Unit 1 Flashcards
(274 cards)
1
Q
Santiago Ramon y Cajal
A
Suggested neurons are the units of the brained he noted the important of neurons in the late 1900s
2
Q
Brains are …
A
Metabolically expensive taking up 20% of the oxygen and nutrients of the body but only contributing to 2% of the body weight
3
Q
The complexity of the brain allows for …
A
Higher intelligence, coordination and function of the body to enhance reproduction and survival rates - complexity of higher executive function, information processing, problem solving for adapting environments to our way of living, communication abilities and movement (bipedalism and specificity of movement)
4
Q
Divisions of the nervous system
A
Central and peripheral nervous system
5
Q
CNS
A
Brain + spinal cord
6
Q
Peripheral nervous system
A
Spinal nerves, autonomic nervous system, (most) cranial nerves
7
Q
Pineal gland alternative name
A
Pine cone gland
8
Q
Formix alternative name
A
Arch
9
Q
Cortex alternative name
A
Bark
10
Q
Cingulate gyrus alternative name
A
The belt
11
Q
Corpus callosum alternative name
A
Hard body
12
Q
Septum pellucidum alternative name
A
Translucent wall
13
Q
Pituitary gland alternative name
A
Slime gland
14
Q
Mammillary bodies alternative name
A
Breast-like things
15
Q
Pons alternative name
A
Bridge
16
Q
Medulla Oblongata alternative name
A
Long-ish marrow
17
Q
Thalamus alternative name
A
Inner chamber
18
Q
Cerebellum alternative name
A
Little brain
19
Q
Vermis alternative name
A
The worm
20
Q
Lingual gyrus alternative name
A
The tongue
21
Q
Cuncus alternative name
A
The wedge
22
Q
Lamina Quadragemina alternative name
A
Layer of the four twins
23
Q
Medial / medius
A
Towards the midline
24
Q
Lateral
A
Towards the side
25
Anterior
Front or near to the head
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Posterior
Back
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Ventral
Front, towards the abdominal
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Dorsal
Back, towards the back of the body
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Dorsal of the brain
Top
30
Ventral of the brain
Bottom
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Dorsal of the spine
Back
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Ventral of the spine
Front
33
Rostral
Towards the nose
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Caudal
Towards the tail
35
Interspecies differences of the brain
Increased size of the cortex with the more advanced / complex species (greater in humans, chimpanzees and dolphins than in rats), larger olfactory bulbs in animals that rely more on smell (rats), larger cortex increases capacity of executive functioning
36
Dorsal view of the brain
Viewing from the top of the brain downward
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Ventral view of the brain
Viewing from the bottom of the brain upward
38
Lateral view of the brain
Looking at the sides of the brain
39
Medial (midsagittal) view of the brain
Looking at the middle of the brain
40
Frontal plane, coronal cut
Anterior and posterior
41
Sagittal plane
Lateral sides, midsagittal if it is in the middle
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Horizontal plane, horizontal cut
Diving into dorsal and ventral
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Coronal cut
Through the brain from left to right
44
Meninges
Membranes that surround the brain and spinal cord
45
Dura Mater
Most outside of the three layers (the most external), known as the hard mother
46
Arachnoid
Has wispy connections with the pia mater, fibrous area
47
Pia mater
Tender mother, sticks right to the brain, follows the curves of the cortex, cannot be separated
48
Subarachnoid space
Between the arachnoid layer (dorsal) and pia mater (ventral), this is where the CSF is found
49
CSF
Cerebrospinal fluid bathes the brain to keep it buoyant and have proper ion concentration to function properly, the fluid of the ventricles has similar density to the brain (while air would be very different), therefore it is able to absorb some of the shock to protect the brain
50
Meningitis
Bacterial, viral or fungal infection of the meninges
51
Ventricles
Four ventricles - two lateral ventricles, third and fourth ventricles, CSF is found in the subarachnoid space and is stored within the ventricles. The ventricles are fluid-filled cavities within the brain
52
Lateral ventricles
Found within the two hemispheres of the brain
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Orientation of the ventricles within the brain
Third ventricles more dorsal than the fourth, but ventral (below) to the lateral ventricles
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Cerebral Aqueduct
Acts as a water channel connecting the third and fourth ventricles, pathway for CSF
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Central Canal
Continuation of the fourth ventricle down within the middle of the spinal cord
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Potential problem with CSF production
CSF is constantly being produced, no nervous or hormonal control on the level of production, just keeps going (no tap). Problem - could be making too much or not enough
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Hydrocephalus
Accumulation of CSF, buildup causes blockage in the drainage system, ventricles swell and head can become enlarged. Pressure buildup must be released with shunt to drain out excess fluid. Not much room for the brain to swell with the cranium's restriction
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Obstruction of CSF flow in hydrocephalus often at ..
