🎯 Objectives
These lessons would familiarize the students with:
- 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, Midbrain, Hindbrain functioning of each anatomical location in the CNS 🎯
🧠 Brain and Spinal Cord
When we study the brain 🧠 and the spinal cord 🦴, we will first study the basics of neuroanatomical structure 🏗️ and systems 🔌. The basic component, like all systems in the body 👤 is the cells 🧫. The building blocks which the Nervous System is composed of are the neurons 🧠, the brain cells and the glial cells 🔬. There is also the cerebrospinal fluid (CSF) 💧 which cleans 🧹 and insulates the brain 🧠.
🧠 Neurons: The Specialized Cells
Neurons are the specialized cells of the Nervous System 🔌. They are critical in the reception 📥, conduction 🔌 and transmission 📤 of information (many things in between, like chemical processes 🧪). There are about 10-12 billion neurons 🧠 in the adult brain (as many stars ⭐ in the Milky Way 🌌). For each neuron 🧫 there are about 10-12 Glial cells 🔬. Make a very crowded brain 🧠👥, especially if their processes are also included 🌿.
We would be discussing these first followed by the cerebrospinal fluid 💧, the blood brain barrier 🛡️ and lastly the neuron 🧠.
🔬 Glial Cells: The Support System
These are the supportive cells found in the brain 🧠. They have many important functions 🎯:
- A) Structural support 🏗️: They hold neuronal systems together 🤝. The synapse 🔗, the neuron 🧫, the synapse 🔗, and the dendrites 🌿 are supported and held in position by the glial cells 🔬.
- B) Housekeeping 🧹: They do the housekeeping chores such as moving out the dead cells 💀 and keep the intra and extracellular space clean of debris 🧹
- C) Nutrient provision 🍽️: They also provide nutrients to the cell 🧫 and its processes 🌿
🔬 Three Major Types of Glial Cells:
The astrocytes ⭐, the oligodendrocytes 🧬, phagocytes 🦠, and microglia 🔬.
⭐ Astrocytes
The Astroglial extensions cover blood vessels 🩸 and capillaries 💉 (as insulation 🛡️); these glial cells form the blood brain barrier 🛡️ (V.IMP ⚠️). Astrocytes cover the neuronal cell bodies 🧫 and their branches 🌿 to keep them in place 📍 as well as separated from the fine branches 🌿. These also provide nutrients 🍽️ and chemicals 🧪 which pass through blood 🩸 into the cell 🧫.
🧬 Oligodendrocytes
This is a glial cell which sends out several layered extensions 🌿 which wrap around the axons 🔌. These are rich in myelin 🧬 which is a fatty sheath 🧈. This sheath provides insulation 🛡️ and support 🏗️ to the axons 🔌 and the dendrites 🌿. This increases the efficiency ⚡ and speed 🏃 of transmission 📡. In axonal processes 🔌, the gaps between the folds are known as the Nodes of Ranvier 🎯, for messages to renew and jump across ⚡. One Oligodendrocyte can send extensions to many axons at the same time 🔗🔗🔗.
🔬 Microglia
Smaller glial cells 🔬 which keep the cells clean 🧹 by moving debris out of the cell ♻️ and move them out of the cell 🗑️.
🦠 Phagocytes
These are like the little Pac-man 👾 eating away all debris unattended 🗑️ and dead cells 💀. They go around eating up and removing them 🔄♻️.
Glial cells not passive providers or caretakers ❌; they are actively involved in chemical transmission 🧪⚡. They control 🎮, establish 🏗️, and maintain 🔧 synapses 🔗.
🛡️ Protection of the Brain
Brain is a highly protected area 🧠🛡️; there are many levels of protection 🏰:
- 1st Layer: The brain is encased by a bony skull case covering 💀🛡️
- 2nd-4th Layers: Additionally three coverings known as the 3 meninges 🧬, which are connective tissues holding the brain in a protective net covering 🕸️
🧬 The Three Meninges:
1️⃣ Dura Mater (Tough Mother 💪)
The outer most meninx is known as the Dura mater 🛡️ (translated from Arabic "tough mother" 💪). This is the tough outer most covering of the brain 🧠, white colored ⚪.
2️⃣ Arachnoid Membrane (Web-like 🕸️)
The second layer lying inside the Dura mater is the arachnoid membrane 🕸️ - a web-like structure (made of spongy filaments like a wire mesh 🧵). Beneath arachnoid membrane lies subarachnoid space 💧 where many large blood vessels 🩸 (part of the vascular system) and the cerebrospinal fluid (CSF) 💧 floating around. This provides protection 🛡️ and the blood supply 🩸. A network of blood vessels 🔗 can be seen if we open this space 🔍.
