Download Central Nervous System

Chapter 10
Functional
Organization of
Nervous Tissue
Neuron Network
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Functions of the Nervous System
•
•
The master controlling and
communicating system of the body
Functions
1. Sensory input: detects external and internal
stimuli
2. Integration: processes and responds to
sensory input
3. Control of Muscles and Glands
4. Homeostasis is maintained by regulating
other systems
5. Center for Mental Activities
Fig. 10.1
Parts of the Nervous System
•
Two anatomical divisions
–
Central Nervous System (CNS)
•
•
–
Peripheral Nervous System (PNS)
•
•
•
Brain and spinal cord
Encased in bone
Nervous tissue outside of the CNS
Consists of sensory receptors and nerves
The anatomical divisions perform different
functions
–
–
PNS detects stimuli and transmits information to the
CNS and receives information from the CNS
CNS processes, integrates, stores, and responds to
information from the PNS
Parts of the Nervous System
•
PNS has two divisions
–
–
Sensory division transmits action potentials from
sensory receptors to the CNS
Motor division carries action potentials away from
the CNS in cranial or spinal nerves (two
subdivisions)
•
•
Somatic nervous system innervates skeletal muscle
Autonomic nervous system (ANS) innervates cardiac
muscle, smooth muscle, and glands (three subdivisions)
–
–
–
Sympathetic division is most active during physical activity
(fight or flight division)
Parasympathetic division regulates resting functions (rest and
digest division)
Enteric nervous system controls the digestive system
Parts of the Nervous System
Central Nervous System
(CNS)
•Brain and Spinal Cord
Peripheral Nervous System
(PNS)
•Nervous tissue outside the CNS
•Sensory receptors and nerves
Sensory Division
•Transmits action potentials from
sensory receptors to the CNS
Motor Division
•Carries action potentials away
from the CNS in cranial nerves or
spinal nerves
Sympathetic Division
Most active during physical activity
Parasympathetic Division
Regulates resting functions
Enteric Nervous System
Controls the Digestive System
Autonomic
Nervous
System (ANS)
•Innervates cardiac
muscle, smooth
muscle, and glands
Somatic
Nervous
System
•Innervates
skeletal muscle
Fig. 10.2
Cells of the Nervous System
• The two principal cell types of the nervous
system are:
– Neurons: excitable cells that transmit
electrical signals
– Non-neural cells (Glial cells): cells that
surround neurons. Account for over half of
the brain’s weight
• Less than 20% is extracellular space
Neurons
• Receive stimuli and transmit action
potentials
• Have three components:
– The cell body (soma) is the primary site of
protein synthesis
– Dendrites are short, branched cytoplasmic
extensions of the cell body that usually
conduct electric signals toward the cell body
– An axon is a cytoplasmic extension of the cell
body that transmits action potentials to other
cells
Fig. 10.3
Neuron Structure
• Cell Body (Soma)
– Contains the nucleus and a nucleolus
– Nissl substance is an aggregate of rough ER and free
ribosomes
• Primary site of protein synthesis
– Golgi apparatus, mitochondria, and other organelles are
present
– Has no centrioles (hence
its amitotic nature)
– Clusters of cell bodies in
the CNS are called nuclei
and in the PNS ganglia
Fig. 10.3
Neuron Structure
• Axons (Nerve Fibers)
– Trigger zone is the part of the neuron where the axon originates
• Action potential is generated from the trigger zone
– Slender processes of uniform diameter and may vary in length
from a few millimeters to more than a meter
– Usually, there is only one unbranched axon
per neuron
• Rare branches, if present, are called collateral
axons
– Presynaptic terminal: branched terminus of
an axon (10,000 or more)
– Synapse: junction between a nerve cell and
another cell
– Bundles of processes are called nerve tracts
in the CNS and nerves in the PNS
Fig. 10.3
Types of Neurons
• Multipolar neurons have several dendrites
and a single axon
– Interneurons and motor neurons
• Bipolar neurons have a single axon and
dendrite
– Components of sensory organs
