Transcript Human Anatomy & Physiology I Overview of the Nervous System Chapter 10

Human Anatomy & Physiology I
Overview of the Nervous System
Chapter 10
Organization of the Nervous System
Central nervous system - CNS
Brain and Spinal Cord (in dorsal body cavity)
Integration and command center – interprets
sensory input and responds to input
Peripheral nervous system - PNS
Paired Spinal and Cranial nerves
Carries messages to and from the spinal cord
and brain – links parts of the body to the CNS
Divisions of the Nervous System
• Central Nervous System
• brain
• spinal cord
• Peripheral Nervous System
• peripheral nerves
• cranial nerves
• spinal nerves
Nervous System
Functions:
Sensory Input – monitoring stimuli occurring inside
and outside the body
Integration – interpretation of sensory input
Motor Output – response to stimuli by activating
effector organs
Divisions Nervous System
Levels of Organization in the
Nervous System
Divisions of Peripheral Nervous System
Sensory Division
• picks up sensory information and delivers it to the CNS
Motor Division
• carries information to muscles and glands
Divisions of the Motor Division
• Somatic – carries information to skeletal muscle
• Autonomic – carries information to smooth muscle,
cardiac muscle, and glands
Functions of Nervous System
Sensory Function
• sensory receptors gather
information
• information is carried to the
CNS
Integrative Function
• sensory information used to
create
• sensations
• memory
• thoughts
• decisions
Motor Function
• decisions are acted
upon
• impulses are
carried to effectors
PNS - Two Functional Divisions
Sensory (afferent) Division
Somatic afferent nerves – carry impulses from
skin, skeletal muscles, and joints to the CNS
Visceral afferent nerves – transmit impulses
from visceral organs to the CNS
Motor (efferent) Division
Transmits impulses from the CNS to effector
organs, muscles and glands, to effect (bring
about) a motor response
Classification of Neurons
Sensory Neurons
• afferent
• carry impulse to CNS
• most are unipolar
• some are bipolar
Interneurons
• link neurons
• multipolar
• in CNS
Motor Neurons
• multipolar
• carry impulses away
from CNS
• carry impulses to
effectors
Motor Division: two subdivisions
Somatic Nervous System (voluntary)
Somatic motor nerve fibers (axons) that conduct
impulses from CNS to Skeletal muscles –
allows conscious control of skeletal muscles
Autonomic Nervous System (ANS) (involuntary)
Visceral motor nerve fibers that regulate smooth
muscle, cardiac muscle, and glands
Two functional divisions – sympathetic and
parasympathetic
Levels of Organization in the Nervous System
Histology of Nerve Tissue
Two principal cell types in the nervous system:
Neurons – excitable nerve cells that transmit
electrical signals
Supporting cells – cells adjacent to neurons or
cells that surround and wrap around neurons
Cell Types of Neural Tissue
• neurons
• neuroglial cells
Neurons (Nerve Cells)
Highly specialized, structural units of the nervous
system – conduct messages (nerve impulses) from
one part of the body to another
Long life, mostly amitotic, with a high metabolic
rate (cannot survive more than a few minutes
without O2)
Structure is variable, but all have a neuron cell body
and one or more cell projections called processes.
Generalized Neuron
Neuron Structure
Nerve Cell Body (Perikaryon or Soma)
Contains the nucleus and a nucleolus
The major biosynthetic center
Has no centrioles
Has well-developed Nissl bodies (rough
ER)
Axon hillock – cone-shaped area where
axons arise
Clusters of cell bodies are called Nuclei in the CNS
and Ganglia in the PNS
Processes
Extensions from the nerve cell body. The CNS
contains both neuron cell bodies and their processes.
The PNS consists mainly of neuron processes.
Two types: Axons and Dendrites
Bundles of neuron processes are called
Tracts in the CNS and Nerves in the PNS
Dendrites
Short, tapering, diffusely branched processes
The main receptive, or input regions of the neuron
(provide a large surface area for receiving signals
from other neurons)
Dendrites convey incoming
messages toward the cell body
These electrical signals are not
nerve impulses (not action
potentials), but are short distance
signals called graded potentials
Axons
Slender processes with a uniform diameter arising
from the axon hillock, only one axon per neuron
A long axon is called a nerve fiber, any branches are
called axon collaterals
Terminal branches – distal ends are called the axon
terminus (also synaptic knob or bouton)
Axons: Function
Generate and transmit action potentials (nerve impulses),
typically away from the cell body
As impulse reaches the axon terminals, it causes
neurotransmitters to be released from the axon terminals
Movement of substances along axons:
Anterograde - toward axonal terminal (mitochondria,
cytoskeletal, or membrane components)
Retrograde - away from axonal terminal (organelles for
recycling)
Anterograde →
←Retrograde
Myelin Sheath
Whitish, fatty (protein-lipoid), segmented sheath
around most long axons – dendrites are unmyelinated
•Protects the axon
•Electrically insulates fibers from one another
•Increases the speed of nerve impulse transmission
Myelin Sheath
Formed by Schwann cells in the PNS
A Schwann cell envelopes
and encloses the axon with
its plasma membrane.
