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Chapter 11
Fundamentals of the Nervous System and Nervous Tissue
J.F. Thompson, Ph.D. & J.R. Schiller, Ph.D. & G. Pitts, Ph.D.
The Nervous System
The Nervous System is the rapid control system of the body
There are two anatomical divisions to the Nervous System: The Central Nervous System (CNS) The Peripheral Nervous System (PNS) They work together as a single coordinated
whole
The Functions of the Nervous System There are three
interconnected functions: sensory input
from millions of specialized receptors
receive stimuli
integration process stimuli interpret
stimuli
motor output cause response at many
effector organs
Organization of the Central Nervous System
the Brain and Spinal Cord
process & integrate information, store information, determine emotions
initiate commands for muscle contraction, glandular secretion and hormone release (regulate and maintain homeostasis)
connected to all other parts of the body by the Peripheral Nervous System (PNS)
Organization of the Peripheral NS anatomical connections
spinal nerves are connected to the spinal cord
cranial nerves are connected to the brain
two functional subdivisions sensory (afferent) division
somatic afferents - skin, skeletal muscle, tendons, joints
special sensory afferents visceral afferents - visceral
organs motor (efferent) division
motor (efferent) neurons muscles/glands
Organization of the PNS (continued) motor (efferent) division
has two parts: Somatic Nervous System
(SNS) voluntary motor neurons output to skeletal muscles
Autonomic Nervous System (ANS) involuntary visceral motor
neurons output to smooth muscle,
cardiac muscles and to glands
two cooperative components:• sympathetic division sympathetic division • parasympathetic divisionparasympathetic division
Autonomic Nervous System Sympathetic
Division – for muscular exertion and for “fight or flight” emergencies
Parasympathetic Division – for metabolic/ physiologic “business as usual” (“feed or breed”)
Nervous Tissue
Review the microanatomy of nervous tissue in Review the microanatomy of nervous tissue in lab and in the PPT with audio: CH11 Histology lab and in the PPT with audio: CH11 Histology of Nervous Tissueof Nervous Tissue
Nerve cell physiology is primarily a cell Nerve cell physiology is primarily a cell membrane phenomenonmembrane phenomenon
Information transmission differs between Information transmission differs between dendrites and axonsdendrites and axons
Neuron Processes - Dendrites
short, tapering, highly branched extensions of the soma
not myelinated contain some cell organelles receptive—initiate and transmit graded potentials (not
action potentials) to the cell body
Neuron Processes - Axons A single process that
transmits action potentials from the soma
Originates from a cone-shaped “axon hillock”
May be long (1 meter) or short (<1 mm) long axons called nerve fibers
Up to 10,000 terminal branches each with an axon terminal
that synapses (joins) with a neuron or an effector (muscle or gland cell)
Axons (continued) Axoplasm: the cytoplasm of the axon Axolemma: the cell membrane of the
axon, specialized to initiate and conduct action potentials (nerve impulses) initiated at the axon hillock (trigger zone),
travels to the axon terminal causes release of neurotransmitter from
terminal neurotransmitters can excite or inhibit transfers a control message to other
neurons or effector cells
Histology of Neurons – Myelin Sheath
lipid-rich, segmented covering on axons
most larger, longer axons are myelinated
dendrites are never myelinated myelin protects & electrically
insulates the axon increases the speed of nerve
impulses
myelinated fibers conduct impulses 10-150x faster than unmyelinated fibers
150 m/sec vs. 1 m/sec
Myelinating Cells neurolemmocytes (Schwann
cells) in the Peripheral NS
oligodendrocytes in the Central NS
Myelination occurs during fetal
development and the first year of life
each myelinating cell wraps around an axon up to 100 times, squeezing its cytoplasm and organelles to the periphery myelin sheath: multiple layers of
the cell membrane neurolemma (sheath of
Schwann): outer layer containing the bulk of the cytoplasm and cell organelles
Myelinated and Unmyelinated Axons Myelinated Fibers
Myelin sheath neurofibril nodes
(Nodes of Ranvier) periodic gaps in the myelin sheath between the neurolemmocytes
Unmyelinated Fibers surrounded by
neurolemmocytes but no myelin sheath present
neurolemmocytes may enclose up to 15 axons (unmyelinated fibers)
