CH. 2. The Neural Basis For Cognition Flashcards
1
Q
Capgras Syndrome
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CAPGRAS SYNDROME – Someone with this syndrome is fully able to recognize the people in her world — her husband, her parents, her friends — but is utterly convinced that these people are not who they appear to be.
- Often, a person with Capgras syndrome insists that there are slight differences between the “impostor” and the person he (or she) has supposedly replaced — subtle changes in personality or appearance.
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What is going on here? The answer lies in the fact that facial recognition involves two separate systems in the brain.
- One system leads to a cognitive appraisal (“I know what my father looks like, and I can perceive that you closely resemble him”), and the other…
- a more global, emotional appraisal (“You look familiar to me and also trigger a warm response in me”).
- When these two appraisals agree, the result is a confident recognition (“You obviously are my father”).
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In Capgras syndrome, though, the emotional processing is disrupted, leading to an intellectual identification without a familiarity response
- Someone with this syndrome is able to recognize a loved one’s face, but with no feeling of familiarity.
- Brain scans suggest a link between Capgras syndrome and abnormalities in several brain areas, indicating that our account of the syndrome will need to consider several elements.
- One site of damage in Capgras patients is in the temporal lobe, particularly on the right side of the head. This damage probably disrupts circuits involving the AMYGDALA – an almond-shaped structure that — in the intact brain — seems to serve as an “emotional evaluator,” helping an organism detect stimuli associated with threat or danger.
2
Q
Amygdala and the Two-System Hypothesis
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AMYGDALA – an almond-shaped structure that — in the intact brain — seems to serve as an “emotional evaluator,” helping an organism detect stimuli associated with threat or danger.
- The amygdala is also important for detecting positive stimuli indicators of safety or of available rewards.
- With damaged amygdalae, therefore, people with Capgras syndrome won’t experience the warm sense of feeling good (and safe and secure) when looking at a loved one’s familiar face.
- This lack of an emotional response is probably why these faces don’t feel familiar to them, and is fully in line with the TWO-SYSTEM HYPOTHESIS – which requires 1) a cognitive response AND 2) an emotional response in order to recognize and accept the identity of a loved one in Capgras syndrome.
- With damaged amygdalae, therefore, people with Capgras syndrome won’t experience the warm sense of feeling good (and safe and secure) when looking at a loved one’s familiar face.
3
Q
Prefrontal Cortex
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PREFRONTAL CORTEX – is an area in the Frontal Lobe especially active when a person is doing tasks that require PLANNING or careful ANALYSIS.
- Conversely, this area is less active when someone is dreaming – reflecting the absence of careful analysis of the dream material, which helps explain why dreams are often illogical or bizarre.
Note on Capgras Syndrome:
- Patients with Capgras syndrome also have brain abnormalities in the frontal lobe, specifically in the right prefrontal cortex.
- With fMRI scans of patients suffering from schizophrenia – neuroimaging reveals diminished activity in the frontal lobes whenever these patients are experiencing hallucinations (same effect as with dreams).
- One interpretation is that the diminished activity reflects a decreased ability to distinguish internal events (thoughts) from external ones (voices) or to distinguish imagined events from real ones.
- With damage to the frontal lobe, Capgras patients may be less able to keep track of what is real and what is not, what is sensible and what is not. As a result, weird beliefs can emerge unchecked, including delusions (about robots and the like) that you or I would find totally bizarre.
4
Q
Recognition and the Two-System Hypothesis
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TWO-SYSTEM HYPOTHESIS – RECOGNITION of ALL STIMULI (not just faces) involves TWO SEPARATE MECHANISMS:
- one that hinges on factual knowledge
- one that’s more emotional
- Capgras syndrome teaches us that many different parts of the brain are needed for even the simplest achievement.
- In order to recognize your father, for example,
- Facts – one part of your brain needs to store the factual memory of what he looks like. Another part of the brain is responsible for analyzing the visual input you receive when looking at a face. Yet another brain area has the job of comparing this now-analyzed input to the factual information provided from memory, to determine whether there’s a match.
