NEUROPHYSIOLOGY

The basic mechanisms underlying the maintenance of resting membrane potentials, action potentials, saltatory conductance etc. together with the basic ideas of neuro transmitter release should be understood.

HISTORICAL: CELL THEORY VERSUS RETICULAR THEORY

In the 19th C., it gradually emerged that plants and animals were composed of cells. However, because of the structure of neurones and the limits of microscopy/tissue stains etc, the cellular nature of the brain was slow to gain acceptance and there was some support for the proposal that the brain was composed of a 3-D net-like structure. Some of the events in the evolution of currently held views are summarised below.

1836 Purkinje (Czechoslovakia) roughly described the structures in the cerebellum which were later to be named "Purkinje cells".

1838 M. Schleiden (Berlin) proposed that plants are composed of cells.

1839 T. Schwann (Berlin).(ie as in Schwann Cells). Animals are composed of cells:this is the basis ot the cell theory.

1863 Deiters (Bonn) described the existence of an unbranched tubular process (axon) extending from some cells in the CNS.

1871 Gerlach proposed the Reticular Theory of Neurones - in this, the nervous system is a net-like structure.

1873 Golgi (as in Golgi Apparatus) discovered the "silver stain" for neurones.

I 887 W His (Leipzig) (ie as in Bundle of His) studied the embryological development of the CNS and concluded that his data was consistent with the "cell theory" and not the "reticular theory".

1888-1891 S. Ramon y Cajal (Barcelona) applied Golgi's staining method and visualized neurones in their entirety. These studies largely discredited the reticular theory.

1891 W. Waldeyer (Berlin). Cells in the CNS should be called "neurones".

1906 Ramon y Cajal and Golgi were awarded the Nobel Prize in Medicine.

I 950’s Electron microscopy finally confirmed the existence of individual cells in the CNS

NERVE CONDUCTION: HISTORICAL

1791 L. Galvani (Bologna) - (ie as in galvanic) frog muscle will contract when stimulated electrically.

1850 H. von Helmholiz (ie as in Gibbs-HeImholtz equation) speed of conduction in frog nerves is 40m/sec, i.e. it is not simply electrical. Although this was established, the nature of synaptic transmission was unclear until later.

1856 R Virchow - described neuroglia le "nerve glue"

1877 du Bois Reynard- On the basis that curare blocked conduction to muscles, he proposed that nerve conduction was chemically mediated.

1904 Elliot showed that the effects of some neurones could be mimicked by adrenergic (trom the adrenal) substances (sympathetic nervous system)

1906 H. Dale -showed that the effects of some reurones could be mimicked by some compounds (parasympathetic nervous system). This was followed the description of nicotinic effects

1924 F G Donnan -formulated the so called "Donnan Eqwlibrium" - this provided an explanation of chemical and charge effects across membranes when a large impermeable organic ion is present.

19 W. Nernst – “Nernst Potential" Allowed calculation of membrane potentials from ion concentrations inside and outside a cell.

1925 Using a 2 heart superfusion preparation, demonstrated chemical transmission. i.e the superfusate from a stimulated frog heart was shown to alter the activity of an unstimulated heart.

1935 Dale's Principle - a neurone releases the same transmitter at all its synapses. (This is somewhat compromised by the fact that it is now clear that co-transmission can occur, i.e. a neurone can release more than one neurotransmitter/neuromodulator).

1952 Hodgkin-Huxley Model -Squid axon studies - established the relationship between Na + IK + fluxes and membrane depolarisation.

1976 Neher. Sak mann and others- Single channel currents recorded from membrane of denervaied frog muscle fibres ie "patch clamping"

1981 CoIquhoun. Sakmarin and others- Measurement of fluctuations in the microsecond time range of the current through single acetylcholine receptor linked ion channels- studies of this type showed that channels may open and close many times during the brief period of receptor occupancy by a agonist.

1982 Cull-Candy Parker and others. - Measurement of the rapid kinetics of opening of channel associated with glutamate receptors i.e. evidence was obtained which demonstrated that some amino acids could have a role in brain which was unrelated to protein synthesis.

CHEMICAL NATURE OF SYNAPTIC TRANSMISSION:

HISTORICAL

During the 19th century, several investigators such as Galvani, Volta and Fontana, identified electrical activity in neuronal tissues.

1897 Sherrington- first used the word "synapse"

Elliott - suggesled that sympathetic neurones release adrenaline

1906 Dale- proposed that parasympathetic neurones release a muscarine-like substance

evidence for conduction between nerves being chemical rather than electrical came from several studies:

1. The time delay in conduction between nerves suggested a non electrical event.

2. Neuronal responses could be inhibitory.

3. No contra flow of electrical impulses between neurones were observed.

4. Amplification of signals could occur.

As mentioned above, the definitive evidence came in 1925 from Loewi's study which demonstrated that a superfusate from a stimulated frog's heart could cause contractions in a second heart preparation.