neurolemmocytes guide regrowth of neuron processes after injury
Myelination In the Central NS Gray matter - unmyelinated cell bodies & processes White matter – myelinated processes in various fiber
tracts
Classification of Neurons
Structural: based on the number of processes extending from the cell body
Functional: based on the direction (location) of nerve impulses
We will focus on functional classification
Afferent (= Sensory) Neurons afferent = towards CNS
nerve impulses from specific sensory receptors (touch, sight, etc.) are transmitted to the spinal cord or brain (CNS)
afferent neuron cell bodies are located outside the CNS in ganglia
Efferent (= Motor) Neurons efferent = away from
CNS
nerve impulses from CNS (brain and spinal cord) are transmitted to effectors (muscles, endocrine and exocrine glands)
efferent neuron cell bodies are located inside the CNS
Association Neurons (= Interneurons)
carry nerve impulses from one neuron to another
99% of the neurons in the body are interneurons
most interneurons are located in the CNS
Neurophysiology - Definitions voltage
the measure of potential energy generated by separated charges
always measured between two points – the inside versus the outside of the cell
referred to as a potential - since the charges (ions) are separated there is a potential for the charges (ions) to move along the charge gradient
Neurophysiology - Definitions current
the flow of electrical charge from one point to another
in the body, current is due to the movement of charged ions
resistance the prevention of the movement of charges
(ions) caused by the structures (membranes)
through which the charges (ions) have to flow
Neurophysiology - Basics Cell interior and exterior have different chemical
compositions Na+/K+ ATPase pumps change the ion concentrations a semi-permeable membrane allows for separation of
ions
Ions attempt to reach electrochemical equilibrium two forces power the movement of ions
individual ion concentrations (chemical gradients) net electrical charge (overall charge gradient)
the balance between concentration (chemical) gradients and the electrical gradient known as the electrochemical equilibrium
the external voltage required to balance the concentration gradient is the equilibrium (voltage) potential
Neurophysiology - Membrane Ion Channels regulate ion
movements across cell membrane
each is specific for a particular ion or ions
many different types may be passive (leaky) may be active (gated)
gate status is controlled gated channels are
regulated by signal chemicals or by other changes in the membrane potential (voltage potential)
Resting Membrane Potential (RMP) electrical charge gradient
associated with outer cell membrane
present in all living cells the cytoplasm within the
cell membrane is negatively charged due to the charge disequilibrium concentrations of cations and anions on either side of the membrane
RMP varies from about -40 to -90 millivolts (a net negative charge inside relative to a net positive charge outside the cell)
Resting Membrane Potential (cont.) RMP is similar to a battery
stores an electrical charge and can release the charge 2 main reasons for this:
ion concentrations on either side of the plasma membrane are due to the action of the Na+/K+ ATPase pumps primarily, Na+ and Cl- are outside; the membrane is polarized primarily, K+, Cl-, proteins- and organic phosphates- are inside
plasma membrane has limited permeability to Na+ and K+ ions
Resting Membrane Potential (cont.) Resting conditions
Na+/K+ ATPase pumps 3 Na+ ions out and 2 K+
ions in per ATP hydrolysis – opposing their concentration gradients concentration gradient drives Na+ to go into the cell concentration gradient drives K+ to go out of the cell
if the cell membrane were permeable to Na+
and K+ ions, then Na+ and K+ ions would diffuse along their electrical and chemical gradients and would reach equilibrium
if the cell was at equilibrium in terms of ion concentrations and charge, their would be no potential energy available for impulse transmission
Resting Membrane Potential (cont.) Neuron Membrane at rest is polarized
the cytoplasm inside is negatively charged relative to the outside
the net negative charge in the cytoplasm attracts all cations to the inside some Na+ leaks in, despite limited membrane
permeability Na+-K+ ATPase keeps working to pump 3 Na+
ions out and 2 K+ ions in, opposing the two concentration gradients (for Na+ and K+)
Resting Membrane Potential (cont.)