The concentric layers of
membrane wrapped
around the axon are the
myelin sheath
Neurilemma – cytoplasm
and exposed membrane of
a Schwann cell
Nodes of Ranvier (Neurofibral Nodes)
Gaps in the myelin sheath between adjacent Schwann
cells
They are the sites where axon collaterals can emerge
Myelination of Axons
White Matter
• contains myelinated
axons
Gray Matter
• contains
unmyelinated
structures
• cell bodies, dendrites
Axons of the CNS
Both myelinated and unmyelinated fibers are present
Myelin sheaths are formed by oligodendrocytes
Nodes of Ranvier are more widely spaced
There is no neurilemma (cell extensions are coiled
around axons)
White matter – dense collections of myelinated fibers
Gray matter – mostly soma and unmyelinated fibers
Classification of Neurons
Bipolar
• two processes
• eyes, ears, nose
Unipolar
• one process
• ganglia
Multipolar
• many processes
• most neurons of
CNS
Classification of Neurons
Structural
Multipolar — three or more processes
Bipolar — two processes (axon and dendrite)
Unipolar — single, short process
Neuron Classification
Functional
Sensory (afferent) – transmit impulses toward the CNS
Motor (efferent) – carry impulses away from the CNS
Interneurons (association neurons) – lie between
sensory and motor pathways and shuttle signals
through CNS pathways
Supporting Cells: Neuroglia
Six types of Supporting Cells - neuroglia or glial
cells – 4 in CNS and 2 in the PNS
Each has a specific function, but generally they:
Provide a supportive scaffold for neurons
Segregate and insulate neurons
Produce chemicals that guide young neurons
to the proper connections
Promote health and growth
Types of Neuroglial Cells
Schwann Cells
• peripheral nervous
system
• myelinating cell
Oligodendrocytes
• CNS
• myelinating cell
Microglia
• CNS
• phagocytic cell
Astrocytes
• CNS
• scar tissue
• mop up excess ions, etc
• induce synapse formation
• connect neurons to blood
vessels
Ependyma
• CNS
• ciliated
• line central canal of spinal cord
• line ventricles of brain
Supporting Cells: Neuroglia
Neuroglia in the CNS
Neuroglia in the PNS
Astrocytes
Satellite Cells
Microglia
Schwann Cells
Ependymal Cells
Oligodendrocytes
Outnumber neurons in the CNS by 10 to 1, about
½ the brain’s mass.
