Vertebrate Physiology Test II

Glial cells

-support neurons structurally and metabolically (regulate pH, ion concentrations) e.g. Schwann cells and oligodendrites
-remove excess K+ from the extracellular fluid, prevent accumulation of extracellular K+ which would otherwise depolarize neurons


-use a mixture of electrical and chemical signals to transmit information
-Excitable, i.e. electrical signals can be generated across plasma membrane
-Have passive and active electrical properties

Efferent neurons

carry signals from CNS to effector regions

Afferent neurons

carry signals towards CNS


complete neural circuit

The neural circuit

is the basic unit with which all animal nervous systems are built

anucleate neurons & ex.

-neurons with no nuclei
-Fairy wasps

Electric charge (q)

-measured in coulombs (C)
�The charge on a single electron is -1.6x10-19 C
�1 mol of electrons has a charge of -96,487 C, or 1 Faraday

Flow of electric charge

-current measured in amperes (A)
�A current of 1 C/sec is one Ampere

Electric potential or electromotive force

voltage measured in volts (V)
�Can be thought of as a measure of tendency for charges to flow


measure of the impediment to current flow, measured in ohms (?)
�A resistance of 1 ? allows 1A of current to flow when a voltage of 1V exists

Ohm's law:



reciprocal of resistance (ease of current flow) measured in siemens (S)


measures ability of a nonconductor to store electric charge. Unit is the farad (F)
� a potential difference can build up across two plates separated by an insulator.
�No charges move across an insulator, but they can nonetheless interact with each other

membrane potential

-All neurons have a stable voltage or potential difference across their cell membrane
-measured by placing microelectrode inside cell and outside
-Voltage diff bt in and out ( -20 -100 mV)
-Cell interiors tend to be negatively charged

threshold potential

-critical number of Na+ channels open; Na+ floods into the neuron down its concentration gradient
- cell becomes rapidly positively charged / depolarized
-Ranges from -30mV to -50mV, but exact value depends on previous electrical activity inside the cell

Application of current in neurons can

-depolarize or hyperpolarize neurons depending on the direction of (+) current flow
-Na+ transport channels are voltage gated, and linked to the membrane potential - so as a cell becomes depolarized (i.e. less negative), those channels begin to open

action potential

- sudden spike in voltage
-Shortly after Na+ channels open, voltage-gated K+ channels open up
K+ rushes out of cell down its concentration gradient; cell becomes repolarized

delayed rectification

-repolarization of the cell after Na+ channels open
-Ion channels can be blocked by various drugs, e.g. tetrodotoxin

overshoot & undershoots

-Brief period of positivity in cell during AP
- After peaking, Vm goes back to resting value or even below

refractory period

During this period, impossible to generate another AP


-Lipid bilayer is impermeable to ions
- Consequently has properties of a capacitor, and can store charges on either side
-Ion channels that allow some particles to cross the membrane give it conductance/resistance

For a given transmembrane voltage (Vm)

-the lower the membrane resistance the more ions can cross the membrane through open ion channels per unit time

Membranes can act as ____?

capacitors by separating charges when a voltage is applied across the membrane

Ions can act _____?

-electrostatically with ions on the opposite side of the membrane
Because cell membrane is thin, charges can interact quite strongly

Capacitance of a membrane limits _____?

how fast the voltage across a membrane can change (because it takes time for charged particles to move)

electrochemical potential

is a Voltage difference across cell membranes

(Donnan equilibrium)

The charges of ions on either side of a membrane can maintain unequal concentrations by opposing the movement of similarly charged ions across the membrane

electromotive force (emf).

The force applied by the charges. is a measure of the work expended per unit of charge to produce an electric potential difference across a membrane

equilibrium potential

The potential difference maintained in this way (specifically for the ion which is able to cross the membrane)

Goldman equation

-takes into account all ion species and their relative permeabilities
-Probability that an ion will cross a membrane is proportional to the product of the concentration of the ion on one side and the permeability of the membrane to that type of ion

Contribution of an individual ion to the overall membrane potential is?

proportional to its own concentration gradient across the membrane
-Vm is close to EK because the permeability of cell membranes to K+ is high

Electrogenic pump

asymmetrical exchange of Na+ and K+ ions gives rise to a potential difference across the cell membrane

relative refractory period

AP can be generated, but require much more intense stimulation than normal. Also, the amplitude of the AP will be less than normal


If several stimuli are delivered in sequence, the membrane becomes progressively less excitable (i.e. threshold potential rises).