Here is the electrochemical gradient at rest: the resting potential
Membrane Potentials As Signals cells use changes in
membrane potential (voltage) to exchange information voltage changes occur by
two means:1. changing the membrane
permeability to an ion; or2. changing the ion
concentration on either side of the membrane
these changes are made by ion channels passive channels – leaky: K+ active channels:
• chemically gated – by neurotransmitters
• voltage gated
Types of Membrane Potentials graded potentials
graded = different levels of strength
dependent on strength of the stimulus
action potentials in response to
graded potentials of significant strength
signal over long distances
all or nothing
Types of Membrane Potentials
graded potentials and action potentials may be either: hyperpolarizing
increasing membrane polarity
making the inside more negative
depolarizing decreasing membrane
polarity making the inside less
negative = more positive
Properties of Action Potentials a nerve impulse (action potential) is
generated in response to a threshold graded potential
depolarization change in the membrane polarization stimuli reach a threshold limit and open
voltage-gated Na+ channels Na+ ions rush into the cell down the Na+
concentration and electrical gradients the cytoplasm inside the cell becomes positive reverses membrane potential to +30 mV
local anesthetics prevent opening of voltage-gated Na+ channels - prevent depolarization
Sequence of Events in Action Potentials
1. Resting membrane potential
Sequence of Events in Action Potentials
2. Depolarizationa) stimulus
strength reaches threshold limit
b) voltage gated Na+ channels open
c) Na+ flows into the cytoplasm
d) More V-gated Na+ channels open
[positive feedback]
Sequence of Events in Action Potentials3. Repolarization
a) voltage gated K+ channels open
b) voltage gated Na+ channels close
Sequence of Events in Action Potentials
4. Hyperpolarization
a)gated Na+ channels are reset to closed
b)membrane remains hyperpolarized until K+ channels close, causing the relative refractory period
Repeat the process:
Sequence of Events in Action Potentials
1. Resting membrane potential
Sequence of Events in Action Potentials
2. Depolarizationa) stimulus
strength reaches threshold limit
b) voltage gated Na+ channels open
c) Na+ flows into the cytoplasm
d) More V-gated Na+ channels open
[positive feedback]
Sequence of Events in Action Potentials3. Repolarization
a) voltage gated K+ channels open
b) voltage gated Na+ channels close
Sequence of Events in Action Potentials
4. Hyperpolarization
a)gated Na+ channels are reset to closed
b)membrane remains hyperpolarized until K+ channels close, causing the relative refractory period
The All-or-None Principle
stimuli/neurotransmitters arrive and open some of the chemically-gated Na+ channels
if stimuli reach the threshold level depolarization occurs voltage-gated Na+ channels open an Action Potential is generated which is constant
and at maximum strength
if stimuli do not reach the threshold level nothing happens
Repolarization Re-establishing the resting membrane
polarization state threshold depolarization opens Na+ channels
Na+ ions flow inward, making the cell interior more positive a few milliseconds later, K+ channels also open
K+ channels open more slowly and remain open longer K+ ions flow out along its concentration and charge
gradients carries positive (+) charges out, making the cell interior
more negative (-) Ion movements drive the membrane potential back
toward resting membrane potential value Na+/K+ ATPase continue pumping ions, adjusting
levels back to resting equilibrium levels hyperpolarization – briefly the exterior of the
membrane is more negative than resting potential voltage level
Refractory Periods Absolute Refractory
Period the time period during
which second AP cannot be initiated
due to closure of voltage-gated Na+ channels
the voltage-gated Na+ channels must be reset before the membrane can respond to the next stimulus
Many physiologists consider this to be the start of the absolute refractory period
Refractory Periods Relative Refractory
Period The time period during
which a second AP can be initiated with a suprathreshold stimulus
K+ channels are open, Na+ channels are closed
the membrane is still hyperpolarized
Propagation of an Action Potential the movement of an Action Potential down an
unmyelinated axon a local electrochemical current, a flow of charged
ions influx of sodium ions attraction of positive charges for negative area of
