The Nervous System
Neurons - Chapter 7
The Nervous System
• Rapid Communication and Control
– Sensation
• receives info. on environmental changes
– Integration
• interprets the changes, integrates signals from
multiple signals
– Response
• induces action from of muscles or glands
Nervous System Organization:
General Anatomy
• Central Nervous System (CNS)
– Brain + Spinal Cord
– control center (integration)
• Peripheral Nervous System (PNS)
– cranial nerves and spinal nerves
– connects CNS to sensory receptors, muscles
and glands
Cell Types
• Neurons
– conduct electrical signals
• Neuroglia
– 80% of all NS cells
– support neurons
Neurons
• Cell Body
– nucleus and organelles
• Dendrites
– receive information
• Axon
– conduct electrical signals (action
potentials)
– axon hillock - site where AP’s
originate
– axon terminals - where chemical
signals are released
Types of Neurons
• Sensory (Afferent) Neuron - input
– part of the PNS
– transmit electrical signals from tissues and
organs to CNS
• detect changes in environment
Types of Neurons
• Motor (Efferent) neuron - output
– part of the PNS
– transmit signals from CNS to effector
tissues (muscle, gland cells)
– somatic motor neurons
• skeletal muscle contraction
• both voluntary and reflexive
– autonomic motor neurons
• smooth muscle, cardiac muscle, and glands
• involuntary
Types of Neurons
• Interneurons = processors & integrators
– 99% of all neurons
– connect afferent to efferent
– located entirely in the CNS
Types of Neuroglia
• Schwann Cells
– surround axons of all PNS
neurons
– form myelin sheath around
axons
• gaps = Nodes of Ranvier
– presence increases the speed
nerves conduct signals
Types of Neuroglia
• Oligodendrocytes
– form myelin sheaths around
CNS axons
– white matter
• area of CNS with high density
of axons
– grey matter
• area of CNS with mostly cell
bodies and dendrites (no
myelin)
Types of Neuroglia
• Microglia
– phagocytose (eat)
foreign and
degenerated material
Types of Neuroglia
• Astrocytes
– cover capillaries within the brain
– control permeability of
capillaries
• regulate exchange of material
between blood and cerebrospinal
fluid
• “blood-brain barrier”
– Deliver nutrients directly to
neurons
– Uptake excess neurotransmitter
and control ion concentrations at
synapses.
Types of Neuroglia
• Ependymal cells
– form epithelial lining
of brain and spinal
cord cavities
– produce cerebrospinal
fluid
Types of Neuroglia
• Satellite Cells
(Ganglionic Gliocytes)
– Form capsules around cell
neuron cell bodies in
ganglia
– Support and protect cell
bodies
Electrical Activity of Neurons:
Resting Potential
• Due to differences in permeability
of membrane to charged particles
– completely impermeable to A-
– relatively permeable to K+
– relatively impermeable to Na+
• Inside of cell negative relative to
the outside (-70 mV)
• At resting potential, neither K+ nor
Na+ are in equilibrium
Electrical Activity of Neurons:
Electrical Signals
• Electrical signals
– changes in membrane potential
– due to changes in membrane permeability and
increased flow of charged particles
– changes in permeability are due to increased
number of open membrane channels.
• Allows ions to flow along electrochemical gradient
Depolarization and
Hyperpolarization
• Depolarize
– reduce charge
difference
• Hyperpolarize
– increase charge
difference
Membrane Proteins Involved in
Electrical Signals
• Non-gated ion channels
– Always open
– specific for a particular ion
Membrane Proteins Involved in
Electrical Signals
• Gated Ion channels
–
–
–
–
open only under particular conditions (stimulus)
voltage gated – changes in membrane potential
chemically gated – binding of a chemical messenger
physically gated – stretching/distortion of the membrane
Membrane Proteins Involved in
Electrical Signals
• Na+/K+ pump
– active (require ATP)
– Na+ pumped out, K+ pumped in (3 Na+ per 2 K+)
Types of Electric Signals:
Graded Potentials
• occur in dendrites and cell body
• small, localized change in
membrane potential
– change of only a few mV
– opening of chemically-gated or
physically-gated ion channels
• changes permeability of membrane
– travels only a short distance (mm)
Types of Electric Signals:
Graded Potentials
• a triggered event (requires stimulus)
– e.g. - light, touch, chemical messengers
• graded
–  stimulus intensity →  change in membrane
potential
Types of Electric Signals:
Action Potentials
• begins at the axon hillock,
travels down axon
• brief, rapid reversal of
membrane potential
– Large change (~70-100 mV)
– Opening of voltage-gated Na+
and K+ channels
– self-propagating - strength of
signal maintained
– transmits electrical signals over
long distances
Types of Electric Signals:
Action Potentials
• triggered
– membrane depolarization at axon hillock
• not graded = "All or none"
– axon hillock must be depolarized a minimum amount
(threshold)
