Transcript Anat3_01_Nervous_Tissue

Dr. Michael P. Gillespie
Nervous System
 The nervous system is an intricate, highly organized
network of billions of neurons and even more
neuroglia.
 The nervous system has a mass of only 2 kg (4.5 lb),
which comprises approximately 3% of total body
weight.
Structures of the Nervous System (CNS)
 Brain (100 billion neurons)
 Spinal cord (100 million neurons)
Structures of the Nervous System (PNS)
 Spinal nerves (31 pairs)
 Cranial nerves (12 pairs)
 Ganglia (Masses of primarily neuron cell bodies)
 Enteric plexuses (networks of neurons in the GI tract)
 Sensory receptors (dendrites of sensory neurons)
Functions of the Nervous System
 Sensory function – afferent neurons
 Sensory receptors detect internal and external stimuli
 Integrative function – interneurons
 The nervous system processes sensory information and
coordinates responses. It perceives stimuli.
 Motor function – efferent neurons
 The cells contacted by these neurons are called
effectors (muscles and glands)
Organization of the Nervous
System
 Central nervous system
 Brain
 Spinal cord
Organization of the Nervous
System
 Peripheral nervous system
 Cranial nerves and their branches
 Spinal nerves and their branches
 Ganglia
 Sensory receptors
 Somatic nervous system
 Autonomic nervous system
 Enteric nervous system
Somatic Nervous System (SNS)
 Sensory neurons.
 Motor neurons located in skeletal muscles.
 The motor responses can be voluntarily controlled;
therefore this part of the PNS is voluntary.
Autonomic Nervous System (ANS)
 Sensory neurons from the autonomic sensory
receptors in the viscera.
 Motor neurons located in smooth muscle, cardiac
muscle and glands.
 These motor responses are NOT under conscious
control; Therefore this part of the PNS is involuntary.
ANS Continued…
 The motor portion of the ANS consists of sympathetic
and parasympathetic divisions.
 Both divisions typically have opposing actions.
Enteric Nervous System (ENS)
 “The brain of the gut”.
 Functions independently of the ANS and CNS, but
communicates with it as well.
 Enteric motor units govern contraction of the GI tract.
 Involuntary.
Types of Nervous Tissue Cells
 Neurons.
 Sensing.
 Thinking.
 Remembering.
 Controlling muscular activity.
 Regulating glandular secretions.
 Neuroglia.
 Support, nourish, and protect neurons.
Neurons
 Have the ability to produce action potentials or
impulses (electrical excitability) in response to a
stimulus.
 An action potential is an electrical signal that
propagates from one point to the next along the
plasma membrane of a neuron.
 A stimulus is any change in the environment that is
strong enough to initiate an action potential.
Parts of a Neuron
 Cell Body
 Dendrites
 Axon
Parts of a Neuron (Cell Body)
 Cell body (perikaryon or soma).
 Contains the nucleus surrounded by cytoplasm which
contains the organelles.
 Clusters of rough ER called Nissl bodies (produce
proteins to grow and repair damaged nerves)
Parts of a Neuron (Nerve Fiber)
 Nerve fiber – any neuronal process that emerges
from the cell body of a neuron.
 Dendrites
 Axon
Parts of a Neuron (Dendrites)
 Dendrites (= little trees).
 The receiving (input) portion of a neuron.
 Short, tapering, and highly branched.
Parts of a Neuron (Axon)
 Axon (= axis).
 Each nerve contains a single axon.
 The axon propagates nerve impulses toward another neuron,
muscle fiber, or gland cell.
 Long, thin, cylindrical projection that often joins the cell body
at a cone-shaped elevation called the axon hillock (= small
hill).
 The part of the axon closest to the hillock is the initial
segment.
 The junction between the axon hillock and the initial segment
is the trigger zone (nerve impulses arise here).
 The cytoplasm of the axon is the axoplasm and is surrounded
by a plasma membrane known as the axolemma (lemma =
sheath).