- Emotions – Another site provides the emotional evaluation of the input.
- Combination – A different site presumably assembles the data from all these other sites — and registers the fact that the face being inspected does match the factual recollection of your father’s face, and also produces a warm sense of familiarity.
- If coordination among these areas is disrupted – yet another area works to make sure you offer reasonable hypotheses about this disconnect and not zany ones. (In other words, if your father looks less familiar to you on some occasion, you’re likely to explain this by saying, “I guess he must have gotten new glasses” rather than “I bet he’s been replaced by a robot.”)
- Unmistakably, this apparently easy task — seeing your father and recognizing who he is — requires multiple brain areas. The same is true of most tasks, and in this way, Capgras syndrome illustrates this crucial aspect of brain function.
- NOTE: That our understanding of Capgras syndrome depends on a combination of evidence drawn from cognitive psychology and from cognitive neuroscience.
- Just as both perspectives can illuminate Capgras Syndrome, both can be illuminated by the syndrome.
- That is, we can use Capgras syndrome (and other biological evidence) to illuminate broader issues about the nature of the brain and of the mind.
- Ex: Capgras syndrome suggests that the AMYGDALA (the “Emotional Evaluator”) plays a crucial role (the emotional side) in supporting the feeling of familiarity.
- That is, we can use Capgras syndrome (and other biological evidence) to illuminate broader issues about the nature of the brain and of the mind.
- This observation gives us a way to think about occasions in which your evaluation of the facts points toward one conclusion, while an emotional evaluation points toward a different conclusion.
5
Q
The Brain
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THE BRAIN:
- The human brain weighs (on average) a bit more than 3 pounds (1.4 kg)
- Male brains weighing about 10% more than female brains
- Estimated to contain 86 billion nerve cells
- 860 trillion connections
- Different parts of the brain perform different jobs.
- Symptoms produced by brain damage depend heavily on the location of the damage.
- Therefore, we need to understand brain functioning with reference to brain anatomy
- The human brain is divided into three main structures: the hindbrain, the midbrain, and the forebrain.
6
Q
Hindbrain, Midbrain, Forebrain
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The human brain is divided into three main structures: the hindbrain, the midbrain, and the forebrain.
HINDBRAIN – is located at the back of the brain at the very top of the spinal cord and includes structures crucial for controlling key life functions.
- Helps to regulate autonomic functions (Breathing, Heart rate)
- Relay sensory information from the body to the brain.
- Coordinate movement
- Maintain balance and Equilibrium
MIDBRAIN in the middle part of the brain and wrapped completely by the Forebrain:
- Helps to coordinate movements
- Processes auditory information
- Processes visual information
- Regulate the experience of pain
FOREBRAIN – This structure surrounds (and so hides from view) the entire midbrain and most of the hindbrain.
- Processes sensory information
- Helps with reasoning and problem-solving
- Regulates autonomic functions (involuntary actions like heart rate and breathing)
- Regulates endocrine functions (Hormones and Biological Processes)
- Regulates motor functions (Glands, reflexes, muscle contractions)
7
Q
Hindbrain
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HINDBRAIN – is located at the back of the brain at the very top of the spinal cord and includes structures crucial for controlling key life functions.
- Regulates the rhythm of heartbeats and the rhythm of breathing.
- Maintains the body’s overall tone.
- Helps maintain the body’s posture and balance.
- Helps control the brain’s level of alertness.
Other parts of the HINDBRAIN include:
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CEREBELLUM – the largest area of the hindbrain. Its role is to:
- coordination of bodily movements and balance.
- spatial reasoning
- discriminating sounds
- integrating the input received from various sensory systems
- PONS – (Latin for “bridge”) is the main connection between the cerebellum and the rest of the brain.
- MEDULLA – controls vital functions such as breathing and heart rate.
8
Q
Midbrain
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MIDBRAIN – Has several functions:
- Coordinates Movements including the precise movements of the eyes
- Relays auditory information from the ears to the forebrain for processing and interpretation.
