Transcript Nervous Tissue

Dr. Michael P. Gillespie
Nervous Tissue
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.
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Structures of the Nervous System
(CNS)
 Brain (100 billion neurons)
 Spinal cord (100 million neurons)
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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)
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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)
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Organization of the Nervous
System
 Central nervous system
 Brain
 Spinal cord
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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
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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.
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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.
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ANS Continued…
 The motor portion of the ANS consists of sympathetic and
parasympathetic divisions.
 Both divisions typically have opposing actions.
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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.
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Types of Nervous Tissue Cells
 Neurons.
 Sensing.
 Thinking.
 Remembering.
 Controlling muscular activity.
 Regulating glandular secretions.
 Neuroglia.
 Support, nourish, and protect neurons.
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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.
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Parts of a Neuron
 Cell Body
 Dendrites
 Axon
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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)
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Parts of a Neuron (Nerve Fiber)
 Nerve fiber – any neuronal process that
emerges from the cell body of a neuron.
 Dendrites
 Axon
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Parts of a Neuron (Dendrites)
 Dendrites (= little trees).
 The receiving (input) portion of a neuron.
 Short, tapering, and highly branched.
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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).
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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.
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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.
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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.
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Classification of Neurons
 Both Structural and Functional features are used to classify
neurons.
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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
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Multipolar Neurons
 One axon and several dendrites.
 Most neurons of the brain and spinal cord are of this type.
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Bipolar Neurons
 Bipolar neurons.
 One axon and one main dendrite.
 Retina of the eye, inner ear, and the olfactory areas of the
brain.
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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.
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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
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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.
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Motor (Efferent) Neurons
 Away from the CNS to effectors (muscles and glands).
 Most are multipolar.
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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.
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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.
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Functions of Neuroglia
 To surround neurons and hold them in place.
 To supply nutrients and oxygen to neurons.
 To insulate one neuron from another.
 To destroy pathogens and remove dead neurons.
 To modulate neurotransmission.
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Types of Neuroglia
 CNS
 Astrocytes
 Oligodendrocytes
 Microglia
 Ependymal cells
 PNS
 Schwann cells
 Satellite cells
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Astrocytes (CNS)
 Star shaped cells with many processes.
 Largest and most numerous of the neuroglia.
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Astrocytes (CNS)
 Functions
 Support neurons.
 Processes wrap around capillaries to create a blood-brain
barrier.
 Regulate growth, migration and interconnection among
neurons in the embryo.
 Maintain chemical environment for impulse transmission
 Influence formation of neural synapses.
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Astrocytes (CNS)
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Astrocytes (CNS)
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Astrocytes (CNS)
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Oligodendrocytes (CNS)
 Similar to astrocytes, but smaller with fewer processes.
 Function
 Form and maintain the myelin sheath around the CNS axons.
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Oligodendrocytes (CNS)
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Microglia (CNS)
 Small cells with slender
processes giving off
numerous spine like
projections.
 Specialized macrophages.
 Function
 Phagocytosis.
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Microglia (CNS)
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Ependymal Cells (CNS)
 Cuboidal to columnar cells.
 They line the cavities of the CNS and make up the walls of
the ventricles.
 Possess microvilli and cilia.
 Functions
 Produce cerebrospinal fluid (CSF)
 Assist in circulation of CSF
 Possibly monitor CSF
 Thought to act as neural stem cells.
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Ependymal Cells (CNS)
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CNS Neuroglia
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Schwann Cells (PNS)
 Encircle PNS axons to forma sheath around them.
 One Schwann cell per axon.
 Function
 Form myelin sheath around PNS neurons
 Assist in axon regeneration
 Have phagocytic activity and clear cellular debris.
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Schwann Cells (PNS)
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Satellite Cells (PNS)
 Small cells that surround neurons in sensory, sympathetic
and parasympathetic ganglia.
 Functions
 Help to regulate the chemical environment.
 Highly sensitive to injury and inflammation and appear to
contribute to pathological states such as chronic pain.
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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.
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Myelination Continued…
 Two types of neuroglial cells produce myelination.
 Schwann cells – located in the PNS.
 Oligodendrocytes – located in the CNS.
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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 re-growth of
the axon.
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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.
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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.
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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.
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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.
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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.
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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)
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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.
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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.
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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.
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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.
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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.
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Types of Ion Channels
 Leakage channels
 Ligand-gated channel
 Mechanically gated channel
 Voltage gated channel
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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.
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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.
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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.
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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.
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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.
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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.
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3 Types of Passive Transport
 Diffusion through the lipid bilayer.
 Diffusion through membrane channels.
 Facilitated diffusion.
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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.
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Factors Influencing Diffusion
 Steepness of the concentration gradient.
 Temperature.
 Mass of the diffusing substance,
 Surface area.
 Diffusion distance.
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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).
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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.
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Electrochemical Gradient
 An electrical difference and a concentration difference
across the membrane.
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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.
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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.
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Inability of most anions to leave the cell.
 The anions are attached to large nondiffusable molecules such as
ATP and large proteins.
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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.
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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.
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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.
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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.
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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).
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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 voltage-gated
Na+ channels and preventing propagation of nerve
impulses.
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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.
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Classification of Nerve Fibers
 A fibers.
 Largest diameter.
 Myelinated.
 Convey touch, pressure, position, thermal sensation.
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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).
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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).
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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.
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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.
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Types of Synapses
 Electrical synapse
 Chemical synapse
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Electrical Synapses
 Action potentials conduct directly between adjacent cells
through gap junctions.
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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.
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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.
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Removal of Neurotransmitter
 Diffusion.
 Enzymatic degradation.
 Uptake by cells.
 Into the cells that released them (reuptake).
 Into neighboring glial cells (uptake).
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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.
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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.
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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).
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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).
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Neural Circuits
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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.
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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.
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