Peripheral Nerves Flashcards
Functions of the nervous system
> Master command + control system
Monitor internal and external environments
integrate sensory input and motor commands
co-ordinate voluntary + involuntary responses
Sensory receptors——–>CNS———>Muscular system
Input ——————–> Intergration———–> Output
PNS————> Brain + Spinal cord (CNS) ———> PNS
Classification
Nervous system is split into CNS + PNS
CNS = brain and spinal cord
PNS = Motor neurones and sensory neurones
Motor neurones = Autonomic and somatic
Autonomic = sympathetic and parasympathetic
Describe the central nervous system
Central Nervous System = brain + spinal cord > Brain - receives + processes sensory info - initiates responses - stores memories - generates thoughts + emotions > Spinal Cord - conducts signals to and from brain - controls reflex activities
Describe the Peripheral Nervous system
PNS = nerves that leave spinal cord
- split into sensory and motor neurons
> Sensory neurons - from sensory organs to CNS
> Motor neurons - CNS to muscles/glands
- split into ANS + SNS
> Somatic nervous system (SNS) - voluntary movements
> Autonomic nervous system(ANS)-involuntary responses
- split into sympathetic and parasympathetic
> Sympathetic = fight or flight responses (nerves = mainly from thoracic and lumbar regions)
> Parasympathetic = rest or digest responses (nerves = mainly from cranial and sacral regions)
Define upper and lower motor neurons
> Upper motor neurons (CNS)
- transmit info from brain to lower motor neurons & interneurons in brainstem and spinal cord
Lower Motor Neurons (PNS)
- cell bodies in spinal cord/brainstem but axons directly innervate skeletal muscle
How many peripheral nerves are there and what types
31 peripheral nerves
> 12 cranial nerves
- carry info to and from brain (axon ends outside brainstem/spinal cord)
- Cranial nerve 7 = facial nerve (bells palsy if damaged)
- Cranial nerve 10 = vagus nerve (controls major systems)
> Spinal nerves
- carry info to and from spinal cord e.g sciatic/radial etc
Describe a peripheral nerves path + structure
Leaves spinal cord as:
- Dorsal root = generally ascending sensory info
+ dorsal root ganglion which contains cell bodies
- Ventral root = generally descending motor info
- then come together to form mixed nerves (carry both afferent/sensory and efferent/motor fibres)
Structure
- Nerve = surrounded by epineurium
- contains blood vessels + smaller bundles - Smaller bundles = surrounded by perineurium
- contain fascicles - Fascicles - surrounded by endoneurium
- with myelinated/unmyelinated fibres
What are the types of neuron
> Bipolar - interneuron - within spinal cord
- connects nerves (cell body in middle)
Unipolar - Sensory neurons (receive info from one pole and transmits to other - cell body not in way)
Multipolar - Motor neurones (many sources to receive info from takes through cell body and along axon to terminals
- transmits as AP to cell body, AP to axon and neurotransmitters to next nerve or muscle fibres
Key components of CNS + PNS (cells)
> CNS
- Astrocytes = link cells together (structure)
- Microglia = immune cells within brain
- Oligodendrocytes = cover CNS nerves for protection
> PNS
- Satellite cells = structure
- Schwann cells = protection + repair (via neurolemma)
Describe how a nerve impulse occurs
- Rest = negative potential of -70mv (maintained by voltage gated ion channels keeping few K+ inside and many Na+ outside)
- Depolarization - stimuli causes Na+ channels to open
- if membrane potential is reduced enough it opens nearby Na+ channels as well = Action Potential - Repolarization-K+ channels open/Na+ channels close
- Hyperpolarization - K+ channels are slow to close so potential goes too far - active ion pumps restore resting potential
Define the all or nothing law and how it is reached
- Threshold level of depolarization to open adjacent Na+ channels
Requires: - Spatial summation (many small signals to many dendrites at same time)
- Temporal summation (series of signals at same synapse)
What are the methods of conduction
- Continuous conduction
- non-myelinated axons
- slower as has to travel along length - Saltatory conduction
- myelinated
- allows impulse to jump = faster
Define chemical synapses and what happens there
> Chemical synapses = where nerves meet (synaptic cleft = space between neurones)
Presynaptic neuron has neurotransmitters in vesicles that are released by arrival of AP
Postsynaptic neuron has receptors for neurotransmitters which will cause/prevent an AP
Neurotransmitters are then removed via diffusion/re-uptake by presynaptic neuron/ enzyme break down
- types of neurotransmitters:
Acetylcholine/dopamine/norepinephrine/serotonin/ GABA
Describe neurons ability to repair (CNS+PNS)
CNS - no ability to repair
- neuroplasticity is creation of new pathways which may bypass damage
PNS - myelinated cells (neurolemma provides structure)
- only if schwann cells and cell body are intact
Describe process of neuron regeneration
PNS Degeneration
- Chromatolysis - shortly after axon damage
- Wallerian degeneration - distal portion degrades to nearest intact myelin gap
- Wallerian regeneration - neurolemma cells divide
- regeneration tube = formed
- new axon reforms - Re-myelination - myelin reforms around axon