- Regulates the experience of pain
9
Q
Forebrain
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FOREBRAIN – the most interesting brain region (and the largest in humans – 80% of the brain by volume.) The forebrain surrounds (and so hides from view) the entire midbrain and most of the hindbrain. It is comprised of:
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CEREBRAL CORTEX – the outer surface of the forebrain. A thin covering on the outer surface of the forebrain; on average, it’s a mere 3 mm thick.
- Cortex refers to an organ’s outer surface, and many organs each have their own cortex. what’s visible in the drawing, then, is the cerebral cortex.
- As thin as it is, it consists of a large sheet of tissue crumpled up and jammed into the limited space inside the skull.
- It’s this crumpling that produces the brain’s most obvious visual feature — the wrinkles, or CONVOLUTIONS.
- Some of the “valleys” between the wrinkles are actually deep grooves (called FISSURES) that divide the brain into different sections, called LOBES.
- The deepest groove is the LONGITUDINAL FISSURE, running from the front of the brain to the back, which separates the left cerebral hemisphere from the right.
- Frontal Lobes – form the front of the brain, right behind the forehead.
- Parietal Lobes – the brain’s topmost part.
- Temporal Lobes – are below the lateral fissure
- Occipital Lobes – are found at the very back of the brain, connected to the parietal and temporal lobes.
- Central Fissure – divides the frontal lobes on each side of the brain
- Lateral Fissure – bottom edge of the frontal lobes
10
Q
Subcortical Structures
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SUBCORTICAL STRUCTURES – (meaning BELOW the CEREBRAL CORTEX) – there are several subcortical structures that are hidden from view underneath the cortex. They are:
- THALAMUS – acts as a relay station for nearly all the sensory information going to the cortex.
- HYPOTHAlAMUS – a structure that plays a crucial role in controlling behaviors that serve specific biological needs — behaviors that include eating, drinking, and sexual activity.
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LIMBIC SYSTEM –This set of structures surrounds the thalamus and hypothalamus and includes a number of subcortical structures that play a crucial role in learning and memory and in emotional processing.
- Included in the LIMBIC SYSTEM are the Amygdala & Hippocampus. Both the amygdala & Hippocampus are located under the Cortex.
- These two structures are essential for learning and memory:
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AMYGDALA – plays a key role in emotional processing.
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Ex: The presentation of frightful faces causes high levels of activity in the amygdala.
- Likewise, people ordinarily show more complete, longer-lasting memories for emotional events, compared to similar but emotionally flat events.
- This memory advantage for emotional events is especially pronounced in people who showed greater activation in the amygdala while they were witnessing the event in the first place.
- Conversely, the memory advantage for emotional events is diminished (and may not be observed at all) in people who (through sickness or injury) have suffered damage to the amygdalae.
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Ex: The presentation of frightful faces causes high levels of activity in the amygdala.
- HIPPOCAMPUS – a brain structure embedded deep into the temporal lobe with a major role in learning and memory.
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AMYGDALA – plays a key role in emotional processing.
11
Q
Lateralization & Contralateral Connections
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LATERALIZATION – Virtually all parts of the brain come in pairs, left and right, including structures (like the hippocampus and the amygdala) and lobes
- In all cases, the left and right structures in each pair have roughly the same shape and the same pattern of connections to other brain areas.
- However, there are differences in function between the left-side and right-side structures.
- That said, the two halves of the brain work together – the functioning of one side is closely integrated with that of the other side.
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COMMISSURES – Connect the two hemispheres. These are what make integration possible. They are thick bundles of fibers that carry information back and forth between the two hemispheres.
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CORPUS CALLOSUM – is the largest commissure.
- In certain cases, though, there are medical reasons to Sever the Corpus Callosum and some of the other commissures. Someone who had this procedure done is said to be a “SPLIT_BRAIN PATIENT”
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CORPUS CALLOSUM – is the largest commissure.
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“SPLIT_BRAIN PATIENT“ – still having both brain halves, but with communication between the halves severely limited.