Synapse
 The synapse is the site of communication between
two neurons or between a neuron and an effector cell.
 Synaptic end bulbs and varicosities contain
synaptic vesicles that store a chemical
neurotransmitter.
Axonal Transport
 Slow axonal transport.
 1-5 mm per day.
 Travels in one direction only – from cell body toward
axon terminals.
 Fast axonal transport.
 200 – 400 mm per day.
 Uses proteins to move materials.
 Travels in both directions.
Structural Diversity of Neurons
 The cell body diameter can range in size from 5
micrometers (μm) (slightly smaller than a RBC) up to
135 μm (barely visible to the naked eye).
 Dendritic branching patterns vary.
 Axon length varies greatly as well. Some neurons have
no axon, some are very short, and some run all the way
from the toes to the lowest part of the brain.
Classification of Neurons
 Both Structural and Functional features are used to
classify neurons.
Structural Classifications of
Neurons
 Structurally, neurons are classified according to the
number of processes extending from the cell body.
 3 Structural Classes
 Multipolar neurons
 Bipolar neurons
 Unipolar neurons
Multipolar Neurons
 One axon and several dendrites.
 Most neurons of the brain and spinal cord are of this
type.
Bipolar Neurons
 Bipolar neurons.
 One axon and one main dendrite.
 Retina of the eye, inner ear, and the olfactory areas of
the brain.
Unipolar Neurons
 Unipolar neurons.
 The axon and the dendrite fuse into a single process that
divides into two branches.
 The dendrites monitor a sensory stimulus such as touch,
pressure, pain, heat, or stretching.
 Called psuedounipolar neurons.
Functional Classification of
Neurons
 Functionally, neurons are classified according to the
direction in which the nerve impulse (action
potential) is conveyed with respect to the CNS.
 3 Functional Classes
 Sensory or afferent neurons
 Motor of efferent neurons
 Interneurons or association neurons
Sensory (Afferent) Neurons
 Either contain sensory receptors or are located
adjacent to sensory receptors that are separate cells.
 Conveyed into the CNS through cranial or spinal
nerves.
 Most are unipolar.
Motor (Efferent) Neurons
 Away from the CNS to effectors (muscles and glands).
 Most are multipolar.
Interneurons (Association
Neurons)
 Mainly located within the CNS between sensory and
motor neurons.
 They process sensory information and elicit a motor
response.
 Most are multipolar.
Neuroglia
 Half the volume of the CNS.
 Generally, they are smaller than neurons, but 5 to 50
times more numerous.
 They can multiply and divide.
 Gliomas – brain tumors derived from glia.
Types of Neuroglia
 CNS
 Astrocytes
 Oligodendrocytes
 Microglia
 Ependymal cells
 PNS
 Schwann cells
 Satellite cells
Astrocytes
 Star shaped cells with many processes.
 Largest and most numerous of the neuroglia.
Astrocytes
 Functions
 Support neurons.
 Processes wrap around capillaries to create a bloodbrain barrier.
 Regulate growth, migration and interconnection among
neurons in the embryo.
 Maintain chemical environment for impulse
transmission
 Influence formation of neural synapses.
Astrocytes
Astrocytes
Astrocytes
Oligodendrocytes
 Similar to astrocytes, but smaller with fewer processes.
 Function
 Form and maintain the myelin sheath around the CNS
axons.
Oligodendrocytes
Microglia
 Small cells with slender processes giving off numerous
spine like projections.
 Function
 Phagocytes.
Microglia
Ependymal Cells
 Cuboidal to columnar cells.
 Possess microvilli and cilia.
 Functions
 Produce cerebrospinal fluid (CSF)
 Assist in circulation of CSF
 Possibly monitor CSF
Ependymal Cells
CNS Neuroglia
Schwann Cells
 Encircle PNS axons to forma sheath around them.
 One Schwann cell per axon.
