Unit 5 Nervous System
ppt1 Organization &Nervous
Tissue
12-1
Nervous Tissue
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• overview of the nervous
system
• Structure and function of
neurons
• supportive cells
(neuroglia)
• electrophysiology of
neurons
Neurofibrils
• synapses
• Neurons and memory
Figure 12.4d
(d)
Axon
12-2
Overview of Nervous System
• endocrine and nervous system maintain internal
coordination
– endocrine system –( pinealgland, pituitary gland,
pancreas, ovaries, testes, thyroid gland, parathyroid gland,
hypothalamus and adrenal glands.) communicates by means of
chemical messengers (hormones) secreted into to the blood
– nervous system -
employs electrical and chemical means to
send messages from cell to cell
12-3
– nervous system carries out its task in
three basic steps:
– 1. sense organs receive information about changes in the
body and the external environment, and transmits coded
messages to the spinal cord and the brain
– 2. brain and spinal cord processes this information, relates
it to past experiences, and determine an appropriate
response to the circumstances
– 3. brain and spinal cord issue commands to muscles and
gland cells to carry out such a response
12-4
Two Major Anatomical
Subdivisions of Nervous System
• central nervous system (CNS)
– brain and spinal cord enclosed in bony coverings
• enclosed by cranium and vertebral column
• peripheral nervous system (PNS)
– all the nervous system except the brain and spinal cord
– composed of nerves and ganglia
• nerve – a bundle of nerve fibers (axons) wrapped in fibrous
connective tissue
• ganglion – a knot-like swelling in a nerve where neuron cell
bodies are concentrated
12-5
Subdivisions of Nervous System
CNS and PNS
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Central nervous
system (CNS)
Peripheral nervous
system (PNS)
Brain
Spinal
cord
Nerves
Ganglia
Figure 12.1
12-6
Sensory Divisions of PNS
• sensory (afferent) division – carries sensory
signals from various receptors to the CNS
– informs the CNS of stimuli within or around the body
– somatic sensory division – carries signals from
receptors in the skin, muscles, bones, and joints
– visceral sensory division – carries signals from the
viscera of the thoracic and abdominal cavities
• heart, lungs, stomach, and urinary bladder
12-7
Motor Divisions of PNS
• motor (efferent) division – carries signals from the CNS
to gland and muscle cells that carry out the body’s
response
• effectors – cells and organs that respond to commands from the CNS
– somatic motor division – carries signals to skeletal muscles
• output produces muscular contraction as well as somatic reflexes –
involuntary muscle contractions
– visceral motor division (autonomic nervous system) - carries
signals to glands, cardiac muscle, and smooth muscle
– involuntary, and responses of this system and its receptors are visceral
reflexes
– sympathetic division
– tends to arouse body for action
– accelerating heart beat and respiration, while
– inhibiting digestive and urinary systems
– parasympathetic division
– tends to have calming effec
– slows heart rate and breathin
– stimulates digestive and urinary systems
12-8
12-9
Organization of Nervous System
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Central nervous system
Brain
Peripheral nervous system
Spinal
cord
Visceral
sensory
division
Figure 12.2
Sensory
division
Somatic
sensory
division
Motor
division
Visceral
motor
division
Sympathetic
division
Somatic
motor
division
Parasympathetic
division
12-10
Universal Properties of Neurons
• excitability (irritability)
– respond to environmental changes called stimuli
• conductivity
– neurons respond to stimuli by producing electrical
signals that are quickly conducted to other cells at
distant locations
• secretion
– when electrical signal reaches end of nerve fiber, a
chemical neurotransmitter is secreted that crosses
the gap and stimulates the next cell
12-11
Functional Types of Neurons
• sensory (afferent) neurons
– specialized to detect stimuli
– transmit information about them to the CNS
• begin in almost every organ in the body and end in CNS
• afferent – conducting signals toward CNS
• interneurons (association) neurons
– lie entirely within the CNS
– receive signals from many neurons and carry out the integrative function
• process, store, and retrieve information and ‘make decisions’ that determine
how the body will respond to stimuli
– 90% of all neurons are interneurons
– lie between, and interconnect the incoming sensory pathways, and the
outgoing motor pathways of the CNS
• motor (efferent) neuron
– send signals out to muscles and gland cells (the effectors)
• motor because most of them lead to muscles
• efferent neurons conduct signals away from the CNS
12-12
Functional Classes of Neurons
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Peripheral nervous system
Central nervous system
1 Sensory (afferent)
neurons conduct
signals from
receptors to the CNS.