- Research with these patients has taught us a great deal about the specialized function of the brain’s two hemispheres.
- It is misleading to claim that we need to silence our “left-brain thinking” in order to be more creative – some elements of creativity depend on specialized processing in the right hemisphere
- Even so, whether we’re examining creativity or any other capacity, the two halves of the brain have to work together, with each hemisphere making its own contribution to the overall performance.
- Therefore, “silencing” one hemisphere wouldn’t allow you new achievements because the skills we display depend on the whole brain.
- Hemispheres are not cerebral competitors, they are an integrated team.
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COMMISSURES – Connect the two hemispheres. These are what make integration possible. They are thick bundles of fibers that carry information back and forth between the two hemispheres.
CONTRALATERAL CONNECTIONS – This refers to the fact that the corresponding area in the right hemisphere receives its input from the left side of the body and the left hemisphere from the right side of the body.
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VISUAL – Likewise for the visual projection areas, although here the projection is NOT contralateral with regard to body parts. Instead, it’s contralateral with regard to physical space.
- Specifically, the visual projection area in the right hemisphere receives information from both the left eye and the right, but the information it receives corresponds to the left half of the visual space.
- The reverse is true for the visual area in the left hemisphere. It receives information from both eyes, but from only the right half of visual space.
- AUDITORY – The pattern of contralateral organization is also evident — although not as clear-cut with auditory signals.
12
Q
Neuropsychology
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NEUROPSYCHOLOGY – the study of the brain’s structures and how they relate to brain function.
CLINICAL NEUROPSYCHOLOGY – Looks at damaged brains to understand the functioning of undamaged brains.
13
Q
Lesion
A
LESION – (a specific area of damage on the brain). Clinical Neuropsychologists look at lesions (damage) on specific parts of the brain to determine what that specific area is used for.
- The damage tends to inhibit or disable the function of that specific part of the brain revealing the primary function of that particular area.
- Ex: A lesion in the occipital cortex produces problems in vision but spares the other sensory modalities.
- The consequences of brain lesions depend on which hemisphere is damaged. These patterns provide a rich source of data that helps us develop and test hypotheses about those functions.
14
Q
Neuroimaging Techniques
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NEUROIMAGING TECHNIQUES – produce precise, three-dimensional pictures of a living brain.
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MAGNETIC RESONANCE IMAGING (MRI scan) – Shows Brain Structure – neuroimaging procedures that provide structural imaging, generating a detailed portrait of the shapes, sizes, and positions of the brain’s components.
- MRI scans provide structural images using magnetic properties of the atoms that make up brain tissue.
- CT SCAN – Computerized Axial Tomography. CT scans rely on X-rays to show the structure of the brain (as does an MRI)
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FUNCTIONAL MAGNETIC RESONANCE IMAGING (fMRI) – Shows Brain Activity by location – tells us the specific areas of activity (as well as the levels of activity) in the brain in response to some stimuli or activity.
- fMRI scans offer an incredibly precise picture of the brain’s moment-by-moment activities.
- PET SCAN – positron emission tomography. PET scans use radiation to determine the activity levels in specific areas of the brain (as does an fMRI).
- Structural imaging (CT or MRI scans) are relatively stable, changing only if the person’s brain structure changes (From injury or disease).
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Activity imaging (PET or fMRI scans), in contrast, are highly variable, because the results depend on what task the person is performing.
- Thus, PET and fMRI are used to explore brain function.
- Ex: to determine which brain sites are especially activated when someone is making a moral judgment or trying to solve a logic problem.
- Thus, PET and fMRI are used to explore brain function.
COMBINING TECHNIQUES – as with most things, combining different techniques allows us to get a fuller picture of the situation. This is because the strengths of one approach often fill in for the weaknesses of another approach.
- CT scan and MRI data tell us about the shape and size of brain structures, but they tell nothing about the activity levels within these structures.
- PET scans and fMRI studies do tell us about brain activity, and they can locate the activity rather precisely (within a millimeter or two). But these techniques are less precise about when the activity took place.
