Basic Neuroanatomy:Brain and spinal cord, Glial cells, Oligodendrocytes

<< DEVELOPMENT OF THE NERVOUS SYSTEM:Oxygen level

Basic Neuroanatomy:Neuron Structure, Cell Soma, Cytoplasm, Nucleolus >>

Neurological Basis of Behavior (PSY - 610)

Lesson24

Basic Neuroanatomy

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, Mid brain, Hind Brain functioning of

each anatomical location in the CNS. Starting from the posterior located areas upto the

anterior-most. Cerebral Cortex: its layers, Corpus Callosum and the two hemispheres.

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 brains cells and the glial cells. There is also the

cerebrospinal fluid (CSF) which cleans and insulates the brain.

Neurons:

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

These are the supportive cells found in the brain. They have many important functions. A) 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) They do the housekeeping chores such as moving out the dead cells

and, keep the intra and extracellular space clean of debris c) they also provide nutrients to the cell and

its processes.

There are three major types of glial cells, the astrocytes, the oligodendrocytes, phagocytes, and

microglia.

Astrocytes:

The Astroglial extensions cover blood vessels and capilliaries (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:

Neurological Basis of Behavior (PSY - 610)

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. Firstly, the brain is encased by a

bony skull case covering, Additionally three coverings known as the 3 meninges, which are connective

tissues holding the brain in a protective net covering? The outer most menix is known as the Dura

mater (translated from Arabic "tough mother"). This is the tough outer most covering of the brain,

white colored. 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. The inner most covering the most delicate membrane is known as the Pia Mater

(Arabic translated into Latin= the soft or pious mother). This sticks to every convulation, 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

connected through a series of opening, 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 cerebrospinal fluid traverses through the brain using the ventricular system. There are four

ventricles. The first two (laterally placed) are very large cavities continuing in both hemispheres. The

third ventricle lies in the mid brain at the level of diencephalic area. The fourth ventricle is 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 milli litres 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 aquaduct is the link between the 3rd and 4rth ventricles. The CSF is made up of water, proteins,

gases glucose, and other chemical ingredients (Carlson, Pinel)

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 glials, the

CSF, the Protection of the brain, and the glial cells. They all work to keep the complex system of the

brain functioning smoothly.

Neurological Basis of Behavior (PSY - 610)

Neurons:

There are many types of neurons which have been identified using the a) silver staining b) electron

microscopic techniques c) the golgi and other histological/cytological techniques.

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:

The neurons fall in three major categories, the Unipolar, Bipolar and Multipolar.

Unipolar: The Unipolar is the neuron which has only one process emerging out of the cell body and

extending to both ends for quicker communication.

ii) Bipolar: the Bipolar neurons are neurons with two poles, 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

iii) Multipolar: This is the most commonly found neuron. This type of neuron has more than more than

two processes i.e. there is always one axon but multiple dendritic connections. These again fall into

different categories, there are neurons with short branches (such as the Astro type cells), these are

the 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) 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) 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:

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 form 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 the Digital code and the Analog code.

Digital code:

Neurological Basis of Behavior (PSY - 610)

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 the 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)

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 crosses 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.

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) 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) In the axonal transmission there is no time delay

c) This is not affected by drugs or other substances

d) This is an all or none firing. Firing begins and ends with stimulation, there is no after discharge

Junctional Transmission:

a) Junctional Impulse travels only in one direction from the presynapse to the post synapse.

b) There is time delay between transmissions. Neuro Transmitter molecules travel across synapse .1 to.2

milli seconds.

c) 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 fire an action

potential.

d) Affected by drugs, as drugs can be used to change the rate of transmission.

e) 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.

Neurological Basis of Behavior (PSY - 610)

References:

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

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

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

4 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.JohnWiley &Sons. N.Y

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

Academic Press New York

Table of Contents: