Nervous system Flashcards
what is the nervous system divided into?
Central nervous system - brain & spinal cord
Peripheral nervous system - somatic (motor), and autonomic (sympathetic and parasympathetic)
what are the 3 regions of the brain?
fore brain
mid brain
hind brain
what are the 3 membranes in the brain?
arachnoid
pia mater
dura mater
Role of the cerebrospinal fluid
lies between the membranes - is a cushion, protective 7 homeostatic layer - prevents brain from knocking against the skull and provides nutrients to it e.g glucose & oxygen
what is the spinal cord?
delicate with a central grey area (2 dorsal & 2 ventral horns), and surrounding white matter (nerve fibres connecting spinal cord to brain).
- Afferent nerve fibres enter via dorsal root e.g sensory pain info via DRG cells
- Efferent nerve fibres leave through the ventral root e.g to skeletal muscle or glands
what is the functional unit of the nervous system
neurone
what are the 2 main types of cell in NS
non neuronal cells & neurons
name 4 non neuronal cells
- astrocytes
- oligodendrocytes
- microglial
- ependymal cells
what are astrocytes?
supportive function for CNS neurons. Contribute to the protective blood brain barrier
what are oligodendrocytes?
form the myelin sheath around axons in the CNS
what are microglial?
phagocytic action in Cns
what are ependymal cells?
epithelial cells in the fluid filled spaces brain and central canal of spinal cord - have microvilli which help circulate CSF around CNS
what are the 2 primary functions of neurons?
1- rapid transmission of information from specific sources to selected targets
- integration (summation) of information from many sources
what is role of dendrites?
receive information from other neurons - “convergence”
role of the axon
transmit information to other neurons, or non neuronal cells
role of axon collaterals
increase the transmission speed of information - divergence 10^3 - 10^4 contacts
what happens at the axon hillock?
action potential generated
what do nodes of ranvier contain
high in Na+ channels
how do nerves signal?
using action potentials
what is the equilibrium potential?
the potential at which the tendency of an ion to move down its concentration gradient is balanced by the resting membrane potential
what is the generation of an AP dependent on?
the pre existence of a resting membrane potential
what 2 major currents determine the action of the nerve AP?
1- Na+ current - inward, depolarising current
2- K+ current - outward, hyperpolarising current
where are Na+ channels activated and inactivated?
activated at M-gate
Inactivated at h-gate
where are K+ channels activated?
n-gate
what happens is if the threshold is exceeded?
APs have a similar magnitude and duration - an all or nothing principle
what is the absolute refractory period?
the period that Na+ channels are inactivated, another AP cannot be generated
what is the relative refractory period?
prolonged K+ channel activation makes it more difficult for an AP to fire (need a stronger stimuli)
how are APS generated?
all or nothing events generated by stimulation above a threshold value at the axon hillock (very rapid <1m/s)
what does myelination do?
speeds conduction velocity by salutatory conduction
what myelinates axons in CNS?
oligodendrocytes
what myelinates axons in the PNS?
Schwann cells
what is myelin?
phospholipid that wraps around and insulates an axon (minimise the dissipation of the signal as it travels down)
How do neurons send and receive signals ?
Specialised junctions called synapses
What are the 2 types of synapse
Chemical (mainly)
Electrical (open channels that conduct directly via gap junctions)
Where do axons of the pre synaptic neurons make contact with target post synaptic cells?
- dendrites (axo-dendritic)
- soma (axo-somatic)
- other axons - axo-axonal
How is NT release regulated ?
Gaps between synapses called synaptic clefts
How is NT release triggered?
By arrival of an action potential at the pre synaptic terminal. Vesicles fuse with the pre synaptic membrane and release the Nt into the cleft
NT act on receptors on the post synaptic membrane to propagate neuronal signals
what are the 2 types of chemical neurotransmitter & describe (give examples )
Excitatory - increase nerve activity, cause depolarisation, initiate exitatory post synaptic potentials EPSPS
Example: glutamate, acetylcholine
Inhibitory- decrease nerve activity, cause hyper polarisation, initiate inhibitory post synaptic potentials IPSPS
Examples: g-aminobutyric acid (GABA), opioids, acetylcholine
What does the generation of an AP depend on?
The balance of EPSPS and IPSPS converging on a neuron at any given moment - synaptic integration
Give example of excitatory synapse and what ion involved
Acetylcholine at nicotinic Ach receptor (nAChR)
Na+
What does Na+ entry cause?
Depolarises the post synaptic membrane
Causes an EPSP
Generates an action potential if the threshold is reached
In comparison to all or nothing potentials - what are EPSPS and IPSPS?
Smaller and are:
- graded (very in amplitude)
- temporal (decay with distance)
- summate (add to eachother)
How is an AP generated at the synapse via Na+ entry?
Transmitter release at 40-80 presynaptic terminals simultaneously needed to pass threshold and generate AP
Example of an inhibitory synapse and what ion is involved
GABA at GABA A receptor
Cl- ion
What happens with Cl- entry?
- hyper polarises post synaptic membrane
- causes IPSP
- inhibits AP generation
How is an AP not generated at the synapse via Cl- entry?
Cl- influx (via inhibitory receptors) and K+ efflux (via leak channels) makes the neuron interior negative and thus inhibits an AP generation
What is synaptic integration?
Neuron receives both excitatory and inhibitory inputs - it must integrate these inputs and decide whether to signal to next neuron or not (balance of these)
What determines whether a neurone will fire or not?