- fMRI data summarize the brain’s activity over a period of several seconds and cannot indicate when exactly, within this time window, the activity took place.
- EEG data give more precise information about timing but are much weaker in indicating where the activity took place.
- So COMBINING the:
- Structural information provided by the CT/MRI with the
- Activity information provided by the PET/fMRI and the
- Timing provided by the EEG…
- …will give us the most accurate picture of what’s going on.
15
Q
Neuron & Neurotransmitter
A
NEURONS – are nerve cells. The brain contains billions of them.
- It is the neurons that do the brain’s main work.
- They communicate with one another via chemical signals called NEUROTRANSMITTER.
- Once a neuron is “activated,” it releases the transmitter, and this chemical can then activate (or, in some cases, de-activate) other, adjacent neurons.
- The adjacent neurons “receive” this chemical signal and, in turn, send their own signal onward to other neurons.
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Neurons have an “input” end and an “output” end.
- The “input” end is the portion of the neuron that’s most sensitive to neurotransmitters.
16
Q
Axial, Coronal, and Sagittal Views
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AXIAL VIEW – a “slice” of the brain viewed from the top of the head.
CORONAL VIEW – shows a slice of the brain viewed from the front.
SAGITTAL VIEW – shows a slice of the brain viewed from the side.
20
Q
Cerebral Cortex
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CEREBRAL CORTEX – This is the region in which an enormous amount of information processing takes place, and so, for many topics, it is the brain region of greatest interest for cognitive psychologists.
- The Cerebral Cortex includes many distinct regions, each with its own function, but these regions are traditionally divided into three categories:
- MOTOR AREAS – contain brain tissue crucial for organizing and controlling bodily movements.
- SENSORY AREAS – contain tissue essential for organizing and analyzing the information received from the senses.
- ASSOCIATION AREAS – support many functions, including the essential (but not well-defined) human activity we call “thinking.”
21
Q
Motor Cortex (of the Cerebral Cortex)
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MOTOR CORTEX – contains brain tissue crucial for organizing and controlling bodily movements.
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MOTOR AREAS:
- PRIMARY MOTOR PROJECTION AREAS – Certain regions of the cerebral cortex serve as the “departure points” for signals leaving the cortex and controlling muscle movement.
- PRIMARY SENSORY PROJECTION AREAS – Other areas are the “arrival points” for information coming from the eyes, ears, and other sense organs.
- In both instances, these areas are called PRIMARY PROJECTION AREAS because these areas seem to form “maps” of the external world, with particular positions on the cortex corresponding to particular parts of the body or particular locations in space.
- Areas of the body that we can move with great precision (e.g., fingers and lips) have a lot of cortical area devoted to them.
CONTRALATERAL CONTROL – stimulation to the left hemisphere leading to movements on the right side of the body, and vice versa.
22
Q
Sensory Areas (of the Cerebral Cortex)
A
SENSORY AREAS – contain tissue essential for organizing and analyzing the information received from the senses. – Information arriving from the skin senses (your sense of touch or your sense of temperature) is projected to a region in the parietal lobe, just behind the motor projection area. This is labeled the SOMATOSENSORY Area.
- The sensory projection areas differ from each other in important ways, but they also have features in common
- Each of these areas provides a “MAP” of the sensory environment. In the somatosensory area, each part of the body’s surface is represented by its own region on the cortex; areas of the body that are near to each other are typically represented by similarly nearby areas in the brain.
- In the VISUAL AREA, each region of visual space has its own cortical representation, and adjacent areas of visual space are usually represented by adjacent brain sites.
- In the AUDITORY PROJECTION area, different frequencies of sound have their own cortical sites, and adjacent brain sites are responsive to adjacent frequencies.
- In each of these sensory maps, the assignment of cortical space is governed by function, not by anatomical proportions.
- Ex: In the parietal lobes, parts of the body that aren’t very discriminating with regard to touch — even if they’re physically large — get relatively little cortical area. Hands and Mouth get large areas dedicated to them, but hips get very little.
