W2: Fundamental Principles of the Brain Flashcards
What is it (4 words) – yes, again, gotta stay sharp
Synaptic Transmission
information transfer at synapse
define + neuron analogy + synapse types list (2)
Synapse
specialised junction where one part of a neuron contacts & communicates with another neuron or cell type (e.g. muscle / glandular cell)
REMEMBER: a neuron is like a fuse that regenerates
- Electrical synapses
- Chemical synapses
what - “keep it simple, stupid”
Electrical Synapses
allow the direct transfer of ionic current from one cell to next
Gap Junction: What
Specialised sites at which electrical synapses occur. Found between cells in nearly every part of the body and interconnect many non-neural cells including epithelial cells, smooth and cardiac myocytes, liver cells, some glandular cells, and glia.
- when gap junctions (GJs) interconnect neurons, they can function as electrical synapses
- ELECTRICALLY COUPLED: cells connected by GJs as electrical current (in the form of ions) can pass through these channels
Gap Junction: Structure / Composition
- plasma membrane of cell 1 and cell 2, the GJs bridging the gap between them (3nm)
- 6 connexin subunits = connexon; 2 connexons meet + combine => gap junction channel
- ca. 20 subtypes of connexins, half of which found in brain
function, direction
Gap Junction: Function
allow ionic current to pass from one neuron to another
most allow current to pass equally well in both directions therefore unlike most chemical synapses, electrical synapses = bidirectional
characterise! how it can function as an escape mechanism
Transmission @ Electrical Synapses
very fast and if synapse is large, nearly fail-safe
- therefore AP in presynaptic neuron can produce, with very little delay, an action potential in the postsynaptic neuron
- some invertebrate species (e.g. crayfish) electrical synapses are sometimes found between sensory and motor neurons in neural pathways mediating escape reflexes thus can be understoof as a mechanism enabling hasty retreat in dangerous situation
definition, 3-step process, nature of PSPs
Postynaptic Potential (PSP)
changes in membrane potential of postynaptic neuron that occurs in response to signals from presynaptic neuron
1) presynaptic neuron generates an AP
2) the electrical current from that AP flows directly to the postsynaptic neuron via the GJ
3) this current engenders a change in the postsynaptic neuron’s membrane potential and vice versa due to the bidirectional travel at electrical synapses
- can be EXCITATORY (depolarising, EPSP) or INHIBITORY (hyperpolarising, IPSP)
- one PSP does not cause an AP, but several PSPs may strongly excite a neuron leading to an AP
- PSPs have a GRADED effect rather than being all-or-nothing like APs
list
Role of Electrical Synapses: Relationship Types
1) NEURON-TO-NEURON
2) NEURON-TO-OTHER
Role of Electrical Synapses: Neuron-to-Neuron
precise role varies from one brain region to another, but often found where normal function requires that the activity of neighbouring neurons be highly synchronised
Role of Electrical Synapses: Neuron-to-Other
GJs between neurons and other cells particularly common early in development. Evidence suggests that during prenatal and psotnatal dev., GJs allow neighbouring cells to share both electrical and chemical signals that may help coordinate their growth + maturation
Chemical Synapses
most synaptic transmission in the mature human nervous system is chemical
Chemical Synapse Universal Characteristic: Structure
- presynaptic and postsynaptoc membranes at chemical synapses separated by synaptic cleft (20-50nm wide, 10x width of separation at GJs)
- cleft is filled with a MATRIX of fiborus extracellular protein, and one function is to serve as “glue” that binds presynaptic and postsynaptic membranes together
- presynaptic side of synapse (aka PRESYNAPTIC ELEMENT) usually an axon terminal which typiclaly contains dozens of small membrane-enclosed spheres storing neurotransmitters (each ca. 50nm in diameter)
- many axon terminals also contain larger vesicles (ca. 100nm wide) called SECRETORY GRANULES containing soluble protein that appears dark in electron miscroscope – thus aka DENSE-CORE VESICLES
Chemical Synapse Universal Characteristic: Membrane Differentiations
