1 lecture #13 – animal nervous systems. 2 key concepts: evolution of organization in nervous...

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Lecture #13 – Animal Nervous Systems

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Key Concepts:

• Evolution of organization in nervous systems

• Neuron structure and function• Neuron communication at synapses• Organization of the vertebrate nervous

systems• Brain structure and function• The cerebral cortex• Nervous system injuries and diseases???

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All animals except sponges have some kind of nervous system

• Increasing complexity accompanied increasingly complex motion and activities

• Nets of neurons bundles of neurons cephalization

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First split was tissues;

next was body

symmetry; echinoderms “went back”

to radial symmetry

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Derived radial symmetry and nerve network

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Cephalization

• The development of a brain• Associated with the development of

bilateral symmetry• Complex, cephalized nervous systems are

usually divided into 2 sectionsCentral nervous system (CNS) integrates

information, exerts most controlPeripheral nervous system (PNS) connects

CNS to the rest of the body

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Critical Thinking

• What is the functional advantage of cephalization???

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Critical Thinking

• What is the functional advantage of cephalization???

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Cephalization

• The development of a brain• Associated with the development of

bilateral symmetry• Complex, cephalized nervous systems are

usually divided into 2 sectionsCentral nervous system (CNS) integrates

information, exerts most controlPeripheral nervous system (PNS) connects

CNS to the rest of the body

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PNS CNS PNS

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Specialized neurons support different sections

• SensoryTransmit information from the sensory

structures that detect the both external and internal conditions

• InterneuronsAnalyze and interpret sensory information,

formulate response• Motor

Transmit information to effector cells – the muscle or endocrine cells that respond to input

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Critical Thinking

• Which type of neuron would have the most branched structure??? Sensory neuronsInterneuronsMotor neurons

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Critical Thinking

• Which type of neuron would have the most branched structure??? Sensory neuronsInterneuronsMotor neurons

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Neuron structure is complex

100 billion nerve cells in

the human brain!

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Basic Neuron Structure

• Cell body• Dendrites• Axons

• Axon hillock• Myelin sheath• Synaptic terminal

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Cell Body

• Contains most cytoplasm and organelles• Extensions branch off cell body

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Dendrites

• Highly branched extensions• Receive signals from other neurons

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Axons

• Usually longer extension, unbranched til end• Transmits signals to other cells

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Axon Hillock• Enlarged region at base of axon• Site where axon signals are generated

Signal is sent after summation

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Myelin Sheath

• Insulating sheath around axon• Also speeds up signal transmission

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Synaptic Terminal• End of axon branches• Each branch ends in a synaptic terminal

Actual site of between-cell signal generation

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Synapse

• Site of signal transmission between cells• More later…

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Supporting Cells - Glia

• Maintain structural integrity and function of neurons

• 10 – 50 x more glia than neurons in mammals

• Major categoriesAstrocytesRadial gliaOligodendrocytes and Schwann cells

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Glia – Astrocytes

• Structural support for neurons• Regulate extracellular ion and

neurotransmitter concentrations• Facilitate synaptic transfers• Induce the formation of the blood-brain

barrierTight junctions in capillaries allow more control

over the extracellular chemical environment in the brain and spinal cord

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Glia – Radial Glia

• Function mostly during embryonic development

• Form tracks to guide new neurons out from the neural tube (neural tube develops into the CNS)

• Can also function as stem cells to replace glia and neurons (so can astrocytes)This function is limited in nature;

major line of research

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Glia – Oligodendrocytes (CNS) and Schwann Cells (PNS)

• Form the myelin sheath around axons• Cells are rectangular and tile-shaped,

wrapped spirally around the axons• High lipid content insulates the axon –

prevents electrical signals from escaping• Gaps between the cells (Nodes of

Ranvier) speed up signal transmission

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The nerve signal is electrical!

• To understand signaling process, must understand the difference between resting potential and action potential

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Resting Potential

• All cells have a resting potentialElectrical potential energy – the separation of

opposite chargesDue to the unequal distribution of anions and

cations on opposite sides of the membraneMaintained by selectively permeable

membranes and by active membrane pumpsCharge difference = one component of the

electrochemical gradient that drives the diffusion of all ions across cell membranes

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Neuron Function – Resting Potential

• Neuron resting potential is ~ -70mVAt resting potential the neuron is NOT actively

transmitting signalsMaintained largely because cell membranes

are more permeable to K+ than to Na+; more K+ leaves the cell than Na+ enters

An ATP powered K+/Na+ pump continually restores the concentration gradients; this also helps to maintain the charge gradient

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Resting Potential Ion Concentrations

1. Cell membranes are more permeable to K+ than to Na+

2. There is more K+ inside the cell than outside

3. There is more Na+ outside the cell than inside

• Both ions follow their [diffusion] gradients

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Critical Thinking

• If both ions follow their diffusion gradients, what is the predictable consequence???