• Unipolar neurons have a single axon
– Most sensory neurons
Fig. 10.4
Glial Cells
• Glial Cells (Supporting Cells):
–
–
–
–
Provide a supportive scaffolding for neurons
Segregate and insulate neurons
Guide young neurons to the proper connections
Promote health and growth
• Glial Cells of the
CNS
–
–
–
–
Astrocytes
Microglial
Ependymal cells
Oligodendrocytes
• Glial Cells of the
PNS
– Satellite cells
– Schwann cells
Glial Cells of the CNS
• Astrocytes
– Most abundant, versatile, and highly branched
– They cling to neurons and their synaptic endings, and
cover capillaries
– Functions:
• Support and brace neurons and blood vessels
– Anchor neurons to their nutrient supplies
• Influence the functioning of the blood-brain barrier
• Guide migration of young neurons
• Process substances
– mopping up leaked potassium ions
– recycling neurotransmitters
• Isolate damaged tissue and limit the spread of inflammation
Astrocytes
Fig. 10.5
Glial Cells of the CNS
• Ependymal cells: range in shape from
squamous to columnar and many are ciliated
– They line the ventricles of the brain and the central
canal of the spinal cord
– Some are specialized (choroid plexuses) to produce
cerebrospinal fluid (CSF)
– Help to circulate CSF using their cilia
Fig. 10.6
Glial Cells of the CNS
• Microglia
– Small, ovoid cells with
spiny processes
– Phagocytes that
monitor the health of
neurons
Fig. 10.7
Glial Cells of the CNS
• Oligodendrocytes: form myelin sheaths
around the axons of several CNS neurons
Fig. 10.8
Glial Cells of the PNS
• Schwann cells: form a myelin sheath around
part of the axon of a PNS neuron
• Satellite cells: support and nourish neuron cell
bodies within ganglia
Fig. 10.9
Myelinated and Unmyelinated Axons
• Myelinated axons
– Plasma membrane of Schwann cells or
Oligodendrocytes repeatedly wraps around a
segment of an axon to form the myelin sheath
– Myelin is a whitish, fatty (protein-lipid), segmented
sheath around most long axons
– It functions to:
• Protect the axon
• Electrically insulate
fibers from one another
• Increase the speed of
nerve impulse transmission
• Node of Ranvier
– Gaps in the myelin sheath
Fig.
10.10
Myelinated and Unmyelinated Axons
• Unmyelinated axons
– Rest in invaginations of Schwann cell (PNS)
or Oligodendrocytes (CNS)
– Conduct action potentials slowly
Fig.
10.10
Fig.
10.10
Organization of Nervous Tissue
• Nervous tissue can be grouped into white matter
and gray matter
– White matter
• Consists of myelinated axons
• Propagates action potentials
• Forms nerve tracts in the CNS and nerves in the PNS
– Gray Matter
• Collections of neuron cell bodies or unmyelinated axons
• Forms cortex and nuclei in the CNS and ganglia in the PNS
• Axons synapse with neuron cell bodies, which
are functionally the site of integration in the
nervous system
Electric Signals
• Electric signals produced by cells are called
action potentials
– When action potentials are received from sensory cells it can
result in the sensations of sight, hearing, and touch
– Complex mental activities, such as conscious thought, memory,
and emotions, result from action potentials
– Contraction of muscles and the secretion of certain glands occur
in response to action potentials
• Electrical properties of cells result from
– Ionic concentration differences across the plasma
membrane
– Permeability characteristics of the plasma membrane
Electric Signals
•
Concentration Differences Across the Plasma
Membrane
–
–
Sodium ions (Na+), calcium ions (Ca2+), and
chloride ions (Cl-) are in much greater concentration
outside the cell than inside
Potassium ions (K+) and negatively charged
molecules, such as proteins, are in much greater
concentration inside the cell than outside
•
Negatively charged proteins are synthesized inside the cell
and cannot diffuse out of it
Electric Signals
• Concentration gradients of ions result mainly from
1. The Na+-K+ pump
• Moves ions by active transport
• Potassium ions are moved into the cell, and Na+ are moved out
of it
2. Permeability characteristics of the plasma membrane
are determined by
• Leak channels (always open)