Cerebral Aqueduct
59
Blood vessels of the brain
Most of the blood goes up the neck through the circle of Willis from the internal carotid and then divides into three main branches - anterior, middle and posterior cerebral arteries. Basilar artery goes posterior as the two posterior cerebral arteries join together
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Problems with blood vessels of the brain - strokes
A stroke is a disruption of blood flow to the brain, the brain needs a constant supply of oxygen and glucose. If this disruption lasts too long there can be defects in cognitive function depending on where the infarct occurs (necrotic tissue accumulation point) or death may result
61
What's an infarct
Accumulation of necrotic tissue in the brain due to hypoxia
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Ischemic stroke
Blood supply is reduced due to a thrombus (local clot at site of development) or embolus (breaks off and causes obstruction somewhere else). Thrombus and embolus caused by accumulation of plaque. Anything downstream will be cut off from blood with complete obstruction
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Hemorrhagic stroke
Caused by the bursting of a blood vessel and therefore blood begins to leak out into the surrounding area. Often caused by aneurysms which are little pouches in the vessels, as they grow, the stretched tissue becomes thin and can rupture. Toxins in the blood that leaks can damage tissue around it and delivery of blood is diminished
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Hemorrhagic strokes can be caused by ...
Due to high blood pressure of a congenital defect (aneurysms, arteriovenous malformation AVM)
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High risk area of blood vessels for hemorrhagic strokes
Tangles of blood vessels
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Symptoms of anterior cerebral artery infarction
Aggression, personality change, aphasia, motor weakness, sensory changes and apraxia (frontal lobe affected)
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Symptoms of middle cerebral artery infarction
Hemiparesis (muscle weakness) on the opposite side of the body to the infarct. Right MCA stroke - left hemiparesis. Left MCA stroke - right hemiparesis and aphasia (difficulty speaking or understanding language)
68
What is aphasia?
Difficulty speaking or understanding language, contralateral function
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What is hemiparesis?
Weakness of the muscle, contralateral function
70
TIA
Transient ischemic attack - mini stroke, may feel dizzy, loose some vision, just feel off. Warning sign for major stroke. Not full blockage, but diminished blood flow to area
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Symptoms of strokes
B.E.F.A.S.T
```
B - balance
E - eyes
F - face
A - arms
S - speech
T - time
```
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Neuron
One of the main cells of the nervous system. Able to make different connections depending on the type of neuron
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Dendrites
The arms of the neurons that spread and make connections
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Cell bodies of the neurons
Can be located in the spinal cord and the terminal axons can go all the way to the tips of the phalanges
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Axo-Somatic connection
Going from the axon to the cell body
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Axo-dendritic connection
Going from the axon to the dendrites
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Glia Cells
Majority of the cells in the brain, they are support cells important in immune function, communication between neurons and blood vessels and act as insulation to the neurons (myelination to propagate signalling between Schwann cells)
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Unmyelinated neurons
Axons are still covered with glia cells but just not to the same extent
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Glioblasts
In the development to become glia
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What can go wrong with glia?
The glia cells are constantly being renewed and therefor at a quite high risk for cancer development because cells are constantly going through the cell cycle in growth and division
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Glioblastoma
Glioblasts are cells that develop into glia
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MS
Multiple sclerosis is an autoimmune disease where the body attacks the myelinated sheath (glia cells) surrounding the axon of the neurons, particularly the motor neurons. This causes slowed movement and poor muscle coordination and function
83
Divisions of the brain in development
The nervous system begins as a neural tube going along the body, then from this tube there is differentiation into the different structures
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The neural tube has four major divisions through development ...
