Nervous system

What you need

A prerequisite or corequisite for this lab (BIOL 347) is the lecture (BIOL 454, or equivalent), and the required text is Silverthorn, Human Physiology (Fourth edition), San Francisco, Pearson - Benjamin Cummings, 2007


The lectures we give are not intended to be thorough. Prof. Russell will cover these topics in Human Cellular Physiology I. Rather, this lecture is to orient you to the neurophysiology you will be seeing in Interactive Physiology. Also, I will use figures from your text.

Lecture

Fig. 8-2 Model Neuron, dendrites, cell body, axon hillock, axon, myelin, presynaptic terminal, postsynaptic dendrite

typical neuron
Connections, from other neurons, created graded electrical potentials at synapses, on dendrites and cell bodies.
Cell body integrates the synaptic excitatory and inhibitory voltages.
If there is net excitation, axon propagates the all-or-none, non -decemental action potential quickly over long distances.

Fig. 8-6b&c Myelin, multiple wrappings of membrane from Schwann cell (1 to 1.5 mm), nodes of Ranvier (1-2 microns)

Membrane is wrapped around and cytoplasm is squeezed out, leaving only alternating bands of electron density and lucency at high magnification.
Each layer of membrane has high resistance, and resistors in series block current flow through membrane.
Each layer of membrae has high capacitance which would leak current, but capacitors in series add reciprocally, decreasing capacitance and leakage.

Fig. 8.7 A graded potential (here shown through a Na+ channel in a postsynaptic membrane) will get smaller with distance (spreads decrimentally)

Introduction. "Action" potential refers to the active voltage-gating that opens the Na+ channel that allows nondecremental propagation. If that did not happen, propagation would be decremental based on the passive spread of current going down the axon and also leaking out the membrane.

Fig. 8-8a&b So a nice sized synaptic potential would not reach threshold for the action potential at the axon hillock

Fig. 8-9 An action potential (voltage as a function of time) is mediated by an increase then a decrease in Na+ permeability (early) and an increase then decrease in K+ permeability late

1950's Sir Alan L. Hodgkin & Sir Andrew F. Huxley (Great Britain)
1963 Nobel Prize in Physiology and medicine for "ionic mechanisms...excitation inhibition...nerve cell membrane"
In general, They showed what was stated above:
For action potential, Na+ channels open then close, K+ channels open (then close)

Fig. 8-10 The Na+ channel activates then inactivates

Fig. 8-12 Refractory period. Another spike cannot be triggered during a spile, and it is hard to trigger after a spike

Fig. 8-15c A spike depolarizes the axon to threshold ahead of it but cannot behind it because of the refractory period.

Fig. 8-16 Since it is not decrimental, a spike is the same size (but slightly later) as you go along the axon.

Fig. 8-17 Spikes are fast in giant axons of invertebrates and in myelinated axons of vertebrates (because of saltatory conduction)

Myelinated axons have faster propagation.
Invertebrates do not have myelin, and that is why they have giant axons.
Here's why: action potential jumps from one node of Ranvier to next, "saltatory" (leaping) conduction

Fig. 8-19 Synaptic vesicles relaease neurotransmitters to receptors

Vesicles are interesting.
Transmitter is very concentrated, there are pumps to move transmitter "up hill" (against gradient) into vesicle.
Sometimes part of transmitter synthesis is in vesicle.
Ca2+ in through voltage gated Ca2+ channel
Note that figure shows that Ca2+ activates calmodulin which activates protein kinase and that "kinase phosphorylates synapsin proteins"
There are vesicle membrane proteins, target (presynaptic) membrane proteins, and cytoplasmic proteins

Fig. 8-201 Voltage gated Ca2+ channels are necessary, and release is complicated (here is shown one process, docking protein).

Fig. 8-23 Receptors are channels or G-protein coupled receptors

Fig. 8-27a If there is enough synaptic excitation, a spike will fire

Sir John C. Eccles 1963 Nobel (with Hodgkin & Huxley) EPSP & IPSP
Postsynaptic potentials (Eccles, using spinal motor neurons)
EPSP - depolarize
increase sodium and potassium conductance
IPSP - hyperpolarize
increase potassium and chloride conductance
Excitatory and Inhibitory integrate before axon hillock "decides" to fire.

Fig. 8-27b But if there is enough inhibition, excitation will not generate a spike

Fig. 11-4 There are 2 synapses in the autonomic nervous system, one in the ganglion and one in the target (gland or smooth muscle)

Fig. 11-7 Nicotinic acetylcholine receptors (channels) are used in ganglia. At the target, adrenergic receptors are used in the sympathetic system while acetylcholine at muscarinic receptors is used in the parasympathetic system

Nicotinic Acetylcholine receptor [More on this later])
Acetylcholine is a ligand (neurotransmitter), nicotine is a pharmnacological agonist.
This receptor is a channel (for ions, giving the membrane electrical conductance [g])
Channel is ligand gated.
Sodium (Na+) and potassium (K+) shown going through pore in membrane that can be open or closed.
Sodium, higher outside the cell, is likely to go in.
Potassium, high inside the cell is likely to go out.