 Function
 Form myelin sheath around PNS neurons
 Assist in axon regeneration
Schwann Cells
Myelination
 The myelin sheath is a lipid and protein covering.
It is produced by the neuroglia.
 The sheath electrically insulates the axon of a
neuron.
 The sheath increases the speed of nerve impulse
conduction.
 The amount of myelin increases from birth on.
 Axons without a covering are unmyelinated.
Axons with a covering are myelinated.
Myelination Continued…
 Two types of neuroglial cells produce myelination.
 Schwann cells – located in the PNS.
 Oligodendrocytes – located in the CNS.
Neurolemma (Sheath of
Schwann)
 The neurolemma (sheath of Schwann) is the outer
nucleated cytoplasmic layer of the Schwann cell.
 It encloses the myelin sheath.
 It is only found around the axons of the PNS.
 If the axon is injured, the neurolemma forms a
regeneration tube that guides and stimulates regrowth of the axon.
Nodes of Ranvier
 The nodes of Ranvier are gaps in the myelin sheath at
intervals along the axon.
 Each Schwann cell wraps one axon segment between
two nodes.
 The electrical impulse jumps from node to node to
speed up the propagation
 Nodes of Ranvier are present in the CNS, but fewer in
number.
Demyelination
 Demyelination is the loss or destruction of the myelin
sheaths around axons.
 It occurs as the result of disorders such as multiple
sclerosis or Tay-Sachs disease.
 Radiation and chemotherapy can also damage the
myelin sheath.
 Demyelination can deteriorate the affected nerves.
Collections of Nervous Tissue
 Neuronal cell bodies are grouped in clusters.
 Axons of neurons are grouped in bundles.
 Nervous tissue is grouped in gray and white matter.
Clusters of Neuronal Cell Bodies
 Ganglion – cluster of neuronal cell bodies in the PNS.
 Associated with the cranial and spinal nerves.
 Nucleus – cluster of neuronal cell bodies in the CNS.
Bundles of Axons
 Nerve – a bundle of axons in the PNS.
 Cranial nerves connect the brain to the periphery.
 Spinal nerves connect the spinal cord to the periphery.
 Tract – a bundle of axons in the CNS.
 Tracts interconnect neurons in the spinal cord and
brain.
Gray and White Matter
 The white matter consists of aggregations of primarily
myelinated and some unmyelinated axons. (Myelin is
whitish in color)
 The gray matter consists of neuronal cell bodies,
dendrites, unmyelinated axons, axon terminals, and
neuroglia. (Nissl bodies impart a gray color)
Electrical Signals in Neurons
 Neurons are electrically excitable and
communicate with one another using 2 types of
electrical signals.
 Graded potentials (short distance communication).
 Action potentials ((long distance communication).
 The plasma membrane exhibits a membrane
potential. The membrane potential is an
electrical voltage difference across the membrane.
Electrical Signals in Neurons
 The voltage is termed the resting membrane
potential.
 The flow of charged particles across the membrane is
called current.
 In living cells, the flow of ions constitutes the
electrical current.
Ion Channels
 The plasma membrane contains many different kinds
of ion channels.
 The lipid bilayer of the plasma membrane is a good
electrical insulator.
 The main paths for flow of current across the
membrane are ion channels.
Ion Channels
 When ion channels are open, they allow specific
ions to move across the plasma membrane down
their electrochemical gradient.
 Ions move from greater areas of concentration to lesser
areas of concentration.
 Positively charged cations move towards a negatively
charged area and negatively charged anions move
towards a positively charged area.
 As they move, they change the membrane potential.
Ion Channel “Gates”
 Ion channels open and close due to the presence of
“gates”.
 The gate is part of a channel protein that can seal the
channel pore shut or move aside to open the pore.
Types of Ion Channels
 Leakage channels
 Ligand-gated channel
 Mechanically gated channel
 Voltage gated channel
Leakage Channels
 Leakage channels – gates randomly alternate between open
and closed positions.