Forebrain, midbrain, hindbrain and spinal cord
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Brainstem basics
Everything above the spinal cord, excluding the cerebellum and cerebrum, the brainstem contains basic, evolutionary function (primal functions)
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Brainstem functions
Breathing, digestion and circulation. A lot of nerves pass through this region to relay information between the spinal cord and the brain to control functions
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Brainstem location and parts
The brainstem contains the midbrain between the parietal and frontal lobes, the pons and the medulla oblongata
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Cerebellum
Involved in movement prediction, found at the inferior aspect of the occipital lobe. It is involved in muscle coordination and balance (helps with controlled motion). The cerebellum has ipsilateral control (same side)
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Cerebrum
Evolutionarily new systems and functions, is the central hemisphere, outermost portion of the forebrain includes grey and white matter and the two hemispheres
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Cerebral cortex
Refers mainly to grey matter
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White matter
Glia cells surround the axons, and glia cells contain a lot of fat which gives them to the appearance of being white in colour (fatty axons = white matter)
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White and grey matter distribution in the CNS
Brain - white on the inside (axons in the middle) and grey on the outside
Spinal cord - grey in the middle and white on the outside as it stretches outward to give up the nerves
93
White matter tracks
Allows axons of a neuron to go across different areas of the brain - including the corpus callosum that goes between the two hemispheres
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Corona Radiata
White matter track that goes between the spinal cord and brain stem
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Basal ganglia
Ganglion (mass of grey matter)
96
New imaging technique of diffusion
Diffusion of water used to figure out which was the water is diffusing - diffuses along the direction of the fibres to construct these white matter tracks
97
Coup concussions
Damage to the brain at the location of the trauma
98
Contrecoupe concussions
Damage to the brain at the location of the trauma plus to the side opposite (bounce back of the brain to the other side)
99
Sheering of tissue from concussions
Sheering of tissue - fibres are damaged by sheering and twisting motion of the brain during the trauma. Sheering can lead to the death of the neurons. Accumulation of necrotic tissue leads to area specific symptoms and can lead to CTE with early dementia
100
4 lobes of the brain
Frontal, occipital, parietal and temporal
101
Insula
Gap between the frontal and parietal, just above the temporal lobe, tissue insulated by the other lobes deep within
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Gyrus
Outward bulge of the brain (the hill) (convexity)
103
Convexity
Gyrus
104
Sulcus
Indentations, the valleys of the brain (concavity)
105
Concavity
Sulcus
106
Fissure
Deep sulcus (deep concavity)
107
Longitudinal fissure
Interhemispheric fissure
108
Sylvian fissure
Huge deep, mostly horizontal, insult is buried within it, separates temporal lobe from parietal and frontal lobe
109
What is the insula hidden i?
The sylvian fissure
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Central sulcus
Divides frontal an parietal lobe. Precentral gyrus anterior and post-central gyrus posterior
111
The cerebral cortex is a ...
Crumpled sheet of 3mm thick. 1,600 cm squared (both hemispheres). About 18" (45 cm diameter) pizza
112
Cerebral cortex =
Mosaic of areas, functions localized within the brain, specialized involvement in functions. Different in cell types (therefore different functions), densities and layerings
113
Efferent neurons
Output, goes from the spinal cord or brain with motor information to the muscles to produce the movement
114
Afferent neurons
Input, goes from the skin or muscles with sensory information to the spinal cord or cranium to be processed
115
Spinal nerve anatomy
Central canal in the middle (with CSF fluid). Spinal nerves surrounded by one vertebrae in the spine. Dorsal root is afferent (sensory information) going in and ventral root is efferent (motor information) is going out. Cell body of the neuron located in the ganglion either dorsal or ventral depending on type
116
How many vertebrae are there?
