Transcript Document

Chapter 43
Lecture Outline
Nervous System Organization
• All animals must be able to respond to
environmental stimuli
• Except for sponges, all animals use a
network of nerves to process and integrate
information.
• Sensory receptors – detect stimulus
• Motor effectors – respond to it
• Nervous system links the two
– Consists of neurons and supporting cells
2
Nervous System Organization
• Receptors respond to stimuli
• Sensory receptors detect the stimulus
• Motor effectors respond to stimulus
• Nervous system divisions
• Central nervous system
– Command center
• Peripheral nervous system
– Collects and responds
3
Nervous System Organization
• Vertebrates have three types of neurons
1. Sensory neurons (afferent neurons) carry
impulses toward central nervous system
(CNS)
2. Motor neurons (efferent neurons) carry
impulses away from CNS to effectors
(muscles and glands)
3. Interneurons (association neurons) provide
more complex reflexes and associative
functions (learning and memory)
4
Nervous System Organization
• Central nervous system (CNS)
• Brain and spinal cord
• Peripheral nervous system (PNS)
• Sensory
• Motor
– Somatic - stimulates skeletal muscles
– Autonomic - stimulates smooth and cardiac muscles, as
well as glands
» Sympathetic and parasympathetic
5
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
PNS
CNS
Sensory Neurons
Interneuron
Cell body
Touch
Direction of
conduction
Cell body
Axon
Dendrites
Dendrites
Taste
Direction of
conduction
Axon
Motor Neurons
Smooth
muscle
Skeletal
muscle
Ganglion
Axon
terminals
Cell
bodies
Direction of
conduction
6
CNS
Brain and Spinal Cord
Motor Pathways
PNS
Sensory Pathways
Sensory neurons
registering external
stimuli
Sensory neurons
registering external
stimuli
Somatic nervous
system
(voluntary)
Sympathetic nervous
system
"fight or flight"
Autonomic nervous
system
(involuntary)
Parasympathetic nervous
system
"rest and repose"
central nervous system (CNS)
peripheral nervous system (PNS)
7
Nervous System Organization
• Neurons have the same basic structure
– Cell body
• Enlarged part containing nucleus
– Dendrites
• Short, cytoplasmic extensions that receive stimuli
– Axon
• Single, long extension that conducts impulses
away from cell body
8
Nervous System Organization
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cell body
Dendrites
Nucleus
Axon
Schwann
cell
Axon
Node of
Ranvier
Myelin
sheath
Myelin
sheath
9
© Enrico Mugnaini/Visuals Unlimited
Nervous System Organization
• Neuroglia
– Support neurons both structurally and
functionally
– Schwann cells and oligodendrocytes produce
myelin sheaths surrounding axons
• In the CNS, myelinated axons form white matter
– Dendrites/cell bodies form gray matter
• In the PNS, myelinated axons are bundled to form
nerves
10
Nervous System Organization
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Axon
Schwann
cell
Nucleus
Myelin sheath
The formation of the myelin sheath around a peripheral axon.
11
Nerve Impulse Transmission
• A potential difference exists across every
cell’s plasma membrane
– Negative pole – cytoplasmic side
– Positive pole – extracellular fluid side
• When a neuron is not being stimulated, it
maintains a resting potential
– Ranges from –40 to –90 millivolts (mV)
– Average about –70 mV
12
Nerve Impulse Transmission
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Extracellular
+
+
+
+
+
+
+
+
+
+
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
+
+
+
+
+
+
+
+
+
+
Electrode
outside axon
--- +40 mV
--- –0 mV
--–70 mV
Electrode
inside axon
Oscilloscope
screen
–
–
–
–
–
Intracellular
–
–
–
proteins and nucleic acids
K+
Na+
13
• The inside of the cell is more negatively charged than
the outside (membrane potential)
• Cell membrane is impermeable to negative ions
(such as Cl-)
• Sodium-potassium pump will transport positive
ions
• Ion channels for K+ are more numerous (allowing
more K+ to transport out of cell)
• Leads to resting potential of ~ -70mV
14
What is the sodium-potassium
pump?