Types of Neuroglial Cells
Astrocytes
Most abundant, versatile, highly branched glial cells
Cling to neurons, synaptic endings, and cover nearby
capillaries
Support and brace neurons
Anchor neurons to nutrient
supplies
Guide migration of young neurons
Aid in synapse formation
Control the chemical environment (recapture K+ ions
and neurotransmitters)
Microglia
Microglia – small, ovoid cells with long spiny
processes that contact nearby neurons
When microorganisms or dead neurons are
present, they can transform into phagocytic cells
Ependymal Cells
Ependymal cells – range in shape from squamous to
columnar, many are ciliated
Line the central cavities of the brain and spinal
column
Oligodendrocytes
Oligodendrocytes – branched cells that line the thicker
CNS nerve fibers and wrap around them, producing
an insulating covering – the Myelin sheath
Schwann Cells and Satellite Cells
Schwann cells - surround fibers of the PNS and form
insulating myelin sheaths
Satellite cells - surround neuron cell bodies within
ganglia
Regeneration of A Nerve Axon
Neurophysiology
Neurons are highly irritable (responsive to stimuli)
Action potentials, or nerve impulses, are:
Electrical impulses conducted along the length
of axons
Always the same regardless of stimulus
The underlying functional feature of the
nervous system
Definitions
Voltage (V) – measure of potential energy between
two points generated by a charge separation
(Voltage = Potential Difference = Potential)
Current (I) – the flow of electrical charge
Resistance (R) – tendency to oppose the current
Units: V (volt), I (ampere), R (ohm)
Insulator – substance with high electrical resistance
Conductor – substance with low electrical resistance
Ohm’s Law
The relationship between voltage, current, and
resistance is defined by Ohm’s Law
Voltage (V)
Current (I) =
Resistance (R)
In the body, electrical current is the flow of ions
(rather than free electrons) across membranes
A Potential Difference exists when there is a
difference in the numbers of + and – ions on either
side of the membrane
Membrane Ion Channels
Types of plasma membrane ion channels
Passive, or leakage, channels – always open
Chemically (or ligand)-gated channels – open
with binding of a specific neurotransmitter (the
ligand)
Voltage-gated channels – open and close in
response to changes in the membrane potential
Mechanically-gated channels – open and close in
response to physical deformation of receptors
Ligand-Gated Channel
Example: Na+-K+ gated channel
Closed when a neurotransmitter is not bound to the
extracellular receptor
Open when a neurotransmitter is attached to the receptor Na+ enters the cell and K+ exits the cell
Voltage-Gated Channel
•Example: Na+ channel
•Closed when the intracellular environment is negative
•Open when the intracellular environment is positive Na+ can enter the cell
Electrochemical Gradient
Ions flow along their chemical gradient when they
move from an area of high concentration to an area
of low concentration
Ions flow along their electrical gradient when they
move toward an area of opposite charge
Together, the electrical and chemical gradients
constitute the ELECTROCHEMICAL GRADIENT
Ion Channels
When gated ion channels open, ions diffuse across the
membrane following their electrochemical gradients.
This movement of charge is an electrical current and
can create voltage change across the membrane.
Voltage (V) = Current (I) x Resistance (R)
Ion movement (flow) along electrochemical
gradients underlies all the electrical phenomena in
neurons.
Resting Membrane Potential
A potential (-70mV) exists across the membrane of
a resting neuron – the membrane is polarized
Resting Membrane Potential
• inside is negative relative to
the outside
• polarized membrane
• due to distribution of ions
• Na+/K+ pump
Resting Membrane Potential
Ionic differences are the consequence of:
•Different membrane permeabilities due to passive
ion channels for Na+, K+, and Cl•Operation of the sodium-potassium pump
Membrane Potentials: Signals
Neurons use changes in membrane potential to
receive, integrate, and send information
Membrane potential changes are produced by:
•Changes in membrane permeability to ions
•Alterations of ion concentrations across the membrane
Two types of signals are produced by a change in
membrane potential:
•graded potentials (short-distance)
•action potentials (long-distance)
Levels of Polarization
•Depolarization – inside of the membrane becomes
less negative (or even reverses) – a reduction in
potential
•Repolarization – the membrane returns to its
resting membrane potential
•Hyperpolarization – inside of the membrane
becomes more negative than the resting potential –
an increase in potential
Depolarization increases the probability of producing
nerve impulses. Hyperpolarization reduces the
probability of producing nerve impulses.
Changes in Membrane Potential
Graded Potentials
Short-lived, local changes in membrane potential
(either depolarizations or hyperpolarizations)
Cause currents that decreases in magnitude with
distance
Their magnitude varies directly with the strength of
the stimulus – the stronger the stimulus the more the
voltage changes and the farther the current goes
Sufficiently strong graded potentials can initiate
action potentials
Graded Potentials
Voltage changes in graded
potentials are decremental,
the charge is quickly lost
through the permeable
plasma membrane
short- distance signal
Action Potentials (APs)
An action potential in the axon of a neuron is called a
nerve impulse and is the way neurons communicate.
The AP is a brief reversal of membrane potential with
a total amplitude of 100 mV (from -70mV to +30mV)
APs do not decrease in strength with distance
The depolarization phase is followed by a
repolarization phase and often a short period of
hyperpolarization
Events of AP generation and transmission are the
same for skeletal muscle cells and neurons
Action Potential: Resting State
Na+ and K+ channels are closed
Each Na+ channel has two voltage-regulated
gates
Activation gates –
closed in the resting
state
Inactivation gates –
open in the resting
state
Depolarization opens the activation gate (rapid)
and closes the inactivation gate (slower) The gate
for the K+ is slowly opened with depolarization.