the collective, densely-accumulated protein adjacent to and within the membranes on either side of the synaptic cleft
Chemical Synapse Universal Characteristic: Active Site
site of neurotransmitter release and synaptic vesicles are clustered in cytoplasm adjacent to the active zones
Chemical Synapse Universal Characteristic: Postsynaptic Density
protein thickly accumulated in and just under postsynaptic membrane
- contains neurotransmitter receptors which convert the intercellular chemical singal (i.e. neurotransmitter) into an intracellular signal (i.e. change in membrane potential / chemical change) in postsynaptic cell
- postynsaptic response can vary based on type of protein receptor activated by neurotransmitter
list 5 types and further 2 subtypes
Chemical Synapses of the CNS
Classification Based on Location of Synapse
- Axodendritic
- Axosomatic
- Axoaxonic
- Axospinous
- Dendrodentritic
Further Classification Based on Appearance
- Gray’s Type I (asymmetrical)
- Gray’s Type II (symmetrical)
Chemical Synapse: Axodendritic
name of synapse when postsynaptic membrane is on a dendrite
Chemical Synape: Axosomatic
name of the synapse when the postsynaptic membrane is on the cell body (soma)
Chemical Synapse: Axoaxonic
name of the synapse when the postsynaptic membrane is on another axon
Chemical Synapse: Axospinous
name of the synapse when the postsynaptic membrane is of a dendritic spine
Chemical Synapse: Dendrodendritic
when dendrites form a synapse between one another in certain specialised neurons
Chemical Synapse: Gray’s Type I
asymmetrical: those in which the membrane differentiation on the postsynaptic side is thicker than that on the presynaptic side
USUALLY EXCITATORY
Chemical Synapse: Gray’s Type II
symmetrical: those in which the membrane differentiations are of similar thickness
USUALLY INHIBITORY
define, characteristics, structure accounting for reliability
Neuromuscular Junction
chemical synapses occuring in the motor axon of motor neurons of the spinal cord + skeletal muscle (and thus much more easily accessible than those in the CNS)
- fast and reliable transmission; an action potential in the motor axon always causes an AP in the muscle it innervates
- this reliability is accounted for by structural specialisations
Structure
- SIZE: one of the largest synapses in the body
- PRESYN. TERMINAL: contains larger number of active zones
- MOTOREND PLATE: in the postsynaptic membrane, contains a series of shallow folds packed with n.transmitter receptors
see W7: Motion
3 primary types
Neurotransmitters
Types: amino acids (AAs), amines, peptides
Neurotransmitter Synthesis + Storage
Synthesised by ribosomes, transported to and packaged within vesicles by so-called transporters (special proteins embedded within vesicle membrane.
2 steps: trigger + consequence
Neurotransmitter Release
1) triggered by the arrival of an AP in the axon terminal
2) dep. of the terminal membrane causes voltage-gated calcium channels on active zones to open
Neurotransmitter Release: Role of Calcium
At rest: gates closed
- Inside: low
- Outside: high
=> steep concentration gradient, allowing calcium to rush in as soon as it is needed (great conc. inside relative to area at actie zones not neuron as a whole)
“Calcium is Necessary and Sufficient for transmitter release”
- N: transmission does not occur without calcium even with an AP
- S: transmission can occur with only an injection of calcium into the presynaptic cell
=> n-transmitters released into synaptic cleft via exocytosis (vesicle fuses with neuronal membrane
Release of Neurostransmitters: the Synaptic Cleft
N-transmitters released into the syn. cleft affect the postsynaptic neuron by binding to specific receptor proteins embedded within the postsyn. density
- when n-transmitter binds, engenders a conformational change to the receptr such that the protein can then function differently
2 primary kinds
Types of N-transmitter Receptors
1) Transmitter-gated ion channels
2) G-Protein-Coupled Receptors
Transmitter-Gated Ion Channels (TGIC)
generally do not show the same level of selectivity as do voltage-gated channels
receptors, membrane-spanning proteins
general rule of thumb:
- if open channels permeable to Na+, net effect = dep.. excitatory therefore EPSP
- If open channels permeable to Ci-, net effect = hyper., inhibitory, therefore IPSP
entering from the extra-cellular environ.
