Category Archives: The Central Nervous System

The Striatum

The striatum is the largest collection of neurons in the basal ganglia. Composed of the caudate nucleus and putamen, the basal ganglia, as the name suggests, sits at the base of the cerebrum. It receives input from regions of the cerebral cortex, the limbic system, and the sensorimotor and motivational systems via the thalamus. In addition to the cerebrum, the striatum receives input from the brainstem including the substantia nigra and the raphe nuclei of the reticular formation. The dopamine and serotonin of these two structures serve a modulatory function. Anatomists organise the striatum on the “basis of differential connectivity and distribution of neurochemical markers” (Redgrave, 2007). Processing strong excitatory input, the striatal neural circuits generate a strong inhibitory output, which controls the output of basal ganglia further along in the motor loop.

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The major cytology of the striatum is GABAergic medium spiny neurons (MSN), making up about 95% of the total cellular structure. MSNs are organised into two groups based on the peptide they contain, substance P and enkephalin and the proportion of dopamine receptors (D1 or D2) they contain. MSNs create dense networks of axon collaterals. As projection neurons, the MSNs create this dense network by forming axon collaterals with one another. Tunstall et al, 2002 found that almost 30% form an axon collateral with a neighbouring MSN. Research has shown that the function of these collaterals is in cellular recognition and “classification of cortical patterns” (Blomeley, et al. , 2009).

The striatum is a vital part of the basal ganglia, and all pathways run through it. From the striatum onwards, the pathway either becomes direct or indirect. As shown in the figure below.

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Citations:

Blomeley, C. P., Kehoe, L. A., & Bracci, E. (2009). Substance P mediates excitatory interactions between striatal projection neurons. The Journal of Neuroscience29(15), 4953-4963.

Redgrave, P. (2007). Basal ganglia. Scholarpedia, 2(6): 1825.

Basal Ganglia: Substance P

As mentioned in my last post on basal ganglia, the majority of the striatum consists of medium spiny neurons. These medium spiny neurons are GABAergic and organised based on the peptide they contain as well the dopamine receptors they contain. One these peptides is called substance P (SP). As a neuropeptide, SP functions as a neurotransmitter as well as a neuromodulator. Other than GABA, SP functions as a neurotransmitter in MSNs. Specifically, SP-releasing neurons mediate “synaptic communication between MSNs” (Blomeley, et al. , 2009).

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Previously it was thought that striatal projection neurons like MSNs only inhibit each other; however, a study by Blomeley, et al. , 2009 has proven that they can also interact in an excitatory manner. Studies have shown the synaptic NK1 receptors, whose major receptor molecule is SP are present in a glutamteric terminals in the stiratum (Jakab et al., 1996). In the study by Blomeley, the importance of these NK1 receptors was investigated. The results suggest that SP plays a crucial role in facilitating the release of glutamate between medium spiny neurons. In other words, communication between the neurons is increased by SP attaching NK1 receptors found on the terminals of glutamate releasing MSNs.

Citations:

Blomeley, C. P., Kehoe, L. A., & Bracci, E. (2009). Substance P mediates excitatory interactions between striatal projection neurons. The Journal of Neuroscience29(15), 4953-4963.

Neuroscience: The Nervous System

he nervous system is the body’s speedy, electrochemical communication system. It consists of all the nerve cells of the peripheral and central nervous systems.

The central nervous system consists of the brain and spinal cord. The peripheral nervous system is the sensory and motor neurons that connect the central nervous system to the rest of the body. Nerves are the neural cables of the nervous system containing many axons. They are part of the peripheral nervous system. They are connected to the central nervous system by muscles, glands, and sense organs.

Sensory neurons carry incoming information from the sense receptors to the central nervous system. Interneurons are part of the central nervous system. They internally communicate and intervene between the sensory inputs and motor inputs. They are the most common type of neuron. Motor neurons carry outgoing information from the central nervous system to the muscles and glands.