– if depolarized to threshold, AP will occur at maximum
strength
– if threshold not reached, no AP will occur
Action Potential:
Depolarization Phase
• Triggering event causes membrane
to depolarize
• slow increase until threshold is
reached
• voltage-gated Na+ channels open
quickly (K+ channels slowly)
–
–
–
–
Na+ enters cell
further depolarization
more channels open
further depolarization
• membrane depolarizes to 0 mV, but
continued flow of Na+ in leads to
reversed polarity (+30 mV)
Action Potential:
Repolarization Phase
• At +30 mV, voltage-gated
Na+ channels close
• Slow opening of voltagegated K+ channels
– reach peak K+ permeability
as Na+ channels close
• K+ rushes out of the cell
– membrane potential restored
• K+ channels close @
threshold
• [Na+] and [K+] restored by
the Na+-K+ pump
Action Potentials
• response of the nerve cell to the
stimulus is “all or none”
– Amt of depolarization
(amplitude) always the same
– differences in stimulus intensity
are detected by
• The number of neurons undergoing
AP in response to the stimulus
• The frequency of action potential
generation
Refractory Period
• time that must pass before the
neuron segment can undergo a
second action potential
• absolute refractory period
– neuron segment is undergoing AP
– cannot respond to a second stimulus
– channels enter an inactive state
• relative refractory period
– neuron segment is repolarizing
– action potential may be produced if
a stronger stimulus is applied
Action Potential Propagation
• Na+ moving into one
segment of the neuron
quickly moves laterally
inside the cell
• Depolarizes adjacent
segment to threshold
Action Potential Propagation:
Myelinated Axons
• Saltatory conduction - increased
speed of the AP produced by
myelination of the axon
– myelin = lipid insulator (PM of
Schwann cells or oligodendrocytes)
– nodes of Ranvier =contain lots of
Na+ channels
• signals “jump” from one node to
the next
– AP conduction speed
Synapses
• Synapse
– functional connection between a neuron and
either an effector cell or another neuron
– allow information to pass from one cell to the
next
Electrical Synapses
(Gap Junctions)
• Present in cardiac and smooth
muscle, and some neurons
• Series of channels crossing
membranes of both cells
• Allow flow of ions from one
cell to the next
• Electrical signals move quickly
from one cell to the next
Chemical Synapses
• Unidirectional info. flow
• presynaptic neuron
– synaptic terminal bouton
– contains synaptic vesicles filled
with neurotransmitter
• synaptic cleft
– space in-between cells
• postsynaptic neuron
– Subsynaptic membrane
– Receptor proteins for
neurotransmitter
Chemical Synapses
• Many voltage-gated Ca2+ channels in
the terminal bouton
– Ca2+ is in higher conc. in the ECF than
the ICF
– AP in bouton opens Ca2+ channels
– Ca2+ rushes in.
• Ca2+ causes vesicles to fuse to plasma
membrane and release contents
• Transmitter diffuses across synaptic
cleft and binds to receptors on
subsynaptic membrane
Chemical Synapses
• Specific ion channels in
subsynaptic membrane open
– chemically-gated ion channels
• Ions enter postsynaptic cell
– graded potential forms
• If depolarizing graded potential
is strong enough to reach
threshold, action potential
generated in postsynaptic cell
Types of Chemical Synapse
• Excitatory chemical
synapse:
– excitatory postsynaptic
potentials (EPSPs)
– Transmitter binding opens
Na+ channels in the
postsynaptic membrane
– Small depolarization of
postsynaptic neuron
• More positive inside the cell
• closer to threshold
Types of Chemical Synapse
• Inhibitory chemical
synapse:
– inhibitory postsynaptic
potentials (IPSPs)
– Transmitter binding opens
K+ or Cl- ion channels
– K+ flows out or Cl- flows in
down gradients
– Small hyperpolarization of
postsynaptic neuron
• More negative inside cell
• further from threshold
Neurotransmitters
• Chemicals that carries the message of the A.P.
from one cell to the next
• Acetylcholine
– somatic MNs – skeletal muscle contraction
– autonomic MNs – slow HR, gland secretion etc.
• Norepinephrine
– autonomic MNs – mental alertness, increases blood
pressure and HR, etc.
• Seratonin + Dopamine
– interneurons – behavioral effects
Neurotransmitters
• types vary between synapses
• response depends on postsynaptic
membrane
• e.g. acetylcholine
– produces EPSPs when applied to
skeletal muscle
– produced IPSPs when applied to
cardiac muscle
Synaptic Integration
• Multiple synaptic events have an additive
effect on membrane potential
• Sum of inputs determines whether axon
hillock depolarized enough for AP to form.
Spatial Summation
• numerous presynaptic
fibers may converge
on a single
postsynaptic neuron
• additive effects of
numerous neurons
inducing EPSPs and
IPSPs on the postsyn.
neuron
Temporal Summation
• additive effects of
EPSPs and IPSPs
occurring in rapid
succession
• next synaptic event
occurs before
membrane recovers
from previous event