– Potassium ion leak channels are more numerous than Na+ leak
channels; thus, the plasma membrane is more permeable to K+
than to Na+ when at rest
• Gated ion channels
– Include ligand-gated ion channels, voltage-gated ion channels, and
other gated ion channels
Electric Signals
• Gated Ion Channels
– Open and close in response to stimuli
• Ligand-gated ion channels
– Open or close with the binding of a specific ligand
(neurotransmitter)
» Ligand is a molecule that binds to a receptor
» A receptor is a protein or glycoprotein that has a receptor site to
which a ligand can bind
– Common in tissues such as nervous and muscle tissue, as well
as glands
• Voltage-gated ion channels
– Open and close in response to small voltage changes across
the plasma membrane
– Common in tissues such as nervous and muscle tissues
• Other gated ion channels
– Open and close in response to physical deformation of
receptors
– Touch receptors (mechanical stimulation) and temperature
receptors (temperature changes) of the skin
Electric Signals
• Establishing the Resting Membrane
Potential
– Resting membrane potential
• Charge difference across the plasma membrane
when the cell is not being stimulated
• The inside of the cell is negatively charged,
compared with the outside of the cell
– Due mainly to the tendency of positively charged K+ to
diffuse out of the cell
– Opposed by the negative charge that develops inside the
plasma membrane
Electric Signals
• Changing the Resting Membrane Potential
– Depolarization is a decrease in the resting membrane
potential caused by
•
•
•
•
A decrease in the K+ concentration gradient
A decrease in membrane permeability to K+
An increase in membrane permeability to Na+ or Ca2+
A decrease in extracellular Ca2+ concentrations
– Hyperpolarization is an increase in the resting
membrane potential caused by
•
•
•
•
•
An increase in the K+ concentration gradient
An increase in membrane permeability to K+
An increase in membrane permeability to ClA decrease in membrane permeability to Na+
An increase in extracellular Ca2+ concentrations
Graded Potentials
• Are small changes in the resting membrane potential
• Confined to a small area of the plasma membrane
– An increase in membrane permeability to Na+ can cause graded
depolarization
– An increase in membrane permeability to K+ or Cl- can result in graded
hyperpolarization
• Decreases in magnitude as the distance from the stimulation increases
•
The term graded potential is used because a
stronger stimulus produces a greater potential
change than a weaker stimulus
Fig. 10.15
•
Graded potentials can summate, or
add together
Action Potentials
• Are larger changes in the resting membrane
potential that spread over the entire surface of the
cell
– Occurs when a graded potential causes depolarization of
the plasma membrane to a level called threshold
– Occur in an all-or-none fashion and are of the same
magnitude, no matter how strong the stimulus
– Occurs in three phases
• Depolarization phase
• Repolarization phase
• Afterpotential
Action Potentials
• Depolarization Phase
– Inside of the membrane
becomes more positive
– Na+ diffuses into the cell through
voltage-gated ion channels
• Repolarization Phase
– Return of the membrane
potential toward the resting
membrane potential
– Voltage-gated Na+ channels
close
– Voltage-gated K+ channels open
and K+ diffuses out of the cell
• Afterpotential
– Brief period of hyperpolarization
following repolarization
Fig. 10.16
Refractory Periods
• Absolute refractory period
– Time during an action
potential when a second
stimulus (no matter how
strong) cannot initiate
another action potential
• Relative refractory period
– Time during which a
stronger-than-threshold
stimulus can evoke
another action potential
Action Potential Frequency
• The number of action potentials produced
per unit of time in response to stimuli
– It is directly proportional to stimulus strength
and to the size of the graded potential
• Subthreshold stimulus: graded potential
• Threshold stimulus: a single action potential
• Submaximal stimulus: action potential frequency
increases as the strength of the stimulus increases