- VISUAL – In the occipital lobes, more cortical surface is devoted to the fovea, the part of the eyeball that is most sensitive to detail.
- AUDITORY – And in the auditory areas, some frequencies of sound get more cerebral coverage than others. These “advantaged” frequencies are those essential for the perception of speech.
23
Q
Association Areas (of the Cerebral Cortex)
A
ASSOCIATION AREAS – support many functions, including the essential human activity we call “thinking.”
- Makes up the largest portion of the Cerebral Cortex.
- Both motor and sensory, make up only a small part of the human cerebral cortex — roughly 25%.
- The remaining cortical areas are traditionally referred to as the ASSOCIATION CORTEX.
ASSOCIATION CORTEX – this large volume of brain tissue can be subdivided further on both functional and anatomical grounds.
- These Subdivisions (Including Visual, Sensory, and Auditory) are perhaps best revealed by the diversity of symptoms that result if the cortex is damaged in one or another specific location.
- APRAXIAS – disturbances in the initiation or organization of voluntary action caused by lesions in the Frontal Lobe.
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AGNOSIAS – disruptions in the ability to identify familiar objects lesions (generally in the occipital cortex, or in the rearmost part of the parietal lobe).
- Agnosias usually affect one modality only – so a patient with visual agnosia, for example, can recognize a fork by touching it but not by looking at it.
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NEGLECT SYNDROME – lesions (usually in the parietal lobe) produce this syndrome, in which the individual seems to ignore half of the visual world.
- A patient afflicted with this syndrome will shave only half of his face and eat food from only half of his plate.
- APHASIA – lesions in areas near the lateral fissure (the deep groove that separates the frontal and temporal lobes) can result in disruption to language capacities, a problem referred to as aphasia
- Damage to the frontmost part of the frontal lobe, the prefrontal area, causes problems in planning and implementing strategies.
- Frontal lobe damage can also (as we mentioned in our discussion of Capgras syndrome) lead to a variety of confusions, such as whether a remembered episode actually happened or was simply imagined.
24
Q
Neurons and Glia
A
NEURONS AND GLIA – the human brain contains many billions of neurons and a comparable number of glia.
GLIA – Support the Neurons – Perform many functions.
- Help to guide the development of the nervous system in the fetus and young infant.
- Support repairs if the nervous system is damaged.
- Control the flow of nutrients to the neurons.
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MYELIN SHEATH – is created by glial cells that wrap around the axons of many neurons.
- This provides a layer of electrical insulation surrounding parts of some neurons; this insulation dramatically increases the speed with which neurons can send their signals.
- Some research suggests the glia may also constitute their own signaling system within the brain, separate from the information flow provided by the neurons.
NEURONS – Carry the main flow of information through the brain.
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Neurons have three major parts:
- CELL BODY – the portion of the cell that contains the neuron’s nucleus and all the elements needed for the normal metabolic activities of the cell.
- DENDRITES – are usually the “INPUT” side of the neuron, receiving signals from many other neurons. In most neurons, the dendrites are heavily branched, like a thick and tangled bush.
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AXON – the “OUTPUT” side of the neuron; it sends neural impulses to other neurons.
- Axons can vary enormously in length — the giraffe, for example, has neurons with axons that run the full length of its neck.
- When the cell fires, neurotransmitters are released from the TERMINAL ENDINGS at the tip of the axon.
- The MYELIN SHEATH is created by glial cells that wrap around the axons of many neurons.
- NODES OF RANVIER – gaps in between the myelin cells
26
Q
Synapse
A
SYNAPSE – communication from one neuron to the next is generally made possible by a chemical signal. This entire site — the end of the axon, plus the gap, plus the receiving membrane of the next neuron — is called a synapse.
- When a neuron has been sufficiently stimulated, it releases a minute quantity of a NEUROTRANSMITTER (a chemical messenger).
- The molecules of this substance drift across the tiny gap (SYNAPTIC GAP) between neurons and latch onto the DENDRITES (Receiving “INPUT” end of a Neuron) of the adjacent cell.