3 Motor (efferent)
neurons conduct
signals from the CNS
to effectors such as
muscles and glands.
2 Interneurons
(association
neurons) are
confined to
the CNS.
Figure 12.3
12-13
Part 2
Structure
of
a
Neuron
soma – the control center of the
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
•
Dendrites
neuron
– also called neurosoma, cell body, or
perikaryon
– has a single, centrally located nucleus with
large nucleolus
– cytoplasm contains mitochondria,
lysosomes, a Golgi complex, numerous
inclusions, and extensive rough endoplasmic
reticulum and cytoskeleton
– cytoskeleton consists of dense mesh of
microtubules and neurofibrils (bundles of
actin filaments)
Soma
Nucleus
Nucleolus
Trigger zone:
Axon hillock
Initial segment
Axon collateral
Axon
Direction of
signal transmission
• compartmentalizes rough ER into dark
staining Nissl bodies
Internodes
Node of Ranvier
– no centrioles – no further cell division
– inclusions – glycogen granules, lipid
droplets, melanin, and lipofuscin (golden
brown pigment produced when lysosomes
digest worn-out organelles)
Myelin sheath
Schwann cell
• lipofuscin accumulates with age
• wear-and-tear granules
• most abundant in old neurons
Terminal
arborization
Figure 12.4a
Figure 12.4a
Synaptic knobs
(a)
12-14
Structure of a Neuron
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Dendrites
• dendrites – vast number of branches
coming from a few thick branches
from the soma
Soma
Nucleus
Nucleolus
Trigger zone:
Axon hillock
Initial segment
– resemble bare branches of a tree in winter
– primary site for receiving signals from
other neurons
– the more dendrites the neuron has, the
more information it can receive and
incorporate into decision making
– provide precise pathway for the reception
and processing of neural information
Axon collateral
Axon
Direction of
signal transmission
Internodes
Node of Ranvier
Myelin sheath
Schwann cell
Terminal
arborization
Figure 12.4a
Figure 12.4a
Synaptic knobs
(a)
12-15
Structure of a Neuron
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Dendrites
• axon (nerve fiber) – originates from a
mound on one side of the soma called
the axon hillock
Soma
Nucleus
Nucleolus
– cylindrical, relatively unbranched for most of
its length
Trigger zone:
• axon collaterals – branches of axon
Axon hillock
Initial segment
– branch extensively on distal end
– specialized for rapid conduction of nerve
signals to points remote to the soma
– axoplasm – cytoplasm of axon
– axolemma – plasma membrane of axon
– only one axon per neuron
– Schwann cells and myelin sheath enclose
axon
– distal end, axon has terminal arborization
– extensive complex of fine branches
Axon collateral
Axon
Direction of
signal transmission
Internodes
Node of Ranvier
Myelin sheath
Schwann cell
• synaptic knob (terminal button) – little
swelling that forms a junction (synapse) with
the next cell
• contains synaptic vesicles full of
neurotransmitter
Terminal
arborization
Figure 12.4a
Figure 12.4a
Synaptic knobs
(a)
12-16
Variation in Neuron Structure
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• multipolar neuron
– one axon and multiple dendrites
– most common
– most neurons in the brain and spinal
cord
Dendrites
Axon
Multipolar neurons
• bipolar neuron
Dendrites
– one axon and one dendrite
– olfactory cells, retina, inner ear
Axon
• unipolar neuron
– single process leading away from the
soma
– sensory from skin and organs to
spinal cord
• anaxonic neuron
– many dendrites but no axon
– help in visual processes
Bipolar neurons
Dendrites
Axon
Unipolar neuron
Dendrites
Anaxonic neuron
Figure 12.5
12-17
Axonal Transport
• many proteins made in soma must be transported to axon
and axon terminal
– to repair axolemma, serve as gated ion channel proteins, as
enzymes or neurotransmitters
• axonal transport – two-way passage of proteins,
organelles, and other material along an axon
– anterograde transport – movement down the axon away from
soma
– retrograde transport – movement up the axon toward the soma
• microtubules guide materials along axon
– motor proteins (kinesin and dynein) carry materials “on their backs”