3️⃣ Pia Mater (Soft Mother 🌸)
The innermost covering – the most delicate membrane is known as the Pia Mater 🌸 (Arabic translated into Latin = the soft or pious mother 💕). This sticks to every convolution 🌊, every groove 🌀, thereby ensuring covering is comprehensive ✅. As can be seen the brain is extremely well protected against injury 🛡️ (protection against jolts 💥).
💧 Cerebrospinal Fluid (CSF)
This fluid fills the arachnoid space 🕸️, the spinal cord 🦴 (central canal 🔌) and the ventricles of the brain 🧠. These are connected through a series of openings 🚪, and the CSF travels through the brain 🧠 and the spinal cord 🦴. Essentially it is one big fluid reservoir 💧🏊. This fluid supports the form 🎯 and shape 📐 of the brain 🧠 (brain is very soft tissue 🧠💫), from inside and outside 🔄.
🏛️ The Ventricular System
The cerebrospinal fluid traverses through the brain using the ventricular system 🏛️. There are four ventricles 💧:
- 1st & 2nd Ventricles: The first two (laterally placed ⬅️➡️) are very large cavities 🏛️ continuing in both hemispheres 🧠
- 3rd Ventricle: Lies in the midbrain ⚡ at the level of diencephalic area 🎯
- 4th Ventricle: Found lower at the brain stem/cerebellar level 🦴. This is connected to the central canal of the spinal cord 🦴🔗
The Choroid plexuses 🧬 in the PIA mater produce the CSF 💧. Small capillaries that get through the PIA mater lining produce the fluid 💉. This is constantly being produced 🔄 and circulated 🌊. It is about 125 milliliters 💧 and replacement of half of it takes place every 3 hours ⏰, indicating a continuous circulation 🔄. Blocking of the fluid 🚫 or any infections 🦠 may alter the level of functioning ⚠️.
Cerebral aqueduct 🌊 is the link between the 3rd and 4th ventricles 🔗. The CSF is made up of water 💧, proteins 🥩, gases 💨, glucose 🍬, and other chemical ingredients 🧪.
🛡️ Blood Brain Barrier
This is not a fence or a barrier which is visible ❌, it is essentially cerebral blood vessels 🩸 and the glial cells 🔬 in the brain very tightly and densely packed together 📦. These provide protection through the insulating glial cells 🛡️, creating difficulty for large molecules to pass through 🚫 such as some Proteins 🥩, but large glucose molecules 🍬 to actively transport through blood vessel walls 🩸. The Blood brain barrier is also selective depending on the locations 🎯. It makes some substances easier to pass than others ✅❌ at some specific locations 📍.
Thus, we see the complexity 🧩 of the brain emerging through the various specialized parts 🎯, the glial 🔬, the CSF 💧, the Protection of the brain 🛡️, and the glial cells 🧫. They all work to keep the complex system of the brain functioning smoothly ⚡✨.
🧠 Neurons: Structure and Types
There are many types of neurons which have been identified using:
- a) Silver staining 🔬🌟
- b) Electron microscopic techniques 🔬⚡
- c) The Golgi and other histological/cytological techniques 🧪
🧫 Main Features of Neurons:
The neuron mainly has three distinct features:
- a) The cell soma 🧫: The cell body
- b) The axon 🔌: One and only output end which carries the commands out of the cell 📤
- c) The dendrites 🌿: Which bring in messages and information to the cell 📥
🔀 Types of Neurons
The neurons fall in three major categories:
1️⃣ Unipolar Neurons
The Unipolar is the neuron which has only one process ① emerging out of the cell body 🧫 and extending to both ends ⬅️➡️ for quicker communication ⚡.
2️⃣ Bipolar Neurons
The Bipolar neurons are neurons with two poles ②, one axon 🔌 (output end 📤) and dendrite 🌿 (input end 📥) and these are mainly for horizontal communication ↔️ as the sensory neuron 📡, found in the eye 👁️ and the ear 👂.
3️⃣ Multipolar Neurons
This is the most commonly found neuron ⭐. This type of neuron has more than two processes ③+, i.e., there is always one axon 🔌 but multiple dendritic connections 🌿🌿🌿. These again fall into different categories:
- Neurons with short branches 🌿 (such as the Astro type cells ⭐)
- Interneurons 🔗 with shorter or no axons ❌🔌, for quick integration and processing of information ⚡
- The Pyramidal cell 📐 which have very long Apical dendrites 🌿📏 but short axon 🔌📉
⚡ Neurons: Special Characteristics
Neurons are different from the cells in the body 👤 because of two main properties:
a) Long-distance Conduction 🏃📏
They can conduct bioelectric signals over long distances 📏 without loss in signal strength 💪, unlike the sound waves 🔊 which become dimmer and dimmer with distance 📉. These carry signals at exactly the same strength 💯 from the point of beginning to the end ⚡.
b) Multiple Intercellular Connections 🔗🔗🔗
The Intercellular connections with other cells 🧫 and tissues such as muscles 💪 and glands 🧬 are multiple 🔗🔗. The information which can be sent out 📤 and which can be received 📥 by the neurons is determined by the connections it has 🔗. There are many connections 🔗 and for each there are specialized neurons 🎯 and groups of neurons 👥. When neurons group together 🤝, they are known as nuclei 🎯. Neurons clustered into bunches of nuclei 🎯; tracts are fiber systems 🔌 connecting these nuclei 🔗.
