Agnosia - A Short Overview

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To understand what agnosia is, we must first understand what gnosia is. Gnosia, according to the dictionary is the perceptive faculty enabling one to recognize the form and the nature of the persons and things.

So agnosia is a loss of ability to recognize different things like: objects, persons, sounds, shapes, and even smells. However, the sense organ is not defective and there is also no significant memory loss involved. The whole thing is actually associated with brain injury and neurological illness, mostly occipitotemporal border (a part of the ventral stream). 

Agnosia will usually result from strokes, dementia, head injury, brain infection, but it can also be hereditary. There are some forms of agnosia that are genetic, for example dyslexia.

There are many types of agnosia which can be divided into 4 groups:

1- Auditory agnosia

This form of agnosia manifests in the inability to recognize or differentiate between sounds, but it has no connection with the hearing organ. There are three main types of auditory agnosia:

Classical auditory agnosia – the person cannot process environmental sounds such as animal noises or industrial noises.

Interpretative or receptive agnosia – the inability to understand music, it is also called amusia.

Linguistic agnosia – the people cannot comprehend words but they are capable of speech and they usually understand through sign language and reading books. However, when they are spoken to, they cannot find meaning in the sounds associated with the words.

2 - Tactile agnosia

This form of agnosia is manifested through an inability to recognize objects when touching, with the eyes closed. They are usually linked to injuries of the parietal lobe and the supramarginal gyrus.

Primary: The patients often cannot recognize the material used for the object or recognize the form and dimension of the object.

Secondary: the patients can recognize all of the above but they cannot recognize the object.

3 – Visual agnosia

The person cannot “see” objects although their sense organ is not injured. There are different types of visual agnosia:

-         objects

-         images

-         spatial

-         graphic symbols

-         associative

-     prosopagnosia or face blindness
-     blind sight

If you’ve read The man who mistook his wife with a hat by Oliver Sacks you will recognize visual agnosia.  

4- Body image agnosia

The person who suffers from this type of agnosia will lose their perception, recognition and representation of the existence of his body. This can be a total body image agnosia or a partial one.

-         asomatognosia – the person cannot recognize parts of their body

-         autotopagnosia - inability to localize and orient different parts of the body (it is linked to the parietal lobe of the left cerebral hemisphere)

-         finger agnosia – inability to distinguish the fingers on the hand (present in lesions of the dominant parietal lobe and a component of Gerstmann syndrome) 

Further reading:

http://www.psychology.sbc.edu/redwine1.htm

http://brainmind.com/Agnosia.html

PS: I wrote a guest post here http://cooking-varieties.blogspot.com/ so check that out too and I also got a blogging award :D which is great. Thanks Wan!  

Glial Cells/Neuroglia

This is another post for those of you trying to study the nervous system.

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What are Glial cells and why are they important?

Glial cells also called neuroglia are non-neuronal cells that maintain homeostasis, form myelin and provide support and protection for the brain’s neurons.

There are two types:

Microglia – protect the neurons of the central nervous system. They are capable of phagocytosis (meaning they are phagocyte). They are found in all regions of the brain and spinal cords. Their main role is to multiply when the brain is damaged. 

Macroglia

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 – contains 4 different types of cells from the central nervous system:

1. astrocytes
2. oligodendrocyts
3. ependymal
4. radial glia

-         contain 3 types of cells from the peripheral nervous system

Astrocytes:

- they have a very important role in the central nervous system

- are star shaped

- they are the most numerous cells in the central nervous system

- they can be found at the exterior of the brain and spinal cord

-they give biochemical support of endothelial cells that form the blood-brain barrier

-during brain damage or radiation they are the first affected cells

Oligodendrocyts:

-         their main function is to form and maintain myelin

      Schwann Cells

-         they have a similar function to oligodentrocytes

-         their main function is to produce myelin

-         between two cells there is a gap which formes the node of Ranvier

-         their role is also to regenerate destroyed neurons (the broken part of the axon is destroyed and the one from the cell body forms a structure that will be the next axon)

     Satellite cells:

-surround neurons in sensory, sympathetic and parasympathetic ganglia

The main function of neuroglia or glial cell is to repair injuries of the central nervous system. Some will occupy the space left by injured neurons, like astrocytes while others will help repair the neuron, like oligodendrocytes. Schwann cells help the nerve regenerate. So you can see that all these cells have a very important role in neuronal organization. 

Epilepsy - a short overview

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Epilepsy is a deterioration of the nervous system that produces sudden and intense seizures. These seizures are caused by abnormal excessive or synchronous neuronal activity in the brain.  It is a common chronic neurological disorder and it seems that about 50 million people worldwide have it. During these seizures speaking and sight are impaired and sometimes the person is not even conscious. People who suffer from epilepsy have regular seizures that occur during their life; unfortunately, without a correct treatment, they become more severe and more frequent. Even people who take the treatment correctly may suffer from seizures during their life.

However, one mustn’t consider that all people with seizures suffer from epilepsy. Sometimes seizures can be the result of a certain traumatism or another disease. Epilepsy’s causes are mostly unknown although there are a few factors people should consider. Also, they should remember that it often starts during childhood or after the age of 60.

There are two types of seizures involved in epilepsy – localized seizures or distributed (generalized seizures). Partial seizures are also divided into two types of seizures: a simple partial seizure or a complex partial seizure. The simple partial seizure happens without affecting the consciousness while the complex partial seizure will affect it.

