Midterm 2 Review Flashcards
(177 cards)
1
Q
Dendrites
A
Gather information from other neurons (input)
2
Q
Dendritic spines
A
protrusions from a dendrite, the usual point of contact with axons of other cells
3
Q
Cell Body or Soma
A
Integration of information
4
Q
Axon
A
Carries information to be passed on to other cells
Axon terminal passes on the message (output)
5
Q
Axon hillock
A
A juncture of soma and axon where the action potential begins
6
Q
Nodes of Ranvier
A
Tiny gaps in the myelin sheath
7
Q
Axon Collaterals
A
branches of an axon, these branches have ends, referred to as Telodendria
8
Q
Terminal buttons (end feet)
A
knob at the tip of the axon, conveys information to other neurons
9
Q
Transmembrane channels
A
Ligand-gated channels
- transmitter-activated channels
- Gated protein channels that open only when specific molecule(s) bind to the channel
- Typically on dendrites
Voltage-activated ion channels
- Gated protein channels that open or close only at specific membrane voltages
- Sodium (Na+ ), potassium (K+ ), calcium (Ca2+)
- Closed at membrane’s resting potential
- Typically on axon
10
Q
Synapses
A
- junctions between one neuron and the next
- Site of information transfer
11
Q
Electrical synapse (Gap junction)
A
Fused presynaptic and postsynaptic membrane that allows an action potential to pass directly from one neuron to the next
12
Q
Chemical synapse
A
The junction where messenger molecules (neurotransmitters) are released from one neuron to excite or inhibit the next neuron
Most synapses in the mammalian nervous system are chemical
13
Q
Electron Microscope:
A
- Projects a beam of electrons through a very thin slice of tissue
- Varying structure of the tissue scatters the beam onto a reflective surface, where it leaves an image, or shadow, of the tissue.
- Much better resolution than the light microscope
- 1950s: revealed the structure of a synapse for the first time
14
Q
Sensory Neurons
A
Bring information to the central nervous system
Structurally, very simple
Bipolar neuron: found in the retina, conduct afferent info to the visual centers of the brain
Somatosensory neuron: afferent info into the spinal cord
15
Q
Interneurons
A
Associate sensory and motor activity within the central nervous system
Pyramidal cell: long axon, two sets of dendrites
Purkinje cell: extremely branched dendrites, info from the cerebellum to the rest of the brain
16
Q
Motor Neurons
A
Send signals from the brain and spinal cord to muscles
Complex dendrites, long axons that connect to muscles
Located in the motor cortex, lower brainstem and spinal cord
Also can be categorized as unipolar (1 extension from cell body), bipolar (2 extensions from cell body), or multipolar (many extensions from cell body)
17
Q
Analog Signal
A
The incoming signal is analog – graded signal (summation of input)
18
Q
Digital Signal
A
The outgoing signal is digital – on or off (binary)
19
Q
Electricity
A
A flow of electrons from a body that contains a higher charge (more electrons) to a body that contains a lower charge (fewer electrons)
20
Q
Negative pole
A
The source of electrons; higher charge
21
Q
Positive pole
A
Location to which electrons flow; lower charge
22
Q
Electrical potential (or electrical charge)
A
Is the ability to do work using stored electrical energy
Electrons flow from the negative pole to the positive pole
Measuring electrical activity in animal tissue
23
Q
Volt
A
A measure of a difference in electrical potential
24
Q
Voltmeter
A
A device that measures the difference in electrical potential between two bodies
25
Cations
Positively charged ions
| Examples: sodium (Na+ ), potassium (K+ )
26
Anions
Negatively charged ions
| Examples: chloride (Cl− ), protein molecules (A− )
27
Diffusion
Movement of ions from an area of higher concentration to an area of lower concentration through random motion
28
Concentration gradient
Differences in concentration of a substance among regions of a container allow the substance to diffuse from an area of higher concentration to an area of lower concentration
29
Voltage gradient
Electrostatic forces/electrostatic pressure
-Difference in charge between two regions that allows a flow of current if the two regions are connected
(Opposite charges attract, Similar charges repel)
-Ions will move down a voltage gradient from an area of higher charge to an area of lower charge
30
Diffusion through semipermeable membranes
- Efflux of chloride ions down the chloride concentration gradient is counteracted by the influx (inward flow) of chloride ions down the chloride voltage gradient
