Communication - The Nervous System Flashcards
What are the main systems of the nervous system and how can they be broken up?
Central nervous system - brain, brain stem and spinal cord
peripheral nervous system - cranial nerves, spinal nerves and all body nerves
- sensory neutrons - something we feel, can also carry autonomic features like from the sensory neutrons in the gut and blood vessels sensing stretching
- motor neutrons - can be broken into
: somatic nervous system - skeletal muscle fibres
: autonomic nervous system - acts on reflex and other triggers, not consciously
Autonomic system made up of
- sympathetic nervous system - fight or flight etc
- parasympathetic nervous system - rest and digest
What plane of the spinal cord are efferent and Afferent nerves radiating from?
Efferent nerves leave the spinal cord from the anterior (front) side of the spinal cord and go to the body to act on it
Afferent nerves enter from the body via the posterior position to pass messages to the brain
How may pairs of cranial nerves and spinal nerves are there?
12 pairs of cranial nerves
31 pairs of spinal nerves
What are the cell types in the nervous system?
CNS
Neurons
Neuralgia
- support cells
- microglia - specific immune cell for the cns
- astrocyte - attach to neurons and capillaries, regulate environment surrounding neurons
- oligodendrocytes - wrap processes around cns nerves for insulation, create myelin
PNS
- neurons
- Schwann cells - wrap body around nerves for insulation, myelin
- satellite cell, support for nerves
Describe dendrites
They are the hair like structures on the end of neurons around the cell body
- receive info - lots and lots of info
- have dendritic spines - increased the surface area for potential synapses with other neurons
Describe axons
- conducting region of a neuron as an electrical signal
- generates and transmits nerve impulses ( action potentials) away from the cell body
- can be up to one meter long
Axon hillock
- point of activation that determines if a neuron transmits a nerve pulse
Axon terminal
- neurotransmitters are releases to act on another neuron or effector tissue
Describe the myelin sheath
Segmented sheath around most long or large diameter axons
Functions
- protect and electrically insulate axon, prevent charge leakage
- increase conduction velocity of nerve impulse
Made by
- oligodentrocytes in cns: myelinate > 60 axons at once - like octopus tentacles
- Schwann cells in pns: dedicate whole self to one axon segment
Node of ranvier are the gaps between myelin sheaths where there are extra sodium channels to keep the action potential going
What is a graded potential?
A localised, short lived change in membrane potential due to a stimulus most often
Depolarisation
- inside the membrane becomes less negative
- a stimulus has cause the gated na+ channels to open and let in some Na+, which makes the cell less negative than resting hence DEpolarised
Hyper polarisation
- inside of the membrane becomes MORE negative,
- due to stimulus opening up cl- channels, or due to too much k+ leaving the cell
- MORE negative = hyper
Describe the axon hillock involvement in generating an action potential
The graded potential needs to be large enough to reach the axon hillock to start an action potential
Graded potentials can summate to give an action potential
Axon hillock has voltage- gated na+ channels
When membrane potential reaches -55mV, an action potential is stimulated - by inflow of Na+
Therefore as a mechanism to stop action potentials firing at any point for any stimulus, the stimulus has to be great enough to cause the graded potential to reach the axon hillock, or there has to be enough of the initial stimulus to cause multiple graded potentials to come together to cause on action potential
describe the steps in generating an action potential
> begins once the membrane potential at the axon hillock reaches -55mV from the depolarising graded potential in the cell via Na+ channels opening and Na coming into the cell. Refer to diagram for below steps
steps:
1) resting membrane potential
- only leaky K+ channels are open, membrane is polarised at -70mV. Na+/K+ ATPase pumps maintain ion gradients
2) Depolarising stimulus in the form of a graded potential
- a signal has been initiated at dendrites to stimulate Na+ channels to open, causing a small, local depolarisation
3) membrane depolarises to threshold at axon hillock
- voltage-gated Na+ channels open quickly and Na+ floods into the cell
4) rapid Na+ entry depolarises the cell
- membrane potential reaches +30mV in 0.5msec
5) Na+ channels close and voltage-gated K+ channels open
- +30mV = overshoot (apex of curve)
- K+ flows out of the cell which starts to repolarise the membrane
6) K + moves out of the cell to the extracellular fluid
7) additional K+ leaves the cell, hyperpolarise membrane (undershoot)
- K+ channels remain open ( in addition to K+ leakage channels)
8) voltage-gated K+ channels close
- less K+ leaks out of the cell
9) Na+/K+ ATPase pump helps to restore and maintain resting membrane potential
how is the action potential signal transmitted in only one direction?
> Na+ channels are double gated and create a refractory period to ensure a one-way direction of electrical flow
Describe how the refractory period is established
COMPONENTS:
> each Na channel has two voltage-sensative gates
a) activation gate: closed at rest; open with depolarisation
b) inactivation gate: open at rest, close/block channel 0.5msec after actiation gate opens
> each K+ channel has one voltage sensative agte
- closed at rest,
- opens slowly with depolarisation
STEPS:
1) at resting membrane potential, activation gate is closed
2) activation gate opens, Na+ enters cell (depolarisation)
3) inactivation gate closes after 0.5msec, stops Na+ entry
4) during repolarisation (K+out), two na+ gates reset (2msec after depolarisation)
describe the difference between absolute refractory period and relative refractory period
Absolute:
- inactivation gate in place
- starts when na channels open (0.5msec) till inactivation gate opens again at 2msec.
- neuron cannot respond to another stimulus
- ensures each AP is an all or nothing response and is seperate
- enforces one way transmission of nerve impulses
Relative refractory period:
- transistion: inactivation gate moving away and activation gate moving into place.
- na channels returning to resting state, repolarisation, some K+ channels still open
- threshold for AP generation is ELEVATED
- only an EXCEPTIONALLY strong stimulus may generate AP
how does the CNS tell the difference between a weak stimulus and a strong one if all AP is the same strength?
- strong stimuli can generate action potentials MORE OFTEN than weak
- CNS determines STIMULUS INTENSITY by the FREQUENCY of IMPULSES
describe how conduction velocities of neurons vary
Axon diameter:
- larger diameter fibres have less resistance to local current flow and have faster impulse conduction
Myelination:
- myelin sheaths insulate and prevent leakage of charge
- continuous conduction in unmyelinated axons is slower than saltory conduction in myelinated axons
