequal distribution
of charges
The membrane is not permeable to negative ions
so there is a buildup of + ions on the outside
and – ions on the inside.
This electrical force of negativity on the inside
pulls the K+ back inside the cell.
Sodium–potassium pump creates significant concentration gradient
Concentration of K+ is much higher inside the cell (more K+ ion channels)
Membrane not permeable to negative ions
Leads to buildup of positive charges outside and negative charges inside cell
Attractive force to bring K+ back inside cell
Equilibrium potential – balance between diffusional force and electrical force
In response to a stimulus, there is a sudden temporary disruptions to the resting membrane potential.
2 types of changes
Graded potentials
Action potentials
Graded potentials—on dendrites and cell
bodies
Small transient changes in membrane potential due to activation of gated ion channels
Each gated channel is selective
Most are closed in the normal resting cell
but open in response to a stimulus
How do they work?
Chemically-gated or ligand-gated channels
Ligands are hormones or neurotransmitters
Induce opening and cause changes in cell membrane permeability
If Na+ flows in—depolarization—makes the membrane potential more positive
ex. -70mV-65mV
If Cl- flows in—hyperpolarization—makes it more negative ex. -70mV-75mV
These small changes result in graded potentials
Can reinforce or negate each other
Action potentials
Result when depolarization reaches the threshold potential (–55 mV)
Depolarizations bring a neuron closer to the threshold
Hyperpolarizations move the neuron further from the threshold
Caused by voltage-gated ion channels—open
and close in response to changes in membrane potential
Voltage-gated Na+ channels
Voltage-gated K+ channels
Voltage-gated Na+ channels
Activation gate and inactivation gate
At rest, activation gate closed, inactivation gate open
When threshold voltage is reached, activation gate opens
Transient influx of Na+ causes the membrane to depolarize
Voltage-gated K+ channels
Single activation gate that is closed in the resting state
In response to threshold voltage, K+ channel opens slowly
Efflux of K+ repolarizes the membrane
The action potential has three phases
Rising, falling, and undershoot
Action potentials are always separate, all-or-none events with the same amplitude
Do not add up or interfere with each other
Propagation of action potentials
Each action potential, in its rising phase, reflects a reversal in membrane polarity
Positive charges due to influx of Na+ can depolarize the adjacent region to threshold
And so the next region produces its own action potential
Meanwhile, the previous region repolarizes back to the resting membrane potential
Signal does not go back toward cell body
hey
Action potential propagation in
unmyelinated neurons—continuous conduction
Nerve Impulse Transmission
Two ways to increase velocity of conduction Axon has a large diameter Less resistance to current flow Found primarily in invertebrates Axon is myelinated Action potential is only produced at the nodes of Ranvier Impulse jumps from node to node Saltatory conduction
Synapses
Intercellular junctions with the dendrites of other neurons, with muscle cells, or with gland cells
Presynaptic cell transmits action potential
Postsynaptic cell receives it
Two basic types: electrical and chemical
Electrical synapses
Involve direct cytoplasmic connections between the two cells formed by gap junctions
Relatively rare in vertebrates
Chemical synapses
Have a synaptic cleft between the two cells
End of presynaptic cell contains synaptic vesicles packed with neurotransmitters
Chemical synapses
Action potential triggers influx of Ca2+
Synaptic vesicles fuse with cell membrane
Neurotransmitter is released by exocytosis
Diffuses to other side of cleft and binds to chemical- or ligand-gated receptor proteins
Produces graded potentials in the postsynaptic membrane
Neurotransmitter action is terminated by enzymatic cleavage or cellular uptake