- Equilibrium occurs when the concentration gradient of chloride ions on the right side of the beaker is balanced by the voltage gradient of chloride ions on the left
- At equilibrium, the concentration gradient is equal to the voltage gradient
31
Electrical stimulation studies
Galvani (eighteenth century)
Electrical current applied to a dissected nerve induced a twitch in the muscle connected to the nerve; Galvani concluded that electricity flows along the nerve
32
Electrical stimulation
Passing an electrical current from the tip of an electrode through brain tissue, resulting in changes in the electrical activity of the tissue
33
Caton (early nineteenth century)
First to attempt to measure electrical currents of the brain using a voltmeter and electrodes on the skull
34
Electroencephalogram (EEG)
Graph that records electrical activity through the skull or from the brain and represents graded potentials of many neurons
35
Von Helmholtz (nineteenth century)
Flow of information in the nervous system is too slow to be a flow of electricity
Nerve conduction: 30–40 meters/second
Electricity: 3 × 108 meters/second
36
Giant axon of the squid
Much larger in diameter than human axons
Humans: 1 to 20 micrometers
Squid: up to 1 millimeter (1000 micrometers)
Easier subject of experiments
Used by Hodgkin and Huxley in the 1930s and 1940s
37
Oscilloscope
A device that serves as a sensitive voltmeter
Used to record voltage changes on an axon
In mV and ms
38
Microelectrodes
A set of electrodes small enough to place on or in an axon
Can be used to:
Measure a neuron’s electrical activity
Deliver an electrical current to a single neuron (stimulation)
39
Resting Potential
Electrical charge across the cell membrane in the absence of stimulation
A store of negative energy on the intracellular side relative to the extracellular side
The inside of the membrane at rest is −70 millivolts relative to the extracellular side
40
4 Particles Producing Resting Potential
-Sodium (Na+ ) and chloride (Cl− ) (Higher concentration outside cell)
Mnemonic: we put salt (NaCl) on the outside of our food
-Potassium (K+ ) and large proteins (A− ) (Higher concentration inside cell)
41
How Resting Potential Happens
1. Membrane is relatively impermeable to large molecules, negatively charged proteins (A-) remain inside cell
2. Ungated potassium and chloride channels - potassium and chloride ions pass freely, gates on sodium channels keep out positively charged sodium ions (leak channels)
3. Na+ –K + pumps extrude Na+ from the intracellular fluid and inject K+
42
Inside the Cell during Resting Potential
- Large protein anions are manufactured inside cells
- No membrane channels are big enough for proteins to leave the cell, their negative charge produces a resting potential
- Cells accumulate positively charged potassium ions (~20 times as many potassium ions cluster inside compared to outside)
- Concentration of potassium higher inside than outside, potassium ions drawn out cell by concentration gradient
43
Outside the Cell during Resting Potential
- Sodium ions kept out (~10 times as many sodium ions outside axon membrane as inside)
- Difference in concentrations of sodium contributes to the membrane’s resting potential
- Gates on sodium ion channels in cell membrane are closed, blocking entry of most sodium ions
- Chloride ions move in and out of cell through open channels in the membrane
- The equilibrium point (Cl-), at which the concentration gradient equals its voltage gradient, ~same as the membrane’s resting potential, chloride ions contribute little to the resting potential
44
Incoming Potentials
If the concentration of any of the ions across the unstimulated cell membrane changes, the membrane voltage changes
These local graded potentials are small voltage fluctuations across the cell membrane
45
Hyperpolarization
Increase in electrical charge across a membrane (more negative)
Usually due to the inward flow of chloride ions or outward flow of potassium ions
46
Depolarization
Decrease in electrical charge across a membrane (more positive)
Usually due to the inward flow of sodium
47
Excitatory postsynaptic potential (EPSP)
Brief depolarization of a neuron membrane in response to stimulation
Depolarized neuron is more likely to produce an action potential
Associated with the opening of sodium channels: allows influx of Na+
48
Inhibitory postsynaptic potential (IPSP)
Brief hyperpolarization of a neuron membrane in response to stimulation
