Nervous System Overview
• The nervous system is the
body’s electrical system
• It is broadly divided into two
parts:
 Central nervous system =
CNS (brain and spinal cord)
 Peripheral nervous system =
PNS (nerves and ganglia)
• The nervous system performs
three important functions:
 Sensory input
 Integration
 Motor output
Nervous System Organization
• The PNS is divided into three parts:
 The somatic nervous system (SNS) = “voluntary” = “skeletal muscles”
 The autonomic nervous system (ANS) = “involuntary” = “smooth muscles”
 The enteric nervous system (ENS) = “brain of the gut” = “GI tract NS”
• All communicate with the CNS
Nervous System Organization
• The ANS is divided into two parts:
 The sympathetic nervous system = “fight or flight”
 The parasympathetic nervous system “rest and digest”
Nervous System Tissue: Neuron
• The neuron is the basic cell of
the nervous system
• Neurons communicate with
other neurons and tissues by
action potentials
• Neurons have special
processes:
 Dendrites receive input
 Axons transmit action
potentials
• The axons of some neurons are
coated with a fatty substance
called myelin
• Gaps in the myelin coatings are
nodes of Ranvier
Nervous System Tissue: Neuron
Nervous System Tissue: Synapses
• A synapse is the
junction between two
neurons
• Axodendritic: axon synapses with dendrite
• Axosomatic: axon synapses with cell body
• Axoaxonic: axon synapses with another axon
Nervous System Tissue: Neuron Types
• Multipolar neurons have many dendrites and one axon. Most CNS neurons are
multipolar.
• Bipolar neurons have one dendrite and one axon. They are found in many
special sense organs.
• Unipolar neurons have one process that diverges from the cell body and
forms dendrites on one end and axon terminals on the other. They are found
in many ganglia of the cranial and spinal nerves.
Nervous System Tissue: Neuron Types
Nervous System Tissue: Neuron Types
Nervous System Tissue: Neuron Types
Nervous System Tissue: Neuron Types
Nervous System Tissue: Glial Cells
• Glial cells (neuroglia) are the
“glue” of the NS
• They also perform many
functions
• There are six basic types:






Astrocytes (CNS)
Microglia (CNS)
Oligodendrocytes (CNS)
Ependymal cells (CNS)
Schwann cells (PNS)
Satellite cells (PNS)
• Glial cells outnumber neurons
about 10 to 1
• Astrocytes provide structural support, form the blood-brain barrier, and regulate ions
• Microglia function as phagocytes
• Ependymal cells line the ventricles and spinal canal, and produce cerebrospinal fluid
• Oligodendrocytes form the myelin sheaths around axons in the CNS
Nervous System Tissue: Glial Cells
Nervous System Tissue: Myelin Sheaths
• Schwann cells form the myelin
sheaths around axons in the
PNS
• Myelin sheaths insulate
neurons and increase the
speed of the action potential
• Schwann cells myelinate a single axon
• Oligodendrocytes myelinate parts of
many axons
• Schwann cells have a neurolemma, which
can guide axon regrowth
Nervous System Tissue: Myelin Sheaths
• Gaps between myelin sheaths are nodes of Ranvier
• Satellite cells provide structural support and regulate ions in the PNS
Nervous System Tissue: Myelin Sheaths
• Schwann cells can guide axon regrowth in the PNS
Nervous System Tissue: Myelin Sheaths
• Oligodendrocytes can myelinate more than one axon
Nervous System Tissue
• Nodes of Ranvier are gaps in the myelin sheath
Nervous System Tissue: Gray & White Matter
• Gray matter consists of cell bodies, unmyelinated axons, dendrites, and glial
cells
• White matter consists of myelinated axons
Overview of Neural Transmission
•
Neurons receive input from other
neurons, especially through their
dendrites
•
Neurons send output in the form of an
action potential (AP) along their axons
•
When an AP arrives at the synaptic end
bulb of the presynaptic neuron,
neurotransmitter (NT) is released
•
NT’s bind to receptors on the
postsynaptic neuron, which often opens
ion channels
•
The flow of ions causes a an electrical
current in the membrane
•
These graded potentials can be either
•
The graded potentials are summed in
the axon hillock
•
If the sum exceeds threshold, then the
postsynaptic neuron will fire an action
potential
•
When the action potential reaches the