while they “crawl” along microtubules
• kinesin – motor proteins in anterograde transport
• dynein – motor proteins in retrograde transport
12-18
Neuroglial Cells
• about a trillion (1012) neurons in the nervous
system
• neuroglia outnumber the neurons by as much
as 50 to 1
• neuroglia or glial cells
– support and protect the neurons
– bind neurons together and form framework for
nervous tissue
– in fetus, guide migrating neurons to their destination
– if mature neuron is not in synaptic contact with
another neuron is covered by glial cells
• prevents neurons from touching each other
• gives precision to conduction pathways
12-19
Six Types of Neuroglial Cells
• four types occur only in CNS
– oligodendrocytes
• form myelin sheaths in CNS
• each arm-like process wraps around a nerve fiber forming an
insulating layer that speeds up signal conduction
– ependymal cells
• lines internal cavities of the brain
• cuboidal epithelium with cilia on apical surface
• secretes and circulates cerebrospinal fluid (CSF)
– clear liquid that bathes the CNS
– microglia
• small, wandering macrophages formed white blood cell called
monocytes
• thought to perform a complete checkup on the brain tissue
several times a day
12-20
• wander in search of cellular debris to phagocytize
Six Types of Neuroglial Cells
• four types occur only in CNS
– astrocytes
• most abundant glial cell in CNS
• cover entire brain surface and most nonsynaptic regions of the
neurons in the gray matter of the CNS
• diverse functions
– form a supportive framework of nervous tissue
– have extensions (perivascular feet) that contact blood capillaries that
stimulate them to form a tight seal called the blood-brain barrier
– convert blood glucose to lactate and supply this to the neurons for
nourishment
– nerve growth factors secreted by astrocytes promote neuron growth and
synapse formation
– communicate electrically with neurons and may influence synaptic
signaling
– regulate chemical composition of tissue fluid by absorbing excess
neurotransmitters and ions
– astrocytosis or sclerosis – when neuron is damaged, astrocytes form
hardened scar tissue and fill space formerly occupied by the neuron 12-21
Six Types of Neuroglial Cells
• two types occur only in PNS
– Schwann cells
• envelope nerve fibers in PNS
• wind repeatedly around a nerve fiber
• produces a myelin sheath similar to the ones produced by
oligodendrocytes in CNS
• assist in the regeneration of damaged fibers
– satellite cells
• surround the neurosomas in ganglia of the PNS
• provide electrical insulation around the soma
• regulate the chemical environment of the neurons
12-22
Neuroglial Cells of CNS
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Capillary
Neurons
Astrocyte
Oligodendrocyte
Perivascular feet
Myelinated axon
Ependymal cell
Myelin (cut)
Cerebrospinal fluid
Microglia
Figure 12.6
12-23
Glial Cells and Brain Tumors
• tumors - masses of rapidly dividing cells
– mature neurons have little or no capacity for mitosis
and seldom form tumors
• brain tumors arise from:
– meninges (protective membranes of CNS)
– by metastasis from non-neuronal tumors in other
organs
– most come from glial cells that are mitotically active
throughout life
• gliomas grow rapidly and are highly malignant
– blood-brain barrier decreases effectiveness of
chemotherapy
– treatment consists of radiation or surgery
12-24
Myelin
• myelin sheath – an insulating layer around a
nerve fiber
– formed by oligodendrocytes in CNS and
Schwann cells in PNS
– consists of the plasma membrane of glial cells
• 20% protein and 80 % lipid
• myelination – production of the myelin sheath
–
–
–
–
begins the 14th week of fetal development
proceeds rapidly during infancy
completed in late adolescence
dietary fat is important to nervous system
development
12-25
Myelin
• in PNS, Schwann cell spirals repeatedly around a single
nerve fiber
– lays down as many as a hundred layers of its own membrane
– no cytoplasm between the membranes
– neurilemma – thick outermost coil of myelin sheath
• contains nucleus and most of its cytoplasm