Hyperpolarized neuron - less likely to produce action potential
Associated with the opening of potassium channels (allows an efflux of K+ ) or with the opening of chloride channels (allows an influx of Cl− )
EPSPs and IPSPs last only a few milliseconds before they decay and the resting potential is restored
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Temporal summation
Pulses from multiple inputs that occur at approximately the same time on a membrane are summed
50
Spatial summation
Pulses from multiple inputs that occur at approximately the same place on a membrane are summed
51
Multiple Inputs to a Single Neuron
EPSPs and IPSPs are summed
Summed ionic inputs exceed threshold potential (approximately −50 mV) at axon hillock, an action potential will be initiated
When they are close in time and space, they sum; when they are far apart in either or both ways, there is no summation
52
Threshold potential
Voltage on a neural membrane at which an action potential is triggered
Opening of Na+ and K+ voltage-activated channels
Approximately −50 mV relative to extracellular surround
53
Sodium and Potassium voltage-activated channels
- Attuned to the threshold voltage of about −50 mV
- If the cell membrane changes to reach this voltage, both types of channels open to allow ion flow across the membrane
- Sodium channels respond faster than potassium channels (Na inlfux before K efflux)
- Influx of Na (2 channels) fast, influx of K, slow and long
54
Steps of Action Potential
1. The voltage-activated sodium channels respond more quickly than the potassium channels
- voltage change - Na+ influx
- voltage change - K+ efflux
2. Sodium channels have two gates
- membrane depolarizes (+30 mV)
- 1 gates closes - Na+ influx quickly
3. The potassium channels open slowly & remain open longer
- efflux of K+ reverses the depolarization by Na+ influx
- hyperpolarizes the membrane
4. Depolarization
- Na channels open
- Na rushes into cell changing membrane potential
5. Repolarization
- Na channels inactivate → Na can not enter
- K channels open → K out changing membrane potential
6. Hyperpolarization
- K channels close slowly leading to overshoot
- Na channels reset
55
Absolute refractory period
The state of an axon in the repolarizing period, during which a new action potential cannot (usually) be elicited because gate 2 of sodium channels, which is not voltage activated, is closed
56
Relative refractory period
The state of an axon in the later phase of an action potential, during which stronger electrical current is required to produce another action potential
Potassium channels are still open
57
Back propagation
Reverse movement of an action potential from the axon hillock into the dendritic field
Signals the dendritic field that the neuron is sending an action potential over its axon and may play a role in plastic changes in the neurons that underlie learning
58
Nerve impulse
Propagation of an action potential on the membrane of an axon
Refractory periods produce a single discrete impulse that travels along the axon in one direction only
Size and shape of action potential remain constant along the axon (all or none)
59
Myelin and Nodes of Ranvier
Voltage-gated channels are located at the nodes
| Enables saltatory conduction
60
Presynaptic membrane/cell (axon terminal)
Where the action potential terminates to release the chemical message
61
Postsynaptic membrane/cell (dendritic spine)
The receiving side of the chemical message, where EPSPs or IPSPs are generated
Sometimes called postsynaptic density
62
Synaptic vesicle (presynaptic)
Small membrane bound spheres that contain one or more neurotransmitters
63
Synaptic cleft (space between)
Small gap where the chemical travels from presynaptic to postsynaptic membrane
64
Postsynaptic receptor (postsynaptic)
Site to which a neurotransmitter molecule binds
65
Tripartite (3 part) synapse
Functional integration and physical proximity of the presynaptic membrane, postsynaptic membrane, and their intimate association with surrounding astrocytes
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Neurotransmitter
Chemical released by a neuron onto a target with an excitatory or inhibitory effect
Outside the CNS, many of these chemicals circulate in the bloodstream as hormones (have distant targets, action slower than that of a neurotransmitter)
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Otto Loewi (1921) - Frog heart experiment
Role of the Vagus nerve and neurotransmitter acetylcholine in slowing heart rate
68
Acetylcholine
The first neurotransmitter discovered in the PNS and CNS
Activates skeletal muscles in the somatic nervous system and may excite or inhibit internal organs in the autonomic nervous system
Subsequent research
69
Epinephrine (adrenaline) & Norepinephrine (noradrenaline)
Important in sympathetic NS → speed up heart rate
70
Anterograde synaptic transmission
Process of transmitting information across a chemical synapse from the presynaptic side to the postsynaptic neuron
1. Neurotransmitter is synthesized
2. Packaged & stored within vesicles at axon terminal
3. Release → into cleft in response to action potential
4. Binds to & activates postsynaptic receptors
5. It is degraded or removed
71
Synthesis in the axon terminal
Building blocks from food are pumped into cell via transporters, protein molecules embedded in the cell membrane
72
Synthesis in the cell body
According to instructions in the DNA (peptide transmitters)
| Transported on microtubules to axon terminal
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Storage of Synthesis
Attached to microfilaments
Stored in granules
Attached to the presynaptic membrane
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Action Potential Release
At the terminal, the action potential opens voltage gated calcium (Ca2+) channels
Ca2+ enters the terminal and binds to the protein calmodulin, forming a complex
The complex causes some vesicles to empty their contents into the synapse and others to get ready to empty their contents
75
Receptor Activation
Post Synaptic Side
1. Depolarize the postsynaptic membrane, causing excitatory action on the postsynaptic neuron (EPSP)
2. Hyperpolarize the postsynaptic membrane, causing inhibitory action on the postsynaptic neuron (IPSP)
3. Initiate other chemical reactions that modulate the excitatory or the inhibitory effect or influence other functions of the receiving neuron
76
Autoreceptor
Self-receptor on the presynaptic membrane that responds to the transmitter that the neuron releases
Negative feedback loop
77
Transmitter-activated receptors
Protein embedded in the membrane of a cell that has a binding site for a specific neurotransmitter
- Ionotropic receptor
- Metabotropic receptor
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Ionotropic receptor
Embedded membrane protein with two parts:
A binding site for a neurotransmitter
A pore that regulates ion flow to directly and rapidly change membrane voltage
Allows the movement of ions such as Na+ , K+ , and Ca2+ across a membrane
79
Metabotropic receptor
Embedded membrane protein with a binding site for a neurotransmitter but no pore
Indirectly produces changes in nearby ion channels or in the cell’s metabolic activity
Linked to a G protein
80
G protein-coupled receptors (GPCRs)
G protein that can affect other receptors or act with second messengers to affect other cellular processes (G protein-coupled receptors (GPCRs)
81
G protein
Consists of three subunits, alpha, beta, and gamma
Alpha subunit detaches when a neurotransmitter binds to the G protein’s associated metabotropic receptor
Detached alpha subunit binds to other proteins in the cell membrane or cytoplasm
82
Second messenger
Binds to a membrane-bound channel, causing the channel to change its structure and thus alter ion flow through the membrane
Initiates a reaction incorporating intracellular (within the cell) protein molecules into the cell membrane, leading to formation of new ion channels
Binds to sites on the cell’s DNA to initiate or cease the production of specific proteins
83
Degradation
After neurotransmitter has passed on message:
| enzymes in the synaptic cleft break down the neurotransmitter
84
Reuptake
After neurotransmitter has passed on message:
Transmitter is brought back into the presynaptic axon terminal; by-products of degradation by enzymes also may be taken back into the terminal to be used again
85
Astrocyte uptake
After neurotransmitter has passed on message:
| nearby astrocytes take up neurotransmitter; can also store transmitters for re export to the axon terminal
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Criteria for identifying neurotransmitters
Transmitter must be synthesized or present in neuron
When released, transmitter must produce a response in target cell
Same receptor action must be obtained when transmitter is experimentally placed on the receptor
There must be a mechanism for removal after the transmitter’s work is done
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Functions of Neurotransmitters