33, each with a set of dorsal and ventral nerves coming off of it
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Sacrum
Sacred bone (formerly thought to house the soul)
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Dermatomes
Map of sensation, each spinal nerve is associated with a specific region of the skin in which it provides sensory control / information (afferent nerve from the dorsal root ganglion of the vertebrae)
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Myotomes
Region of the body innervated by the efferent, motor neuron
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Spinal cord injury
Symptoms / dysfunction downward to the injury of the nerve. Compression of the spinal cord due to the displacement of the vertebrae. Different myotomes and dermatomes affected
121
Shingles
Reactivation of chicken pox virus, can infect spinal nerve. Infected roots of specific spinal nerves - rash will follow corresponding dermatome
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Neuron
Specialized cell of the nervous system that processes information, shape of the neuron dictates function, senses environmental changes and receives and transmits signals to other neurons
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Glia
From Greek 'glue' these specialized cells support functions of the neurons. There is a 10:1 ratio of glia to neurons in the nervous system
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Histology
Microscopic study of structure of tissues, use of antibodies or other substances to label the brain and see how the brain looks like within
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3 stain types
Nissl stain, myelin statins, golgi stains
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Nissl stain
Distinguishes neurons from glia
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Myelin stain
Visualizes fibres by staining myelin on axons
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Golgi stain
"La reazione nera" - the black reaction stain cell body (soma, perikaryon), neuritis (axon, dendrites)
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Differences in neurons with different stains
Different stains can be used to study the nervous system as there are different types of neurons that are made up of different specialized structures and components that can be tagged and targeted to measure and identify
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Golgi was rewarded the Nobel Prize in ...
1906
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Cytoarchitecture
The study or arrangement of neurons in different parts of the brain, an example of stain mechanisms
132
Soma
Central processing units, many of the organelles and processes related to shape of the neuron and other important processes, transmits information in communication
133
Reticular theory
Nervous fibres form a diffuse continuous network, neural communication occurs by continuity (everything was connected to everything), if something is touching your finger, eventually every cell in the nervous system would know this (but this is metabolically expensive)
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Reticular theory developed by
Camillo Golgi but has been disproved
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Reticular means
"Net-like"
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___ would not be possible through the reticular theory
Multi-tasking
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Cell theory
Tissues composed of individual cellular elements, in neural terms, neuritis do not form a continuous network. Each nervous element an "absolutely autonomous canton"
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Cell theory developed by ...
Cajal, he shared the Nobel prize in Golgi in 1906
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Cajal initially wish to be an ..
Artist
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___ theory came from cell theory
Neuron theory came from cell theory and this is what we still agree upon today
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One of the first animals with a nervous system
Jellyfish
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Neuron doctrine
The neuron is the anatomical and physiological unit of the nervous system. Neurons communicate by contact, not continuity
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Principle of dynamic polarization
Electrical signals within a nerve cell flow in a unidirectional flow of information from the receptive surface of the dendrites through to the trigger region at the axon terminals
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Principle of connectional specificity
Nerve cells make specific connections, at specific contact points with certain target cells and not others
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Cytosol
Fluid inside the cell salty and potassium rich, contained in the soma
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Cytoplasm
Everything inside the neuron, except the nucleus, contained in the soma
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Neuronal membrane
Encloses neuron, separates it from the outside world, contained in the soma
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Organelles
Specialized membrane-enclosed structures within the soma, contained in the soma
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Nucleus
Gene expression and protein synthesis, DNA made up of nucleotides, ribosomes are actually responsible for the translation of DNA into proteins
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Rough endoplasmic reticulum
Major site of protein synthesis
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Smooth endoplasmic reticulum
Heterogenous function, protein folding, calcium regulation
152
Golgi apparatus
Protein sorting site
153
Mitochondria
Cellular powerhouse
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Neuronal membrane
The function of neurons cannot be understood without understanding the structure and function of the membrane and its associate proteins. 5 nm thick, protein studded - contains proteins involved in determining what goes in an out of the cell (active pumps and pores - protein channels)
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Cytoskeleton
Scaffolding of the neuron - gives it shape
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Three elements of the cytoskeleton
Microtubules, neurofilaments, microfilaments
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Cytoskeleton and Alzheimer's disease
Related to the accumulation of extracellular amyloid plaques and intracellular tangles
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The axon
Found only in neurons, specialized for transmission of information over long distances, parts (axon hillock, axon and collaterals and axon terminals)
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Diameter of axons
Highly variable (0.2 to 20 um) - important, because conduction velocity is linked
160
At the axon terminal is the
Terminal bouton
161
Point of contact with other neurons and site of communication is the
Synapse
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Synapse term coined by ..
Charles Sherrington
163
Sit of action of many drugs and toxins
Synapse
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Terminal contains ..
