The Peripheral Nervous System Flashcards
What is the PNS?
PNS is composed of nerves (cranial and spinal) and ganglia outside brain and spinal cord
What are the divisions of the peripheral nervous system?
Sensory and motor
motor=> autonomic and somatic
autonomic= para and symp
What carries information to the CNS?
Sensory (afferent) nerves carry information towards the CNS
- back of cord
What carries information away from the CNS?
Motor (efferent) nerves carry information away from the CNS
How many cranial nerves?
12 pairs of cranial nerves
How many spinal nerves?
31 pairs of spinal nerves
What are somatic nerves?
Somatic afferent nerves convey information from skin, skeletal muscle and joints
Somatic efferent nerves convey information to skeletal muscles
What is a dermatome?
An area of skin that is supplied by a single spinal nerve
- bones don’t match up with the segments
What is a myotome?
Group of muscles innervated by a single spinal nerve
What are visceral autonomic nerves?
Visceral afferent nerves carry information from the viscera (thoracic, abdominal and pelvic organs)
Visceral efferent nerves can be divided into sympathetic and parasympathetic
What is the difference between the para and symp visceral efferent autonomic nerves?
Sympathetic efferent nerves innervate the viscera (organs) and periphery (vasculature and sweat glands)
Parasympathetic efferent nerves innervate the viscera (organs) only
What is a ganglion?
A ganglion is a collection of cell bodies outside the CNS
What is a nucleus?
A nucleus is a collection of cell bodies inside the CNS
What is a plexus?
A plexus is a network of interconnecting nerves
What is the similarity between all afferent nerves (somatic and visceral)?
All afferent (somatic and visceral) fibres have their cell bodies in spinal ganglia.
Where do visceral nerves synapse?
In the peripheral ganglion
What is an overview of direction of afferent and efferent nerves and where they go in the peripheral nervous system?
How are peripheral nerves arrange?
in fasciculli
What are the layers of connective tissue in peripheral nerves?
3
External vascular layer - epineurium
Individual fascicles covered in perineurium
Individual axons covered in endoneurium
How are peripheral nerves classified?
Two classification systems
- one based on conduction velocity
- one based on axonal diameter (sensory only)
How does the conduction velocity method of classification of peripheral nerves work?
uses letters A,B, and C (A is the fastest)
- e.g., the one that is most myelinated would be the fastest so would be A
How does the axonal diameter method of classification of peripheral nerves work?
sensory only
Uses roman numerals I-IV with I being the largest diameter
How would the classification system help us see the function of nerves?
Myelinated Aa would be motor to skeletal muscle
unmyelinated C IV would be postganglionic autonomic fibres; sensory from free nerve endings for pain and temperature; smell
What do sensory receptors do?
Detect external or internal information
How are sensory receptors classified?
by source of stimulus
-external or internal
Or by mode of detection
What are the external sources of stimulus?
Exteroceptors
- pain
- temperature
- touch
- pressure
What are internal sources of stimulus?
Proprioceptors
- movement
- joint position
Enteroceptors
- movement through gut
- blood ph
What are the modes of detection for sensory receptors/ how they transduce?
Chemoreceptors
Photoreceptors
Thermoreceptors
Mechanoreceptors
Nociceptors (receptors for pain)
What do chemoreceptors do?
Detector molecules which bind to receptor, e.g. in olfactory bulb
What do photoreceptors do?
Detect light in retina
What do thermoreceptors do?
Detect temperature in skin
What do mechanoreceptors do?
Mechanical opening of ion channels, e.g. touch receptors in skin
What do nociceptors do?
Detect tissue damage, interpreted as pain
What are the proprioceptors?
Muscle spindles
Golgi tendon organs
Joint receptors
What do muscle spindles do?
Detect changes in muscles length
What do golgi tendon organs do?
Detect changes in tension in tendons
what do joint receptors do?
Found in joint capsules – detect start and end of movement
What is a neuromuscular junction?
Specialised synapse between a motor neuron and a muscle fibre
What is a motor unit?
A single motor neuron together with all the muscle fibres that it innervates. It is the smallest functional unit with which to produce force.
Humans have approximately 420,000 motor neurons and 250 million skeletal muscle fibres.
What does stimulation of one motor unit cause?
Stimulation of one motor unit causes contraction of all the muscle fibres in that unit.
On average how many muscle fibres does one motor neuron supply?
600
Can 2 motor units innervate the same muscle fibre?
NO
What is a reflex?
An involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli.
What is the reflex action steps?
- Stretching stimulates SENSORY RECEPTOR (muscle spindle)
- SENSORY NEURON activated
- Within INTEGRATING CENTRE (spinal cord) sensory neuron activates motor neuron
- MOTOR NEURON activated
- EFFECTOR (same muscle) contracts and relieves the stretching
What are the parts of the autonomic nervous system?
Has visceral afferent and efferent parts
Efferent part has sympathetic (S) and parasympathetic (PS)
- Have broadly opposing actions (flight & flight, rest & digest)
Have pre- and post-ganglionic neurons
What is released at the pre-ganglionic synapses?
ACh (Acetylcholine)
What is released at post-ganglionic neurones?
Release noradrenaline (S) or acetylcholine (PS) at post-ganglionic neurons
When are there exceptions for what is being released at a synapse?
- There are exceptions – e.g. sweat glands and blood vessels, where acetylcholine is the neurotransmitter released instead of noradrenaline
Where are the visceral sensory neurones in the autonomic NS?
Relay sensory information from the core
Pain, fullness, blood pressure
T1-L2, S2-S4 and cranial nerves IX and X
Where are the visceral motor neurones of the autonomic nervous system?
Outflow to core and body wall
Controls pupils, sweat glands, salivary glands, heart muscle, airways
Thoracolumbar (T1-L2) and craniosacral outflow (cranial nerves III, VII, IX, X)
What is the anatomy of sympathetic outflow to periphery? (think descending and aascending)
Some can go up or down before they synapse
What does the grey ramus have?
Unmyelinated fibres
What does the white ramus have?
Myelinated fibres= pre-ganglionic neurones
What is the anatomy of sympathetic outflow to heart?
preganglionic neurones synapse in the ganglia (same for supply of periphery and heart)
What is the anatomy of sympathetic outflow to viscera?
what is the anatomy of parasympathetic outflow to viscera?
cranial nerves-> 4 ganglion in head
vagus nerve= longest
What are the overall functions of the autonomic nervous system e.g., heart rate?
Describe the overview of the autonomic nervous system.
Afferent and efferent parts
Sympathetic (S) and parasympathetic (PS)
Has pre- and post-ganglionic neurons
Utilises acetylcholine and noradrenaline as neurotransmitters
Innervate core (S & PS) and periphery (S)
describe peripheral nerves.
Arranged in bundles (fasciculi)
Have connective tissue surrounding them
Classified in two ways
PNS= Nerves and ganglia outside spinal cord
How are sensory receptors classified? (2)
Classified by source of stimulus or mode of detection
How are sensory receptors classified? (2)
Classified by source of stimulus or mode of detection
What is the nervous system divided into?
Central and peripheral
