“I like nonsense; it wakes up the brain cells.” --Dr. Seuss Chapters 48, 49, and 50: Sensory and...
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“I like nonsense; it wakes up the brain cells.” --Dr. Seuss Chapters 48, 49, and 50: Sensory and Motor Mechanisms Part I travismulthaupt.com
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Transcript of “I like nonsense; it wakes up the brain cells.” --Dr. Seuss Chapters 48, 49, and 50: Sensory and...
“I like nonsense; it wakes up the brain cells.”
--Dr. Seuss
Chapters 48, 49, and 50: Sensory and Motor Mechanisms Part I
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Nerve Systems
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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.
Nerve Systems
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Simple animals have nerve systems classified in nerve nets-very diffuse organization.Example: Cnidarian
Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.
Nerve Systems
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Increasing in their complexity, nerve nets are also associated with nerves.
These assist with more complex movements.
Example: Sea stars
Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.
Nerve Systems
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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
Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.
Nerve Systems
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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
Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.
Nerve Systems
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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
Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.
Nerve Systems
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Fast moving molluscs such as the cephalopods have more highly organized nervous systems.Example: Squids and Octupi
Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.
Nerve Systems
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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
Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.
Supporting Cells of the Nervous System
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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.
1. Schwann Cells
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Schwann cells are associated with the PNS as are glia, and they form myelin sheaths around the axons of many vertebrate neurons.
2. Oligodendrocytes
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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.
3. Radial Glia
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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.
4. Astrocytes
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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.
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
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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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Near its end, an axon divides into several branches, each ending in a synaptic terminal.
Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.
What is a nerve?—Form Fitting Function
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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
What is a nerve?—Form Fitting Function
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 TransmissionWhen 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 TransmissionAn 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 PotentialMembrane 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 ChannelsWhen 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.
Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.
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Ion Channel Stimulation
Depolarization reduces the magnitude of the membrane potential—the inside becomes less negative.
Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.
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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.
Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.
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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
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.
• Na+ channels have 2 gates—an activation gate and an inactivation gate. Both must open for Na+ to get through.
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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.
Action PotentialsSchwann 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--SynapsesWhen 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--Electrical1. 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 post-synaptic neuron.
Synaptic TransmissionThe 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?
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Information processing by the nervous system consisting of 3 stages:1. Sensory input2. Integration3. Motor output
How Do Nerve Systems Work?
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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.
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 SystemThe 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 SystemAdditionally, 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 SystemThe 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 TransductionWhen 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 TransductionSensory 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 photonA single molecule
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Receptor PotentialThe 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 PotentialThe 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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AmplificationThe 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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