Vesicles, covered with proteins, lots of mitochondria
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Anterograde transport
Movement of material from soma to terminal
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Retrograde transport
Movement of material from terminal to soma
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Metabolic vs. electrical signalling
Metabolic signalling is more slow than the electrical signalling - different levels of frequency of conductance for different processes
168
Cashing in on axonal transport
Inject HRP into the brain - the neurons will take it and you can cut into the brain and see which neurons are coloured and therefore took up the drug, affected, can see the outlines of the superior colliculus or whatever structures uses the drug
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Superior colliculus
Grows within the cortex in mammals, involved in sensory transport
170
Cashing in on axonal transport ocular dominance in cortex
Inject in one eye and it goes into cortex, injection in only one eye, there is stripes in the cortex in one region that are unique and are different to the stripes in the same region from the other eye. This is ocular dominance, each eye has unique pattern of neurons in the cortex, specific regions / neurons used by each eye
171
Dendrites
Are the antennae of the neuron - collect and integrate signals from other neurons. Used in communication and receive the information from the axon terminals from adjacent neurons
172
Collection of dendrites for a neuron is called the
Dendritic tree
173
Dendritic spines
Covered in many, many synapses some with specialized structures
174
Pyramidal neurons
Neurons have long tails, axons and then terminals that stretch far in the body to its target
175
Stellate neurons
Stretched out in every direction to its target, more localized
176
Classifying neurons by number of neurites
Unipolar, bipolar and multipolar
177
Classifying neurons by dendritic structure
Pyramidal cell (stretches far out into the nervous system, stellate cell (spreads out in many directions, but not far)
178
Classifying neurons by connectivity
Sensory, motor or interneurons
179
Classifying neurons by axon length
Golgi type I (long axon, projection neurons), golgi type II (short axon, local circuit neurons)
180
Classifying neurons by NTM
Ach containing - cholingeric, dopamine containing - dopaminergic
181
Neurons are organized in circuits
Reflective response of the patella and the lifting of the leg. Receptor is in the ligament and tendon and then there is signalling in the spinal cord that receives the afferent sensory information and transmits it into efferent motor information to produce movement of the leg
182
Circuit principles - divergence
Axonal branches of one neuron reach many neurons, one signal reaches many targets
183
Circuit principles - convergence
Axonal branches of many reach one target neuron, many signals reaches one target, for comparison or integration
184
Glia have several important jobs ...
Support cells, myelinations, scavenging, housekeeping (mopping up transmitters), formation of blood-brain barrier (astrocytes)
185
Astrocytes
Control the chemical content of the extracellular medium (blood brain barrier and blood flow), important for axon guidance and synaptic support
186
Myelinating glia cells
Provides layers of membrane that insulate axons and speed action potential transmission (oligodendrocytes, Schwann cells), affected in demyelinating disorders like multiple sclerosis
187
First brains
250 million years ago
188
First human brains
4 million years ago
189
Vertebrae brains
Ganglia's found in the head of the flat worm, more of these nerve clusters in squid. Frogs or animals with an actual backbone, this is when we start seeing nervous system organization similar to ours
190
Bipedalism allows ..
Our hands to be free which opened up many opportunities of development of tools and other uses beneficial to survival - hands needed more cortex space for more fine detailed movement
191
Hominid =
Human-like ape
192
Homo
Man
193
Australopithecus
Walked upright, brain size similar to apes (1/3 human brain size)
194
Plotting brain weight as a function of body weight
Gives better representation of brain size compared to body size. Humans and apes are seen above the line (more advanced species in function and capabilities), their brain is larger than you would expect to see relative to our body
195
Size of cerebellum across species
Stays relatively constant
196
Size of medulla across species
Gets smaller with more advanced brains
197
Lissencephalic =
Things like rats and mice have smooth brains
198
Gyrencephalic =
More complex brains with brains (gyruses), convolutions in the cortex
199
Enkephalon =
Brain
200
The more convolutions you have..
The more surface area of the cortex (holds more), as you fold things you have more connections (axon) - faster neural connections, less space wasted on wiring
201
Brains are arranged from..
Left to right, top to bottom in order of increasing number of neurons according to average species values from rodents, non-human primates, insectivores and human brain
202
Rodents hemispheric side
To the right
203
Primates hemispheric side
To the left
204
How many neurons in the human brain?