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– The Peripheral Nervous System –

The peripheral nervous system is made up of the somatic nervous system and the autonomic nervous system. The somatic nervous system controls the body’s skeletal muscles. The autonomic nervous controls the glands and muscles of the internal organs. It is broken down into the sympathetic and the parasympathetic nervous system. The sympathetic nervous system arouse the body, mobilising its energy in stressful situations (fight or flight response). The parasympathetic nervous system calms the body, conserving its energy.

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– The Central Nervous System –

The spinal cord is the information highway connecting the peripheral nervous system to the brain. Ascending neural tracts send up sensory information. Descending neural tracts send down motor control information. Reflexes are the body’s autonomic response to stimuli controlled by the spinal cord. They are composed of 1 sensory neuron and 1 motor neuron that communicate through one interneuron. Because they only run through your spinal cord, they react automatically without your brain being involved in the process. The spine sends information back to the brain. Bodily pain or pleasure is controlled by the brain.

The brain receives information, interprets it and then decides on a response. It functions like a computer, receiving slightly differing images of an object from the eyes, it computes the differences and infers how far the object must be to project such a difference.

Neural networks are interconnected neural cells which, with experience, can learn, as feedback strengthens or inhibits connections that produce certain results. Stephen Kosslyn and Oliver Koening proposed to think of neural networks as networks of people. Neuron network with nearby neurons with which the can have short, fast connections. Each layer of a neural connects with various cells in the next layer. Learning occurs as feedback strengthens connections that produce certain results. New computer models simulate this process plus the excitatory and inhibitory conniptions to mimic the brain’s capacity for learning.

 

The Brain: Introduction, Brain Scans and Imagery

Scanning of a human brain by X-rays

Scanning of a human brain by X-rays

The brain enables the mind: seeing, hearing, remembering, thinking, feeling, speaking and dreaming.

Science now enables us to know about the living brain through lesions. Lesions are destroyed tissue. A brain lesion is naturally or experimentally caused destruction of brain tissue, which selectively removes tiny clusters of normal or defective brain cells without harming the surroundings. We can also probe the brain with tiny electrical pulses. Scientists can look upon on the messages of individual neurons and on mass action of billions of neurons. We can see colour representations of the brain’s energy – their consuming activity. These tools facilitated the neuroscience revolution.

The oldest method of studying the brain-mind connection is to observe the effects of brain disease and injuries. This has been going on for more than five thousand years. In the past two centuries, physicians have been recording the results of damage to specific brain areas. Some noticed that damage to one side of the brain often caused numbness or paralysis on the opposite side of body. This suggested that that somehow the right side of the body is wires to the left side and vice versa.

Other scientists noticed that damage of the back of brain disrupted vision and that damage to the left front part of the brain caused speech difficulties. These discoveries have helped scientists map the brain. Today scientists are able to electrically, chemically or magnetically stimulate various parts of the brain to record the effects. Modern electrodes are so small that they can detect the electro pulse in a single neuron.

scan2An electroencephalogram or EEG is an amplified recording of the waves of electrical activity that travels across the brain’s surface. These waves are measured by electrodes placed on the scalp when presented with a stimulus.

A positron emission tomography or PET scan is a visual display of brain activity. It detects where a radioactive form of glucose travels to whists the brain performs a given task.

A magnetic resonance imaging system or an MRI is a technique that uses magnetic fields and radio waves to produce computer generated images that distinguish among different types of soft tissue. It also allows us to see structures within in the brain. MRIs align the spinning atoms in our brain through the use of a magnetic field as well as causing a pulse of radio waves that disorients them momentarily. When the atoms return to normal spin the release detectable signals. MRIs can also detect oxygen-laden blood flow.

Citation

Myers, David G. Psychology . 6. Worth Publishers, 2001. Print.Myers, David G. Psychology . 6. Worth Publishers,2001. Print.

The Brain: The Cerebral Cortex

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The cerebral cortex is the intricate fabric of interconnected neural cells that covers the cerebral hemisphere. It serves as the ultimate control and information processing centre. Humans have larger cortexes which enables us to be more adaptable, which gives us the ability to learn and think beyond basic survival instincts.