Depolarization Phase
Na+ activation gates open quickly and Na+ enters
causing local depolarization which opens more
activation gates and cell interior becomes
progressively less negative. Rapid depolarization and
polarity reversal.
Threshold – a critical level of depolarization
(-55 to -50 mV) where
depolarization becomes
self-generating
Positive Feedback?
Repolarization Phase
Positive intracellular charge opposes further Na+ entry.
Sodium inactivation gates of Na+ channels close.
As sodium gates close, the slow voltage-sensitive K+
gates open and K+ leaves the cell following its
electrochemical gradient and the internal negativity of
the neuron is restored
Hyperpolarization
The slow K+ gates remain open longer than is needed
to restore the resting state. This excessive efflux causes
hyperpolarization of the membrane
The neuron is
insensitive to
stimulus and
depolarization
during this time
Role of the SodiumPotassium Pump
Repolarization restores the resting electrical
conditions of the neuron, but does not restore the
resting ionic conditions
Ionic redistribution is accomplished by the
sodium-potassium pump following repolarization
Potential Changes
• at rest
membrane is
polarized
• threshold
stimulus reached
• sodium
channels open
and membrane
depolarizes
• potassium leaves
cytoplasm and
membrane
repolarizes
Phases of the Action Potential
Impulse Conduction
Action Potentials
Propagation of an Action
Potential
The action potential is self-propagating and
moves away from the stimulus (point of
origin)
Stimulus Intensity
All action potentials are alike and are independent
of stimulus intensity
How can CNS determine if a stimulus intense or
weak?
Strong stimuli can generate an action potential
more often than weaker stimuli and the CNS
determines stimulus intensity by the frequency of
impulse transmission
Threshold and Action Potentials
Threshold Voltage– membrane is depolarized by 15
to 20 mV
Subthreshold stimuli produce subthreshold
depolarizations and are not translated into APs
Stronger threshold stimuli produce depolarizing
currents that are translated into action potentials
All-or-None phenomenon – action potentials
either happen completely, or not at all
Stimulus Strength and AP
Frequency
Absolute Refractory Period
When a section of membrane is generating an AP and
Na+ channels are open, the neuron cannot respond to
another stimulus
The absolute refractory period is the time from
the opening of the Na+ activation gates until the
closing of inactivation gates
Relative Refractory Period
The relative refractory period is the interval
following the absolute refractory period when:
Na+ gates are closed
K+ gates are open
Repolarization is occurring
During this period, the threshold level is elevated,
allowing only strong stimuli to generate an AP
(a strong stimulus can cause more frequent AP
generation)
Refractory Periods
Axon Conduction Velocities
Conduction velocities vary widely among neurons
Determined mainly by:
Axon Diameter – the larger the diameter, the faster
the impulse (less resistance)
Presence of a Myelin Sheath – myelination
increases impulse speed (Continuous vs. Saltatory
Conduction)
Saltatory Conduction
Current passes through a myelinated axon only at
the nodes of Ranvier
Voltage-gated Na+ channels are concentrated at
these nodes
Action potentials are triggered only at the nodes
and jump from one node to the next
Much faster than conduction along unmyelinated
axons
Saltatory Conduction
Saltatory Conduction
Current passes through a myelinated axon only at the
nodes of Ranvier (Na+ channels concentrated at nodes)
Action potentials occur only at the nodes and jump
from node to node
Synapse
A junction that mediates information transfer from
one neuron to another neuron or to an effector cell
Presynaptic neuron – conducts impulses toward
the synapse (sender)
Postsynaptic neuron – transmits impulses away
from the synapse (receiver)
Types of Synapses
Axodendritic – synapse between the axon of one
neuron and the dendrite of another
Axosomatic – synapse between the axon of one
neuron and the soma of another
Other types:
Axoaxonic (axon to axon)
Dendrodendritic (dendrite to dendrite)
Dendrosomatic (dendrites to soma)
Synapses
Electrical Synapses
Less common than chemical synapses
Gap junctions allow neurons to be electrically
coupled as ions can flow directly from neuron to
neuron - provide a means to synchronize activity of
neurons
Are important in the CNS in:
Arousal from sleep
Mental attention and conscious perception
Emotions and memory
Ion and water homeostasis
Abundant in embryonic nervous tissue
Chemical Synapses
Specialized for the release and reception of chemical
neurotransmitters
Typically composed of two parts:
Axon terminal of the
presynaptic neuron containing
membrane-bound synaptic
vesicles
Receptor region on the
dendrite(s) or soma of the
postsynaptic neuron
Synaptic Cleft
Fluid-filled space separating the presynaptic and
postsynaptic neurons, prevents nerve impulses from
directly passing from one neuron to the next