1. Carry a message from one neuron to another by influencing the voltage on the postsynaptic membrane
2. Change the structure of a synapse
3. Communicate by sending messages in the opposite direction.
These retrograde (reverse-direction) messages influence the release or reuptake of transmitters on the presynaptic side
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Types of Neurotransmitters
- Small-molecule transmitters
- Amino acids, amines
- (Neuro) Peptide transmitters
- Lipid transmitters*
- Gaseous transmitters
- Ion transmitters* *newer categories; some controversy
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Small-molecule transmitters
Class of quick-acting neurotransmitters
| Synthesized from dietary nutrients and packaged ready for use in axon terminals
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Acetylcholine
- Acetylcholine synthesis: Choline – comes from breakdown of fats, Acetate – found in acidic foods
- Breakdown of acetylcholine - Enzyme: acetylcholinesterase (AChE)
- Choline and acetate are taken back up into the presynaptic neuron and can be reused
91
Norepinephrine
Important in Sympathetic NS
In CNS, involved in arousal and attention
Dysfunction in ADHD and depression
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Epinephrine
Important in SNS
93
Dopamine
Involved in movement, motivation, and reward
Imbalances involved in Parkinson’s disease, schizophrenia, ADHD
-Almost all addictive drugs either directly or indirectly activate the dopamine system
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Serotonin (5-HT)
Made from tryptophan (found in pork, turkey, milk, bananas)
| Regulates mood and aggression, appetite and arousal, respiration, and pain perception
95
Amino acid transmitters
```
Glutamate: main excitatory transmitter
-Involved in learning and memory
GABA: main inhibitory transmitter
-GABA is formed by a simple modification of a glutamate molecule.
-Glycine, histamine
```
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Neuropeptides
Synthesized through translation of mRNA from instructions in the neuron’s DNA
Most are assembled on the neuron’s ribosomes, packaged in a membrane by Golgi bodies, and transported by the microtubules to the axon terminals
Act slowly and are not replaced quickly
97
Functions of Peptide Transmitters
Act as hormones that respond to stress
Enable a mother to bond with her infant
Regulate eating and drinking, pleasure and pain
Contribute to learning
Opioids such as morphine and heroin mimic the actions of natural brain peptides
98
Opioids
Any endogenous or exogenous compound that binds to opioid receptors to produce morphine-like effects
99
Classes of Opioids
Five classes of opioid peptides: dynorphins, enkephalins, endorphins, endomorphins, and nociceptin
Sleep-inducing (narcotic) and pain-relieving (analgesic) properties
100
Four classes of receptors
delta, kappa, mu, and nociceptin
101
Lipid transmitters
Main example: endocannabinoids (endogenous cannabinoids)
A class of lipid neurotransmitters synthesized at the postsynaptic membrane to act on receptors at the presynaptic membrane
Synthesized on demand
Act as retrograde neurotransmitters
102
Gaseous transmitters
Neither stored in synaptic vesicles nor released from them
Synthesized in cell as needed
Easily cross cell membrane
103
Ion transmitters
Recent evidence has led researchers to classify zinc (Zn2+) as a transmitter
Actively transported, packaged into vesicles—usually with another transmitter like glutamate—and released into the synaptic cleft
104
Cannabinoids
Drug that acts on the endocannabinoid system (Marijuana/cannabis)
Tetrahydrocannabinol (THC) is one of 84 cannabinoids and the main psychoactive constituent in cannabis
THC alters mood primarily by interacting with the cannabidiol 1 (CB1) receptor found on neurons
Also binds with the CB2 receptors found on glial cells and in other body tissues
105
Activating systems
Neural pathways that coordinate brain activity through a single neurotransmitter
Cell bodies lie in a nucleus in the brainstem, and their axons are distributed through a wide region of the brain
106
4 Activating Systems
Cholinergic, dopaminergic, noradrenergic, and serotonergic
107
Event-Related Potentials (ERPs)
Largely the graded potentials on dendrites that a sensory stimulus triggers
Complex electroencephalographic waveforms are related in time to a specific sensory event
To counter noise effects, the stimulus is presented repeatedly, and the recorded responses are averaged
-Used to detect which brain areas and regions process stimuli
108
Epilepsy
Characterized by recurrent seizures, which register on an electroencephalogram (EEG) as highly synchronized neuronal firing indicated by a variety of abnormal waves
About 1 person in 20 has at least one seizure in his or her lifetime, usually associated with an infection, temperature, or hyperventilation during childhood
109
Circuits
All behaviour is governed by specific circuits in the brain