•
•
•
Helps to maintain ion imbalance needed to have resting potential
Uses ATP
Leads to build up of positive outside cell and negative inside cell
–
More + ions going out, - ions not able to cross
15
• Sudden temporary disruptions to resting membrane potential occur
in response to stimuli
• 2 types of changes:
• Graded potentials – small continuous changes
• Ligand-gated channels
• Respond to hormones and neurotransmitters
• Action potentials – transient disruptions, signals that
propagate down the neuron
• Voltage-gated channels (Na+ channel and K+ channel)
• Action potential “jumps” from node of Ranvier to next
node
16
• Disruptions in membrane potential
result in
• Depolarization – membrane
becomes less negative
• Hyperpolarization – membrane
becomes more negative
• With graded potentials change in
membrane potential is much less
• With action potentials it is greater
Nerve Impulse Transmission
• Uniqueness of neurons compared with other
cells is the production and maintenance of the
resting membrane potential
18
• Ligand-gated channels
• Ligands are hormones
or neurotransmitters
• Induce opening and
cause changes in cell
membrane permeability
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Synaptic cleft
Na+
Na+
Na+
Acetylcholine
Cell
membrane
Receptor
protein
Ion
channel
Na+
Na+
Cytoplasm in postsynaptic cell
19
• The action potential has three phases
– Rising, falling, and undershoot
• Action potentials are always separate, allor-none events with the same amplitude
20
Nerve Impulse Transmission
• Two ways to increase velocity of conduction
– Axon has a large diameter
• Less resistance to current flow
• Found primarily in invertebrates
– Axon is myelinated
• Action potential is only produced at the nodes of
Ranvier
• Impulse jumps from node to node (see next slide)
• Saltatory conduction
23
Nerve Impulse Transmission
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Saltatory
conduction
Action
potential
Action
potential
Na+
Na+
Myelin
Axon
+
+
Na+
+
+
+
+
–
24
Synapses
• Intercellular junctions with the dendrites of
other neurons, with muscle cells, or with
gland cells
• Presynaptic cell transmits action potential
• Postsynaptic cell receives it
• Two basic types: electrical and chemical
25
• Electrical synapses
– Involve direct cytoplasmic connections
between the two cells formed by gap
junctions
– Relatively rare in vertebrates
• Chemical synapses
– Have a synaptic cleft between the two
cells
– End of presynaptic cell contains synaptic
vesicles packed with neurotransmitters
26
Synapses
• Chemical synapses
– Action potential triggers influx of Ca2+
• Causes synaptic vesicles fuse with cell membrane
• Vesicles contain neurotransmitter, released by
exocytosis
• Diffuses to other side of cleft and binds to
chemical- or ligand-gated receptor proteins
• Produces graded potentials in the postsynaptic
membrane
• Neurotransmitter action is terminated by enzymatic
cleavage or cellular uptake of neurotransmitter
27
Synapses
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Neurotransmitter (ACh)
Action potential
Inward diffusion of Ca2+
Terminal branch
of axon
Ca2+
Synaptic
cleft
Synaptic
vesicle
Na+
Receptor
protein
28
Synaptic Integration
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Axon
a.
b.
50 µm
b: © Science VU/Lewis-Everhart-Zeevi/Visuals Unlimited
29
Drug Addiction
• Habituation
– Prolonged exposure to a stimulus may cause
cells to lose the ability to respond to it
– Cell may decrease the number of receptors
because there is an abundance of
neurotransmitters
• In long-term drug use, means that more of the drug
is needed to obtain the same effect
30
Drug Addiction
• Cocaine
– Affects neurons in the brain’s “pleasure
pathways” (limbic system)
– Binds dopamine transporters and prevents
the reuptake of dopamine
– Dopamine survives longer in the synapse and
fires pleasure pathways more and more
31
Drug Addiction
• Nicotine
– Binds directly to a specific receptor on
postsynaptic neurons of the brain
• Actually binds to a receptor for acetylcholine
– Explains “excitation” by initial nicotine use
– Brain adjusts to prolonged exposure by
“turning down the volume” by
• Making fewer receptors to which nicotine binds
• Altering the pattern of activation of the nicotine
receptors
32
The Central Nervous System
• Sponges are only major phylum without nerves
• Cnidarians have the simplest nervous system
– Neurons linked to each other in a nerve net
– No associative activity
• Free-living flatworms (phylum Platyhelminthes) are
simplest animals with associative activity
– Two nerve cords run down the body
– Permit complex muscle control
• All of the subsequent evolutionary changes in nervous
systems can be viewed as a series of elaborations on
the characteristics already present in flatworms
33
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cnidarian
Earthworm
Human
Central nervous
system
Peripheral
nerves
Cerebrum
Cerebellum
Spinal cord
Nerve
net
Cervical
nerves
Thoracic
nerves
Arthropod
Echinoderm
Lumbar
nerves
Sacral
nerves
Femoral
nerve
Radial
nerve
Brain
Nerve ribs
Ventral
nerve cords
Mollusk
Sciatic
nerve
Flatworm
Nerve
cords
Associative
neurons
Tibial
nerve
Giant
axon Brain
34
Vertebrate Brains
• All vertebrate brains have three basic
divisions:
