Basic Neuroanatomy:Control of molecules, Electrical charges, Proximal-distal

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Neurological Basis of Behavior (PSY - 610)

Lesson26

Basic Neuroanatomy

Objectives:

The student would learn about the ionic and molecular movement of the neurons and how the

electrophysiological properties of neurons change

· Systems, structure, Cells of the NS Neurons, Types of neurons, axonic and dendritic

communications,

· Neuronal conduction and functioning, ionic and electrophysiological properties,

· Localizing brain areas planes of reference (anterior-posterior etc).

· The Brain and the Peripheral systems: Brain: Forebrain, Mid brain, Hind Brain functioning of

each anatomical location in the CNS.

We have studied in our earlier lesson how the neuronal membrane is structured, and how the

phospholipids form a tight mesh from which substances and molecules have difficulty leaving or

entering. We would discuss this more in detail

Control of molecules:

In the Phospholipid layers, the movement of lipid molecule through the membrane is easier and also that

of smaller molecules. The cell membrane also allows materials to move in and out depending on the

changes in the membrane permeability. Increased Permeability mean that membrane can allow those

materials to pass which had earlier not been able to pass through, and decreased permeability means that

the gates of passing in/out are closed

Membrane permeability is determined by ionic state of membrane:

The most important task of the neurons is to communicate, and we have seen that neurons are active as

living systems. There is a constant movement of ions in the intracellular and the extracellular

membrane. This constant state of flux in which these ions (Ions are molecules which are negatively

charged, or positively charged depending on the number of electrons they carry) are moving generates

electrical charges which then enable neurons to communicate and to send out electrical signals.

Electrical charges are measured in terms of volts (milli volts in the case of neurons) and the difference

of electrical charge between the intracellular membrane and the extracellular membrane is known as the

Potential

Using a voltmeter by which we can place one electrode on the intracellular and one on the extracellular

membrane we would find that the inside has a large concentration of negatively charged ions whereas

the extracellular membrane has more positively charged ions. Thus, the inside of the cell is negative as

compared to outside and the difference in potential is recorded at -70 mV (this is about 1/15th of the

difference of charges in the household battery). This is known as the Resting Potential of the neuron. At

this stage the cell is at a Resting state. When positively charged ions enter the cell, the inside becomes

positively charged as compared to the outside, and the charge is recorded at +50 mV, the cell will fire an

action potential. The voltage difference is about 120 mV to get to an Action Potential (How?).

ION

Concentration Concentration Cell State

Inside

outside

Resting, (impermeable to NA+ inside the

460

Sodium NA

(

large

cytoplasm

molecule)

Potassium K+

400

Resting, small molecule, moves in and out

(Small

molecule)

Neurological Basis of Behavior (PSY - 610)

Cloride CL-

Resting, small molecule moves in and out of the

560

cell

(Small

molecule)

Anions A-

Resting (impermeable to A- outside the

345

(large molecule)

cytoplasm)

From Brown and Wallace (1980), and Carlson (1988), page 18

As we can see there is a high concentration of negatively charged molecules inside the cell, and these

ions are trying to equalize the two sides of the cellular membrane.

Ionic movement follows two processes to maintain equilibrium and thereby causing the movement of

electrical charge. Ions move along their osmotic/concentration gradient and electrostatic gradient. When

molecules move from areas of high concentration to areas of low concentration to create equilibrium

especially in a permeable or a semi permeable membrane this process is known as osmosis (nature

strives for equilibrium). Therefore if the concentration of ions is low on one side the ions would move to

equalize the balance on both sides. From the above table we can see that all the four would move to

equalize concentrations. This is known as the osmotic gradient.

Similarly, the law in electricity is that like charges repel and unlike charges attract, therefore molecules

would move towards balancing the electrostatic gradient.

Both the forces of osmosis and electrostatic gradient are working together continuously to create a

constant state of movement of ions.

