Module 3+4: nervous system Flashcards
nervous system
coordinates voluntary and involuntary actions transmits signals to and from different parts of its body
afferent
carry information towards CNS
efferent
carry information away from CNS
central nervous system (CNS)
Brain
spinal cord
peripheral nervous system (PNS)
nerve tissue outside the CNS: cranial nerves and branches, spinal nerves and branches, ganglia, plexuses and sensory receptors
cells of the nervous system: neurons
the basic signalling units of the nervous system
cells of the nervous system: gila
support cells
neurons carry ______ ______
electrical signals
parts of a neuron
-a cell body (soma), considered the control centre, with processes that extend outward; dendrites and axons
-shape number and length of axons and dendrites vary from neuron to neuron
-dendrites receive incoming signals fro neighbouring cells
-axons carry outgoing signals from neighbouring cells
-presynaptic terminals contain transmitting elements
neurons
Afferent: sensory
-carry information about temperature, pressure, light and other stimuli to the CNS
-specialized receptor converts stimulus to electrical energy
Pseudounipolar
neurons that have a single process called the axon. during development, the dendrite fused with the axon
Neurons
Interneurons
-complex branching neurons that facilitate communication between neurons
bipolar
neurons that have two relatively equal fibres extending off the central cell body
Neurons
Efferent: motor and autonomic
-motor: controls skeletal muscles
-autonomic: influences many internal organs
-sympathetic and parasympathetic
-usually have axon terminal or varicosities
anaxonic
Anaxonic CNS interneurons have no apparent axon
nerves
bundle of peripheral neurons
-efferent (motor)
-afferent (sensory)
-mixed
multipolar
multipolar CNS interneurons are highly branched but lack long extensions
axonal transport
the axon is specialized to convey chemical and electrical signals that require a variety of different types of proteins
-the axon contains many types of fibres and filaments but lacks ribosomes and ER necessary for protein production, therefor proteins must be produced in the cell body na transported down the axon
fast axonal transport
-membrane bound proteins and organelles (vesicles or mitochondria)
-anterograde: cell body to axon terminal, up to 400 mm/day
-retrograde: axon terminal to cell body 200mm/day
slow axonal transport
-cytoplasmic proteins (enzymes) and cytoskeleton proteins
-anterograde, up to 8mm/day some evidence for retro
-not well characterized, may be slower due to frequent periods of pausing of movements
kinesins
anterograde transport
dyneins
retrograde transport
kinesins and dyneins: motor proteins
fast axonal transport
ATP hydrolysis drives movement of proteins to “walk” along filaments
synapses
majority are chemical synapses
-space contains extracellular matrix (proteins and carbohydrates) that hold the pre and post synaptic cells in close proximity
how do billions of neurons in the brain find correct targets during development?
-depends on chemical signals
-axons of embryonic nerve cells contain growth cones that sense and move towards particular chemical signals
-growth cones depends on growth factors, molecules in the extracellular mantric and membrane proteins
-once reaching target cell a synapse forms
synapses must then be maintained through repeated use “use it or lose it”
gila support for neurons
initially believed to outnumber neurons by 10-50 too 1 more recently 1-4 to 1
fro years thought to be simply for physical support
now known to communicate with neurons and provide important biochemical support
Glia found in the CNS
ependymal cells
astrocytes
microgila (modified immune cells)
oligodendrocytes
Gila in PNS
Shawn cells
satellite cells
what are the myelin forming glia ?
CNS- oligodendrocytes
-wraps the saxons of multiple neurons
PNS- Schwann cells
1 cell wraps a segment of 1 neuron
non-myelinating Schwann cells
myelin forming glia
what is myelin and what does it do ?
