🎯 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
| ION | Concentration Inside | Concentration Outside | Cell State |
|---|---|---|---|
| Sodium NA+ ➕ (Large molecule 🔷) | 50 | 460 | Resting 😴, (impermeable to NA+ inside the cytoplasm 🚫) |
| Potassium K+ ➕ (Small molecule 🔹) | 400 | 10 | Resting 😴, small molecule, moves in and out ↔️ |
| Cloride CL- ➖ (Small molecule 🔹) | 40 | 560 | Resting 😴, small molecule moves in and out of the cell ↔️ |
| Anions A- ➖ (Large molecule 🔷) | 345 | 0 | Resting 😴 (impermeable to A- outside the cytoplasm 🚫) |
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 and Equilibrium
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 🔄.
🥛 Osmosis Example: Salt in Water
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 ⚡.
🔋 Resting and 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 receive 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 💪.
⚡ 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 up to +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 ⏳.
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 ✅.
📡 Excitatory and Inhibitory Post Synaptic Potentials
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) they 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 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 Axes
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 Axis
Anterior 🔼 towards the front: the nose end 👃, and posterior 🔽 is towards back: the tail end, so all structures in the front would be anteriorly 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 Axis
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 Axis
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 👉.
🔝 Important Terms of Reference
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 Fibers
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 fibers or projections which lie on the top, whereas the lower structures, projections, fibers, 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 fibers, or nerves or structures which are on the same side, whereas the contra lateral 👈👉 would be structures, fibers or areas on the opposite side 🔄. The ipsilateral fibers 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 📥 is 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 🔼🔽.
📚 References
- Kalat, J.W. (1998). Biological Psychology. Brooks/Cole Publishing Company.
- Carlson, N. R. (2005). Foundations of physiological psychology. Pearson Education New Zealand.
- Pinel, J. P. (2003). Biopsychology. (5th ed). Allyn & Bacon Singapore.
- Bloom, F., Nelson., & Lazerson. (2001), Behavioral Neuroscience: Brain, Mind and Behaviors. (3rd ed). Worth Publishers New York
- Bridgeman, B. (1988). The Biology of Behavior and Mind. John Wiley & Sons, New York
- Brown, T.S. & Wallace, P.S. (1980). Physiological Psychology. Academic Press, New York