• external to neurilemma is basal lamina and a thin layer of fibrous
connective tissue – endoneurium
• in CNS – oligodendrocytes reaches out to myelinate
several nerve fibers in its immediate vicinity
– anchored to multiple nerve fibers
– cannot migrate around any one of them like Schwann cells
– must push newer layers of myelin under the older ones
• so myelination spirals inward toward nerve fiber
– nerve fibers in CNS have no neurilemma or endoneurium
12-26
Myelin
• many Schwann cells or oligodendrocytes are needed to
cover one nerve fiber
• myelin sheath is segmented
– nodes of Ranvier – gap between segments
– internodes – myelin covered segments from one gap to the next
– initial segment – short section of nerve fiber between the axon
hillock and the first glial cell
– trigger zone – the axon hillock and the initial segment
• play an important role in initiating a nerve signal
12-27
Myelin Sheath in PNS
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Axoplasm
Axolemma
Schwann cell
nucleus
Neurilemma
Figure 12.4c
(c)
Myelin sheath
nodes of Ranvier and internodes
12-28
Myelination in CNS
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Oligodendrocyte
Myelin
Nerve fiber
Figure 12.7b
(b)
12-29
Myelination in PNS
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Schwann cell
Axon
Basal lamina
Endoneurium
Nucleus
(a)
Neurilemma
Myelin sheath
Figure 12.7a
12-30
Diseases of Myelin Sheath
• degenerative disorders of the myelin sheath
– multiple sclerosis
• oligodendrocytes and myelin sheaths in the CNS deteriorate
• myelin replaced by hardened scar tissue
• nerve conduction disrupted (double vision, tremors, numbness, speech
defects)
• onset between 20 and 40 and fatal from 25 to 30 years after diagnosis
• cause may be autoimmune triggered by virus
– Tay-Sachs disease - a hereditary disorder of infants of Eastern
European Jewish ancestry
• abnormal accumulation of glycolipid called GM2 in the myelin sheath
–
–
–
–
normally decomposed by lysosomal enzyme
enzyme missing in individuals homozygous for Tay-Sachs allele
accumulation of ganglioside (GM2) disrupts conduction of nerve signals
blindness, loss of coordination, and dementia
• fatal before age 4
12-31
Unmyelinated Axons of PNS
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Neurilemma
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Myelin sheath
Unmyelinated
nerve fibers
Myelinated
axon
Schwann
cell cytoplasm
Basal
lamina
Neurilemma
Unmyelinated
axon
(c)
3µm
Schwann cell
© The McGraw-Hill Companies, Inc./Dr. Dennis Emery, Dept. of Zoology and Genetics, Iowa
State University, photographer
Figure 12.7c
Basal lamina
Figure 12.8
• Schwann cells hold 1 – 12 small nerve fibers in grooves on its surface
• membrane folds once around each fiber overlapping itself along the
edges
12-32
• mesaxon – neurilemma wrapping of unmyelinated nerve fibers
Electrical Potentials and Currents
• electrophysiology – cellular mechanisms for producing electrical
potentials and currents
– basis for neural communication and muscle contraction
• electrical potential – a difference in the concentration of charged
particles between one point and another
• electrical current – a flow of charged particles from one point to
another
– in the body, currents are movement of ions, such as Na+ or K+ through
gated channels in the plasma membrane
– gated channels are opened or closed by various stimuli
– enables cell to turn electrical currents on and off
• living cells are polarized
• resting membrane potential (RMP) – charge difference across the
plasma membrane
– -70 mV in a resting, unstimulated neuron
– negative value means there are more negatively charged particles
on the inside of the membrane than on the outside
12-33
Resting Membrane Potential
• RMP exists because of unequal electrolyte
distribution between extracellular fluid
(ECF) and intracellular fluid (ICF)
• RMP results from the combined effect of
three factors:
– ions diffuse down their concentration gradient
through the membrane
– plasma membrane is selectively permeable
