Constant Theme of the Course

COGS 17 – Neurobiology of Cognition

Handout #2, Wednesday, October 8th, 2008

Constant theme of the course:

BRAIN à MIND

STRUCTURE à FUNCTION

There is a physical basis for the mind, & we are studying it.

It’s really easy to get caught up in the little details of the neurobiology, but what will help you is if you constantly try to understand WHAT this means for cognition. We’re studying the structure… so what is the function? We’re studying the brain… so how does it define the mind?

Chapter 1 reading (Week 1, Tuesday)

Consciousness: an awareness of self, & requires the ability to communicate with self and others about self.

Three pieces of experimental evidence for consciousness:

Phenomenon / What is it? / What does this tell us about consciousness? / How?
Blindsight / This occurs when one is unaware of a certain perception of a sensation, and the corresponding sensory information is able to guide that person’s behavior. / Consciousness (awareness of an object in the visual field) requires a special area of the brain, the “mammalian visual system”; it is not a general property for all parts of the brain. / What is sensed by the eye is fed into two separate visual systems.
Primitive: no awareness, simple behavior. Mammalian: awareness, speech.
Unilateral neglect / Damage to the R parietal cortex, which is normally used for awareness of body and space, causes a failure to both notice and remember what is in the L visual field. / Consciousness (awareness of body and space) requires the R parietal cortex. / The R parietal cortex receives visual input from the R occipital cortex, which receives sensory input from the L field of both eyes. Memories are also lateralized.
Split brain / In attempts to treat epilepsy (spreading & excessive neuronal activity), the corpus callosum is cut. Now, each hemisphere can still work, but has no idea what is going on with the other.
* Patient will say, “I see (R)”, but cannot see (L).
* Patient will point with L hand to (L). / Consciousness (awareness of object in left or right visual field) requires visual information to reach the verbal areas in the left side. / Each hemisphere:
- receives visual input from the opposite side
- controls movement for the opposite side
Left hemisphere:
- understands verbal instruction
- produces words

Chapter 2 reading (Week 1, Thursday)

Information is gathered from the environment by sensory neurons. Movements are carried about by muscle contractions which are controlled by motor neurons. There are also interneurons that communicate between the sensory neurons and motor neurons, located entirely within the central nervous system. There are two types of interneuron: local, which form circuits with nearby neurons and are responsible for small pieces of information, and relay, which connect circuits of local interneurons in different brain regions.

The nervous system is the system in the body that receives input from the environment, makes sense of it, and controls behaviors as a response. It is divided into the central nervous system and the peripheral nervous system. Of the central nervous system, there is the brain and the spinal cord. The basic functional unit of the nervous system is the neuron. Here is a neuron:

The neuron, or brain cell, has several key features. The soma contains the nucleus, which essentially controls the cell. Extending from the soma are the dendrites, which receive input from surrounding neurons. The axon is the long structure that transmits information along the cell in the form of an electrical signal known as the action potential. Surrounding the axon is myelin, which is provided by the Schwann (for the PNS) or oligodendrocytes (for the CNS). Myelination is needed in order to increase the speed of electrical conduction. Myelination works because there are gaps in the myelin, called nodes of Ranvier, which allow the electrical signal to jump from gap to gap, which is much faster than having the electrical signal travel down the entire length of the axon. This type of electrical conduction is called “saltatory". Once the electrical signal reaches the terminal button, it causes the release of neurotransmitter into the synapse, which is the connection between two neurons. The released neurotransmitter binds to receptors on the dendritic spines of other neurons, which recreate the electrical signal that is sent off down new neurons.

The internal structure of a neuron, as with all cells in the human body, is important to understand if we want to know how the neuron carries out its functions. The important organelles of the neuron are contained within the soma. The main bulk of the soma is the cytoplasm, which is a jelly-like liquid that fills the entire cell, and contains the organelles. One of the organelles, the nucleus, is very important because it contains DNA in the form of chromosomes, and in these chromosomes are genes which control what proteins are made by the neuron. Proteins are needed by all cells and have several different cellular functions; these include providing structure and serving as enzymes. Enzymes are proteins that catalyze biological reactions, and these biological reactions are needed for the neuron to live. Another important organelle is the mitochondria, which provides energy to the neuron.