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Critical Thinking

• If both ions follow their diffusion gradients, what is the predictable consequence???

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• A dynamic equilibrium is predictable, but is prevented by an ATP powered K+/Na+ pump

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• ATP powered pump continually transfers 3 Na+ ions out of the cytoplasm for every 2 K+ ions it moves back in to the cytoplasm

• This means that there is a net transfer of + charge OUT of the cell

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• Thus, the membrane potential is maintained

• Cl- and large anions also contribute to the net negative charge inside the cell

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Cl-, other anions, and Ca++ also affect resting potential

REVIEW

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Gated Ion ChannelsWhy Neurons are Different

• All cells have a membrane potential• Neurons can change their membrane

potential in response to a stimulus• The ability of neurons to open and close

ion gates allows them to send electrical signals along the extensions (dendrites and axons)Gates open and close in response to stimuli

Only neurons can do this!

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• Gated ion channels manage membrane potentialStretch gates – respond when membrane is

stretchedLigand gates – respond when a molecule

binds (eg: a neurotransmitter)Voltage gates – respond when membrane

potential changes

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• Hyperpolarization = inside of neuron becomes more negative

• Depolarization = inside of neuron becomes more positiveEither can occur, depending on stimulusEither can be graded – more stimulus = more

change in membrane potential• Depolarization eventually triggers an

action potential = NOT graded

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Depolarization eventually triggers an action potential – action potentials

are NOT graded

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Action Potentials ARE the Nerve Signal

• Triggered whenever depolarization reaches a set threshold potential

• Action potentials are all-or-none responses of a fixed magnitudeOnce triggered, they can’t be stoppedThere is no gradation once an action potential

is triggered• Action potentials are brief depolarizations

1 – 2 milliseconds• Voltage gated ion channels control signal

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Critical Thinking

• If the action potential is of a fixed magnitude, how do we sense different levels of a stimulus???

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Critical Thinking

• If the action potential is of a fixed magnitude, how do we sense different levels of a stimulus???

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Action Potentials ARE the Nerve Signal

• Triggered whenever depolarization reaches a set threshold potential

• Action potentials are all-or-none responses of a fixed magnitudeOnce triggered, they can’t be stoppedThere is no gradation once an action potential

is triggered• Action potentials are brief depolarizations

1 – 2 milliseconds• Voltage gated ion channels control signal

49Fig. 48.13; p. 1019, 7th Ed.

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Voltage Gate Activity

1. Resting Potential – Na+ and K+ activation gates closed; Na+ inactivation gate open on most channels

2. Depolarization – Na+ activation gates begin to open – Na+ begins to enter cell

3. Rising Phase – threshold is crossed, Na+ floods into the cell, raising the membrane potential to ~ +35mV

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1. Resting Potential – Na+ and K+ activation gates closed; Na+

inactivation gate open on most channels

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4. Falling Phase – Na+ inactivation gates close, K+ activation gates open – Na+ influx stops, K+ efflux is rapid

5. Undershoot – K+ activation gates close, but not until membrane potential has gone a little bit below resting potential

6. Refractory Period – the Na+ inactivation gates remain closed during stages 4 and 5, limiting the maximum frequency of action potentials

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Membrane repolarizes

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4. Falling Phase – Na+ inactivation gates close, K+ activation gates open – Na+

influx stops, K+ efflux is rapid

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6. Refractory Period – the Na+ inactivation gates remain closed during stages 4

and 5, limiting the maximum frequency of action potentials

67Fig. 48.13, 7th Ed.

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Conduction of Action Potential

• Electrical signal moves along the axon by depolarizing adjacent regions of the membrane past the threshold

• The depolarization effect is NOT directional – the cytoplasm becomes more + in both directions

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Critical Thinking

• If the depolarizing effect is bilateral, why does the signal travel in one direction only???

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Critical Thinking

• If the depolarizing effect is bilateral, why does the signal travel in one direction only???