There is another kind of receptor, the G-protein-coupled receptor
For cholinergic transmission, the muscarinic receptor is an example

Fig. 11-11 This figure relates the autonomic nervous system to the somatic motor system and to the hormonal system of adrenalin from the adrenal medulla.

Orientation for today's lab and demonstrations

Much of this laboratory will involve demonstrations, Excel, Web of Science, EndNote, PhotoShop, etc. Also traditional (historical) physiology equipment will be demonstrated to give you a greater appreciation for the shortcomings that have veen overcome with newer equipment.

Students will work in groups on an exercise to become acquainted with the use of modern computer with interface for physiology. The tutorial utilizes the plethysmograph. This is perhaps the simplest devise to use and is most commonly used to measure the pulse in the fingers. The plethysmograph has many other creative uses in other labs.

The iWorx units are newly purchased and replace older PowerLab units that fed into system 9 Macs via a SCSI cable. These iWorx interfaces come with software and manuals and are much friendlier. However, all the future labs will be difficult without first becoming acquainted with the use of this equipment.

Exam questions from the last few years related to this lab

What component of the Schwann cell actually insulates the myelinated axon?

Tightly wound cell membrane after the cytoplasm has been squeezed out

In what part of the neuron is the action potential generated?

Axon hillock

What are the gaps between regions of myelination called?

Nodes of Ranvier

During the resting potential, what is the status of the voltage-gated sodium channels?

Closed

When acetylcholine binds the nicotinic channel, which ions will move, and in which direction will they move?

Sodium moves into the cell, and potassium moves out

Which direction does the sodium-potassium pump move potassium?

into the cell

What transmitter receptor is used in the ganglion of both sympathetic and parasympathetic nervous systems?

Nicotinic

An action potential depolarizes the membrane ahead of it to threshold, and hence the action potential propagates. Why doesn't the action potential cause an action potential to propagate behind it?

refractory period (sodium channel inactivation

Because of the high resistance and capacitance of a glass micropipette, fast signals like action potentials can be missed or greatly distorted. In other words, the micropipette acts like what kind of filter?

low pass

What does it mean to say that ion channels are "selective?"

for size, charge, etc

After the action potential arrives at the presynaptic membrane, what does the voltage gated calcium channel in this membrane do?

calcium influx involved in vesicle release

Multiple layers of tightly wound membrane are a hallmark of what neural structure?

myelin, Schwann cell

Ca+2 channels at the axon terminal open via which signal?

Action Potential/Depolarization

Which ion is most responsible for the resting potential of neurons?

K+

This phenomena ensures the unidirectionality of action potentials.

Refractory period

Giant squid axons are larger due to the lack of _____________?

Myelin

Will an IPSP usually fire an action potential?

No

All adrenergic receptors use which type of signaling pathway?

G-protein Coupled

On a standard Cartesian coordinate graph, which axis is the independent variable?

X axis

In making fine tipped microelectrodes, a very small capillary tube in heated and pulled apart. What inside of the pulling apparatus is doing the pulling?

Magnets

If fast noise is interfering with recordings on the physiograph, which type of filter would you
use?

High cut off (low pass) filter

Both the sympathetic and parasympathetic system use which receptor and neurotransmitter
at the first synapse?

Nicotinic receptor and Acetylcholine

Graded potentials decrease in size with increasing ______________.

Distance

The jump of action potentials from node to node is referred to as what?

Saltatory Conduction

The inside of the cell is __________ in respect to outside of the cell.

Negative

For most intents and purposes, Ramon y Cajal won out over Golgi, on the basis of staining techniques concerning what issue?

whether cells were separate (and hence needed (what turned out to be synaptic) communication

Myelin is wrapped around the axon many times to prevent current leakage. What happens to resistance due to these multiple wrappings?

increases

When multiple layers of myelin are added in series, what happens to capacitance?

decreases

Sodium channels in the axon have activation and inactivation gates. Give the activation and inactivation status when the neuron is at its resting membrane potential.

it is closed but not inactivated and, of course, it is not activated

Through axon membrane channels, which ion goes into the cell and which ion leaves the cell?

Na+ enters, K+ leaves

Which ion(s) can pass through a cholinergic nicotinic receptor?

Na+, K+

What causes Ca2+ channels to open in the axon terminal?

depolarization

Do action potentials decrease in size as they move down the axon?

no

If graded potentials are summed and are above threshold at the axon hillock, what will be generated?

an action potential

What type of gradient causes Na+ to rush into the cell when an action potential is present?

chemical (electrochemical) gradient

What two gradients are responsible for the membrane's resting potential?

concentration and electrical

What type of system is the autonomic division of the nervous system?

motor

Adrenergic receptors bind what hormone and what neurotransmitter?

epinephrine and norepinephrine

This page was last updated 1/23/08

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