86 billion neurons
205
Electrical transmission in neurons
Neurons communicate y means of electrical transmission, but must overcome some obstacles. Information is encoded by the frequency of all or none action potentials. Action potentials are digital pulses that travel along axons, they can be generated only by those cells with an excitable membrane (property also found in myocytes and skeletal muscle cells). Understanding how charges are distributed across the membrane is critical to understanding neuronal signalling
206
Neurons differentially activated by different stimuli
Put small electrode and shows different levels of action potentials (excitability) depending on the orientation of the bar, cells responds to the electrode stimulation, selectivity for different types of stimulus - this is the basis of perception (toning curve)
207
Resting membrane potential
Different in electrical charge across the membrane when not generating an action potential
208
Determinants of resting membrane potential
Cytosol and extracellular fluid, ions, phospholipid membrane, protein channels and ion pumps
209
Cytosol and extracellular fluid
Cell is an enclosed system, water is the primary ingredient, along with ions, the distribution of ions is the basis of the resting and action potentials - important to have membrane around the cell and its organelles, exchange system with outside environment for energy retrieval
210
Ions
Atoms or molecules with an electrical charge
211
Cations
Net positive charge ion
212
Anions
Net negative charge ion
213
Phospholipid membrane
Phospholipid bilayer, barrier to water soluble ion flow, hydrophilic polar head and two hydrophobic tails
- lipid-soluble molecules (hormones, vitamins) can enter into the cell (no regulation), an isolated system will die if nothing can get into it because it requires energy
214
Protein channels
Membrane-spanning membranes that form a pore, different subunit composition gives different properties (may be ion-selective - sodium, potassium, may be gated - open / closed), proteins can be chemically heterogeneous with non-polar and polar regions - hydrophobic and hydrophilic and will therefore more readily with the membrane or the fluid
- organization / sequence of amino acids within the protein channel will dictate functionality and use, the specificity of the protein channel
- open / closed gated channels can charge the rate of ion entry
215
Ion pumps
Some membrane spanning proteins form ion pumps which actively transport ion across the membrane - play a critical role in membrane potential
216
Movement of ions
Movement of ions across the neuronal membrane is responsible for changes in membrane potential. Can be influenced by two driving forces - diffusion, electricity
217
Diffusion
Ions tend to move from high to low concentration - down the concentration gradient
- concentration gradient = how the concentration of something (ions) changes from one place to another (opposite sides of a membrane), movement will occur until each side is equal in charge or concentration - equilibrium
218
Electricity
Electrical current, movement of electrical charge, unit of measure amperes (symbol)
219
Two factors determine current flow ..
Electrical potential - force exerted on a charge particle, reflects different in charge between anode and cathode, more current as different increases, measured in volts
Electrical conductance - relative ability of a charge to move from one area to another (symbol, g, inverse of resistance) dictated by number of particles and ease of travel - electrical gradient formed through changing the permeability of certain cations or anions to change the electrical charge across the membrane
220
Ohm's law
Quantifies the relationship between potential, conductance and current flow. Lower resistance will result in higher flow
```
I = V / R
I = Vg
R = 1 / g
```
221
For key factors
Ions in solution on both sides of the membrane
Ions can cross the membrane only via protein channels
Protein channels are highly selective for certain ions
Ion movement depends on concentration gradient and electrical potential across the membrane
222
Membrane potential
The voltage across the neuronal membrane at any movement (Vm), resting membrane potential is negative relative to the outside (-65 mV for the neuron) - higher concentration of potassium and a high concentration of sodium outside the cell
223
Large concentration gradient, no permeability =
No movement, charges balanced
224
Electrical potential
Selective potassium channel, diffusion rules - inside becomes more negative (electrical gradient is formed), electrical force pulls potassium back through channel, equilibrium state is reaches when diffusional and electrical forces equal and opposite
225
Equilibrium potential
Electrical potential difference that exactly balances ionic concentration gradient (Vm = -80 mV), difference that makes electrical and chemical gradients equal in magnitude
226
Proteins within the membrane and electrical charge
There are lots of protein within the cell which tend to be negative and therefore this may be why there is so much potassium in the cell - to balance out the charges
227
Steady state of potassium
When there is a lot of potassium on the inside and little on the outside
228
Balance of electrical gradient and concentration gradient =
Equilibrium potential, two gradients actually equal each other, movement prevented when they deviant
229
Critical points of the cell membrane
Large changes in membrane potential are caused by very small changes in ionic concentrations. The net difference in electrical charge occurs at the inside and outside surfaces of the membrane, capacitance - charge storage. Ions driven across the membrane at a rate proportional to the difference between the membrane potential and equilibrium potential
230
Ionic driving force
Difference between the real membrane potential and th equilibrium potential for a particular ion is the ionic driving force (Vm-Eion)
231
Distribution of ions across the membrane
Membrane potential depends on the ionic concentrations on either side of the membrane. Potassium is more concentrated on the inside of the neuron. Sodium and calcium are more concentrated on the outside of the neuron
232
Nerst equation derived ..
