Transcript Chapter 49 and 50 Presentations-Sensory and Motor Mechanisms

Chapters 48, 49, and 50: Sensory
and Motor Mechanisms Part I
“I like nonsense; it wakes up the brain cells.”
--Dr. Seuss
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Nerve Systems
 A neuron is a nerve cell, and there are 100 billion in the
brain.
 Except for sponges, all animals have some type of nervous
system. The thing that sets them apart is their organization.
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Nerve Systems
 Simple animals have nerve
systems classified in nerve
nets-very diffuse
organization.
 Example: Cnidarian
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Nerve Systems
 Increasing in their
complexity, nerve nets are
also associated with nerves.
 These assist with more
complex movements.
 Example: Sea stars
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Nerve Systems
 Nerve systems with greater
complexity involve
cephalization.
 This included the clustering
of neurons in the head and
bilaterally symmetrical
bodies. These are simple
CNS’s.
 Example: Planarians
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Nerve Systems
 The more complex brains as
well as ventral nerve cords and
clusters of nerve cells called
ganglia are seen in more
complex invertebrates.
 These systems have a peripheral
nervous system that connects
with the CNS.
 Example: Annelids
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Nerve Systems
 The structure of nerve system
organization is closely related
to function.
 For example: molluscs are
slow moving and don’t have a
very highly organized nervous
system.
 Example: Clams and Chitons
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Nerve Systems
 Fast moving molluscs such as
the cephalopods have more
highly organized nervous
systems.
 Example: Squids and Octupi
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Nerve Systems
 Vertebrates have a CNS
consisting of a brain and
spinal cord running along the
dorsal side of the body, along
with nerves and ganglia
comprising the PNS.
 Example: Salamander
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Supporting Cells of the Nervous
System
 Glia are the supporting cells of the nervous system.
 There are several different types, among them are:
 1. Schwann cells
 2. Oligodendrocytes
 3. Radial glia
 4. Astrocytes.
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1. Schwann Cells
 Schwann cells are associated with the PNS as are glia, and
they form myelin sheaths around the axons of many
vertebrate neurons.
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2. Oligodendrocytes
 Oligodendrocytes are associated
with the CNS and do the same
thing as Schwann cells.
 The myelin sheath generated by
these cells forms an insulation
blanket. This aids in nerve
conduction.
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3. Radial Glia
 In an embryo, radial glia form tracks along which newly
formed neurons and migrate from the neural tube during
development.
 Radial glia and astrocytes act as stem cells and give rise to
new neurons and glia.
 Animations.
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4. Astrocytes
 These provide structural support,
regulate extracellular ion
concentrations and neurotransmitter
concentrations.
 They are involved in dilating blood
vessels, increasing blood flow to
neurons, and they facilitate information
transfer.
 They induce tight junction formation in
the course of development of the CNS
helping form the blood-brain barrier.
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What is a nerve?
 A nerve is a bundle of neurons wrapped in connective tissue
that transfers information about the environment to and
from the CNS using electrical signals.
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What is a nerve?—Form Fitting Function
 The organelles of a neuron are located in the cell body. Two
extensions arise from the cell body.
 1. Axons—longer, transmit signals.
 2. Dendrites—highly branched, receive signals.
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What is a nerve?—Form Fitting Function
 Near its end, an axon divides into several branches, each
ending in a synaptic terminal.
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What is a nerve?—Form Fitting Function
 The synapse is the site of
communication between one
nerve and another.
 Neurotransmitters transmit
the signal from a pre-synaptic
cell to a post-synaptic
neuron.
http://biologyclass.neurobio.arizona.edu/images/synapse2.jpg
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Synaptic Transmission
 The transmission of
information from the
presynaptic neuron to the
postsynaptic neuron due to
an action potential can
trigger short and long term
changes—membrane
potential or signal cascades.
http://biologyclass.neurobio.arizona.edu/images/synapse2.jpg
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Synaptic Transmission
 When the organism receives information from the outside,
this information must be relayed quickly so that an
appropriate response can occur.
 This relay of information is accomplished via an action
potential which synapses with a series of other neurons along
the way—sometimes up to 100,000!
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Synaptic Transmission
 An action potential is the result of a series of events which
triggers changes in membrane voltage that has been set up by
sodium potassium pumps within the neuron’s cell
membrane.
 Disruptions in the resting membrane potential result in
propagation of the action potential.
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Membrane Potential
 Membrane potential is the basis of nearly all electrical signals
in the nervous system.
 The membrane potential can change from its resting value
when the membrane’s permeability to a particular ion
changes—due to the opening/closing of ion channels.
 Na+, K+, Ca2+, and Cl- all play major roles in nerve signal
transmission (as well as muscle contraction).
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Ion Channels
 When ion channels are always open, they are said to be ungated.
 Gated ion channels switch open and closed to one of three kinds
of stimuli:
 Stretch gated ion channels sense stretch.
 Ligand gated ion channels open and close in response to specific
signals.
 Voltage gated ion channels open and close due to changes in
membrane potential.
 Stimulating gated ion channels can trigger hyperpolarization or
depolarization.
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Ion Channel Stimulation
 Hyperpolarization results in an
increased magnitude of
membrane potential—the inside
of the membrane becomes more
negative.
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Ion Channel Stimulation
 Depolarization reduces the
magnitude of the membrane
potential—the inside becomes
less negative.
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Ion Channel Stimulation
 In most neurons, depolarizations
are graded up to a certain
threshold.
 Once a stimulus has reached a
threshold, an action potential is
triggered.
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Action Potentials
 Action potentials are all or none. They carry signals over a long
distance along axons. They are very brief, and can thus be
generated at a high frequency.
 Both Na+ and K+ voltage-gated ion channels are involved in the
production of an action potential.