• Maximal or a supramaximal stimulus: produces a
maximum frequency of action potentials
Propagation of Action Potentials
• An action potential generates ionic currents
– Currents stimulate voltage-gated Na+ channels in
adjacent regions of the plasma membrane to open
– Producing new action potentials
• Reversal of the direction of action potential
propagation is prevented by the absolute
refractory period
• Occurs most rapidly in myelinated, largediameter axons
Propagation of Action Potentials
• In an unmyelinated axon, action potentials are generated
immediately adjacent to previous action potentials
• In a myelinated axon, action potentials are generated at
successive Nodes of Ranvier
Fig. 10.21
The Synapse
• The synapse is the junction between two
cells where communication takes place
– Presynaptic cell: transmits signal towards a
synapse
– Postsynaptic cell: receives the signal
• Two types of synapses
– Electrical synapse
– Chemical synapse
Electrical Synapses
• Gap junctions in which tubular proteins called
connexons allow ionic currents to move between
cells
• An action potential in one cell generates an ionic
current that causes an action potential in an
adjacent cell
• Action potentials are conducted rapidly between
cells allowing for synchronized activity
• Common in cardiac muscle and in many types of
smooth muscle where coordinated contractions
are essential
Chemical Synapses
• Have three anatomical
components
– The enlarged ends of the
axon are the presynaptic
terminals containing
synaptic vesicles
– The postsynaptic
membranes contain
receptors for the
neurotransmitter
– The synaptic cleft, a space,
separates the presynaptic
and postsynaptic
membrane
Fig. 10.22
Chemical Synapse Activity
1. Action potentials arriving at the
presynaptic terminal cause voltagegated Ca2+ channels to open
2. Calcium ions diffuse into the cell
and cause synaptic vesicles to
release neurotransmitters
3. Neurotransmitters diffuse from the
presynaptic terminal across the
synaptic cleft
4. Neurotransmitters combine with
their receptor sites and cause
ligand-gated ion channels to open.
Ions diffuse into the cell (shown) or
out of the cell (not shown) and
cause a change in membrane
potential
Fig. 10.22
Chemical Synapse Activity
• The effect of the neurotransmitter on the
postsynaptic membrane is stopped in two ways
– The neurotransmitter is broken down by an enzyme
– The neurotransmitter is taken up by the presynaptic
terminal
• A list of neurotransmitters and neuromodulators is
presented in Table 10.4
– Neuromodulators are substances released from
neurons that can presynaptically or postsynaptically
influence the likelihood that an action potential will be
generated
Chemical Synapse Activity
• Excitatory and inhibitory postsynaptic potentials
– An excitatory postsynaptic potential (EPSP) is a
depolarizing graded potential of the postsynaptic
membrane
– An inhibitory postsynaptic potential (IPSP) is a
hyperpolarizing graded potential of the postsynaptic
membrane
– Presynaptic inhibition decreases neurotransmitter
release
– Presynaptic facilitation increases neurotransmitter
release
Spatial and Temporal Summation
• Presynaptic action potentials through
neurotransmitters produce graded potentials in
postsynaptic neurons. The graded potential can
summate to produce an action potential at the
trigger zone
– Spatial summation occurs when two or more presynaptic
terminals simultaneously stimulate a postsynaptic neuron
– Temporal summation occurs when two or more action
potentials arrive in succession at a single presynaptic
terminal
• Inhibitory and excitatory presynaptic neurons can
converge on a postsynaptic neuron
– An action potential is produced at the trigger zone when
the graded potential is produced as a result of the sum of
the EPSPs and IPSPs reaching threshold
Neuronal Pathways and Circuits
• Convergent pathways have many
neurons synapsing with a few neurons
• Divergent pathways have a few
neurons synapsing with many neurons
• Oscillating circuits have collateral
branches of postsynaptic neurons
synapsing with presynaptic neurons