In 1888 by German Physical Chemist Walter Nerst
233
Nerst value for calcium
30.77 mV
234
Nerst value for potassium
-80 mV
235
Nerst value for sodium
61.54 mV
236
How are the concentration gradients established?
Two pumps actively maintain potassium, sodium and calcium concentrations ..
1 - sodium-potasisum pum,p
2 - calcium pump
237
Relative ion permeabilities of membrane at rest
Pumps establish ionic concentration gradients across membrane. We can compute equilibrium potentials for different ions using Nerst equation, BUT this is the membrane potential that results if membrane is selectively permeable to single ion only, resist potential results from relative permeabilities of ions determined mainly by potassium and sodium (but there are different permeabilities for the different channels which the Nerst equation doesn't take into account, this is why people were getting different measures for membrane potential)
238
Goldman equation
Quantifies dependence of membrane potential on relative ionic permeability and concentration. Applies only when Vm is not changing (not during an action potential)
239
Membrane permeability of the membrane - sodium vs. potassium
Permeability of the membrane to potassium is 40 times higher than it is to sodium - therefore potassium leaves the cell more easily and the resting membrane potential is going to be negative and closer to the charge of that ion (-80 mV) (resting membrane potential = -65 mV). Membrane potential is going to oscillate between charges close to the ions that move in and out, changing the potential
240
Extracellular potassium must be controlled
Membrane is highly permeable to potassium at rest, so membrane potential is determined largely by potassium concentration. This means that small changes in potassium can have large effects on membrane potential, and neuronal function
241
Increasing extracellular potassium by a factor of 10?
Resting membrane potential goes to Vm = -16.69 mV
(increase from -65 to -17 mV)
Increases in extracellular potassium depolarizes neurons, this can cause aberrant activity - hyper excitability, conduction block
242
Astrocytes
Mop of extracellular potassium. These glia cells have their own mechanisms and machinery in order to help maintain extracellular space and concentration of ions. Mops the potassium up (then stores it inside, goes into the CSF and is pumped into the blood flow to be excreted by the kidneys). Helps to maintain resting membrane potential
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The action potential
When a neuron is said to fire we really mean that an action potential has been generated. The generation of an action potential is like an ON / OFF switch - this is called the all or none law
Action potentials are necessary to transmit information over long distances, very short duration of less than 2 seconds
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Scientists who contributed to action potential
Eccles, Hodgkin and Huxley won the Nobel Prize in 1963 for their discoveries regarding the ionic basis of action potential generation and conduction
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Rising phase
Rapid depolarization of the membrane, caused by rapid influx of sodium ions. Ascending phase due to permeability of sodium. Increase in the positive charge within the cell because of the influx of sodium (sodium channels are open to allow this movement), depolarization of the cell. Cell becomes depolarized until zero and then goes up even further in an overshoot until the calcium channels shut and then the permeability of potassium increases and it leaves the cell making it more negative again
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Rising phase and Nerst potential
Sodium is trying to reach the Nerst potential level of this ion (reaching closer to 61.54) - this is the driving force
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Permeability changes of potassium and sodium during phases
Permeability is higher for potassium at the beginning in resting state, then sodium is higher at the middle depolarization phase and then potassium again is more permeable during the depolarization phase as the cell becomes more negative again. Triggered by changes in the voltage of the membrane (remember these are voltage-gated channels)
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Voltage-gated channels
When internal membrane potential reaches -40 mV sodium channels pop open, and they close at +20 mV. Voltage of the membrane acts like an ON / OFF switch for sodium channels. Deflection below means that a channel is open (because current is moving so the channel is open), no movement when the channel is closed and therefore no deflection as there is no movement
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Opening of voltage-gated sodium channels
Intrinsic mechanisms within the channel that cues changes in structure and allows the channel to become open and allows molecules to bind and get through
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Voltage-gated sodium channels
Open with little delay, stay open briefly (1 ms), cannot be opened again until membrane potential goes back to negative
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Overshoot phase
Portion of the action potential in which the inside of the membrane becomes positively charge, overshoots into positive mV potential. Overshoots is necessary to give the action potential its intensity - requires a significant change enough to be recognized. In the overshoot, the sodium is beginning to slow down and then there is an increase in potassium permeability
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Falling phase
Period of rapid repolarization caused by rapid efflux of potassium across the membrane