MFMBRANE POTENTIALS

When two electrodes are placed such that one is inside and the other is outside a cell it can be seen that a potential difference exists such that the inside of the cell is close to -40mV with respect to the outside: this degree of polarisation exists in virally all cell types. Membrane potentials exist and are maintained for the following reasons:

Organic acids (HA) in the neuronal cytoplasm dissociate to form H + and A-.

A- cannot freely difflise out of the cell.

Intracellular INa+ I is low relative to the extenial concentration (12OmM) due to the cell membrane being relatively impermeable to this ion.

The cell has high permeability to K+ and Cl-

The resting membrane potential is largely dependent on K + le it is the balance between the electrostatic and the concentration gradients for this ion and is approximately - 7OmV (EK) ie the inside of the cell is relatively negative Hence, the resting membrane potential of a cell can be shifted by increasing extracellular 1K + J and this fact is used in many experimental protocols.

Notes

· The transport of Na+ across membranes is much lower than K+ because it is highly hydrated and because of the Na + pump which moves Na + out of cells. 2. The membrane potential can be calculated from the Nernst Equation (above), if the ionic concentrations are known. In fact, a more appropriate equation is the Goldman Constant Field Equation which involves ionic permeabilities rather than concentrations.

ACTION POTENTIALS

Neurones differ from other cells in that they are excitable i.e. unlike a normal cell when a nerve cell membrane. such as that of the giant axon of the squid, is depolarised from its resting value (- 7OmV) to -10 to lSmV, a rapid self-limiting process occurs by which the transmembrane potential is reduce and oversh(x)ts zero, so that the inside of the membrane bec~)mes po~itive relative to the outside. This is an action potential. When the cell begins to depolarise t#om stimulation current, the current flow across the membrane is carried by K+. A~ the membrane becomes more depolarised, the resistance to Na+ decreases and more enter, the cell along its electrical and chemical gradients. As Na+ enters, the membrane becomes even more depolarised. which further increases the conductance to Na+. These changes are voltage-dependent. and self-perpetuating. They are driven by the flow of Na + t()ward.% its equilibrium distribution, which should be proportional to its original extracellular and intracellular concentrations. However, the Peak of the action potential does not attain the equilibrium potential predicted on the basis of transmembrane Na+ concentrations because of a second phase of events. The voltage dependent increase in sodium conductance and the consequent depolarisation also activates a voltage-dependent K + conductance, which causes potassium eftiux along its concentration gradient. This maintains the inside negativity ot the cell and begins to reduce the membrane conductance to sodium, thus making the action potential a self-limiting phenomenon. There is also calcium current i.e. due to C~ influx into the cell However. although calcium entry into the cell can precipitate a multitude of events, It does not contribute significantly to the action potential. In most axons, the action potential last for 0.2 - 0.5 ms. Re-establishment of the pre-action potential ionic balance across the membrane is achieved by virtue of the action of the Na/K ATPase pump which extrudes M)dlum whilst accumulating ~potassium within the cell.

Inhibitory postsynaptic polentials (IPSP's) inhibit by keeping membrane potential from Teaching threshold for spike generation. IPSP's achieve this by rapid HYPERPOLARISATION or increases in membrane potential, which reduce the probability that a cell will trigger an action potential.

NERVE CONDUCTION

There are several conce~\1Iacts which should be known in relation to nerve conduction 1) IPepolarisationlhypepolarisauon (EPSPI I PS P). 2) Miniature end plate potential (MEPP) 3) latency. 4) The spcpl of pw~gation is 2-224 mph. 5) Myelinationlnon myclination; the former condu~i impulses faster than the latter; the myelination on neurones is due to Schwann cells wrapping themselves around th axon. 6) Saltatory conductance between the Nodes of Ranvier of myelinated neurones is the reason they conduct impulses so rapidly. 7) Refractory periods (absolute and relative) during the absolute refractory period following an action potential, stimulation will not result in an action potential; during the relatively refractory period, it is difficult to generate an action potential. 8) Quantal releasel vesicular hypothesis; there is a large body of evidence eg anatomical and physiological, that neurotrarismitters are stored in presynaptic vesicles, that these vesicles fuse with the neuronal plasma membrane during depolarisation and that this results in quantal or "packet~ release of the transmitter substance(s). In addition, there is also biochemical evidence that neurotransmitters may be present in the cytosol of the nerve ending and that release can occur from this fraction: in this context, it has also been demonstrated that neurotransmitters are pr&~ably present in several discrete intracellular pools and that release from these is not uniform ic '&:)me may function as long term storage pools.