Localized seizures are characterized by abnormal excessive or synchronous neuronal activity in a certain part of the brain and they can generalize and affect the whole body creating a secondary generalization. They all involve loss of consciousness and they are divided according to their effect on the body in: absence, clonic, myoclonic, tonic-clonic, tonic and atonic seizures.

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Risk factors that may lead to epilepsy:

-         having somebody in your family with epilepsy

-         a serious brain traumatism (fracture or a profound cerebral lesion with the loss of consciousness or amnesia)

-         cerebral tumor

-         encephalitis or meningitis

-         lead poisoning

-         Alzheimer’s disease

There are a few types of epilepsy that can cause seizures:

Benign centrotemporal lobe epilepsy of childhood or benign Rolandic epilepsy

–        occurs in children between the ages of 3 and 13 years;

–        simple, partial and often nocturnal seizures in brief episodes;

Benign occipital epilepsy of childhood

-         occurs in children with ages between 3 and 5 with late onset between 7 and 10 years;

-         convulsions involving one half of the body or hemiconvulsions

Temporal lobe epilepsy

-         the most common epilepsy of adults

-         seizures begin in late childhood and adolescence and they are complex partial seizures preceded by aura  

West syndrome

-         infantile spasm

-         onset between three months and 2 years with the most common cause to be tuberous sclerosis

Juvenile myoclonic epilepsy 
-occurs between ages 8 and 20 with myoclonic jerks and tonic-clonic seizures but also some absence seizures.

The diagnosis of epilepsy is made using an Electroencephalography (EEG)

The Neuron - Part 1 and 2

The Neuron - Part I - Introduction 

I want to make a few posts that will help psychology students or other people interested in the brain and how it works. These are just basic so don't expect them to be extremely detailed because they are only meant for beginners (like me); also, they are not meant for learning brain surgery :) When I will finish the neuron series I will also post a map that will help you even more with your recap.

The neuron is the core component of the nervous system. It is an electrically excitable cell that processes and transmits the information through chemical and electrical signaling. It is formed of a cell body and protoplasmic processes that extend from that cell body.

There are three types of neuron: unipolar, bipolar, and multipolar.

Unipolar neurons (sensory neurons which conduct impulses into the central nervous system) have one process from the cell body which splits into longer processes. They are found in the spinal ganglia and cranial nerves.

Bipolar neurons (also sensory neurons) have two processes: one axon and one dentrite. For example, bipolar neurons are found in the retina of the eye, the ganglia of the vestibulocochlear nerve and bipolar cells that transmit motor signals to control muscles. 

Multipolar neurons have multi processes and they have many dentrites and a single axon. Their function is motor or as associations (they conduct impulses and permit communications between neurons in the central nervous system)

The cell body contains

-         the nucleus (with nuclear envelope containing chromatin, nucleolus, nucleoplasm, ribosomes and nuclear pore)

-         cytoplasm (a thick liquid between cell membrane holding all the cell’s internal sub structures which are called organelles) which contains mitochondria, Golgi apparatus, lysosome, rough and smooth endoplasmic reticulum, vacuole, vesicle and cytoskeleton.

-         cell membrane

The protoplasmatic processes have dentrites and axons. Dentrites are short and transport the impulses to the cell body.

Axons have varied length to a few nanometers to 1 meter (like the axons of the sciatic nerve); they make contact with other cells like other neurons, but also muscles or gland cells through synapses.

There are two types of axons:

-         myelinated axons and unmyelinated axons

Myelin is a dielectric material composed of 40 % water; however the dry mass is about 70-85% lipids and 15%-30% proteins. If the myelin layer is think the transport is faster.  

Axons contain two types of axoplasmic transport:

-         anterograde transport (from the cell body to the synapse)

-         retrograde transport (sends chemical messages and endocytosis products  headed to endolysosomes from the axon back to the cell) is practically the way toxins and vitamins are transported to the central nervous system 

Neuron Part 2 – Synapses

A synapse is a junction that permits a neuron to pass an electrical or chemical signal to another cell. 

Two neurons are connected through a synapse. The synaptic cleft is what separates the presynaptic neuron and the postsynaptic one. Synthesized neurotransmitters are sent in the synaptic cleft and this produces feedback at a postsynaptic level. When there is an action potential at the synaptic terminal (a bulb at the end of an axon in which neurotransmitter molecules are stored and released), it releases neurotransmitters into the synaptic cleft. The receptors in the membrane or the cell body of the postsynaptic neuron receive the neurotransmitters from the synaptic cleft.

The axon terminal contains synaptic vesicles (about 39.5 nanometers) that play an important role for propagating nerve impulses between neurons and are constantly recreated by the cell. 

In most synapses the presynaptic part is located on the axon but there are some presynaptic parts located on a dendrite or soma.

There are many types of synapses and here you can see them grouped into 5 types:

Excitatory Ion Channel Synapses – synapses with neurorecepters that are sodium channels.

Inhibitory Ion Channel Synapses – synapses with neuroreceptors with chloride channels/ typical neurotransmitters are glucine or GABA

Non Channel Synapses – neuroreceptors that are not channels but are membrane- bound enzymes

Neuromuscular Junctions – synapses formed between motor neurons and muscle cells

Electrical Synapses – the membranes of the two cells actually touch and share proteins in these synapses so the action potential passes directly from one membrane to the next (can only be found in the heart and eye)

Tomorrow we'll learn about neurotransmitters and I will post the final recap map.