 More potassium ion (K+) leakage channels than sodium
(Na+) leakage channels.
 The potassium ion leakage channels are leakier than the
sodium ion leakage channels.
Ligand-gated Channel
 Ligand-gated channels – open and close in response to a
specific chemical stimulus.
 Neurotransmitters, hormones, and certain ions can act as
the chemical stimulus that opens or closes these channels.
Mechanically Gated Channel
 Mechanically gated channels – opens or closes in response
to mechanical stimulation.
 Vibration, touch, pressure, or tissue stretching can all
distort the channel from its resting position, opening the
gate.
Voltage-gated Channel
 Voltage-gated channels – opens in response to a change in
membrane potential (voltage).
 These channels participate in the generation and
conduction of action potentials.
Gradients
 Concentration Gradient – A difference in the
concentration of a chemical from one place to another.
 Electrochemical Gradient – The combination of the
effects of the concentration gradient and the
membrane potential.
Transport Across the
Membrane
 Passive Transport – does not require cellular energy.
 Substances move down their concentration or
electrochemical gradients using only their own kinetic
energy.
 Active Transport – requires cellular energy in the form
of ATP.
3 Types of Passive Transport
 Diffusion through the lipid bilayer.
 Diffusion through membrane channels.
 Facilitated diffusion.
Diffusion
 Materials diffuse from areas of high concentration to
areas of low concentration.
 The move down their concentration gradient.
 Equilibrium – molecules are mixed uniformly
throughout the solution.
Factors Influencing Diffusion
 Steepness of the concentration gradient.
 Temperature.
 Mass of the diffusing substance,
 Surface area.
 Diffusion distance.
Resting Membrane Potential
 The resting membrane potential occurs due to a
buildup of negative ions in the cytosol along the inside
of the membrane and positive ions in the extracellular
fluid along the outside of the membrane.
 The potential energy is measured in millivolts (mV).
Resting Membrane Potential
 In neurons, the resting membrane potential ranges
from –40 to –90 mV. Typically –70 mV.
 The minus sign indicates that the inside of the cell is
negative compared to the outside.
 A cell that exhibits a membrane potential is
polarized.
 The potential exists because of a small buildup of
negative ions in the cytosol along the inside of the
membrane and positive ions in the extracellular fluid
along the membrane.
Electrochemical Gradient
 An electrical difference and a concentration difference
across the membrane.
Factors Producing the Resting
Membrane Potential
 Unequal distribution of ions in the ECF and cytosol.
 Inability of most anions to leave the cell.
 Electrogenic nature of the Na+/K+ ATPases.
Unequal distribution of ions in
the ECF and cytosol.
 ECF is rich in Na+ and CL- ions.
 Cytosol has the cation K+ and the dominant anions are
phosphates attached to ATP and amino acids in proteins.
 The plasma membrane has more K+ leakage channels than
Na+ leakage channels.
Inability of most anions to leave
the cell.
 The anions are attached to large nondiffusable molecules
such as ATP and large proteins.
Electrogenic nature of the
+
+
Na /K ATPases.
 Membrane permeability to Na+ is very low because
there are very few sodium leakage channels.
 Sodium ions do slowly diffuse into the cell, which
would eventually destroy the resting membrane
potential.
 Na+/K+ ATPases pump sodium back out of the cell and
bring potassium back in.
 They pump out 3 Na+ for every 2 K+ they bring in.
Graded Potentials
 A graded potential is a small deviation from the resting
membrane potential.
 It makes the membrane either more polarized (more
negative inside) or less polarized (less negative inside).
 Most graded potentials occur in the dendrites or cell
body.
Graded Potentials
 Hyperpolarizing graded potential make the
membrane more polarized (inside more negative).
 Depolarizing graded potential make the
membrane less polarized (inside less negative).
 Graded potentials occur when ligand-gated or
mechanically gated channels open or close.