Phasic neurons _____?

exhibit accommodation
e.g. Pacinian corpuscles (mechanoreceptors in skin that sense vibration and pressure)

Tonic neurons

fire repeatedly, albeit with gradually reduced frequency
e.g. some olfactory neurons


Reduction in frequency

Action potentials are an example of?
Limited by what 2 things?

-positive feedback
-1. the driving force (emf) on Na+ is reduced as Vm approaches ENa
-2. Na+ channels become automatically deactivated (closed) after a certain period of time

Hodgkins cycle

-cycle of channel opening and closing and flow of Na+ ions

Voltage clamping allowed Hodgkin and Huxley to?

-to depolarize a cell to a specific voltage and hold it there
-By bringing the cells to the equilibrium potential of the individual ions, able to measured the conductance of those ions across the membrane
- work out the charges on each membrane surface by

Patch clamping

allowed researchers to record the currents passing through individual ion channels

Tetrodotoxin (TTX)

enabled researchers to estimate the density of ion channels on a given membrane surface (because TTX binds to Na+ channels, so they radioactively labeled it)

Voltage gating is driven by

a conformational change in the ion channels themselves in response to the change in Vm, and the change in charge in particular

Molecular structure of K+ channels

Four alpha protein subunits, 4 beta protein subunits for channel modulation

Molecular structure of Na+ channels

Single large alpha protein with 4 membrane-linked domains

Ca2+ channels

-extremely similar to Na+
-Ca2+ important in crustacean neurons, vertebrate smooth muscle and cardiac muscle, embryonic neurons, synapses and dendrites
-Ca2+ channels are so ubiquitous, it is thought that APs were originally driven by Ca2+ rather than Na+

Passive signals are ?

electrotonic (degrade over distance)
For long-distance conduction, graded sensory

Graded signals must ?

- make it to the spike-initiating zone to become APs
Alternate APs and passive signals along the length of a neuronal circuit


Between neurons, signals are transferred across

Chemical synapses:

APs propagated along axons prompt release of neurotransmitters. These in turn cause a change in Vm in the postsynaptic membrane (dendrites and soma of the next neuron in the circuit). This is called a post-synaptic potential. This response is graded, but

Electrical synapses:

-Much faster than chemical synapses, but subject to signal attenuation
-Current can flow in either direction through an electrical synapse
-In rectifying synapses, current flows more readily in one direction
- Electrical synapse activity can be recorded b

Passive spread of signals depends on what 2 things?

-1.movement of positive charges (ions) along the two faces of the plasma membrane
-2. resistance and capacitance of the cell - called cable properties
-These properties affect the distance of signal conduction

Why plasma membranes are insulator?

-The cytoplasm generally has less resistance than plasma membranes
-Signals decrease because current (ions) leaks out of cell through plasma membrane

Distance of propagation depends on_____?

leakiness of plasma membrane, as well as cell and membrane resistance and capacitance

length constant

How far a signal can spread

non-spiking neurons

-neurons so small that generation of APs isn't possible
-Transmit signals entirely via passive, electrotonic potentials
-Must still rely on APs to ensure signal propagation
-Signals must still be large enough to elicit release of neurotransmitters

Short or absent axons

-high membrane resistance, and hence very high length constants.
-Highly efficient and undecremented spread of signals through these neurons.
- Found in places like the vertebrate retina

APs must be

- regenerated
- Vm in one region is transmitted electrotonically (i.e. passively) to a neighboring region
-Current produced by Na+ crossing the membrane spreads longitudinally through the axon, because entering positive charge pushes charges in cytosol aw

Electrotonic transmission is not directional, yet APs travel in one direction only - why?

Because of the refractory period following AP generation - remember that neurons become temporarily hyperpolarized because of K+ conductance, and that the Na+ channels become temporarily deactivated.
Consequently, membrane that has just produced an AP can

safety factor

-to ensure the AP signal makes it to the next site of AP generation even if signal decreases because of electrotonic transmission
- Total depolarization during AP is around 100mV
- Typical threshold potential about 20mV
AP produces signal boost of around

Speed of AP propagation depends on?

cable properties of cells

Speed increases under two conditions:

(1) Internal cell resistance decreases/cross sectional area increases. Many fast-conducting invertebrate neurons are very large e.g. squid giant axons
(2) Membrane resistance increases (i.e. greater insulation). Prevents current leaking out of cell. This

Myelinated cells wrapped in what? Explain two consequences of this.

- 200 layers of the cell membranes of glial cells (oligodendrites and Schwann cells)
-1. extra lipid layers increase resistance of the neuron membrane, enhance speed and efficiency of electronic signal transmission
- 2. decrease membrane capacitance (i.e.