🔀 Neurons Types: Heterogeneous
Neurons are heterogeneous with respect to cell size 📏, shape 📐 etc. because the kind of work the cell has to do 🎯 depends on the location 📍, the systems 🔌, the connections 🔗 and the neurotransmitters 🧪 that it is specialized in. (Motor neurons 💪 are different from the sensory neurons 📡, visual cells 👁️ are different from the auditory cells 👂)
Cells in all cases are composed of the soma - perikaryon 🧫 (surrounding the nucleus), the axon 🔌 (efferent: output 📤), the dendrites 🌿 (receiving and input 📥), and majority of the neurons are multipolar 🔀.
💻 Neuronal Codes of Communication
How does information get processed 🧠 and transmitted 📡 by the neurons? Information is coded in two different codes 💻:
💻 Digital Code
This is the code used by the neuron to pass information from one end to of the neuron to the other ⬅️➡️. This is like the Morse code 📟, where the changing number of dots and dashes 📊 change the message sent out 📤. In this the Rate of change is constant ⚡ and in the same unit when the axonal end 🔌 talks to the soma 🧫 and the dendritic ends 🌿. This determines how and what message sent 📨. This is electrical in nature ⚡; the electrical impulse is generated and sent ⚡📤.
🧪 Analog Code
This is the code used when two neurons are communicating with each other 🧫↔️🧫. This is a biochemical signal 🧪, varies with the intensity of the message 📊. The more intense the message 💪 the more Neurotransmitter released 🧪⬆️ (Chemical 🧪).
🔄 Code Transformation
Can the two codes be linked/transformed one into other 🔄? Yes ✅, this is happening constantly 🔄. Neurons are communicating to each other 🧫↔️🧫 as well as passing information within the neuron 🧫. The frequency or rate of information in the digital 💻 would lead to amount of neuro-chemical released in the synapse 🧪 and when the neurotransmitter molecules cross over to the other neuron 🔗, their contact transforms into an electrical signal ⚡. Therefore, it is a continuous Electrical⚡---chemical🧪---electrical⚡ change taking place 🔄. The neuron can communicate effectively in both information systems 📡✅.
📡 Types of Transmission
The transmission of a neuron takes place when the axon sends in the impulse to the cell soma 🔌➡️🧫, and then the cell responds by triggering a message to the dendritic synapse 🔗 (with axons/somas of other cells 🧫). This is known as the axonal transmission ⚡ and the junctional transmission 🔗.
⚡ Axonal Transmission
- a) Bidirectional ⬅️➡️: In this transmission the impulse travels both ways i.e., from the cell body to terminal 🧫➡️🔌 is known as orthodromic or anterograde ➡️, and if the impulse travels from the Terminal to cell body 🔌➡️🧫 it is known as antidromic or retrograde ⬅️.
- b) No time delay ⚡: In the axonal transmission there is no time delay ⏰❌
- c) Drug resistant 🛡️: This is not affected by drugs 💊 or other substances 🧪
- d) All or none firing 💯: This is an all or none firing 💥. Firing begins and ends with stimulation ⚡, there is no after discharge ❌
🔗 Junctional Transmission
- a) Unidirectional ➡️: Junctional Impulse travels only in one direction ➡️ from the pre-synapse to the post synapse 🔗
- b) Time delay ⏰: There is time delay between transmissions ⏳. Neuro Transmitter molecules travel across synapse 🧪🔗 .1 to .2 milliseconds ⏱️
- c) Summation ➕: Spatial and temporal summation 📊. In this transmission, the messages get summated at the axonal hillock 🎯 before a decision is made to fire or not 🔥 i.e., they reach threshold where the cell fires an action potential ⚡
- d) Drug sensitive 💊: Affected by drugs 💊, as drugs can be used to change the rate of transmission 📈📉
- e) Graded response 📊: It is not all or none transmission ❌💯 it is graded response 📊⬆️⬇️
In the next lesson we would discuss more about the neuron 🧠 and the processes which take place within the neuron 🔬.
📚 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