 Mechanically gated and ligand-gated channels are
present in sensory neurons.
 Ligand-gated channels are present in interneurons and
motor neurons.
Graded Potentials
 Graded potentials are graded because they vary in
amplitude (size) depending on the strength of the
stimulus.
 The amplitude varies depending upon how many
channels are open and how long they are open.
 The opening and closing of channels produces a flow
of current that is localized.
Graded Potentials
 The charge spreads a short distance and dies out
(decremental conduction).
 The charge can become stronger and last longer by
adding with other graded potentials (Summation).
Types of Graded Potentials
 Post-synaptic potentials – a graded potential that
occurs in the dendrites or cell body of a neuron in
response to a neurotransmitter.
 Receptor potentials and generator potentials – graded
potentials that occur in sensory receptors and sensory
neurons.
Action Potentials
 An action potential or impulse is a sequence of
events that decrease and reverse the membrane
potential and eventually restore it to its resting state.
 Depolarizing phase – the resting membrane
potential becomes less negative, reaches zero, and
then becomes positive.
 Repolarizing phase – restores the resting membrane
potential to -70 mV.
Threshold
 Threshold – depolarization reaches a certain level
(about –55 mV), voltage gated channels open.
 A weak stimulus that does not bring the membrane to
threshold is called a sub-threshold stimulus.
 A stimulus that is just strong enough to depolarize a
membrane is called a threshold stimulus.
 Several action potentials will from in response to a
supra-threshold stimulus.
 Action potentials arise according to an all or none
principal.
Depolarizing Phase
 A depolarizing graded potential or some other
stimulus causes the membrane to reach threshold.
 Voltage-gated ion channels open rapidly.
 The inflow of positive Na+ ions changes the
membrane potential from –55mv to +30 mV.
 K+ channels remain largely closed.
 About 20,000 Na+ enter through the gates.
Millions are present in the surrounding fluid.
 Na+/K+ pumps bail them out.
Repolarizing Phase
 While Na+ channels are opening during
depolarization, K+ channels remain largely closed.
 The closing of Na+ channels and the slow opening of
K+ channels allows for repolarization.
 K+ channels allow outflow of K+ ions.
Refractory Period
 The refractory period is the period of time after
an action potential begins during which an
excitable cell cannot generate another action
potential.
 Absolute refractory period – a second action potential
cannot be initiated, even with a very strong stimulus.
 Relative refractory period – an action potential can be
initiated, but only with a larger than normal stimulus.
Propagation of Nerve Impulses
 Unlike the graded potential, the impulse in the action
potential is not detrimental (it does not die out).
 The impulse must travel from the trigger zone to the
axon terminals.
 This process is known as propagation or
conduction.
 The impulse spreads along the membrane.
 As Na+ ions flow in, they trigger depolarization which
opens Na+ channels in adjacent segments of the
membrane.
2 Types of Propagation
 Continuous Conduction – step by step depolarization
and repolarization of each segment of the plasma
membrane.
 Saltatory Conduction – a special mode of action
potential propagation along myelinated axons.
 The action potential “leaps” from one Node of Ranvier to
the next.
Continuous and Saltatory
Conduction
 Few ion channels are present where there is myelin.
 Nodes of Ranvier – areas where there is no myelin –
contain many ion channels.
 The impulse “jumps” from node to node.
 This speeds up the propagation of the impulse.
 This is a more energy efficient mode of conduction.
Neurotoxins & Local
Anesthetics
 Neurotoxins produce poisonous effects upon the
nervous system.
 Local anesthetics are drugs that block pain and other
somatic sensations.
 These both act by blocking the opening of voltagegated Na+ channels and preventing propagation of
nerve impulses.
Factors That Affect Speed of
Propagation
 1. Amount of myelination - Myelinated axons conduct
impulses faster than unmyelinated ones.
 2. Axon diameter - Larger diameter axons propagate
impulses faster than smaller ones.