Entire axon must be?

unmyelinated spaces (Nodes of Ranvier) where charges can move across the membrane so that APs can be regenerated

saltatory conduction

-APs "jump" from one node of Ranvier to the next
- Electrotonic signals spread rapidly through internodes which lack voltage-gated ion channels entirely
-Conduction speed of myelinated fibres ranges from a few meters per second to more than 120 m.s-1

chemical synapses:(1) Fast or direct chemical transmission

- found in the neuromuscular junction and other places in the central nervous system.
-Release of synaptic vesicles via exocytosis into the synaptic cleft.
-Neurotransmitter molecules are very small.

chemical synapses:(2) Slow or indirect chemical transmission

-neurotransmitter release is the first of several steps that result in a change in Vm.
-Neurotransmitter molecules here are large, and synthesized from amino acids. Biogenic amines (single aa), neuropeptides (several aa).
-Postsynaptic response is slow, a

neuromuscular junction

- Where fast chemical synapses occur. Also called motor endplates or motor terminals

junctional folds

- Where Axons terminals lie in the muscle membrane
- Clusters of active zones above the folds in each membrane
Axon terminals innervating a single muscle fibre contain around
1 000 000 synaptic vesicles

Acetylcholine (ACh)

- diffuses into synaptic cleft down concentration gradient towards active zones
- Binds to ACh-specific receptors in the postsynaptic membrane of the neuromuscular junction, causing Na+ and K+ selective channels to open

ACh is then hydrolyzed by

acetylcholinesterase (AChE). This limits time during which ACh is active

endplate potential (epp)

Depolarization of motor endplate associated with ACh

Application of curare to neuromuscular junction

-prevents muscle from contracting
- binds ACh receptors
- reduces the magnitude of the epp to below threshold potential so no AP can be generated

postsynaptic current

The change in rate of ion flow across the postsynaptic membrane

miniature endplate potentials (mepps)

Spontaneous "miniature" depolarizations are found to occur near postsynaptic membrane at motor endplates,

quanta or individual vesicles

represent "units" of transmitter release

Quantal release

100-300 vesicles each containing around 10,000 neurotransmitter molecules typically released from axon terminals, which activate around 2000 postsynaptic channels.

Readily releasable pool

fraction of vesicles in axon terminal that are able to be released at a given time

Synaptic delay -

time taken for neurotransmitters to cross synaptic cleft and bind with active zones

Non-spiking release

some neurons which do not generate APs are nonetheless able to release neurotransmitter


Binding to postsynaptic membrane is mediated by proteins called

Fast direct:

-acetylcholine is excitatory at the sympathetic neuromuscular junction; gamma-aminobutyric acid (GABA) and glycine are inhibitory in the central nervous system; glutamate is excitatory in CNS. Neurons releasing ACh are called cholinergic

Slow indirect:

-act via secondary messengers within the cell called G-proteins. Biogenic amines (amino acid derivatives) include epinephrine, norepinephrine, dopamine, catecholamines and serotonin.
- Neurons releasing norepinephrine are called adrenergic. Neuropeptides

Nicotinic ACh receptors

-bind nicotine, are responsible for fast, direct neurotransmission
-Nicotinic receptors occur in neuromuscular junction.
-Permeable to both Na+ and K+; Erev somewhere in between ENa and EK


bind receptors and inhibit their action (e.g. curare).

Muscarinic ACh receptors

-bind muscarin (mushroom toxin)
- responsible for slow, indirect neurotransmission

electroplax organs

Muscarinic ACh is studied through theses organs of teleost fish

G-protein alpha subunit

Alpha portion binds guanosine diphosphate and guanosine triphosphate
-GDP bonds= inactive
- GTP bonds=active
Receptor binding causes GDP to be replaced with GTP

muscarinic receptors & G-proteins

tend to open K+ channels which hyperpolarize the cell and are thus inhibitory


-Slow indirect transmission takes longer, but much longer-lasting
- slow transmission can modulate effects of fast synaptic transmission
- slow transmitter neuromodulation can affect more than one target neuron

Presynaptic inhibition

1.reduces the amount of neurotransmitter released from excitatory neuron terminal
2.May increase conductance of K+ or Cl+ or render Ca2+ channels less responsive to depolarization
3.Net effect postsynaptic membrane receives less neurotransmitter


any one signal is extremely small and often not enough to generate an AP by itself

Integration at synapses

-Each neuron integrates all input it receives (excitatory, inhibitory, slow and fast) from hundreds or thousands and will either fire to produce an AP or not
-Processes linking synaptic potentials are complex
-Depends on electrotonic spread of postsynapti

Inhibitory neurons tend to?

lie close to spike-initiating zone; are able to block excitatory signals effectively and ensure APs only fire when needed

Signals in dendrites and soma spread ? & Why is it important?