- If the dendrites receive enough of this substance, the next neuron will “fire,” and so the signal will be sent along to other neurons.
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Neurons usually don’t touch each other directly. Instead, at the end of the AXON (The ‘OUTPUT’ end of the Neuron) there is a gap separating each neuron from the next (The Synaptic Gap).
- The bit of the neuron that releases the transmitter into this gap is the PRESYNAPTIC MEMBRANE.
- The bit of the neuron on the other side of the gap, affected by the transmitters, is the POSTSYNAPTIC MEMBRANE.
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When the neurotransmitters arrive at the postsynaptic membrane, they cause changes in this membrane that enable certain ions to flow into and out of the postsynaptic cell.
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If the ionic flows are large enough, they trigger a response in the postsynaptic cell.
- If these ionic flows are relatively small, then the postsynaptic cell quickly recovers and the ions are transported back to where they were initially – i.e. they don’t trigger a response.
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If the ionic flows are large enough, they trigger a response in the postsynaptic cell.
- ONLY IF the incoming signal reaches the postsynaptic cell’s THRESHOLD, then the cell fires.
- That is, it produces an ACTION POTENTIAL — a signal that moves down its axon, which in turn causes the release of neurotransmitters at the next synapse, potentially causing the next cell to fire.
- In some neurons, the action potential moves down the axon at a relatively slow speed.
- For other neurons, specialized glial cells are wrapped around the axon, creating a layer of insulation called theMYELIN SHEATH.
- Because of the MYELIN, ions can flow in or out of the axon only at the gaps between the myelin cells. As a result, the signal traveling down the axon has to “jump” from gap to gap, and this greatly increases the speed at which the signal is transmitted.
- For neurons without myelin, the signal travels at speeds below 10 m/s; for “myelinated” neurons, the speed can be ten times faster.
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Neurons depend on two different forms of information flow.
- CHEMICAL NEUROTRANSMITTER – Communication from one neuron to the next is (for most neurons) mediated by a chemical signal (neurotransmitters).
- ELECTRICAL ION– In contrast, communication from one end of the neuron to the other (usually from the dendrites down the length of the axon) is made possible by an electrical signal, created by the flow of ions in and out of the cell.
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Postsynaptic neuron’s response – If the signal is sent, it is always of the same magnitude, a fact referred to as the ALL-OR- NONE LAW.
- ALL-OR- NONE LAW – Just as pounding on a car horn won’t make the horn any louder, a stronger stimulus won’t produce a stronger action potential. A neuron either fires or it doesn’t; there’s no in-between.
- Crucially, though, once these inputs reach the postsynaptic neuron’s firing threshold, there’s no variability in the response – either a signal is sent down the axon or it is not.
- So when researchers talk about the strength of a neuron signal, they are not talking about the strength of a single firing – that is always the same. Instead, they are referring to how rapidly the neuron is firing.
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The brain relies on many different neurotransmitters.
- A hundred transmitters have been cataloged so far.
- This diversity enables the brain to send a variety of different messages.
- Some transmitters have the effect of stimulating subsequent neurons; some do the opposite and inhibit other neurons.
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The Central role of Synapse.
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The synaptic gap is actually quite small — roughly 20 to 30 nanometers across.
- For contrast’s sake, the diameter of a human hair is roughly 80,000 nanometers.
- Each neuron receives information from (i.e., has synapses with) many other neurons, and this allows the “receiving” neuron to integrate information from many sources.
- This pattern of many neurons feeding into one also makes it possible for a neuron to “compare” signals and to adjust its response to one input according to the signal arriving from different inputs.
- In addition, communication at the synapse is adjustable. This means that the strength of a synaptic connection can be altered by experience, and this adjustment is crucial for the process of learning — the storage of new knowledge and new skills within the nervous system.
- Remember that the STRENGTH of a synaptic connection can be altered and measured only in terms of how RAPID the neuron fires, since the strength of EACH firing is the same.
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The synaptic gap is actually quite small — roughly 20 to 30 nanometers across.