The cerebral cortex is made up of a sheet of cells that is 1/8 of an inch think and contains approximately 30 billion nerve cells. Glial cells or glue cells as they are commonly called, hold the nervous system together. They are NOT neurons but their own category of cells. Glial cells serve to support, nourish and protect neurons by communicating with them. Scientists are currently attempting to find connection between glial cells and information transmission and memory.

brain lobes

Folds of the brain increase the brain’s surface area allowing for maximised function and activity. As most people know, the brain’s cerebral cortex consists of four lobes: the parietal lobe, the occipital lobe, the temporal lobe and the frontal lobe. The frontal lobe is the front portion of the cerebral cortex, lying right behind the forehead. The frontal lobe is involved in speaking, muscle movement, high level cognition (planning, judgment, reasoning). Damage to the frontal lobe can result in changes in social skills, libido, attention and risk-taking. The parietal lobe is the part of the cerebral cortex at the top of head, behind the frontal lobe towards the back. It includes the sensory cortex. This means the parietal lobe processes sensory information such as pain, touch and pressure. Damage to the parietal lobe results in sensory problems such as impaired verbal memory and language skills. The occipital lobe lies at the base of te head and includes the visual areas; it receives visual information from the opposite visual field. This means that what is seen by our right is processed by the left side of our occipital lobe and vice versa. The temporal lobe lies above the ears and includes the auditory areas. These two areas receive auditory information from the opposite ear much like how the eye and occipital lobe work. 

– Functions of the Cerebral Cortex –

German physicians Fritsch and Hitzig electrically stimulated the cerebral cortexes of dogs. Through their experiments, Fritsch and Hitzig found that they could make different parts of the dogs’ bodies move. However, their ability to make the dogs move through stimulation was selective. Movement was only observable when a specific arch-shaped area of the back of the frontal lobe was stimulated. This area is know known as the motor cortex. Furthermore, the physicians discovered that the parts of the body that were moved, corresponded to stimulation on the opposite side of the brain.

Neurosurgeons Foerster and Denfield also investigated the functions of the cerebral cortex through stimulation. They found that precise control requires the greatest amount of cortical space. Furthering this idea, Jose Delgado found that specific parts of the cerebral cortex correspond with certain actions. Today it is evident, through the use of MRI scans, that precise actions require overlapping cortical sites.

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The cerebral cortex specialises in receiving information from the skin senses and the movement of body parts. The greater the area devoted to specific body region, the more sensitive this area becomes. As a paradigm, our lips are far smaller than our back; however, relative to size, the cerebral cortex dedicates far greater area to our lips making them far more sensitive and kisses so enjoyable. It also explains why our backs are far less sensitive to pain than say our stomachs.

– Association Functions – 

The association areas consist of 3/4 of the cerebral cortex. Association areas are uncommitted to sensory of muscular activity. They associate with various sensory inputs with stored memories. The functions of the association areas cannot be triggered by stimulation or any other forms of probing. The existence of these areas are vital in disproving the popular belief that 90% of our brain is dormant. Our brain relies heavily on these unassociated areas for interpretation, integration and acting on processed sensory information.

Citations:

Cherry, Kendra. “The Anatomy of the Brain.” The Four Lobes (2012): n. pag. About.com Psychology. Web. 03 Sept. 2012. <http://psychology.about.com/od/biopsychology/ss/brainstructure_2.htm&gt;.

Myers, David G. Psychology . 6. Worth Publishers, 2001. Print.Myers, David G. Psychology . 6. Worth Publishers,2001. Print.

The Brain: Language

brain-diagram-1

Our language abilities result from intricate coordination of many brain areas. Learning more about the function of each of these areas allows physicians to find the root of language impairments. Aphasia is the impaired use of language but not necessarily all aspects of it.

Norman Gershwind, an American behavioural neurologist, noted the order and way in which we interpret language. First we hear or read language, which is received by the visual cortex as written words. These written words serve as visual stimulation. Then, the angular gyrus transforms the visual stimulation into an auditory code. Next, Wernicke’s area interprets the auditory code. Broca’s area, controlling our speech muscles via the motor cortex enables us to reply based on the interpreted information.

Citation:

Myers, David G. Psychology . 6. Worth Publishers, 2001. Print.Myers, David G. Psychology . 6. Worth Publishers,2001. Print.