| Connections exist between neurons in different brain structures/nuclei
110
How did/do we learn about circuits
Animal studies
Human studies
Functional Magnetic Resonance Imaging (fMRI)
Resting-State MRI (rs-MRI)
111
Animal studies
Lesions: remove a brain region and see what behaviour(s) is affected
Tract tracing: inject a specific chemical into a brain region, wait, then stain to determine where the chemical ended up (stay inside neuron, travel backwards from axon to cell body (retrograde) or forwards from dendrites/cell body to axon terminal (anterograde), some can also cross synapses to examine multiple connections)
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Human studies
EEG: can provide a vague idea of what regions are active in what order
fMRI
113
Functional Magnetic Resonance Imaging (fMRI)
When human brain activity increases, the increase in oxygen produced by increased blood flow actually exceeds the tissue’s need for oxygen
Amount of oxygen in an activated brain area increases
Changes in the oxygen content of the blood alter the magnetic properties of the water in the blood
114
Resting-State MRI (rs-MRI)
The living brain is always active
Used to infer brain function and connectivity by studying fMRI signals when participants are “resting,” (i.e., not engaged in any specific task)
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Purpose of the nervous system
Responsible for communicating to the rest of the body
Regulating
Allows us to interact with the environment
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Vision:
Light energy produces chemical energy
117
Auditory:
Air pressure produces mechanical energy
118
Somatosensory:
Mechanical energy
119
Taste and olfaction:
Chemical molecules
120
Sensory receptors
They respond best to a specific part of the sensory world
overlap in the receptive fields of each cell
Density of sensory receptors is important for determining the sensitivity of a sensory system
Differences in receptor density determine the special abilities
121
Neural relays/labeled lines
All receptors connect to the cortex through a sequence of three or four intervening neurons
Information can be modified at various stages in the relay, allowing the sensory system to mediate different responses
122
Input for Sensory Circuits
sensory neuron - small receptive field
123
Relays
Sensory neurons synapse w/ neurons in a relay station (brain region), some sensory systems have multiple stops before primary cortex, each neuron receives input from multiple sensory neurons
124
Primary Cortex
the main cortical area that processes a specific sense
125
Secondary Cortex
most sensory input is further processed in secondary regions, receives input from multiple areas of primary cortex, can involve integration of multiple sensory systems
126
Topographic map
Neural–spatial representation of the body or of the areas of the sensory world perceived by a sensory organ
Frequently found at every step of the relay
Homunculus: representation of the human body
Maintained throughout the visual circuit
127
How do action potentials encode various kinds of sensations
Different sensations are processed in different areas of the cortex
We learn to distinguish the senses through experience
Each system has distinct wiring set up at all levels of neural organization
In mammals, each sensory system has at least one primary cortical area
128
Encoding Sensory Info
Info encoded by action potentials that travel along peripheral nerves to the CNS
Presence of a stimulus can be encoded by an increase or decrease in discharge rate (Increase or decrease can encode stimulus intensity)
129
Encoding features of particular sensations
Activity in different neurons
| In some cases, different rates of firing of the same
130
Range of electromagnetic energy visible to humans
About 400 nanometers (violet) to 700 nanometers (red)
| Nanometer (nm): one-billionth of a meter
131
Rods
More numerous than cones
Sensitive to low levels of light (dim light)
Used mainly for night vision
One type of pigment only
132
Cones
Highly responsive to bright light
Specialized for color and high visual acuity
In the fovea only
Three types of pigment
133
Retinal Ganglion Cells
Photoreceptors connect to other cells in the retina then to the ganglion cells
Retinal Ganglion Cells (RGC) exit the eyes as the optic nerve
134
Optic Chiasm
Junction of the optic nerves from each eye
Axons from the nasal (inside) half of each retina cross over to the opposite side of the brain
Axons from the temporal (outer) half of each retina remain on the same side of the brain
Information from left visual field goes to the right side of the brain; information from the right visual field goes to the left side of the brain
135
Three Routes to the Visual Brain
Geniculostriate system
Tectopulvinar system
Retinohypothalamic tract
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Geniculostriate system
Projections from the retina to the lateral geniculate nucleus to the visual cortex
137
Tectopulvinar system