– Hindbrain or rhombencephalon
– Midbrain or mesencephalon
– Forebrain or prosencephalon
• In fishes,
– Hindbrain – largest portion
– Midbrain – processes visual information
– Forebrain – processes olfactory information
35
Vertebrate Brains
• Relative sizes of different brain regions
have changed as vertebrates evolved
• Forebrain became the dominant feature
Mammals
Birds
Reptiles
Amphibians
Bony Fish
Cartilaginous Fish
Jawless Fish
Lancelets
Tunicates
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chordate Ancestor
37
Vertebrate Brains
• Forebrain is composed of two elements
– Diencephalon
• Thalamus – integration and relay center
• Hypothalamus – participates in basic drives and
emotions, controls pituitary gland
– Telencephalon (“end brain”)
• Devoted largely to associative activity
• Called the cerebrum in mammals
38
Cerebrum
• The increase in brain size in mammals reflects
the great enlargement of the cerebrum
• Split into right and left cerebral hemispheres,
which are connected by a tract called the corpus
callosum
• Each hemisphere receives sensory input from
the opposite side
• Hemispheres are divided into: frontal, parietal,
temporal, and occipital lobes
39
Cerebrum – don’t have to know each separate part
Thalamus Pineal gland
Corpus
callosum
Parietal lobe of
cerebral cortex
Frontal lobe
of cerebral
cortex
Occipital
lobe of
cerebral
cortex
Lateral
ventricle
Optic
recess
Optic chiasm
Pons
Temporal
lobe of cerebral
cortex
Pituitary
gland
Hypothalamus
Cerebellum
Medulla
oblongata
40
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Motor speech area
(Broca's area)
Central
sulcus
Frontal lobe
Motor areas involved with the
control of voluntary muscles
Sensory areas involved with
cutaneous and other senses
Parietal lobe
General interpretative
area (Wernicke's area)
Occipital lobe
Auditory area
Lateral
sulcus
Temporal
lobe
Interpretation of sensory
experiences, memory of
visual and auditory patterns
Combining
visual images,
visual recognition
of objects
Cerebellum
41
Hand
Fingers
Forefinger
Eye
Nose
Face
Gums
Teeth
Lips
Jaw
Tongue
Wrist
Leg
Genitals
HIp
Trunk
Arm
Elbow
Forearm
Hand
Fingers
Toes
Knee
Hip
Trunk
Shoulder
Arm
Elbow
Each of these regions of the cerebral cortex is
associated with a different region of the body – don’t have to know
all of these, just a reference
Thumb
Neck
Brow
Eye
Face
Lips
Jaw
Tongue
Sensor
Motor
Pharynx
42
Other Brain Structures
• Thalamus
– Integrates visual, auditory, and
somatosensory information
• Hypothalamus
– Integrates visceral activities
– Controls pituitary gland
• Limbic system
– Hypothalamus, hippocampus, and amygdala
– Responsible for emotional responses
43
Complex Functions of the Brain
• Language
– Left hemisphere is “dominant” hemisphere
• Different regions control various language activities
• Adept at sequential reasoning
– Right hemisphere is adept at spatial
reasoning
• Primarily involved in musical ability
44
Complex Functions of the Brain
• Memory
– Appears dispersed across the brain
– Short-term memory is stored in the form of
transient neural excitations
– Long-term memory appears to involve
structural changes in neural connections
– Two parts of the temporal lobes, the
hippocampus and the amygdala, are involved
in both short-term memory and its
consolidation into long-term memory
45
Complex Functions of the Brain
• Alzheimer disease
– Condition where memory and thought
become dysfunctional
– Two causes have been proposed
1. Nerve cells are killed from the outside in
– External protein: b-amyloid
2. Nerve cells are killed from the inside out
– Internal proteins: tau ()
46
Spinal Cord
• Cable of neurons
extending from the
brain down
through the
backbone
• Enclosed and
protected by the
vertebral column
and the meninges
47
Spinal Cord
• 2 zones
– Inner zone is gray matter (opposite of brain)
• Primarily consists of the cell bodies of
interneurons, motor neurons, and neuroglia
– Outer zone is white matter
• Contains cables of sensory axons in the dorsal
columns and motor axons in the ventral columns
48
Spinal Cord
• It serves as the body’s “information
highway”
– Relays messages between the body and the
brain
• It also functions in reflexes
– The knee-jerk reflex is monosynaptic
– However, most reflexes in vertebrates involve
a single interneuron
49
The Peripheral Nervous System
• Consists of nerves and
ganglia
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
– Nerves are bundles of axons
bound by connective tissue
– Ganglia are aggregates of
neuron cell bodies
• Function is to receive info
from the environment,
convey it to the CNS, and to
carry responses to effectors
such as muscle cells
5 µm
© E.R. Lewis/Biological Photo Service
50
The Autonomic Nervous System
• Composed of the sympathetic and
parasympathetic divisions, plus the
medulla oblongata
51
Don’t have to know, just for reference
Parasympathetic
Sympathetic
Dilate
Constrict
Stop secretion
Secrete saliva
Dilate bronchioles
Constrict bronchioles
Speed up heartbeat
Slow down heartbeat
Spinal cord
Sympathetic
ganglion
chain
Adrenal gland
Stomach
Secrete adrenaline
Increase secretion
Decrease secretion
Large intestine
Decrease motility
Increase motility
Small intestine
Empty colon
Retain colon contents
Empty bladder
Delay emptying
Bladder
52