How Nerves Work Flashcards
Describe the anatomical organisation of the nervous system
CNS - brain, spinal chord
PNS - nerves
Describe the structure of the brain
meninges (protective membrane) - dura, arachnoid + pia mater
cerebellum
cerebrum - frontal, temporal, pariental, occipital lobes
diencephalon - thalamus, hypothalamus
brainstem - midbrain, pons, medulla oblongata
Describe the various spinal nerves
31 pairs of spinal nerves
8 cervical - neck, shoulders, arms
12 thoracic - chest, abdomen
5 lumbar - genitalia, gastrointestinal tract
1 coccygeal
describe the structure of the spinal chord
white matter - myelinated axons
grey matter - neuronal cell bodies
dorsal horn, ventral horn
dorsal root afferent (sensory)
ventral root efferent (motor)
Describe the general structure of Neurons
cell body (soma) - contains the nucleus
dendrites - receive information
initial segment (axon hillock) - triggers action potential
axon - sends action potential
axon (presynaptic) terminals - release transmitter
describe the main cells in the nervous system
neurons - sensory (sensory receptor), inter (CNS), motor (acts on muscles, glands or neuron)
glia - 90% of cells in CNS
astrocytes (maintain the cell external environment for neuron, surrounds blood vessels & produce blood brain barrier)
obligondendrocytes (form myelin sheaths in the CNS - schwann cells in the PNS)
microglia (phagocytic hoovers mopping up infection)
Ependymal cells (produce the cerebrospinal fluid)
Describe the ionic basis of the resting membrane potential
Phospholipid bilayer impermeable to water and ions, equal conc. of NaCl and KCl inside and outside cell = no membrane potential
now Na/K pump has evolved, uses ATP to pump K ions into cell and Na ions out 1:1, charge still balanced = no membrane potential
now leaky K ion channels leak out some K ions down conc. gradient, builds up electrical gradient, an equilibrium is reached when the electrical gradient is equal and opposite to conc. gradient = resting membrane potential
Describe the properties of graded potentials
generator potentials - at sensory receptor
Postsynaptic potential - at synapse
Endplate potentials - at neuromuscular junction
Pacemaker potentials - in pacemaker tissues
small stimulus will trigger small response and open few channels
large stimulus will trigger large response and open many channels
only useful for short distance as axon is “very leaky” as signal gets smaller as travel along membrane
Describe the function of graded potentials
to depolarise the cell to threshold and make it fire an action potential
Describe the ionic basis of action potentials
K ions leak out of cell
voltage gated Na ion channels open - Na ions rush into cell
voltage gated channels close cell repolarises and voltage gated K ion channels open
Describe the properties of action potentials
have a threshold
are all or nothing - action potential produced or not
stimulus intensity in their firing frequency, not amplitude
have a refractory period - always goes forward
self-propagating - grows action potentials as it goes along membrane
travels slowly
How to increase conduction velocity
large axons - electrical current flows more easily down large axon than small axon (lower axial resistance), allows the Na channels to be wider spaced along the membrane - depolarisation spreads further
myelination - schwann cells in PNS and oligodendrytes in CNS, wrap myelin and sections of axon, increases membrane resistance and reduces membrane capacitance - less current wasted, action potential spread passively from node to node (saltatory conduction)
Describe the consequences of demyelination
multiple sclerosis in CNS & Guillain - Barre syndrome in PNS
attack myelin sheath
membrane resistance is decreased and capacitance is increased
more current lost between nodes
conduction fails
Describe the structure of the neuromuscular junction
synapse between motor neuron and skeletal muscle
contains: voltage gated Na channel, voltage gated Ca channel, Ach receptor, acetylcholinesterase enzyme, vesicle containing Ach (mEPPs and quanta)
Describe the process of the neuromuscular junction
- action potential in motor neuron
- opens voltage gated Ca channels in presynaptic terminal
- ACh binds to ACh (nicotrinic) receptors
- Opens ligand-gatedNa/K channels
- Evokes graded (local) potential (end plate potential) - always depolarises adjacent membrane to threshold
- Opens voltage gated Na channels - evokes new AP
- ACh removed by acetylcholinesterase
Describe the classification of nerve fibre types
classed into three types:
group A nerve fibers, group B nerve fibers, and group C nerve fibers
Groups A and B are myelinated, and group C are unmyelinated.
These groups include both sensory fibers and motor fibers.
Describe the ultrastructure of synapses between the neurons
1) the presynaptic membrane which is formed by the terminal button of an axon
2) the postsynaptic membrane which is composed of a segment of dendrite or cell body
3) the space between these two structures which is called the synaptic cleft.
Describe the function of synapses between the neurons
allows a signal to pass from one neuron to the next
Describe the process of synaptic transmission in the CNS
Range of neurotransmitters- acetylcholine, noradrenaline, dopamine, serotonin, histamine, glutamate, GABA, glycine etc
range of postsynaptic potentials, small postsynaptic potentials
several receptors - fast EPSPs, slow EPSPs, fast IPSPs, slow IPSPs - generally small and enables complex synaptic integration
synaptic connectivity - convergence, divergence, feedback inhibition, monosynaptic & polysynaptic pathways
Explain the role of synapses in integration of neuronal function
initial segment crucial to determining if cell reaches threshold (fire action potential)
axo-axonic synapse - manipulates the effects of a postsynaptic neuron’s firing on the neurons further downstream in the network = pre synaptic inhibition
axo-somatic synapse -formed by one neuron projecting its axon terminals onto another neuron’s axon = post synaptic inhibition
axo-dendritic synapse -synapses that one neuron makes onto the dendrite of another neuron = spatial summation
describe common excitatory and inhibitory neurotransmitters
Excitatory - epinephrine and norepinephrine
inhibitory - serotonin and gamma-aminobutyric acid (GABA)
Explain the basic properties of signal transduction in neurons
Neurons are connected by synapses in biological neural networks
When presynaptic neuron emits a spike, then neurotransmitters are released from the synapses and bind to receptors located in the postsynaptic cell to excite or inhibit the postsynaptic neurons
This process facilitates the transmission of information
What is meant by EPSP and IPSP
EPSP - excitatory postsynaptic potential (opening Na/K channels or closing leaky K ion channels)
IPSP - inhibitory postsynaptic potential ( opening Cl ion channels or opening K ion channels)