Projections from the retina to the superior colliculus to the pulvinar (thalamus) to the parietal and temporal visual areas
Role in orienting towards visual stimuli in the environment
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Retinohypothalamic tract
Synapses in the tiny suprachiasmatic nucleus in the hypothalamus
Role in regulating circadian rhythms and in the pupillary reflex
139
Lateral geniculate nucleus (thalamus)
Right LGN: input from right half of each retina
| Left LGN: input from left half of each retina
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Primary visual cortex (V1)
Aka striate cortex (striped)
Organized into cortical columns
Info from each eye is kept separate producing alternating columns for each eye (ocular dominance columns)
Info about colour, form, and motion is kept segregated
Different regions receive different info
141
Secondary visual cortex (V2 to V5)
Extrastriate cortex
Segregation maintained in V2
Starts to combine in V3 onwards
142
Dorsal visual stream
Pathway that originates in the occipital cortex and projects to the parietal cortex
The how pathway (how action is to be guided toward objects)
projects to the secondary somatosensory cortex then to the frontal cortex
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Ventral visual stream
Pathway that originates in the occipital cortex and projects to the temporal cortex
The what pathway (identifies an object)
Secondary somatosensory cortex interacts with the ventral stream by providing conscious haptic information about the identity of objects and completed movements
144
In temporal lobe
Faces
| Scenes
145
In parietal lobe
Eye movements
Visual control of grasping
Visually guided grasping
146
Injury to the Ventral Pathway
Agnosia: inability to recognize…
Objects, colours, faces, etc.
Still able to appropriately shape their hand when reaching to grasp an object they can’t identify
147
Injury to the Dorsal Pathway
Optic ataxia
Deficit in the visual control of reaching and other movements
Damage to parietal cortex
Retention of ability to recognize objects normally
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Somatosensory system
Tells us what the body is up to and what’s going on in the environment by providing bodily sensations such as:
Touch, temperature, pain, position in space, and movement of the joints
149
Somatosensory receptors
- Hapsis
- Proprioception
- Nociception
150
Hapsis
Perceive fine touch and pressure; identify objects that we touch and grasp
Activated by mechanical stimulation of hair, tissue or capsule
151
Proprioception
Perception of the location and movement of the body
| Sensitive to the stretch of muscles and tendons and the movement of joints
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Nociception
Perception of pain, temperature and itch
| Free nerve endings activated by chemicals
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Rapidly Adapting Receptor
Body (Somatosensory) sensory receptor that responds briefly to the beginning and end of a stimulus on the body
154
Slowly Adapting Receptor
Body (Somatosensory) sensory receptor that responds as long as a sensory stimulus is on the body
155
Proprioceptive and Haptic Neurons
Carry information about location and movement (proprioception) and touch and pressure (hapsis)
Large, well-myelinated axons (fast)
Ascend the spinal cord ipsilaterally
156
Posterior Spinothalamic Tract
The haptic and proprioceptive axons for touch and body awareness ascend the spinal cord ipsilaterally
Axons from dorsal-root ganglion neurons enter spinal cord & ascend ipsilaterally synapsing in the dorsal column nuclei
Axons from dorsal column nuclei cross to opposite side of brain and project up as part of a pathway - medial lemniscus
Axons synapse with neurons located in the ventrolateral nucleus of the thalamus, which projects to the somatosensory cortex and motor cortex
157
Primary Somatosensory Cortex
Receives projections from the thalamus
| Begins the process of constructing perceptions from somatosensory information
158
Secondary Somatosensory Cortex
Aka nonprimary or association cortex
Located behind the primary somatosensory cortex
Refines the construction of perceptions, projects to the frontal cortex
159
Damage to Somatosensory Cortex
Damage does not disrupt plans for making movements, but does disrupt how the movements are performed, leaving their execution fragmented and confused
160
How are movements performed
Movement categories/motor plans are evoked by electrical stimulation of the monkey cortex
Reaching, jumping, climbing, chewing
Studies on humans using MRI suggest that the human motor cortex is organized in terms of functional movement categories
161
Frontal lobes
Prefrontal cortex: plans complex behavior
Premotor cortex and supplementary motor area: produces the appropriate complex movement sequences
Primary motor cortex (M1): specifies how each movement is to be carried out
162
Corticospinal Tract
Main efferent pathways from the motor cortex to the brainstem to the spinal cord
