W5a: the peripheral nervous system Flashcards
what are the features of nerves
transmit info via electrical signals (action potentials)
communicate via synapses (elec and chem)
can occur in different shapes and sizes, they contain common structural elements
electrically excitable cells- neuronal membrane is polarised
describe neuron structure
cell body- contain nucleus
dendrite- recieve the messages between neurons
axon- link cell body and terminal bouton
terminal bouton- message is passed over to another neuron across the synapse
myelin sheath- insulating fatty layer and speed the transmission
how is the myelinate sheath formed
schwann cells
types of neurons
sensory
interneuron
motor
brain cell type diversity
bipolar
multipopar
unipolar
what does a nerve cell membrane have
an electrical potential between the inside and outside of cell (abt -70 mv) = resting potential
due to movement of charged ions across membrane
either by diffusion thru selective ion channels
or selective pumps to move ions against the concentration gradient
describe the propagation of action potential
1) resting membrane potential- na ion channels are shut
2) action potential is generated as na ions flow inwards across the membrane= na ions channels open
3) depolarisation of action potential spreads to neighbouring region of membrane= reinitiates action potential there and na channels open
4) to the left of the AP area re polarisation occurs as k flows out, the return the cell back to resting state= na channels shut and k channels open
depolarisation and repolarisation is repeated and local current of ions across plasma membrane casues the AP to be PROPAGATED along the axon
describe the initiation of AP
- Resting Membrane Potential (-70 mV): inside of the neuron is negatively charged compared to the outside due to the action of the sodium-potassium pump
- Stimulus and Local Depolarization: When a stimulus reaches the neuron, it causes the membrane pot to become slightly less negative in the region where the stimulus was applied as na channels start to open
- Reaching the Threshold: If incoming signal is strong enough to bring membrane potential to around -55 mV (threshold potential)= this activates na channels at the axon hillock (the trigger zone of the neuron) and the neuron is now “primed” for the AP
- Rapid Depolarization (+30 mV): Once threshold is reached, na channels open fully, causing a massive influx of Na⁺= inside of the neuron quickly becomes positively charged (~ +30 mV). (ALL OR NOTHING PRINCIPLE)
- hyperpolariation: membrane potential becomes briefly more negative than 70 before stabilisation
once peak is hit, na channel close because its now inactivated but k channel open to allow k to leave (repolarisation)= so resting pot can be reached again
what is saltatory conduction
occurs in myelinated neurons (motor)
depolarisation passes along sheath to the next node
allows to conduct impulses faster than non myelinated nerves
what are synapses
occur at the boundary of each nerve cell, where it meets another cells
nerve impulse can be transmitted by elec or chem signal
substances which act as chem signallers are neurotransmitters
what are the 2 main modalities of synaptic transmission
chemical synapse
electrical synapse
describe electrical synapse
gap junctions (pores in membrane), na and k etc can cross
cell is electrically coupled= direct transfer of ionic current across membrane
conduction speed is very fast
found in neuronal pathways associated with escape reflexes or in neurones that need to be synchronised (retina)
BIDIRECTIONAL
describe a chemical synapse
en elec single reaches the axon terminal of a neuron, it stimulates the release of neurotransmitters
asymmetric morphology with distinct features found in pre and post synaptic parts
chem synapses are UNIDIRECTIONAL
what are the steps of a chemical synapse
- action potential arrives at terminal
- ca ions channel open
- ca enters pre synaptic neurone
- ca signals to neurotransmitter vesicles
- vesicle move to membrane and dock
- neurotransmitters releases via exocytosis
- neurotransmitters bind to receptors on post
- single initiated in postsynaptic neurone
what are some common neurotransmitters
dopamine (+/-)
serotonin (+/-)
norepinephrine (+/-)
acetylcholine (+/-)
glutamate +
gaba -
+= sympathetic
-= parasympathetic
describe the PNS
made up of nerves derived from CNS (12 cranial nerve pairs and 31 spinal nerve pairs)
can be subdivided into afferent and efferent nerves
afferent nerves are sensory nerves= transmit impulse from body to CNS
efferent nerves are motor nerves= transmit impulse from CNS to effector organs
spinal cord anatomy
continuous with medulla
extends from upper boarder of C1 to lower boarder of L1
45cm long, width of little finger
nerves converting impulses from brain descend down and at appropriate levels, leave the cord
what is the reflex arc
stimulus detected by receptor, info passed to cns from sensory neuron
cns interpret and integrate message
motor neuron carries mesage to effectors
effectors (muscle or gland) then responds
what are sympathetic and parasympathetic nerves
most organs have both and the 2 systems have opposing effects
S: fight or flight
P: rest and digest (return to homeostasis)
how do ganglion assist with the connection of PNS and CNS
ganglion- group of neuron cell bodies where synapses occur between preganglionic and postganglionic nerve cells
act as communication hubs, the info received from the sensory neurons, or info received from motor neurons are processed here
describe the SNS
preganglionic fibres of the SNS produce acetylcholine (short fibres because close to spinal cord)
most postganlionic fibres produce norepinephrine (long fibres because needs to reach target organs)
location of ganglia is within few cm of CNS along vertebral column
sympathetic fibres orignate from thoracolumbar region of spinal cord (T1-L2)
describe the PNS
inhibits the effector organ (expect in digestive tract)
all pre and post ganglionic fibres produce acetylcholine
location of ganglia is in or near effector organ effectors
preganglionic fibres arise from CNS and sacral region of spinal cord (S2-S4)
how can spinal cord injury affect the body
result from trauma or disease
level of neurological injury depends by the lowest point on the spinal column below which here is a decrease of feeling
C4= quadriplegia (complete paralysis below neck)
C6= partial paralysis of hands/arms/lower body
T6= paraplegia (below chest)
L6= paraplegia (below waist)
dermatomes
an area of skin that provides sensory input to CNS via single spinal root
30 dermatomes that relay sensation from particular region of skin to brain
dysfunction/damage to spinal nerve can triggers symptoms in corresponding dermatome