and allows some ions to pass easier than
others
– electrical attraction of cations and anions to
each other
12-34
Creation of Resting Membrane Potential
• potassium ions (K+) have the greatest influence on RMP
– plasma membrane is more permeable to K+ than any other ion
– leaks out until electrical charge of cytoplasmic anions attracts it back in
and equilibrium is reached and net diffusion of K+ stops
– K+ is about 40 times as concentrated in the ICF as in the ECF
• cytoplasmic anions can not escape due to size or charge
(phosphates, sulfates, small organic acids, proteins, ATP, and RNA)
• membrane much less permeable to high concentration of sodium
(Na+) found outside the cell
– some leaks and diffuses into the cell down its concentration gradient
– Na+ is about 12 times as concentrated in the ECF as in the ICF
– resting membrane is much less permeable to Na+ than K+
• Na+/K+ pumps out 3 Na+ for every 2 K+ it brings in
– works continuously to compensate for Na+ and K+ leakage, and
requires great deal of ATP
• 70% of the energy requirement of the nervous system
– necessitates glucose and oxygen be supplied to nerve tissue (energy
needed to create the resting potential)
– pump contributes about -3 mV to the cell’s resting membrane
potential of -70 mV
12-35
Ionic Basis of Resting Membrane
Potential
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
ECF
Na+ 145 mEq/L
K+
Na+
channel
K+
channel
4 mEq/L
Figure 12.11
Na+ 12 mEq/L
ICF
K+ 150 mEq/L
Large anions
that cannot
escape cell
• Na+ concentrated outside of cell (ECF)
• K+ concentrated inside cell (ICF)
12-36
Synapses
• a nerve signal can go no further when it reaches the end of the axon
– triggers the release of a neurotransmitter
– stimulates a new wave of electrical activity in the next cell across the
synapse
• synapse between two neurons
– 1st neuron in the signal path is the presynaptic neuron
• releases neurotransmitter
– 2nd neuron is postsynaptic neuron
• responds to neurotransmitter
• presynaptic neuron may synapse with a dendrite, soma, or axon of
postsynaptic neuron to form axodendritic, axosomatic or
axoaxonic synapses
• neuron can have an enormous number of synapses
– spinal motor neuron covered by about 10,000 synaptic knobs from other
neurons
• 8000 ending on its dendrites
• 2000 ending on its soma
• in cerebellum of brain, one neuron can have as many as 100,000
synapses
12-37
Synaptic Relationships Between
Neurons
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Soma
Synapse
Axon
Presynaptic
neuron
Direction of
signal
transmission
Postsynaptic
neuron
(a)
Axodendritic synapse
Axosomatic
synapse
(b)
Figure 12.18
Axoaxonic synapse
12-38
Discovery of Neurotransmitters
• synaptic cleft -gap between neurons was discovered by Ramón y Cajal
through histological observations
• Otto Loewi, in 1921, demonstrated that neurons communicate by
releasing chemicals – chemical synapses
–
–
–
–
he flooded exposed hearts of two frogs with saline
stimulated vagus nerve of the first frog and the heart slowed
removed saline from that frog and found it slowed heart of second frog
named it Vagusstoffe (“vagus substance”)
•
later renamed acetylcholine, the first known neurotransmitter
• electrical synapses do exist
– some neurons, neuroglia, and cardiac and single-unit smooth muscle
– gap junctions join adjacent cells
• ions diffuse through the gap junctions from one cell to the next
– advantage of quick transmission
• no delay for release and binding of neurotransmitter
• cardiac and smooth muscle and some neurons
– disadvantage is they cannot integrate information and make decisions
• ability reserved for chemical synapses in which neurons communicate by
releasing neurotransmitters
12-39
Synaptic Knobs
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Axon of
presynaptic
neuron
Synaptic
knob
Soma of
postsynaptic
neuron
© Omikron/Science Source/Photo Researchers, Inc
Figure 12.19
12-40
Structure of a Chemical Synapse
• synaptic knob of presynaptic neuron contains synaptic
vesicles containing neurotransmitter
– many docked on release sites on plasma membrane