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Voltage gated potassium channels
When internal membrane becomes positive, voltage-gated potassium channels open and these ions rapidly exit the neuron. The driving forces for this movement are the concentration gradient and electrical forces generated by the internal positivity of the neuron. Potassium channels will open during the times of higher charge and they open more slowly therefore action is delayed relative to sodium
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Restoration phase
Resting potentials gradually restored, as sodium-potassiump pump re-establishes concentration gradients
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Absolute refractory period
Once an action potential has been discharged, a neuron cannot discharge again for approximately 1 msec (voltage-gated sodium channels inactivated), action potentials don't normally ride on each other (summate intensities) because of refractory periods, channels respond to certain voltages which are not reached until after this period again
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Relative refractory period
In the next few msec, more current is required to initiate another action potential, potassium channels remain open, can have another action potential, but may be a bit delayed (takes more intensity of current to get it going again)
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Tetradotoxin (TTX)
Binds to sodium channels and prevents them from opening
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Saxitoxin
Blocks sodium channels
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Batrachotoxin
Interferes with timing of opening and closing of channels
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Toxins can affect the sodium channels
Some of the most dangerous toxins are related to the function of these sodium channels. Silencing some part of the brain (locally) to interfere with the channels and slow down the action potentials for some with epilepsy
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Hodgkin-Huxley Model
Nobel Prize 1963
The principle ionic currents:
- the sodium current (I Na)
- the potassium current (I K)
- the leakage current (I L) - the other ions and their movement in the cell
Potassium and sodium - important to calculate m, n and h - describes the dynamics of the connetics
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Orthodromic conduction
Once initiated, action potentials travel from the axon hillock to the axon terminal, action potential initiated close to the body of the neurone and then travels down the axon and the terminals to the next neuron (usually unidirectional)
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Antidromic conduction
Action potentials can also be induced by artificial means and travel up the axon in the opposite direction. Not sure if there is a biological function to antidromic conduction where the signal goes backwards
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Axon diameter and conductance
Larger = faster
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Myelination and conductance
Myelination speeds conduction
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Nodes of Ranvier
Isolation portion of the membrane, where the membrane potential can jump between these spaces, the sodium channels are located in the spaces to speed up conductance
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When you get hit with pain
The initial pain that it quick is conducted by myelinated fibres and the slower, more duller pain that is more persistent is actually conducted by the unmyelinated fibres
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Squid giant axon
Giant axons have evolved in separate animal groups, common in systems that required ultra-fast conduction, such as systems mediating escape behaviours. Have evolved in several groups. Evolutionary convergence suggests that selective pressure have given rise to this adaptation for fast processing repeatedly. Diameters of the squid axons are huge. Neurons tend to be quite similar across different species
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Saltatory conduction
Presence of insulation with intervening nodes causes action potential to skip from one node to the next, and speeds action potential conduction velocity. This process is referred to as saltatory conduction
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Multiple sclerosis
Autoimmune disease where the body attacks and breaks down the myelin sheath of the axons causing weakness, lack of coordination and impaired speech and vision. Sclerosis - hardening (describes the lesions of the axon)
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Intrinsically bursting
Action potentials do not fire at proper speed, they are intermittent and slow
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Rate of firing can change based on ..
Strength and frequency of stimulation of the neuron (with more current stimulation there is greater rate of firing of action potentials)
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Threshold of action potential
Around -50 mV, some level of increase in charge from resting to -50 mV to get an action potential
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Membrane currents and conductance
Potassium 20 times inside cell, sodium 10 times outside the cell. In natural conditions, there is a certain permeability for potassium (movement is occurring), increasing permeability of a certain ion there is going to be a new flow of that ion
1. net movement of potassium is an electrical current
2. number of channels open is proportional to an electrical conductance
3. current flows only when Vm is not equal to Ek