-a substance composed of multiple concentric layer of phospholipid membrane wrapped around an axon
-provide structural stability, acts as insulation around the axon to speed up electrical signals (saltatory conduction), supply trophic factors
multiple sclerosis (MS)
disorder resulting from demyelination in brain and spinal cord
MS symptoms: sneery motor and cognitive issues
cause is unclear but the underlying mechanism appear to be:
-autoimmune (immune cells attack myelin)
-reduced ability of myelin producing cells
-genetic and environmental factors
satellite glial cells
surround the cell bodies in the PNS
-form a supportive capsule around the cell bodies of neurons (sensory and autonomic)
-supply nutrients
-structural support, proved a protective cushion
Astrocytes (glial cell)
highly branched glial cell sin CNS believed to make up half of all cells in the brain
-several subtypes, from a functional network
functions of Astrocytes
-take up and release chemicals at synapses
-provide neurons with substrates for ATP production
-help maintain homeostasis in the extracellular fluid (take up K+ and H2O )
-surrounds vessels
-part of the blood brain barrier
-influence vascular dynamics (increase/decrease regional blood flow)
Microgila (glial cell)
specialized immune cells that reside in the CNS
-serve to protect and preserve neuronal cells from pathogens and facilitate recovery from metabolic insults
if the signals that activate microglia pass a threshold, with respect to intensity, or microglia remain activated past a certain time period, these cells start to display detrimental properties
-Alzheimer’s disease, ALS
if the membrane potential of a cell changes form - 70mV to +10mV the cell is
depolarizing
(becoming more positive = depolarizing)
ependymal cells (glial cell)
line fluid cavities in the brain and spinal cord
-help to circulate cerebral spinal fluid that fills these cavities and surrounds the brain and spinal cord
-chemical stability
-protection
-clearing wastes
-possibly a source of neural stem cells
peripheral neuron injury
CNS repair less likely to occur naturally, glia tend to seal off and form scar tissue (prevents reformation)
Schwann cells can create a tube to guide the regenerating axon
1 mm/day in small nerves and 5mm/day in large nerves
electrical signals in neurons
neurons and muscle cells are “excitable” due to their ability to propagate electrical signals over long distance in response to a stimulus
two factors that influence membrane potential
- the uneven distributions of ions across the cell membrane (concentrations gradients)
- membrane permeability to those ions
the ________ describes the membrane potential that would result if the membrane were completely permeable to only one ion (the equilibrium potential for that ion)
Nernst equation
Goldman-Hodgkin-Katz equation (GHK equation)
-predicts membrane potential that results from the contribution of all ions that can cross the membrane
-determines as the combined contribution of each ion (concentration x permeability) to the membrane potential
-different form Nernst equation, which calculates the equilibrium potential for a single ion
gas, temperature, faraday constant
electrical signals in neurons
resting membrane potential in most neurons is -70mV
-mainly due to potassium
-Na+ contributes slightly ( very few Na+ leak channels)
-CI- minimally, equilibrium potential close to resting membrane potential
ion movements create electrical signals
a change in the K+ concentration gradient or change in permeability to ions alters the membrane potential
gated channels control ion permeability in neurons
ion permeability is primarily altered by opening or closing channels in the membrane
-new open channels can be added or removed from the membrane to alter permeability (slow)
the ease with which ions flow through a channel is known as the channels
conductance
-varies with the gating state of the channel
-channel protein isoform
gated channels:
mechanically gated channels
open in response to physical forces (pressure of stretch) found in sensory neurons
gated channels
chemically gated ion channels
in neurons respond to ligands including extracellular neurotransmitters and neuromodulators or intracellular or intracellular signalling molecules
gated channels
voltage gated channels
respond to changes in the cells membrane potential
gated channels
there is much variation in gated channels
-voltage for channel opening can vary from channel to channel
-the speed at which channels open or close varies
-many channels that open to depolarizations will close during repolarization
-some channels spontaneously inactivate
each major channel has subtypes, some have many
-varying properties between subtypes
-multiple isofroms that expressed different gating kinetics
-modified by different proteins and pathways
channelopathies
-can disrupt how ions normally flow through the ion channel
-can alter channel activation
-can alter channel inactivation
cystic fibrosis, cogentital insensitivity to pain, muscle disorders
current flow
the opening of ion channels allow. ions to move in or out of a cell
-the flow of electrical charge carried by an ion is the ions current
current flow follows ohms law
-current flow (I) is directly proportional to the electrical potential difference (v) between two points an inversely proportional to the resistance
two sources of resistance in a cell
membrane resistance
- resistance of phospholipid bilayer
internal resistance of the cytoplasm
-cytoplasmic composition and size of the cell
-resistance will determine how far current will flow in a cell before the energy is dissipated
electrical signals in neurons
voltage changes across the membrane can be classified in to two types of electrical signals
graded potentials
variable strength signals that travel over short distance and lose strength as they travel. can be depolarizing or hyper polarizing if graded potentials create a large enough depolarization it can induce an action potential
action potential
very brief, large depolarizations that travel for long distances through a neuron without losing strength. rapid signals over long distances
graded potentials (how does it work)
-graded because amplitude (size) is directly proportional to the strength of the stimulus and can vary
-decrease in strength as they spread out from the point of origin
-generated by chemically gated (ligand gated) ion channels or closure of leak channels (CNS and efferent neurons)
-chemical, mechanical, thermal gated sensory neurons
grade potentials reflect stimulus strength
-local current flow is a wave of depolarization or hyper polarization that moves through the cell
graded potentials lose strength as they move through the cell due to
current leak: open channels allow ions to leak out
cytoplasmic: open channels allow ions to leak out
cytoplasmic resistance: when there is a depolarizing substance it causes positive ions to be repelled, potassium will be repelled
there will be a decrease in the polarization
excitatory versus inhibitory
depolarization: excitatory postsynaptic potential (EPSP)
hyper polarization: inhibitory postsynaptic potential (IPSP)
trigger zone (axon hillock)
high concentration of voltage gated Na+ channels
-if membrane potential is 55mV an AP will be generated
action potential
electrical signals of uniform strength (all or none) that travel form the trigger zone to the axon terminals
sequential opening of voltage gated ion channels in the axon membrane as electrical current moves down
movement if AP along axon is referred to as conduction of AP