membrane nearby depolarizes nearby membrane – opening V-gated Na+
channels
Propagation of an Action Potential
destabilizing the adjacent membrane makes the Action Potential self-propagating and self-sustaining
the Action Potential renews itself at each region of the membrane – a relatively slow process because so much is happening at the molecular level
Conduction Velocity physical factors may influence impulse
conduction heat increases conduction velocity cold decreases conduction velocity
2 structural modifications can increase impulse velocity: increase neuron diameter - decreases
resistance insulate the neuron - myelin sheath
myelinated fibers may conduct as rapidly as 150 m/sec
unmyelinated may conduct as slowly as 0.5 m/sec
Saltatory Conduction
not a continuous region to region depolarization instead, a “jumping” depolarization myelinated axons transmit an Action Potential differently
the myelin sheath acts as an insulator preventing ion flows in and out of the membrane
neurofibral nodes (node of Ranvier) interrupt the myelin sheath and permit ion flows at the exposed locations on the axon membrane
the nodes contain a high density of voltage-gated Na+ channels
Saltatory Conduction in a myelinated fiber, the ionic current flows in at
each node and travels through the axoplasm to the next node
each node depolarizes in sequence, renewing the Action Potential at that node
the Action Potential jumps to next node very rapidly
energy efficient – the membrane only has to depolarize and repolarize at the nodes
less Na+/K+ ATPase activity is required, therefore, less energy is required
The Synapse Function
there must be a means of communication between each neuron and the next target cell
the synapse is the connection
Organization presynaptic neuron postsynaptic neuron separated by synaptic
cleft
The Two Types of Synapses (1) electrical synapses
gap junctions – found in cardiac muscle and in some smooth muscle tissues
direct, rapid electrochemical connections between neurons
may be bidirectional; useful for coordinated contraction rare in adults
(2) chemical synapses specialized for synthesis, release, reception and
removal of neurotransmitters neurotransmitters
chemical signal molecules released from a presynaptic neuron function to open or close chemically-gated ion channels effect membrane permeability and membrane potential
Action of a Chemical Synapse Presynaptic Events
an action potential reaches the axon terminal and depolarizes the terminal voltage gated Ca2+ channels open; Ca2+ ions enter the axoplasm neurotransmitter is released by exocytosis
neurotransmitter molecules diffuse across the cleft
Action of a Chemical Synapse (cont.) Postsynaptic Events
1) the neurotransmitters bind to specific postsynapticreceptors
2) gated ion channels open as a result 3) neurotransmitter molecules are eliminated quickly
a) degraded by extracellular enzymes in the synapse, with the products re-uptaken and recycled by the axon terminal
b) diffuse away from the synapse to the blood circulation
Postsynaptic Potentials EPSP
excitatory postsynaptic potential
provides a small local depolarization
generally results from opening Na+ channels
IPSP inhibitory postsynaptic
potential provides a small local
hyperpolarization generally results from
opening K+ or CL- channels
Summation of Postsynaptic Potentials temporal – rapid repeated stimulation from 2
or more presynaptic neurons spatial – simultaneous stimulation at 2 or
more different places on the neuron by presynaptic neurons
EPSPs and IPSPs counteract each other
End Chapter 11
The Nernst Equation
Ex= lnRT [X]out
zF [X]in
EX= Equilibrium potential of ion X in voltsR = gas constantT = temperature in kelvinsz = charge of each ionF = Faraday’s constant (96,500 coulombs/gram-equivalent charge[X] = ion concentration
At 38°C, (the standard temperature of many mammals) & converting ln:
Ex= log 61 [X]out
z [X]in
The Goldman-Hodgkin-Katz Equation
ENa,K,Cl= logRT PK[K+]out + PNa[Na+]out + PCl[Cl-]in
F PK[K+]in + PNa[Na+]in + PCl[Cl-]out
EX= Equilibrium potential of all ions in voltsR = gas constantT = temperature in kelvinsF = Faraday’s constant (96,500 coulombs/gram-equivalent charge
PERMEABILITY CHANGES DEPENDING UPON NEURON STATUS
At rest: PK:PNa:PCl=1/0.04/0.45
At Action Potential Peak: PK:PNa:PCl=1/20/0.45
The Goldman-Hodgkin-Katz Equation
ENa,K,Cl= logRT PK[K+]out + PNa[Na+]out + PCl[Cl-]in
F PK[K+]in + PNa[Na+]in + PCl[Cl-]out
At rest: PK:PNa:PCl=1/0.04/0.45
Ion Species Extracellular (mM)
Intracellular (mM)
K+ 5 150
Na+ 150 15
Cl- 120 10