chapter 3 - nerve cells and nerve impulses Flashcards
1
Q
what is a neuron / nerve cells
A
- basic structural / functional units of the NS
- S: cell body, dendrites (many branches, receive messages from other neurons carry towards cell body), axon (long, unbranched, carry impulses away from cell body, often myelinated)
- > myelin sheath: myelinated fibres (have sheath, white matter, cell body) unmyelinated fibres (no sheath, grey matter, cell body)
- > Schwann cells: wrap around axon forming the sheath, neurilemma (outermost part of cell, repair of damaged fibres)
- > nodes of ranvier: gaps in myelin sheath (act as insulator, protect axon from damage, speeds up movement of impulses along axon)
2
Q
what are the functional types of neurons
A
- sensory (receptor): carry messages from receptors in sense organs / skin to CNS
- motor (effector): carry messages from CNS to muscle / gland
- interneuron (relay): located in spinal chord, link between sensory and motor neurons
3
Q
what are the structural types of neurons
A
- multipolar: one axon, multiple dendrites extending from cell body, most common (most interneuron + motor neuron)
- bipolar: one axon, one dendrite (may have branches on ends), occur in eye, ear, nose (impulses from receptor cells to neurons)
- unipolar: have one axon, cell body is to one side (most sensory neurons)
4
Q
what is a synapse
A
- small gap / junction between two neurons
- action potential reaches the synaptic knob of pre-synaptic nerve
- opening of Ca gated channels, Ca rushes into cells
- synaptic vessels containing neurotransmitters migrate towards the synaptic knob and fuse with membrane of axon terminals
- released via exocytosis as a chemical message
- diffuse across synapse (synaptic cleft), taken up by receptors on dendrite / cell body of post synaptic cell
- neuromuscular junction: tiny gap between axon and muscle
5
Q
what is a nerve impulse
A
- an electrochemical charge that travels along a nerve fibre
- electrochemical charge: involves a change in electrical voltage that is bought by changes in the concentration of ions outside and inside the cell membrane of neuron
6
Q
explain attraction of electrical charges
A
- attraction: +Ve and -Ve charges attract, when they come together energy is released, when they are separated they have the potential to come together and release energy
- potential / potential difference: between two places that can be measured (voltage = mV)
7
Q
explain the composition of extra / intra cellular fluid
A
- extra: high in NaCl (Na+) (Cl-), low in K+
- intra: low in NaCl, high in K+ and other negative ions
8
Q
what is membrane potential
A
- the difference in concentration of ions inside and outside the cell causes potential difference called membrane potential
- resting membrane potential: of unstimulated nerve cells, measured at -70 mV (inside potential is 70 mV less than outside)
- > mainly due to differences in distribution of Na+ and K+ ions
- > Na+: concentration is 10x higher outside neuron, membrane is only slightly permeable to Na+ (impermeable to large -Ve ions)
- > K+: concentration is 30x higher inside neuron, membrane is highly permeable to K+ and (Cl-), tendency for K+ to leave cell
9
Q
how is resting membrane potential difference maintained
A
- NaK pump: active movement of ions across membrane (2 K+ ions into cell, 3 Na+ ions out of cell), against diffusion / concentration gradient
- polarisation: membrane is not equally permeable to (large # of -Ve charged ions trapped inside cell, not enough K+ ions to counteract this charge)
- > inside is therefore maintained negative in relation to the positive outside (this state is considered polarised)
10
Q
what is depolarisation
A
- when a strong stimulus is introduced to a nerve the membrane becomes more permeable to Na+, this causes the membrane to slowly depolarise
- threshold: depolarisation only occurs if the level of stimulation exceeds a certain threshold (15mV or -55mV), once this is met more Na+ channels open
- action potential: depolarisation continues along axon (affects voltage gated channels making inside of neuron more positive), triggers repolarisation
11
Q
what is repolarisation
A
- membrane voltage rises to ~30mV (despite strength / size of stimulus, proceeds independently once reaching threshold)
- Na+ channels close and K+ channels open at an attempt to repolarise the membrane (more negative inside)
- hyper polarisation: membrane potential reaches ~80mV
12
Q
what is resting potential / refractory period
A
- resting potential: NaK pump helps return the membrane to resting potential once K+ channels close, this is done through active transport (Na leave cell, K enter cell)
- refractory period: during action potential (depolarisation + repolarisation) nerve fibre cannot be stimulated to respond to another stimulus (important in transmission of an impulse along a fibre)
13
Q
describe transmission along an unmyelinated fibre
A
- depolarisation of one area of membrane causes a local current flow between neighbouring areas on membrane
- depolarisation occurs immediately adjacent to stimulus
- action potential moves away from stimulus (action potential doesn’t travel along nerve, nerve impulse does)
- refractory period prevents nerve impulse from going backwards
14
Q
describe transmission along a myelinated fibre
A
- sheath insulates fibre from extracellular fluid so ions cannot flow between inside / outside (action potential can’t form)
- nodes of ranvier are where the myelin sheath is absent, action potential jumps between nodes (salatory conduction)
- speed: move along myelinated fibres faster than unmyelinated, at 140m/s and 2m/s respectively
- size: of nerve impulse that travels along fibre (provided it exceeds threshold) is always the same size and has the same magnitude
15
Q
why are there few synapses in a reflex arc
A
- synapses slow the speed of nerve impulses
- fewer synapses = shorter overall time to respond to stimuli
- advantageous as this protects the body from injury when in relation to the reflex arc