e.g. ACh-gated channels, glutamate-gated channels, GABA,
G-Protein-Coupled Receptors
[steps + overview]
Autoreceptors
[steps + overview]
Neurotransmitter Recovery & Degradation
[overview]
define + 2 types of effects
Neuropharmacology
the effect of drugs on nervous system tissue
- Receptor Antagonists
- Receptor Agonists
Neuropharmacology: Receptor Antagonists
inhibitors of neurotransmitter receptors, bind to receptors and block normal action of neurotransmitters
Neuropharmacology: Receptor Agonists
mimic the actions of the naturally occurring neurotransmitter
deine, types, analogy
Synaptic Integration
process by which multiple synaptic potentials combine with one postsyn. neuron
EPSPs & IPSPs
leaky hose analogy
simple definition
Synaptic Integration
how neurons ‘add up’ input from presynaptic neurons
quantum, quantisation, EPSP size formula
Synaptic Integration: EPSPs - Quantal Analysis
- Quantum: elementary unit of neurotransmitter release, contents of a single postsyn. vesicle (each vesicle contains ca. the same no. of transmitter molecules usually several thousand)
- Quantisation: total amount of n.transmitter released is some multiple of this number
=> EPSP Size = no. vesicles released x mini amplitude
e.g. if one vesicle creates a mini of 0.2mV, then 200 vesicles cause: 200 x 0.2mV = 40mV
mini < PSP < AP
(the mini works as a random release, miniature postsyn. potential)
what + state 2 types
Synaptic Integration: EPSPs - Summation
represents the simplest form of synaptic integration in the CNS
- Spatial Summation
- Temporal Summation
Synaptic Integration: EPSPs - Spatial Summation
adding together of EPSPs generated simultaneously at many different synapses on a dendrite
Synaptic Integration: EPSPs - Temporal Summation
adding together of EPSPs generated at the same synapse if they occur in one rapid succession, within about 1-15msec of one another
leaky hose analogy + 2 properties
Synaptic Integration: Cable Properties
Leaky Hose Analogy
- the farther the water travels, the more water is lost through the leaks so less water reaches the end
- the farther an EPSP travels, the more charge leaks out so its amplitude decreases
The effectiveness of an excitatory synapse in triggering an AP depends on how far the synapse is from the spike-initation zone and on the properties of the dendritic membrane:
- Internal Resistance (Ri)
- Membrane Resistance (Rm)
Synaptic Integration: Cable Properties - Ri
resistance to current flowing longitudinally down the dendrite
- Lower Ri = easier current flow, EPSP travels faster
- Higher Ri = harder current flow, EPSP dies out quicker
Synaptic Integration: Cable Properties - Rm
resistance to current flowing across the membrane
- Lower Rm = more leaks, EPSP weakens faster
- Higher Rm = fewer leaks, EPSP stays strong longer
Synaptic Integration: IPSPs
[shunting, ]
Synaptic Integration: Modulation
[chain reactions within the neurons]
Anatomical Name
NS Terms
Anatomical References
Anatomical Names
Slice / Slicing
Section / Sectioning
NS Anatomy
[]
- anterior / rostral
- posterior / caudal
- dorsal
- ventral
- medial
- midline
- lateral
NS Anatomy: Ipsilateral
two structure on the same side of the midline
NS Anatomy: Contralateral
two structures on opposite sides of the midline
NS Anatomy: Anatomical Planes
- (Mid)sagittal Plane
- Horizontal / Axial Plane
- Coronal Plane
3
CNS: Components
all encased in bone
- Cerebrum
- Cerebellum
- Brain Stem
CNS: Cerebrum
rostal-most and largest part of the brain and split down the middle into 2 cerebral hemispheres separated by deep sagittal fissure
sensation in left -> processed by right; sensation in right -> processed by left
CNS: Cerebellum
Latin: “little brain”
lying behind cerebrum and while dwarfed by it, contains as many neurons as both cerebral hemispheres combined
- primarily a movement control center
- also a site of regulation of vital functions like breathing, consciousness, body temp.
sensation in left -> processed by left; sensation in right -> processed by right
CNS: Brain Stem
forms the stalk from which the cerebellum and cerebral hemispheres sprout
- relays info. from cerebrum to spinal cord and cerebellum and vice versa
- also site of regulation of vital functions like breathing, consciousness, body temperature
considered the most primitive and thus damage to it is usually fatal
- tectum (“roof”)
- tegmentum (“covering”)
- basilar pons
CNS: Spinal Cord
encased in bony vertebral column and attached to brain stem.
- Major conduit of info. from skin, joints, muscles of body in the brain, and vice versa
- Spinal cord communicates with the body via the spinal verves, which are part of the PNS
- Spinal nerves exit the spinal cord through notches between each vertebra of the vertebral column
- Each spinal nerve attaches to the spinal cord by means of two branches - the dorsal root and ventral root
2
PNS: Components
All other parts of NS other than brain and spinal cord
- Somatic PNS
- Visceral PNS
- (ANS)
PNS: Somatic PNS
all the spinal nerves that innervate the skin, the joints, muscles that are under VOLUNTARY control
- somatic motor axons, which command muscle contraction, derive from motor neurons in the ventral spinal cord
- cell bodies of the motor neuron lie within the CNS, but their axons mostly in the PNS
- somatic sensory neurons, which innervate and collect information from the skin, muscles, joints enter the spinal cord via the dorsal roots. the soma of these neurons lie outside the spinal cord in clusters called dorsal root ganglia (there is a dorsal root ganglion for each spinal nerve)
PNS: Visceral PNS
aka involuntary, vegetative, automatic nervous system (ANS), consisting on neurons that innervate the internal organs, blood vessels, glands
- visceral sensory axons bring info. about visceral function to CNS e.g. pressure and oxygen content of the blood in the arteries
- Visceral motor fibers command the contraction and relaxation of muscles forming the walls of intestines and blood vessels (smooth muscles), the rate of cardiac muscle contraction, secretory function of various glands
when one speaks of emotional reactions beyond volumtary control (like ‘butterflies in stomach’) usually mediated by visceral PNS & ANS
Afferent / Efferent Axons
- AFFERENT: “carry to”
- EFFERENT: “carry from”
=> indicates whether axons are transporting info. toward or away from a particular point (considering axons in pNS relative to point of reference in CNS)
toward CNS = afferent; away from CNS = efferent
Cranial Nerves
12 pairs that arise from brain stem + innervate mostly the head
each cranial nerve has a name and number
mnemonic!