synaptic end bulb, NT’s are released,
and the cycle begins again
 Positive/Depolarization/Excitatory
 Negative/Hyperpolarization/Inhibitory
The Action Potential (AP)
•
The AP propagates along the
axon
•
As the wave of depolarization
moves along the axon, Na+ and
K+ channels open in sequence
•
Eventually the AP reaches the
synapse and neurotransmitters
are released
Graded or Local
Potentials (GP)
•
Small deviations from resting
potential of -70mV due to the opening
of ion channels
 Depolarization = membrane has become
more positive
 Hyperpolarization = membrane has
become more negative
•
The signals are graded, meaning they
vary in amplitude (size), depending on
the strength of the stimulus
•
The signals are localized
•
Graded potentials occur most often in
the dendrites and cell body of a
neuron
Graded (Local) Potentials:
Excitatory & Inhibitory Signals
Threshold and the Trigger Zone
•
•
•
•
•
•
Graded potentials that arise in
dendrites converge in the axon
hillock
Graded potentials are summed
If the sum exceeds the threshold
value of the neuron (usually about
-55 mV), then an action potential
arises in the trigger zone
Unlike graded potentials, the action
potential is all-or-none
When an action potential arises, it
propagates along the axon
When it reaches the axon terminals
it may synapse with another neuron
and cause graded potentials in that
neuron
Speed of Impulse Propagation
•
The propagation speed of a nerve impulse is not related to stimulus
strength
 Larger fibers conduct impulses faster due to size (less resistance)
 Myelinated fibers conduct impulses faster due to saltatory conduction
•
Fiber types
 A fibers largest (5-20 microns & 130 m/sec)
 myelinated somatic
sensory & motor to skeletal muscle
 B fibers medium (2-3 microns & 15 m/sec)
 myelinated visceral
sensory & autonomic preganglionic
 C fibers smallest (0.5-1.5 microns & 2 m/sec)
 unmyelinated
sensory & autonomic motor
Nervous System Tissue:
Fiber Diameter and Conduction Velocity
The fatter the fiber, the faster it flies
Continuous versus Saltatory Conduction
•
Continuous Conduction (unmyelinated axons)
 Step-by-step depolarization of each portion of the length of the axolemma
•
Saltatory Conduction (myelinated axons)
 Depolarization only at nodes of Ranvier where there is a high density of voltagegated ion channels
 The myelin sheath and nodes of Ranvier act like a capacitor, and the impulse
“leaps” down the axon, regenerating itself at each node
Continuous versus Saltatory Conduction
•
•
From the Latin verb saltare, which means “to leap”
The saltarello was a dance of the Middle Ages
The Saltarello
Nerve Transmission: The Story So Far
•
An action potential (AP) propagates over the surface of the axon membrane
 Na+ flows into the cell causing a dramatic depolarization
 In response to depolarization, adjacent voltage-gated Na+ and K+ channels open, selfpropagating along the membrane
 K+ flows out of the cell causing a dramatic hyperpolarization, the resting potential of the
membrane is gradually restored, following a refractory period
•
The traveling AP is called a nerve impulse
•
When the nerve impulse reaches the synaptic end bulbs of the axon terminals,
neurotransmitters are released
•
When neurotransmitters bind to receptors on the dendrites and soma of the
postsynaptic neuron, they produce graded potentials
•
The graded potentials are summed in the axon hillock (trigger zone)
•
If the sum exceeds threshold, then an action potential is produced
Comparison of Graded & Action Potentials
•
•
•
•
•
•
Origin
 GPs arise on dendrites and cell bodies
 APs arise only at the trigger zone on the axon hillock
Types of Channels
 AP is produced by voltage-gated ion channels
 GP is produced by ligand or mechanically-gated channels
Conduction
 GPs are localized (not propagated)
 APs conduct (propagate) over the surface of the axon
Amplitude
 amplitude of the AP is constant (all-or-none)
 graded potentials vary depending upon stimulus strength
Duration
 The AP is always the same
 The duration of the GP is as long as the stimulus lasts
Refractory period
 The AP has a refractory period due to the nature of the
voltage-gated channels, and the GP has none.