Axons descend into the brainstem, sending collaterals to a few brainstem nuclei, and eventually emerge on the brainstem’s ventral surface where they form a large bump on each side (pyramidal tracts)
163
The Basal Ganglia:
Receive input from all areas of the neocortex and allocortex, including motor cortex,
the nigrostriatal dopaminergic system from the substantia nigra
Project back to the motor cortex and substantia nigra
Subserve a wide range of functions, including association or habit learning, motivation, emotion, and motor control
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Volume Control Theory
The internal globus pallidus (Gpi) acts like a volume control on the motor cortex
If it is turned up, movement is blocked; if it is turned down, movement is allowed
Direct & indirect pathways of basal ganglia
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Two Pathways within the Basal Ganglia
Direct: When activated, the GPi is inhibited and the pathway is freed to produce movement
Indirect: When activated, the GPi is activated and inhibits the thalamus, thus blocking movement
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Seeing & Identifying an Object
Light into the eyes onto the rods and cones → retinal ganglion cells → lateral geniculate nucleus → primary visual cortex → ventral stream to temporal lobe region involved with recognizing mugs
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Location of body vs object
Proprioceptive information coming from our joints → dorsal column nuclei → brainstem → ventrolateral nucleus of the thalamus → primary somatosensory cortex → secondary somatosensory cortex
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Reaching for an Object
Involves the dorsal visual stream for visually guided movements
Also involved the prefrontal cortex (planning) → premotor cortex → primary motor cortex → info travels down the spinal cord and out to muscles
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Grasping the Object
Involves haptic feedback from our fingers → dorsal column nuclei → brainstem → ventrolateral nucleus of the thalamus → primary somatosensory cortex → secondary somatosensory cortex
Involves basal ganglia (volume control hypothesis)
(bringing mug to mouth - motor system)
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Cerebellum
Timing
| Accuracy
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Damage to the Motor System
-Spinal cord damage → paralysis
-Dysfunction of the basal ganglia
Direct pathway dysfunction → Parkinson’s disease
Indirect pathways dysfunction → Huntington’s disease
-Damage to the cerebellum → difficulty making corrections for errors in movement
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Why is Pain Necessary?
The occasional person born without pain receptors experiences body deformities through failure to adjust posture, and acute injuries through failure to avoid harmful situations
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Nociceptive Neurons
Pain, temperature and itch information, nerve fibers synapse with neurons whose axons cross to the contralateral side of the spinal cord before ascending to the brain
- A delta fibers: Larger, myelinated axons, AP travels quickly, Fast/sharp pain
- C fibers: Small axons with little or no myelination, AP travels slowly, Dull, throbbing pain
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Anterolateral / Spinothalamic Tract
Axons from the dorsal-root ganglion neurons enter the spinal cord and cross over, synapsing onto neurons on the contralateral side
Axons from contralateral spinal cord ascend where they join other axons forming the medial lemniscus, synapsing with neurons in the ventrolateral nucleus of the thalamus
Neurons from the thalamus then project to the somatosensory cortex and cingulate cortex
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Gate Theory of Pain
Activities in different sensory pathways play off against each other and so determine whether and how much pain is perceived as a result of an injury
Haptic-proprioceptive stimulation can reduce pain perception, whereas the absence of such stimulation can increase pain perception through interactions at a pain gate
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Gate theory
Interneuron that is the gate uses an endogenous opiate as an inhibitory neurotransmitter (Opioids relieve pain by mimicking the actions of the endogenous opioid)
Electrical stimulation at a number of sites in the brainstem can reduce pain, perhaps by closing brainstem pain gates
Perceptions might be lessened through descending pathways from the forebrain and the brainstem to the spinal-cord pain gate
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Periaqueductal Gray Matter (PAG)
Electrical stimulation of the PAG suppresses pain
PAG neurons produce their pain-suppressing effect by exciting pathways in the brainstem that project to the spinal cord where they inhibit neurons that form the ascending pain pathways