Cranial Nerves: Names
“Ooh, Ooh, Ooh, to touch and feel very good velvet. Such heaven!”
Name: Ooh, Ooh, Ooh, to touch and feel very good velvet. Such heaven!
- Olfactory
- Optic
- Oculomotor
- Trochlear
- Trigeminal
- Abducens
- Facial
- Auditory-Vestibular
- Glossopharyngeal
- Vagus
- Spinal Accessory
- Hypoglossal
mnemonic!
Cranial Nerves: Functional Information
“Some say marry money but my brother says big brains matter more”
Functional Info: Some say marry money but my brother says big brains matter more.
- Sensory
- Sensory
- Motor
- Motor
- Both
- Motor
- Both
- Sensory
- Both
- Both
- Motor
- Motor
3 Layers
Meninges
1) Dura mater
2) Arachnoid Membrane
3) Pia Mater
Meninx: Dura Mater
outermost, “hard mother”
leather-like consistency, forming a touch bag that surrounds the brain and the spinal cord
Meninx: Arachnoid Membrane
just below, “spider”
has appearance + consistency of resembling spider web
Real-Life Example
- usually no space here b/w the 2 meningeal layers
- however, if blood vessels passing through the dura are ruptured, blood can collect here and form a subdural hematoma
- the buildup of fluid in this subdural space can disrupt brain function by compressing parts of the CNS
- the disorder is treated by drilling a hole in the skull + draining the blood
Meninx: Pia Mater
innermost, “gentle mother”
thin membrane that adheres closely to the surface of the brain
- along the pia run many blood vesself that ultmiately dive into the substance of the underlying brain
- pia is separated from the arachnoid by a fluid-filled space containing a salter clear liquid called cerebrospinal fluid (CNS)
Ventricular System
fluid-filled (CSF) caverns and canals indise brain constitute the ventricular system
- CSF is produce in choroid plexus in ventricles of cerebral hemispheres
- flows from paired ventricles of cerebrum
- through series of connected, central cavities at core of brain stem
- exits ventricular system
- enters subarachnoid space
- CSF absorbed by blood vessels at arachnoid villi
if normal flow of CSF disrupted, brain damage can result
3 components
Primary Brain Vesicles
1) Prosencephalon / Forebrain
2) Mesencephalon / Midbrain
3) Rhomencephalon / Hindbrain
Collections of Neurons
Grey Matter
Generic term for a collection of neuronal cell bodies in the CNS. When a freshly dissected brain is cut open, neurons appear gray.
Collections of Neurons
Cortex
Any colection of neurons that form a thin sheetm usually at the brain’s surface. Latin for “bark”.
e.g. cerebral cortex = the sheet of neurons found just under the surface of the cerebrum
Collections of Neurons
Nucleus
A clearly distinguishavle mass of neurons, usually deep in the brain
From Latin “nut”
Collections of Neurons
Substantia
A group of related neurons deep within the brain but usually with less distinct borders than those of nuclei.
Collections of Neurons
Locus
Plural: Loci
A small, well-defined group of cells
Collections of Neurons
Ganglion
Plural: Ganglia
A collection of neurons in the PNS.
Latin for “knot”
Collections of Axons
Nerve
A bundle of axons in the PNS. Only one collection of CNS axons is called a nerve: the optic nerve
Collections of Axons
White Matter
A generic term for a collection of CNS axons. When a freshly dissected brain is cut open, axons appear white.
Collections of Axons
Tract
A collection of CNS axons having a common site of origin and a common destination.
e.g. corticospinal tract = originates in cerebral cortex and ends in the spinal cord
Collections of Axons
Bundle
A collection of axons that run together but do not necessarily have the same origin and destination.
Collections of Axons
Capsule
A collection of acons that connect the cerebrum with the brain stem.
Collections of Axons
Commissure Lemniscus
A tract that meanders through the brain like a ribbon.