The Brain: Plasticity

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Brain plasticity is the brain’s capacity for modification as evident in brain reorganization following immense learning, birth or damage. The younger we are, the greater our brain plasticity. As infants our brain’s capacity to change in response to stimuli is the greatest. This is due to the basic fact that as infants we have the immense responsibility to organise the world around us. Our neuroplasticity enables us to learn and begin to take in our surroundings. This plasticity is also seen in response to the learning and memorisation we experience through adulthood. Finally, however, brain plasticity is also seen following great brain damage. Plasticity enables our brains to compensate for lost function by emphasising remaining function. Infants and children have the greatest capacity to rebuild after brain damage as they still hold greater plasticity.

Master “stem cells” than can develop into any type of brain cells have been found in fetal brains. This discovery has raised hopes immensely of recovery mechanisms that would be able to mend damaged brains.

The Brain: Lateralisation of Brain Function

“The great pleasure and feeling in my right brain is more than my left brain can find the words to tell you.” – Roger Sperry

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The left hemisphere of our brain functions in reading, writing, speaking, arithmetic, reasoning and understanding. It is considered the major hemisphere because it has proven far easier to comprehend and study. Our right brain to this day is still very much misunderstood.  We do know that the right hemisphere is immensely important to our creativity, expression and social skills such as recognising faces and tone of voice. However, diseases that afflict the right side of our brain still befuddle scientists today.

Popular psychology and self-help books discuss the left versus brain dominance. Creative, free-spirited people who have excellent social skills but are poor at maths are considered right brain dominant; whereas, logical, analytical, linear people lacking creativity are considered to be left brain dominant. Like most self-help fads, there is some truth behind these claims. Most people generally rely on one side of the brain more than the other whilst thinking. This is known as brain lateralisation, a term that grew out of work by American neurobiologist, Roger W. Sperry. That is to say that, people that rely more on their left brain whilst thinking do posses a more logical, linear, objective perspective than those who rely on their right brain more. However, it is important to understand that all humans rely on both hemispheres for day to day activities and hardly anyone displays solely the characteristics of one hemisphere.

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The left and right hemispheres are connected by a large band of neural fibres called the corpus callosum. It allows for messages to be carried between the two hemispheres. Sperry along with other psychologists, Myers and Gazzinga, concluded through the splitting of the corpus callosum of animals left them relatively normal. These findings were then applied as a form of treatment for extreme cases of epilepsy – a neurological disorder marked by sensory disturbance, loss of consciousness and convulsions as a result of excessive nerve firing in the brain. In these extreme cases of epilepsy the excessive firing would start in one hemisphere but cascades into a storm of firing across the corpus callosum to the other hemisphere. A surgery in which the corpus callosum was split was seen as the only alternative to treat the worst symptoms. The surgery enabled the patients to carry out normal life without the constant life-threatening symptoms; however, through these experimental surgeries the function of the corpus callosum was discovered.

Sperry and Gazzangia found that the corpus callosum did, in fact, have significance.  It enables communication between the two hemispheres. Each hemisphere continues to learn after the operation; however, the two hemispheres remain unaware of any learning and experience of the other side.

Citations:

Cherry, Kendra. “The Anatomy of the Brain.” The Four Lobes (2012): n. pag.About.com Psychology. Web. 03 Sept. 2012. <http://psychology.about.com/od/biopsycholog

Myers, David G. Psychology . 6. Worth Publishers, 2001. Print.Myers, David G. Psychology . 6. Worth Publishers,2001. Print.

The Split Brain Experiments”. Nobelprize.org. 8 Sep 2012 http://www.nobelprize.org/educational/medicine/split-brain/background.html

 

The Cranial Nerves

cranial nerves

 

The majority of the cranial nerves emerge from the brain stem on the ventral surface of the brain. Cranial nerves I and II emerge from the forebrain and cranial nerve XI from the brain stem as well as the spinal cord. As they are nerves they are classified as part of the peripheral nervous system; however, two of the cranial nerves are also considered part of the central nervous system, I and II. Each nerve often has fibers performing many different functions. Also they have associated cranial nerve nuclei in the midbrain, pons and medulla of the brainstem.