Transmission across the synaptic cleft:
Is a chemical event (as opposed to an electrical
one)
Ensures unidirectional communication between
neurons
Synaptic Cleft: Information
Transfer
Nerve impulses reach the axon terminal of the
presynaptic neuron and open Ca2+ channels
Neurotransmitter is released into the synaptic cleft via
exocytosis
Neurotransmitter crosses the synaptic cleft and binds
to receptors on the postsynaptic neuron
Postsynaptic membrane permeability changes due to
opening of ion channels, causing an excitatory or
inhibitory effect
Synaptic Cleft: Information
Transfer
Termination of Neurotransmitter Effects
Neurotransmitter bound to a postsynaptic neuron
produces a continuous postsynaptic effect and also
blocks reception of additional “messages”
Terminating Mechanisms:
Degradation by enzymes
Uptake by astrocytes or the presynaptic
terminals
Diffusion away from the synaptic cleft
Synaptic Delay
Neurotransmitter must be released, diffuse across
the synapse, and bind to receptors (0.3-5.0 ms)
Synaptic delay is the rate-limiting step of neural
transmission
Postsynaptic Potentials
Neurotransmitter receptors mediate graded changes
in membrane potential according to:
The amount of neurotransmitter released
The amount of time the neurotransmitter is
bound to receptors
The two types of postsynaptic potentials are:
EPSP – excitatory postsynaptic potentials
IPSP – inhibitory postsynaptic potentials
Excitatory Postsynaptic
Potentials
EPSPs are local graded depolarization events that
can initiate an action potential in an axon
Na+ and K+ flow in opposite directions at the
same time
Postsynaptic membranes do not generate action
potentials. The currents created by EPSPs
decline with distance, but can spread to the axon
hillock and depolarize the axon to threshold
leading to an action potential
Inhibitory Postsynaptic
Potentials
Neurotransmitter binding to a receptor at inhibitory
synapses reduces a postsynaptic neuron’s ability to
generate an action potential
Postsynaptic membrane is hyperpolarized due to
increased permeability to K+ and/or Cl- ions. Na+
permeability is not affected.
Leaves the charge on the inner membrane face
more negative and the neuron becomes less likely
to “fire”.
EPSPs and IPSPs
Summation
A single EPSP cannot induce an action potential
EPSPs must summate (add together) to induce an AP
Temporal Summation – presynaptic neurons
transmit impulses in quick succession
Spatial Summation – postsynaptic neuron is
stimulated by a large number of terminals at the
same time
IPSPs also summate and can summate with EPSPs.
Summation
Neurotransmitters
Chemicals used for neuron communication with
the body and the brain
More than 50 different neurotransmitters have
been identified
Classified chemically and functionally
Neurotransmitters
Neurotransmitters – Chemical
classification
•Acetylcholine (ACh)
•Biogenic amines
•Amino acids
•Peptides
•Novel messengers: ATP and dissolved gases
NO and CO
Neurotransmitters: Acetylcholine
Released at the neuromuscular junction
Enclosed in synaptic vesicles
Degraded by the acetylcholinesterase (AChE)
Released by:
– All neurons that stimulate skeletal muscle
– Some neurons in the autonomic nervous
system
Neurotransmitters: Biogenic
Amines
Include:
– Catecholamines – dopamine,
norepinephrine, and epinephrine
– Indolamines – serotonin and histamine
Broadly distributed in the brain
Play roles in emotional behaviors and our
biological clock
Synthesis of Catecholamines
Enzymes present in
the cell determine
length of biosynthetic
pathway
Norepinephrine and
dopamine are
synthesized in axon
terminals
Epinephrine is
released by the
adrenal medulla
Neurotransmitters: Amino Acids
Include:
– GABA – Gamma ()-aminobutyric
acid
– Glycine
– Aspartate
– Glutamate
Found only in the CNS
Neurotransmitters: Peptides
Include:
– Substance P – mediator of pain signals
– Beta endorphin, dynorphin, and
enkephalins
Act as natural opiates, reducing our perception
of pain
Bind to the same receptors as opiates and
morphine
Gut-brain peptides – somatostatin and
cholecystokinin (produced by non-neural
tissue and widespread in GI tract)
Neurotransmitters: Novel
Messengers
ATP
– Is found in both the CNS and PNS
– Produces excitatory or inhibitory responses
depending on receptor type
– Induces Ca2+ wave propagation in astrocytes
– Provokes pain sensation
Nitric oxide (NO)
– Activates the intracellular receptor guanylyl
cyclase
– Is involved in learning and memory
Carbon monoxide (CO) is a main regulator of cGMP in
the brain
Functional Classification of
Neurotransmitters
Two classifications: excitatory and inhibitory
– Excitatory neurotransmitters cause
depolarizations
(e.g., glutamate)
– Inhibitory neurotransmitters cause
hyperpolarizations (e.g., GABA and glycine)
Some neurotransmitters have both excitatory and
inhibitory effects (determined by the receptor type of
the postsynaptic neuron). ACh is excitatory at
neuromuscular junctions with skeletal muscle and
Inhibitory in cardiac muscle.