Although neurons are the functional units of the nervous system and the brain, there are many other cell types that are needed for neurons to work. In the brain, there are four types of neuroglial cells:

Astrocytes / Remove excess NT and ions / “Moppers” / Astromop!
Oligodendrocytes / Myelinate neurons / “Warmers” / Oligodendrowarmer!
Ependymal / Line ventricles, release CSF / “Squirters” / Epensquirt!
Microglia / Fight pathogens / “Security guards” / Microguard!

Chapter 3 reading (Week 2, Tuesday)

Because the nervous system and the brain are so complex, researchers have come up with different ways to study the different parts of the body and the brain.

There are three different axes by which we study the body and the brain.

Anterior-posterior: refers to near the head/near the tail.

Ventral-dorsal: refers to toward belly/back, or toward the bottom/toward the top.

Lateral-medial: refers to toward the side/toward the middle.

We can also describe different body parts or brain regions relative to one another by saying they are ipsilateral (on the same side) or contralateral (on opposite sides).

Since the brain is such a complex structure with many substructures, researchers need to slice the brain in different ways, in order to visualize the structures deep within the brain. There are three different planes by which we can slice the brain.

Horizontal: looking at it from the top/bottom

Frontal: looking at it from the front/back

Saggital: looking at it from either side

Support structures

Meninges: protective tissue around the brain and spinal cord

-  Dura mater (“tough mother”) thick layer

-  Arachnoid mater (“spider mother”) spider-web-y and spongy layer

-  Pia mater (“pious mother”) glad wrap layer

à If meninges are inflamed, meningitis L

Ventricles: hollow inter-connected cavities in the brain that produce CSF

o  Forebrain: lateral ventricles (first & second), third ventricle

o  Midbrain: cerebral aqueduct

o  Hindbrain: fourth ventricle

à If flow from ventricles is blocked, hydrocephalus (“water on brain”) L which can be drained J

CSF: cerebrospinal fluid, made by ependymal cells along the ventricles

-  cushions and supports the fragile brain

-  provides a reservoir of hormones and nutrients

-  contained in subarachnoid space (between the two arachnoid membranes)

-  half-life = ~3 hours, drains from subarachnoid space and is reabsorbed by veins

Blood vessels: arteries bring blood in, veins bring blood away

-  main purpose is to bring oxygen and glucose for the brain to function

-  the brain is less than 2% of body weight, but it requires 20% of continuous blood supply

-  cut blood supply for 6 seconds à unconscious; 4-6 minutes à permanent brain damage

Blood-brain barrier: semi-permeable membrane that prevents most chemicals from entering brain

Brain development

There are two components to brain development: genetic and environmental. The genetic component comes into play after the eighteenth day of conception. The neural plate fuses such that it becomes the neural tube, which in turn produces the forebrain, midbrain and hindbrain. The tube also develops three chambers, which become the ventricles. As development progresses, the individual ventricles form from the dividing chambers. Sometime later, there is a switch, and then the brain becomes susceptible to environmental influence: chemicals and outside signals can change the brain structure.

Brain organization

Forebrain

-  Telencephalon

·  Limbic system emotions

·  Basal ganglia control of movement

·  Basal forebrain arousal

-  Diencephalon

·  Thalamus receives all sensory input

·  Hypothalamus 4 F’s: feeding, fighting, fleeing, sex

Midbrain

-  Mesencephalon

·  Tectum part of sensory pathways to brain

·  Tegmentum part of motor pathways from brain

Hindbrain

-  Metencephalon

·  Cerebellum coordination of fine movement, balance

·  Pons sleep and arousal

-  Myelencephalon

·  Medulla oblongata vital reflexes: e.g. breathing, heart rate, vomiting, coughing

Four lobes

Frontal à executive functioning, defines personality

*Primary motor cortex (M1) produces movement

*Primary somatosensory cortex (S1) receives & combines input from sensory organs

Parietal à understanding space and own relationship to it

End of “where” pathway

Occpital à visual processing

*Primary visual cortex (V1)

Temporal à auditory and visual processing

*Primary auditory cortex (A1)

End of “what pathway”