I. Olfactory: a special sensory cranial nerve responsible for our sensation of smell.

II. Optic: a special sensory cranial nerve responsible for our sensation of vision.

III. Oculomotor: a somatic and visceral motor cranial nerve responsible for the movement of the eye and eyelid as well as the parasympathetic control of pupil size.

IV. Trochlear: a somatic motor cranial nerve responsible for the movements of the eye.

V. Trigeminal: a somatic sensory and motor cranial nerve responsible for the sensation of touch on the face and the muscles of mastication.

VI. Abducens: a somatic motor cranial neuron responsible for the movements of eye.

VII. Facial: a somatic and special sensory cranial neuron responsible for the movement of muscles of facial expressions as well as the sensation of taste in the anterior 2/3 of the tongue.

VIII. Auditory-vestibular: a special sensory motor neuron responsible for the sensation of hearing and balance.

IX. Glossopharyngeal: a somatic and visceral motor cranial neuron as well as a visceral and special sensory cranial neuron responsible for the movement of muscles in the throat and the parasympathetic control of salivary glands in addition to the detection of blood pressure changes in aorta and the sensation of taste in posterior one third of tongue.

X. Vagus: a visceral motor and sensory cranial neuron as well as a somatic motor neuron responsible for the parasympathetic control of heart, lungs and abdominal organs as well as the sensation of pain associated with the viscera and movement of muscles in the throat.

XI. Spinal accessory: somatic motor neuron responsible for the movement of muscles in the throat and neck.

XII. Hypoglossal: a somatic motor cranial neuron responsible for the movement of the tongue.

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Citation:

Bear, Mark F., Barry W. Connors, and Michael A. Paradiso. Neuroscience: Exploring the Brain. Philadelphia, PA: Lippincott Williams & Wilkins, 2007. Print.

Introduction to Basal Ganglia: Anatomy and the Motor Loop

To begin with…

Anatomy 

Studies have shown that the motor loop through the basal ganglia helps initiate conscious movement. One model has shown that furthered inhibition of the thalamus via the basal ganglia underlies what is known as hypokinesia or the reduction of movement. Contrarily, decreased output by the basal ganglia leads to hyperkinesia or the excess of movement.

Now the basal ganglia consists of several structures ,which includes the caudate nucleus, the putamen, the globus pallidus and the subthalamic nigra. Some neuroscientists also include the substantia nigra as part of the basal ganglia even though technically it is part of the midbrain. This is because the substantia nigra plays a quintessential role in the control of movement. Together with the putatmen, the caudate nucleus makes up the striatum, which is the target of cortical input to the basal ganglia. The globus pallidus controls output to the thalamus, which helps create a loop of information from the cortex back to the cortex.

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Motor Loop 

A simplified version of the motor loop:

Cortex -> Striatum -> Globus pallidus -> Thalamus -> Cortex

The impulse that drives the motor loop originates from the cortex (the frontal, parietal and prefrontal) and forms an excitatory connection with the putamen. The putamen cells then form an inhibitory connection with neurons in the globus pallidus, which then forms an inhibitory connection with the thalamus. More specifically, a part of the thalamus known as the ventral lateral nucleus or VLo. The VLo then forms the thalamocortical connection with the supplementary motor area or SMA, which is a medial region of cortical area 6 that directly sends axons to motor units.

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This may seem counter-intuitive. Why would an inhibitory signal cause the activation of a motor unit? Well, basically at rest, neurons in the globus pallidus are active. Because the neurons globus pallidus are active, this inhibits the activity of the neurons in the thalamus, specifically in the VLo. So, when the impulse from the cortex excites the putamen, the neurons globus pallidus are inhibited. The inhibition of these neurons allows the VLo to become active or excited. The activation of the neurons in the VLo sends neural activity via the thalamocortical connection to the SMA.

References:

Bear, Mark F., Barry W. Connors, and Michael A. Paradiso. Neuroscience: Exploring the Brain. Philadelphia, PA: Lippincott Williams & Wilkins, 2007. Print.