• ready to release neurotransmitter on demand
– a reserve pool of synaptic vesicles located further away from
membrane
• postsynaptic neuron membrane contains proteins that
function as receptors and ligand-regulated ion gates
12-41
Structure of a Chemical Synapse
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Microtubules
of cytoskeleton
Axon of presynaptic neuron
Mitochondria
Postsynaptic neuron
Synaptic knob
Synaptic vesicles
containing neurotransmitter
Synaptic cleft
Neurotransmitter
receptor
Postsynaptic neuron
Neurotransmitter
release
Figure 12.20
• presynaptic neurons have synaptic vesicles with
neurotransmitter and postsynaptic have receptors and
ligand-regulated ion channels
12-42
Neurotransmitters and Related
Messengers
• more than 100 neurotransmitters have been identified
• fall into four major categories according to chemical
composition
– acetylcholine
• in a class by itself
• formed from acetic acid and choline
– amino acid neurotransmitters
• include glycine, glutamate, aspartate, and -aminobutyric acid (GABA)
– monoamines
• synthesized from amino acids by removal of the –COOH group
• retaining the –NH2 (amino) group
• major monoamines are:
– epinephrine, norepinephrine, dopamine (catecholamines)
– histamine and serotonin
– neuropeptides
12-43
Categories of Neurotransmitters
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Acetylcholine
CH3
O
+
H3C N CH2 CH2 O C CH3
CH3
Catecholamines
HO
OH
CH CH2 NH CH2
Amino acids
HO
HO
C CH2 CH2 CH2 NH2
Tyr
Enkephalin
Arg Pro Lys
OH
CH CH2 NH2
Norepinephrine
HO
HO
C CH2
HO
NH2
Glycine
O
O
C CH CH2 C
HO
OH
NH2
C
Dopamine
HO
O
OH
Glutamic acid
CH2 CH2 NH2
Ser
Thr Met Phe
Glu
Glu Phe
SO4
Gly
N
Substance P
Phe
Cholecystokinin
Tyr
ß-endorphin
Serotonin
Glu
N
Leu Met
Gly
Lys
Ser
N
Aspartic acid
C CH CH2 CH2
HO
NH2
CH2 CH2 NH2
HO
Pro
Glu
Phe Gly
Asp Tyr Met Gly Trp Met Asp
GABA
O
O
Met Phe
Gly Gly
Epinephrine
HO
O
Neuropeptides
Monoamines
CH2 CH2 NH2
Histamine
Thr
Ala
Pro
Asn
Lys
Leu
Val
Thr Leu
Phe
IIe
IIe
Lys Asn Ala Tyr
Lys
Lys
Gly
Glu
Figure 12.21
12-44
Neuropeptides
• chains of 2 to 40 amino acids
– beta-endorphin and substance P
• act at lower concentrations than
other neurotransmitters
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Neuropeptides
Met Phe
Gly Gly
Tyr
Enkephalin
Arg Pro Lys
Pro
Glu
Glu Phe
Phe Gly
Leu Met
Substance P
• longer lasting effects
• stored in axon terminal as larger
secretory granules (called
dense-core vesicles)
• some function as hormones or
neuromodulators
• some also released from
digestive tract
– gut-brain peptides cause food
cravings
Ser
Thr Met Phe
Glu
Asp Tyr Met Gly Trp Met Asp
SO4
Gly
Phe
Cholecystokinin
Gly
Tyr
Lys
ß-endorphin
Ser
Glu
Thr
Ala
Pro
Asn
Lys
Leu
Val
Thr Leu
Phe
IIe
IIe
Lys Asn Ala Tyr
Lys
Lys
Gly
Glu
Figure 12.21
12-45
Function of Neurotransmitters
at Synapse
• they are synthesized by the presynaptic
neuron
• they are released in response to
stimulation
• they bind to specific receptors on the
postsynaptic cell
• they alter the physiology of that cell
12-46
Effects of Neurotransmitters
• a given neurotransmitter does not have
the same effect everywhere in the body
• multiple receptor types exist for a
particular neurotransmitter
– 14 receptor types for serotonin
• receptor governs the effect the
neurotransmitter has on the target cell
12-47
Synaptic Transmission
• neurotransmitters are diverse in their action
– some excitatory
– some inhibitory
– some the effect depends on what kind of receptor the
postsynaptic cell has
– some open ligand-regulated ion gates
– some act through second-messenger systems
• three kinds of synapses with different modes of action
– excitatory cholinergic synapse
– inhibitory GABA-ergic synapse
– excitatory adrenergic synapse
• synaptic delay – time from the arrival of a signal at the
axon terminal of a presynaptic cell to the beginning of an
action potential in the postsynaptic cell
– 0.5 msec for all the complex sequence of events to occur
12-48
Excitatory Cholinergic Synapse
• cholinergic synapse – employs acetylcholine (ACh) as its
neurotransmitter