- electrotonically
- size of signal decreases with distance
-Location of synapses therefore important
-synapses further away from spike-initiating zone are less effective


When several postsynaptic potentials add together to affect postsynaptic Vm,

spatial summation

when signals come from two or more different locations

Temporal summation

when high-frequency inputs occur at a specific rate. Successive potentials can "piggy-back" onto each other

Nervous system

-Nervous systems present in all animals
-Basic principles of neural function are the same in all taxa

central nervous system

-As nervous systems become more complex, neurons are compacted
- Within the CNS, cell bodies of neurons are placed in close proximity with each other, increasing opportunities for interconnections

reflex arc & 3 types

Basic pathway for information flow in the nervous system. Single cell, monosynaptic reflex, polysynaptic reflex

single cell

may be both receptor and effector neuron

Monosynaptic reflex arc

comprises a sensory nerve synapsing onto an effector

Polysynaptic reflex arc

includes one or more interneurons

Increasing number of interneurons ?

presents greater potential for behavioral complexity via intergration and neuromodulation

parallel processing & route of information

-permits the system to analyze information more rapidly than a linear fashion.
- convergence of information into clusters of neurons allows comparison and integration of information from various kinds of sensory input and from multiple processing areas of

comparative method

-method used to understand nervous system evolution, cannot be studied through the fossil record

Nervous system evolution based on____ ? & what trends?

- elaboration of the basic reflex arc
-Trend towards collection of neurons into CNS and ganglia
-Trend towards cephalization - grouping together of similar functional neurons in anterior part of body
-Relative size of brain regions in each species reflect

Spinal cord def. & segment regions

-major dorsal nerve cord in all vertebrates
1.cervical 2.thoracic 3. lumbar 4.sacral

a) white matter b) gray matter

a) after shiny white color of myelin sheaths on axons
b)contains soma and dendrites of interneurons and motor neurons

spinal canal

Fluid-filled central cavity in spinal cord

cerebrospinal fluid

continuous with that of the cerebral ventricles in the brain

Afferent neurons enter

CNS through dorsal roots

Efferent neurons carry information

out via ventral roots

1.ventral horn
2. dorsal horn
3.dorsal root ganglia

1. Soma of spinal motor neurons located in ventral gray matter
2. Soma of afferent neurons in dorsal gray matter
3. where sensory neurons synapse onto afferent interneurons

neural tube

-Forerunner of nervous system
-develops during gastrula-stage
-From this, three vesicles (precursors of fore-, mid- and hindbrain) develop,regions later subdivide further

Medulla oblongata

-where the brain joins spinal cord
-Contains centers controlling respiration and autonomic function
-Relays messages from sensory system (e.g. equilibrium and hearing)
- relays messages to motor centers


-dorsal to medulla oblongata
-Pair of smooth (in lower vertebrates) or convoluted (in higher vertebrates) hemispheres
-Contributes to coordination of output
-Compares and integrates information from semi-circular canals and proprioceptors
-Controls postur


- a relay station
- consists of fiber tracts connecting different regions of the brain
-Contains tectum or optic lobe


-receives and integrates visual input, but also auditory and tactile
-Information organized into "maps" from different modalities


modalities become confused (e.g. can see sounds as colors)


-a major coordinating center for both sensory and motor signalling
-Sensory information sent from thalamus to sensory regions of cerebral cortex
-Motor information received by thalamus from cortex as well


-processes information and organizes output relating to emotions
-It is linked to hypothalamus


-includes centers controlling temperature regulation, thirst and appetite
- also involved in emotional responses such as excitement, pleasure and rage

Neuroendocrine cells

-inside hypothalamus control water and electrolyte balance, pituitary gland action

Olfactory region

- Anterior of brain
-especially large in those vertebrates that rely heavily on smell
- only sensory modality that is not processed through the hypothalamus in mammals, but rather travels directly to the cerebrum

Cerebral cortex

-Surface highly convoluted - increases surface area for neurons
-Surface gray matter organized into sublayers
-organized into functional regions (e.g. sensory, motor)
-In "higher" vertebrates, association regions are neither clearly sensory nor clearly mo

Autonomic nervous system

-sympathetic and parasympathetic
- When animal is relaxed or asleep, parasympathetic pathway is dominant
-When excited or frightened, sympathetic is dominant

Autonomic reflex arc

-processed in brain - otherwise afferent automatic reflexes identical to somatic

Efferent reflex arc

- motor output carried by chains of neurons