The relative balance of IPSPS and EPSPS
In a neuron with aXo-somatic and axo-dendritic synapses- which axon requires the most pulses to depolarise the post synaptic cell to threshold?
Distal dendrites
Why does the axon on the distal-dendrites require more stimulus pulses to depolarise the post synaptic cell to threshold?
Synaptic potentials dissipate as they travel along the membrane (EPSPS-temporal- decay with distance) so the further away the input the lower it is when it gets there
What happens if for example at 16 synapses firing, there is an AP generated- (+20Mv)
and the number of synapses firing increases to 24, what is the maximum voltage?
APs are all or nothing - after an AP is generated, the Na+ channels inactive in what is called he absolute refractory period - where no AP can be generated. So -
We can’t go much higher than the voltage that is already achieved
What is the peripheral nervous system comprised of?
Autonomic - non voluntary control
Somatic - conscious - motor control
what are the 3 regions of the ANS
1 parasympathetic - cranial sacral output. Synapse at ganglia and close to innervated tissue
- sympathetic - throracic-lumbar output. Synapse at ganglia
- Enteric NS - neurones with cell bodies in wall of intestine which innervates the GI tract, pancreas & gall bladder
what is the ANS and what does it regulate?
Visceral. it regulates - cardiac muscle Smooth muscle contraction& relaxation exocrine gland function intermediate metabolism
What is acetylcholine
released from parasympathetic ns.
- all preganglionic neurons of both sympathetic and para
- All postganglionic neurons in just para
Results in cholinergic transmission
What is noradrenaline
released from sympathetic ns.
- Most postganglionic neurons in sympathetic
- results in noradrenergic transmission
what are postganglionic neurones?
either excitatory or inhibitory
Name the 3 neurotransmitter receptors in the PNS and what they are stimulated by
- sympathetic effects are due to noradrenaline action - all postganglionic fibres release NA to act on alpha or beta adrenoreceptors
- Parasympathetic effects due to acetylcholine action - all postganglionic nerves release Each to act on muscarinic Act receptors - mAChRs
- Somatic NS effects are also due to acetylcholine action but motor nerves release ACh which act on nicotinic Each receptors - nAChRs
What is the exception of the sympathetic innervation by noradrenaline?
sympathetic innervation of sweat glands is due to Acetylcholine not NA.
Act works on muscarinic ach receptors
what is the enteric ns?
controls activity of the GI tract, pancreas and gall bladder. It has 2 plexuses
what are the 2 enteric plexuses?
- myenteric (auerbachs)- controls GI movement
2. Submucosal (meissners) - controls GI secretion and local blood flow
While it is true that the Enteric nervous system can function alone. What is the sympathetic and parasympathetic regulation:?
Para- vagal & splanchnic nerves
Symptoms- pre vertebral neves
what is the somatic NS and what is is controlled by?
This directs goal directed movement and maintenance of posture. Controlled by:
- Spinal
- Central
How does spinal and central control somatic NS?
spinal - reflexes are unconscious and inherited - for fast, survival orientated effects. Maintain posture
Central - control goal directed movement by acting via spinal motor neurons
what are the characteristics of spinal control of the somatic ns?
They don't depend on consciousness There aren't many synapses so very fast protective and survival functions inherited hard wired systems modulated by the CNS
what are the 2 neutrons in spinal reflexes?
afferent/sensory
efferent/motor
what are the 3 types of reflex?
- Monosynaptic - 2 neurons & 1 synapse e.g stretch reflex
- Disynaptic - 1 interneuron and 2 synapses e.g flexor withdrawal reflex
- Polysynaptic - >1 interneuron and >2 synapses e.g crossed extensor reflex & Golgi tendon
what is a stretch reflex?
dynamic (phasic) - transient stretching of muscle produces powerful, rapid reflex contraction to oppose sudden change in muscle length
tonic (static) - maintained stretching produces weaker, prolonged reflex responses to small changes in muscle activity
What is a flexor reflex?
stimuli detected by pain receptor - pain nociceptor.
Flexor muscles in the affected area contract via a disynaptic pathway
What is a crossed extensor reflex?
As a consequence of the flexor reflex in one limb, the opposite limb frequently extends. Polysynaptic response to allow body to escape noxious stimuli
what is the Golgi tendon reflex?
tendon detects changes in muscle tension - causes a protective feedback inhibition in active muscle by causing relaxation , and reciprocal contraction of antagonistic muscle
Name 4 types of spinal reflex
- Stretch (monosynaptic)
- flexor (disynaptic)
- Crossed extensor (polysynaptic)
- Golgi tendon (polysynaptic)
define proprioception
The perception of self - where the body is in relation to the surroundings
What is the proprioception process?
2 way:
- proprioceptors signal to spinal cord/CNS (afferent)
- Motor neurons signal to skeletal muscle (efferent)
what are the 2 major proprioceptors?
- Muscle spindles - within muscles & respond to stretch
- Golgi tendon organs - in tendons, respond to tension
what are the contractions in skeletal, smooth and cardiac?
skeletal - neurogenic
smooth - myogenic
cardiac - myogenic (doesn’t rely on neuronal input)
what are the nerve controls in each muscle type?
Skeletal - alpha-motor neuron
Smooth - ANS
Cardiac - ANS
what is needed in order for skeletal muscles to contract?
Each cell must be stimulated by a motor neuron
what blocks the NMJ? (3)
- paralysing poisons e.g curare blocks nAChRs
- Agents acting at nAChRs during general anaesthesia to control movement during surgery
- Local injection of botulinum toxin (botox) blocks vesicle fusion with the pre synaptic membrane. (treats muscle spasms)