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Conduction of Action Potential

• Electrical signal moves along the axon by depolarizing adjacent regions of the membrane past the threshold

• Depolarization zone travels in one direction only due to the refractory period (Na+ gates locked)

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Speed!• Diameter of axon

Larger = less resistance faster signalFound in invertebratesMax speed ~ 100 m/second

• Nodes of RanvierSignal jumps from node to nodeFound in vertebratesSaves space – 2,000 myelinated axons can fit

in the same space as one giant axonMax speed ~ 120 m/second

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Synapses – the gaps between cells

• Electrical synapses occur at gap junctionsAction potential is transmitted directly from cell

to cellEspecially important in rapid responses such as

escape movements Also with controlling heart beat (but with specialized muscle tissue)

• Most synapses are chemicalThe signal is converted from electrical

chemical electricalNeurotransmitters cross the synapse and carry

the signal to the receiving cell

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Chemical Synapses

• A multi-stage processNeurons synthesize neurotransmitters, isolated

into synaptic vesicles located at the synaptic terminal

The action potential triggers the release of neurotransmitters into the synapse

Neurotransmitters diffuse across the synapseNeurotransmitter binds to a receptor,

stimulating a response (more later)

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Chemical Synapses

1. Action potential depolarizes membrane at synaptic terminal

2. Depolarization in this region opens Ca++ channels

3. Influx of Ca++ stimulates synaptic vesicles to fuse with neuron cell membrane

4. Neurotransmitters are released by exocytosis

5. Neurotransmitters bind to the receiving cell membrane

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Chemical Synapses

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Chemical Synapses

1. Action potential depolarizes membrane at synaptic terminal

2. Depolarization in this region opens Ca++ channels

3. Influx of stimulates synaptic vesicles to fuse with neuron cell membrane

4. Neurotransmitters are released by exocytosis

5. Neurotransmitters bind to the receiving cell membrane

REVIEW

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Chemical Synapses

• Direct synaptic transmissionNeurotransmitter binds directly to ligand-gated

channelsChannel opens for Na+, K+ or both

• Indirect synaptic transmissionNeurotransmitter binds to a receptor on the

membrane (not to a channel protein)Signal transduction pathway is initiatedSecond messengers eventually open channelsSlower but amplified response

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Chemical synapses allow more complicated signals

• Electrical signals pass unmodified at electrical synapses

• Chemical signals are modified during transmissionType of neurotransmitter variesAmount of neurotransmitter released variesSome receptors promote depolarization; some

promote hyperpolarizationSignals are summed over both time and spaceRemember that many, many neurons are

responding to any given stimulus

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• Responses are summed at the axon hillockAction potential is generated and sent down

axon; or not

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• Summation is over both time and space• Excitory and inhibitory signals can “cancel”

each other

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Neurotransmitters – review text and table, but don’t memorize

Table 48.1, 7th ed.

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CNS Organization in Vertebrates• Brain – integrates• Spinal cord – 1o transmits• Both derived from hollow, dorsal embryonic

nerve cordHollow remnants remain in ventricles of brain

and central canal of spinal cordSpaces are filled with cerebrospinal fluid that

helps circulate nutrients, hormones, wastes, etcFluid also cushions CNS

• Axons are aggregated = white matter

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PNS Organization in Vertebrates

• Major role – transmitting information from sensory structures to the CNS; and from the CNS to effector structuresNerves always in left/right

pairs that serve both sides of the body

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• Cranial nerves originate in brain and connect to the head and upper bodySome have only sensory

neurons (eyes, nose)• Spinal nerves originate in

spinal cord and connect to the rest of the bodyContain both sensory and

motor neurons

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Critical Thinking

• Can the eyes do anything besides see???• Can the nose do anything besides smell???• Can the ears do anything besides hear???

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Critical Thinking

• Can the eyes do anything besides see???• Can the nose do anything besides smell???• Can the ears do anything besides hear???

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• Cranial nerves originate in brain and connect to the head and upper bodySome have only sensory

neurons (eyes, nose)• Spinal nerves originate in

spinal cord and connect to the rest of the bodyContain both sensory and

motor neurons

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PNS – Sub-divisionsAll work together to

maintain homeostasis and

respond to external stimuli

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PNS - Somatic

• Nerves that transmit signals to and from skeletal muscles

• Respond primarily to external stimuli• Largely under voluntary control

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PNS - Autonomic

• Nerves that control the internal environment• Respond to both internal and external

signals• Largely under involuntary control• Three sub-divisions

Sympathetic – stress responsesParasympathetic – opposes sympatheticEnteric – controls digestive system

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PNS – Autonomic

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Autonomic - Sympathetic

• Activates flight or fight responses

• Promotes functions that increase sensory perception and ATP levels

• Inhibits non-essential functions such as digestion and urination

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Autonomic – Parasympathetic

• Returns body systems to base-line function

• Promotes digestion and other normal functions

• Usually antagonistic to sympathetic division

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Autonomic – Enteric

• Specifically controls the digestive system• Regulated by both the sympathetic and

parasympathetic divisions

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Brain Development

1 lecture #13 – animal nervous systems. 2 key concepts: evolution of organization in nervous...

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