– ACh excites some postsynaptic cells
• skeletal muscle
– inhibits others
•
describing excitatory action
– nerve signal approaching the synapse, opens the voltage-regulated calcium gates
in synaptic knob
– Ca2+ enters the knob
– triggers exocytosis of synaptic vesicles releasing ACh
– empty vesicles drop back into the cytoplasm to be refilled with ACh
– reserve pool of synaptic vesicles move to the active sites and release their ACh
– ACh diffuses across the synaptic cleft
– binds to ligand-regulated gates on the postsynaptic neuron
– gates open
– allowing Na+ to enter cell and K+ to leave
• pass in opposite directions through same gate
– as Na+ enters the cell it spreads out along the inside of the plasma membrane and
depolarizes it producing a local potential called the postsynaptic potential
– if it is strong enough and persistent enough
– it opens voltage-regulated ion gates in the trigger zone
12-49
– causing the postsynaptic neuron to fire
Memory and Synaptic Plasticity
• physical basis of memory is a pathway through
the brain called a memory trace or engram
– along this pathway, new synapses were created or
existing synapses modified to make transmission
easier
– synaptic plasticity – the ability of synapses to
change
– synaptic potentiation - the process of making
transmission easier
• kinds of memory
– immediate, short- and long-term memory
– correlate with different modes of synaptic potentiation
12-50
Immediate Memory
• immediate memory – the ability to hold
something in your thoughts for just a few
seconds
– essential for reading ability
• feel for the flow of events (sense of the
present)
• our memory of what just happened
“echoes” in our minds for a few seconds
– reverberating circuits
12-51
Short-Term or Working Memory
• short-term memory (STM) - lasts from a few
seconds to several hours
– quickly forgotten if distracted
– calling a phone number we just looked up
– reverberating circuits
• facilitation causes memory to last longer
– tetanic stimulation – rapid arrival of repetitive signals
at a synapse
• causes Ca2+ accumulation and postsynaptic cell more likely to
fire
– post-tetanic potentiation - to jog a memory
• Ca2+ level in synaptic knob stays elevated
• little stimulation needed to recover memory
12-52
Long-Term Memory
• types of long-term memory
– declarative - retention of events that you can put into
words
– procedural - retention of motor skills
• physical remodeling of synapses
– new branching of axons or dendrites
• molecular changes - long-term potentiation
– changes in receptors and other features increases
transmission across “experienced” synapses
– effect is longer-lasting
12-53
Alzheimer Disease
• 100,000 deaths/year
– 11% of population over 65; 47% by age 85
• memory loss for recent events, moody, combative, lose ability to
talk, walk, and eat
• show deficiencies of acetylcholine (ACh) and nerve growth factor
(NGF)
• diagnosis confirmed at autopsy
– atrophy of gyri (folds) in cerebral cortex
– neurofibrillary tangles and senile plaques
– formation of beta-amyloid protein from breakdown product of plasma
membranes
• genetics implicated
• treatment - halt beta-amyloid production
– research halted due to serious side effects
– Give NGF or cholinesterase inhibitors
12-54
Alzheimer Disease Effects
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Neurons with
neurofibrillary
tangles
Shrunken
gyri
Wide sulci
Senile plaque
(b)
(a)
© Simon Fraser/Photo Researchers, Inc.
Custom Medical Stock Photo, Inc.
Figure 12.31a
Figure 12.31b
12-55
Parkinson Disease
• progressive loss of motor function beginning in 50’s or 60’s no recovery
– degeneration of dopamine-releasing neurons
• dopamine normally prevents excessive activity in motor centers (basal
nuclei)
• involuntary muscle contractions
– pill-rolling motion, facial rigidity, slurred speech,
– illegible handwriting, slow gait
• treatment - drugs and physical therapy
– dopamine precursor (L-dopa) crosses brain barrier – bad side effects
on heart & liver
– MAO inhibitor slows neural degeneration
– surgical technique to relieve tremors
12-56