 Both open due to depolarization of the membrane.
 Na+ opens 1st, K+ 2nd.
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Action Potentials
• Na+ channels have 2 gates—an activation gate and an
inactivation gate. Both must open for Na+ to get through.
 1. At resting potential, the activation gate is
closed, inactivation gate is open. (For Na+).
 Depolarization rapidly opens the activation gate
and slowly closes the inactivation gate.
 For K+, the activation gate is closed at resting
potential.
 Depolarization slowly opens the gate.
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Action Potentials
 2. When a stimulus depolarizes
the membrane, the activation
gates open on some channels
allowing some Na+ in.
 Na+ influx causes depolarization
opening more activation gates
and so on (positive feedback).
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Action Potentials
 3. When the threshold is
crossed, this positive
feedback cycle brings the
membrane potential close
to ENa (equilibrium
potential) during the rising
phase.
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Action Potentials
 4. ENa is not reached:
 -Activation gates close most
Na+ channels halting Na+
influx.
 -K+ activation gates open
causing efflux of K+
decreasing the membrane
potential.
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Action Potentials
 5. Undershoot occurs as too
much K+ leaves the cell.
Eventually, K+ activation gates
close and the membrane
returns to its membrane
resting potential.
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Action Potentials
 Schwann cells myelinate axons and contribute to rapid
transmission of an action potential—termed saltatory conduction.
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Action Potentials
 An absolute refractory period occurs when the Na+ channels
remain open which prevents the triggering of another action
potential.
 This is what prevents the backflow of a stimulus.
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Action Potentials
 A relative refractory period occurs
when the Na+ channels are newly
closed and a very strong stimulus
is needed for an action potential to
occur.
 This relative refractory period
occurs during hyperpolarization.
 This, too, works to prevent the
backflow of a stimulus.
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Action Potentials--Synapses
 When action potentials reach the ends of axons, they
contribute one of 2 general mechanisms of information
transfer.
 1. Electrical synapse.
 2. Chemical synapse.
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Synapses--Electrical
 1. Electrical synapses contain gap junctions which allow
electric current to flow from one neuron to the next.
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Synapses--Chemical
 2. Chemical synapses make
up the vast majority of
synapses.
 They involve the release of
chemical neurotransmitters
from the pre-synaptic
neurons via synaptic
vesicles.
 The neurotransmitters from
the synaptic vesicles interact
with the dendrites of a postsynaptic neuron.
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Synaptic Transmission
 The release of neurotransmitters to the neighboring cell acts
to transmit the signal and elicit the appropriate response.
Some responses are excitatory, others are inhibitory.
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How Do Nerve Systems Work?
 Information processing by the nervous system consisting of 3
stages:
 1. Sensory input
 2. Integration
 3. Motor output
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How Do Nerve Systems Work?
 These three stages are handled
by specialized neurons.
 1. Sensory neurons transmit
information from sensors that
detect external stimuli and
internal conditions.
 These receptors are usually
specialized neurons or epithelial
cells.
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How Do Nerve Systems Work?
 2. Interneurons integrate and
analyze sensory input. They
allow the spinal cord to work
independently of the brain
and provide reflexes.
 This reflex is an automatic
response to certain stimuli
and acts to protect the body
from harm—think about
touching something hot.
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How Do Nerve Systems Work?
 2. These interneurons provide
inhibitory signals to opposing
muscles allowing the reflex to
produce the desired result.
 The CNS also provides the
integrative power for the
organism—specifically the
brain.
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How Do Nerve Systems Work?
 3. Motor output leaves the
CNS via motor neurons which
communicate with effector
cells eliciting a change.
 These motor neurons can be
due to voluntary control, or
involuntary control.
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The Peripheral Nervous System
 The PNS transmits information to and from the CNS to
regulate the organism’s interaction with the enviroment.
 The PNS has afferent and efferent neurons.
 Afferent neurons bring information from the environment to
the CNS for processing.
 Efferent neurons carry the CNS response to the target tissues
(muscles, glands, organs).
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The Peripheral Nervous System
 Additionally, the PNS had two functional components: the
motor system and the autonomic nervous system.
 The motor system carries signals to skeletal muscles in response
to external stimuli.
 The autonomic system controls the internal environment by
sending signals to cardiac and smooth muscles, glands, and
organs.
 The ANS has three divisions—sympathetic, parasympathetic,
and enteric.
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The Peripheral Nervous System
 The sympathetic and parasympathetic divisions are largely
antagonistic.
 The sympathetic division regulates arousal and energy
transformations—the “fight or flight” response.
 The parasympathetic division regulates calming and a return to
normal—the “rest and digest” function.
 The enteric division controls the neural network of the
digestive tract, pancreas, and gall bladder. It can function by
itself, but it is largely under the control of the sympathetic
and parasympathetic divisions.
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Sensory Transduction
 When the body moves, this stimulates stretch-sensitive
dendrites in these receptors.
 Ion channels in the membranes of the dendrites open and
close in response to the stimuli.
 The flow of ions across the membranes of these receptors
results in a change in the membrane potential.
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Sensory Transduction
 Sensory transduction involves converting this
physical/chemical stimulus into response by the organism.
 These receptors are very sensitive.
 Usually receptive to the smallest possible amount of input.
 A single photon
 A single molecule
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Receptor Potential
 The magnitude of the input controls the frequency of the
action potentials produced.
 A large scale input results in a high frequency of action
potentials and an appropriate response.
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Receptor Potential
 The magnitude of the input controls the frequency of the
action potentials produced.
 A small scale input results in a lower frequency of action
potentials and an appropriate response.
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Amplification
 The sensory stimuli needs to be amplified during
transduction.
 This often involves signal transduction pathways, second
messengers, enzyme catalyzed reactions, and accessory
structures.
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