Neurotransmitter Receptor
Mechanisms
Direct: neurotransmitters that open ion
channels
– Promote rapid responses
– Examples: ACh and amino acids
Indirect: neurotransmitters that act
through second messengers
– Promote long-lasting effects
– Examples: biogenic amines, peptides, and
dissolved gases
Channel-Linked Receptors (ligandgated ion channel)
Mediate direct neurotransmitter action, action is
immediate, brief, and highly localized
•Ligand binds to the receptor and ions enter the cells
•Excitatory receptors depolarize membranes
•Inhibitory receptors hyperpolarize membranes
G Protein-Linked Receptors
Responses are indirect, slow, complex,
prolonged, and often diffuse
These receptors are transmembrane
protein complexes
Examples: muscarinic ACh receptors,
neuropeptides, and those that bind
biogenic amines
G Protein-Linked Receptors:
Mechanism
Neurotransmitter binds to G protein-linked
receptor
G protein is activated and GTP is hydrolyzed to
GDP
The activated G protein complex activates
adenylate cyclase
Adenylate cyclase catalyzes the formation of
cAMP from ATP
cAMP, a second messenger, brings about
various cellular responses
G Protein-Linked Receptors:
Mechanism
G Protein-Linked Receptors:
Effects
G protein-linked receptors activate intracellular
second messengers including Ca2+, cGMP,
diacylglycerol, as well as cAMP
Second messengers:
– Open or close ion channels
– Activate kinase enzymes
– Phosphorylate channel proteins
– Activate genes and induce protein
synthesis
Neural Integration: Neuronal Pools
Functional groups of neurons that:
Integrate incoming information received from
receptors or other neuronal pools
Forward the processed information to its
appropriate destination
Simple neuronal pool
Input fiber – presynaptic fiber
Discharge zone – neurons most closely associated with
the incoming fiber
Facilitated zone – neurons farther away from
incoming fiber
Simple Neuronal Pool
Types of Circuits in Neuronal Pools
Divergent – one incoming fiber stimulates ever increasing number
of fibers. These circuits are often amplifying circuits. (an impulse
from a single brain neuron can activate 100 or more motor neurons
in the spinal cord and → 1000s of skeletal muscle fibers)
Divergence
• one neuron sends
impulses to several
neurons
• can amplify an
impulse
• impulse from a
single neuron in
CNS may be
amplified to
activate enough
motor units
needed for muscle
contraction
Types of Circuits in Neuronal Pools
Convergent – opposite of
divergent circuits,
resulting in either strong
stimulation or inhibition
Convergence
• neuron receives input from
several neurons
• incoming impulses represent
information from different
types of sensory receptors
• allows nervous system to
collect, process, and respond
to information
• makes it possible for a
neuron to sum impulses from
different sources
Types of Circuits in Neuronal Pools
Reverberating or oscillating– chain of neurons
containing collateral synapses with previous
neurons in the chain. Involved in the control of
rhythmic activities (sleep-wake cycle, breathing)
Types of Circuits in Neuronal Pools
•Parallel after-Discharge – incoming neurons
stimulate several neurons in parallel arrays
Clinical Application
Multiple Sclerosis
Symptoms
• blurred vision
• numb legs or arms
• can lead to paralysis
Treatments
• no cure
• bone marrow transplant
• interferon (anti-viral drug)
• hormones
Causes
• myelin destroyed in
various parts of CNS
• hard scars
(scleroses) form
• nerve impulses
blocked
• muscles do not
receive innervation
• may be related to a
virus