 3. Temperature – Axons propagate action potentials at
lower speeds when cooled.
Classification of Nerve Fibers
 A fibers.
 Largest diameter.
 Myelinated.
 Convey touch, pressure, position, thermal sensation.
Classification of Nerve Fibers
 B fibers.
 Smaller diameter than A fibers.
 Myelinated.
 Conduct impulses from the viscera to the brain and
spinal cord (part of the ANS).
Classification of Nerve Fibers
 C fibers.
 Smallest diameter.
 Unmyelinated.
 Conduct some sensory impulses and pain impulses from
the viscera.
 Stimulate the heart, smooth muscle, and glands (part of
ANS).
Encoding Intensity of a Stimulus
 A light touch feels different than a firmer touch
because of the frequency of impulses.
 The number of sensory neurons recruited (activated)
also determines the intensity of the stimulus.
Signal Transmission at Synapses
 Presynaptic neuron – the neuron sending the signal.
 Postsynaptic neuron – the neuron receiving the
message.
 Axodendritic – from axon to dendrite.
 Axosomatic – from axon to soma.
 Axoaxonic – from axon to axon.
Types of Synapses
 Electrical synapse
 Chemical synapse
Electrical Synapses
 Action potentials conduct directly between adjacent
cells through gap junctions.
Electrical Synapses
 Tubular connexons act as tunnels to connect the
cytosol of the two cells.
 Advantages.
 Faster communication than a chemical synapse.
 Synchronization – they can synchronize the activity of a
group of neurons or muscle fibers. In the heart and
visceral smooth muscle this results in coordinated
contraction of these muscle fibers.
Chemical Synapses
 The plasma membranes of a presynaptic and
postsynaptic neuron in a chemical synapse do not
touch one another directly.
 The space between the neurons is called a synaptic
cleft which is filled with interstitial fluid.
 A neurotransmitter must diffuse through the
interstitial fluid in the cleft and bind to receptors
on the postsynaptic neuron.
 The synaptic delay is about 0.5 msec.
Removal of Neurotransmitter
 Diffusion.
 Enzymatic degradation.
 Uptake by cells.
 Into the cells that released them (reuptake).
 Into neighboring glial cells (uptake).
Spatial and Temporal Summation
of Postsynaptic Potentials
 A typical neuron in the CNS receives input from 1000
to 10,000 synapses.
 Integration of these inputs is known as summation.
Spatial and Temporal Summation
of Postsynaptic Potentials
 Spatial summation – summation results from buildup
of neurotransmitter released by several presynaptic
end bulbs.
 Temporal summation – summation results from
buildup of neurotransmitter released by a single
presynaptic end bulb 2 or more times in rapid
succession.
Neural Circuits
 Diverging circuit –single presynaptic neuron
influences several postsynaptic neurons (i.e. muscle
fibers or gland cells).
 Converging circuit – several presynaptic neruons
influence a single post-synaptic neuron (results in a
stronger signal).
Neural Circuits
 Reverberating circuit – Branches from later neurons
stimulate earlier ones (may last for seconds to hours)
(breathing, coordinated muscular activities, waking
up, short-term memory).
 Parallel after-discharge circuit – a presynaptic neuron
stimulates a group of neurons that all interact with a
common postsynaptic cell (quick stream of impulses)
(mathematical calculations).
Neural Circuits
Neurogenesis in the CNS
 Birth of new neurons.
 From undifferentiated stem cells.
 Epidermal growth factor stimulates growth of neurons
and astrocytes.
 Minimal new growth occurs in the CNS.
 Inhibition from glial cells.
 Myelin in the CNS.
Damage and Repair in the PNS
 Axons and dendrites may undergo repair if the cell
body is intact, if the Schwann cells are functional, and
if scar tissue does not form too quickly.
 Wallerian degeneration.
 Schwann cells adjacent to the site of injury grow
torwards one another and form a regeneration tube.