As an example let's take a glass of water, divide it with a fine muslin cloth (or sieve) drop a teaspoon of

salt (sodium chloride = NA+ CL-) on one side only. There would be diffusion as the molecules move to

equalize both sides as one side has both NA and CL and other does not. Therefore both NA and CL ions

would move to equalize both sides of the glass moving according to their Osmotic gradient i.e. to

equalize and balance concentration. However, the sieve does not allow large ions to pass, therefore large

ions get stuck on side and the small ions move to other side, leaving CL- on one side and NA+ on the

other. Now we see the electrostatic gradient come into action, as there are negatively charged

molecules on one side and the positively charged on the other. This leads to attraction and movement of

ions again. However, only the smaller positively charged molecules can cross over. Thus, in turn

osmotic gradient moves ions to equalize, then negatively charged attract to move ions again. This

movement across the sieve causes flux in the glass.

This is the same kind of action taking place in the neuronal/axonal membrane leading to the resting and

the action potential.

In the resting state of the axon the membrane is impermeable to both large ions a) positively charged

sodium ions (which are outside) and the Anions (which are inside) and the smaller chloride (negative)

and Potassium (positive) are continuously moving back and forth according to the osmotic and

electrostatic gradients. However, this changes when the axons receives inputs from the cell soma to fire,

there is a change in the concentration of ions as the cell membrane becomes permeable and large

sodium ions rush in, making in the inside of the cell positively charged.

Sodium potassium pump:

When the cell permeability changes, large ions rush in NA+, inside becomes positively charged. The

cell becomes impermeable again, but it is stuck with the large sodium ions inside. Then, the cell

membrane uses a biological pump known as the sodium- potassium pump to push out the NA+ and

carry molecules of potassium back inside the cell. This uses up to 40% of the cell's energy as the cell is

pushing them against their osmotic gradients.

Neurological Basis of Behavior (PSY - 610)

How does the resting potential change to an action potential. The cell at the resting state is receiving

inputs form all over which are being summated at the axonal hillock. There are changes in the cell's

electrical threshold that are taking place.

The inside is negative as compared to the outside membrane, and the difference is of -70 mV. This

negativity can increase resulting in Hyperpolarization is where there is an increase negativity from -70

to -80. On the other hand the Depolarization are decreases in negativity from -70 to -65, or -60 ( these

are small depolarization) but a larger depolarization of leads to crossing the threshold and going upto

+50mV. This is an action potential which leads the cell to fire. Once the peak AP is reached, the inside

electrical charge starts becoming negative, to the point that it drops below the -70 mV.

After action potential has been fired, the cell goes into a refractory state- hyperpolarized- to about -75

mV. It will not fire, till it returns to the resting state

The action potential lasts for about 1/1000th of a second, and the refractory period can continue for about

some milliseconds

Neurological Basis of Behavior (PSY - 610)

Firing of the action potential leads to the conductance of the signal. The rate and speed of conductance

is equivalent to 224 miles/hour which is equal to 100meters per sec in cat brain, in humans it is about 60

meters per second.

The axonal conduction is an all- or- none phenomenon, the cell would fire an action potential once the

threshold is reached. The action would be completed once it begins.

Once the axonal transmission has crossed to the postsynaptic site, it can lead to two types of action:

The Excitatory Post Synaptic Potentials (EPSPs), this would cause the post synaptic site to fire an action

potential. This stimulates action in the post synaptic site.

Inhibitory Post Synaptic Potential (IPSPs) inhibits ongoing firing of the cell that it synapses on to. So

activity of the cell would be brought to a resting state.

Since there are multiple synapses on each cell ( at the dendrites, the cell soma), there may be some

which are IPSP and some which are EPSP's, these stimulations are summated and if the stimulation

crosses the excitatory threshold to arouse the cell , it would fire otherwise it would stay in the resting

state.

Spatial and temporal summation: multiple synapses are continuously adding together the EPSP's and

IPSPs received by them. There are two kinds of summations of stimulation that are carried out at the

cell soma and the axonal hillock,

A) Spatial summation: When a neuron receives inputs from several locations these can EPSP's which

create depolarization and IPSP's which lead to hyper polarization. These spread across the cell

membrane and reach the axonal hillock at the same time they are integrated and summated algebraically

if the sum is slightly negative then a small hyperpolarization would take place and the cell would go

from -70mV to -75 mV.

B) Temporal Summation: When a neuron receives input from the same location but repeatedly over

time (could be EPSP's or IPSP's) the are summed together received one after another ( how can this

happen one stimulation is received and has still not faded away, the 2nd one received adds up as does

Neurological Basis of Behavior (PSY - 610)

the third and the fourth one). After summation at the axonal hillock, the neuron may either depolarize

further or hyperpolarize

Basic Neuroanatomy: Anatomical Axis, Directions and Planes of Reference

Before we study the brain we have to understand the basic concepts of the locations, sites and their

relationship to each other is defined. Just as we use the directional reference of North-South, and East-

West in Geography, we also have specialized terms for identifying the directions in the brain

Basic neuroanatomical axis: Anterior- posterior, dorsal- ventral, lateral -medial;

In humans we follow the same system that is followed for all other animals, especially the vertebrates.

Anterior-posterior: Anterior towards the front: the nose end, and posterior is towards back,: the tail

end, so all structures in the front would be anteriorlly located and the structures in the back would be

posteriorly located. This is also known as the rostral-caudal axis (rostral: towards the face and caudal:

towards the tail, easier in animals which have tails!)

Dorsal- Ventral: This axis is easier to understand with a four legged animal or the fish than in humans.

Dorsal means towards the back for example the dorsal fin of shark of head and body, ventral is towards

the chest /stomach region or the bottom of the head. In humans the dorsal surface becomes the back side

as we stand. The top of the head, the back side facing the vertebral column are then the dorsal areas

Medial- Lateral: The third axis in which medial is used as reference for areas towards the center or the

mid line. The nose is medially located with reference to the face and ear are laterally located that is they

are located toward the sides. Therefore the brain areas towards the outside are laterally located. It is

important to remember the other terms of reference which are continuously being used with reference to

the brain and various neuroanatomical sites

Ascending- descending fibres: Descending refers to the groups of nerves/ processes which travel down

from the higher areas to lower areas: from cortex, the nerves descend to the Thalamus and from the

thalamus to the lower areas. Ascending refers to the nerves and the projections which carry messages up

to the higher brain areas.

Superior-inferior: Superior is those structures, nerve fibres or projections which lie on the top, whereas

the lower structures, projections, fibres, areas are referred to as inferior (because they lie lower than, not

because their functioning is lower).

Proximal-distal: proximal areas are those which lie closer to the brain or to each other. Those areas

which are farther are known as distally located areas. Ipsilateral-contralateral: Ipsi means the same side

and contra means the opposite side. Therefore ipsilateral would means those areas, or fibres, or nerves

or structures which are on the same side, whereas the contra lateral would be structures, fibres or areas

on the opposite side. The ipsilateral fibres would travel from the left side occipital cortex to the left eye;

contralateral would cross over at the optic chiasm to the right eye. Afferent- Efferent: afferent are those

which are bringing messages into the brain: these refer to the nerves which carry information to the

brain form the sensory areas Efferent taking info out of the brain or carry commands messages from the

brain to motor areas.

Planes of reference: When brain is dissected for studying the sections are cut and referred to in planes

of reference. Horizontal sections are cut slicing the brain through from the dorsal to the ventral areas.

(or vice-versa) the sagittal cuts are made when we move in the lateral to the medial- lateral direction.

The mid sagittal section is made through the middle of the two hemispheres at the level of the point of

joining. The frontal section is cut from the front of the brain towards the back.

Neurological Basis of Behavior (PSY - 610)

References:

Kalat J.W (1998) Biological Psychology Brooks/ Cole Publishing

Carlson N.R. (2005) Foundations of Physiological Psychology Allyn and Bacon, Boston

Pinel, John P.J. (2003) Biopsychology (5th edition) Allyn and Bacon Singapore

Bloom F, Nelson and Lazerson (2001), Behavioral Neuroscience: Brain, Mind and Behaviors (3rd

edition) Worth Publishers New York

5. Bridgeman, B (1988) The Biology of Behaviour and Mind. John Wiley and Sons New York

6. Brown,T.S.and Wallace. (1980) P.M Physiological Psychology0

Academic Press New York

Note: References 2, 3, 4, 7 more closely followed in addition to the references cited in text.

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