Physiology Flashcards
List all regions of a neurone
Dendrites Cell body - soma Axon hillock Axon Synapse
What are the functions of the dendrites
Receive inputs from other neurones and convey graded electrical signals passively to the soma
What are the functions of the cell body
Contains the nucleus, mitochondria, ribosomes etc
Integrates the incoming electrical signals and conducts them passively to the axon hillock
What is the function of the axon hillock
Site of initiation of the ‘all or none’ action potential
What is the function of the axon
Conducts the action potentials to the presynaptic terminal
Mediates transport of materials between the soma and presynaptic terminal
What is the function of the synapse
It is the point of chemical communication between neurones
Describe unipolar neurones
Has one process (neutrite) arising from the cell body
Peripheral autonomic neurones have this structure
Describe pseudounipolar neurones
One process from the cell body but it bifurcates and can supply different areas
Dorsal root ganglion neurones have this structure
Describe bipolar neurones
Has two processes from the cell body - receiving and outgoing
Retinal bipolar neurones are an example
Describe multipolar neurones
Have 3 or more processes contacting the cell body
Larger neurones that integrate info from larger areas
Lower motor neurones have this structure
Describe the initiation of an action potential
All neurones have a resting potential
A depolarising stimulus occurs which makes the potential more positive
If it reaches the threshold the AP is triggered
Rapid sodium influx occurs = depolarization and upward stroke
Then K+ channels allow K efflux and the potential becomes negative again - downstroke
The neurone then rebalances itself to resting potential
Action potentials have constant amplitudes - true or false
TRUE
What governs the distance travelled by an AP
The strength of the signal which is itself determined by the ion movement
Membrane resistance must be high and axial resistance must be low in order to increase the length the signal travels
What causes the change in membrane potential as you travel further from the source
Current leaking back out into the extracellular space
This is a passive process and occurs exponentially
The further away, the less the difference in charge
Longer AP’s travel slower - true or false
False
Increased length potential means the signal will travel at a greater speed
What factors can increase the speed of nerve conduction
Thicker axons - less axial resistance
Insulate the axons - myelin sheath
Which cells produce the myelin sheath
Schwann cells in the PNS
Oligodendrocytes in the CNS
What are the nodes of ranvier
Unmyelinated regions along the axons
Sodium channels cluster here
Describe how neurotransmitters are released and returned
Calcium enters the presynaptic area and triggers the fusion of the neurotransmitter vesicles with the membrane
This allows the neurotransmitters to be released into the cleft
They bind to receptors to the other side and trigger either efflux or influx which can create an impulse
NT’s are taken back up into the neurone or broken down to ensure that the signal only lasts as long as it needs to
How do the pre and post synaptic membranes stay close to each other
A matrix of fibrous extracellular protein within the cleft holds them in place
List the different classes of synapse
Axodendritic - from axon of one neurone to dendrite of another
Axosomatic - from axon of one to soma of another
Axoaxonic - from axon of one to another axon
What is the most common excitatory neurotransmitter
Glutamate
Describe the action of glutamate as an excitatory neurotransmitter
It is released and activates postsynaptic, selective receptors
This generates an excitatory depolarising response e.p.s.p
What are the most common inhibitory neurotransmitters in the CNS
GABA and glycine
Describe the action of inhibitory neurotransmitters
GABA or glycine bind to selective post-synaptic receptors
This generates a hyperpolarising response so that an AP is not produced
For GABA it does this by causing Cl- influx
Describe summation
A neuron can receive multiple influences from different neurones – must compute all inputs at the axon hillock
These inputs are added together to determine the response
List the different classes of NT
Amino acids - e.g. glutamate
Amines - e.g. dopamine and NA
Peptides - CCK
What is an ionotropic receptor
This receptor is itself the channel - opens when NT binds
Direct gating
What is a metabotropic receptor
The receptor influences a nearby channel (signals it to open, usually via G-proteins)
This path is triggered when the NT binds
Describe the action of ACh on ionotropic and metabotropic receptros
Can bind to both
The ionotropic receptor causes a fast response– Na/K channel opens
The metabotropic has a slower response - triggers closure of K+ channel
How can glutamate have an inhibitory effect
If it acts on metabotropic receptors
This has a role in modulation of neurotransmission
What type of channels can glutamate act on
Non-NMDA - mediate fast transmission to CNS
NMDA - contribute a slow component to the excitatory potential
What are the functions of the somatosensory system
Mediates sensory modalities of: Fine touch Proprioception Temperature Pain - nociception Itch - pruriception
What is the function of the exteroceptive division of the somatosensory system
It registers info from the surface of the body from numerous types of receptor
What are the 3 divisions of the somatosensory system
Exteroceptive
Proprioceptive
Enteroceptive
What is the function of the proprioceptive division of the somatosensory system
Monitors posture and movement
Has sensors in the muscles, tendons and joints
What is the function of the enteroceptive division of the somatosensory system
Reports upon the internal state of the body
Closely related to autonomic functions
Describe the general somatosensory pathway
Usually 3 neurons in sequence
1st has its cell body in the dorsal root ganglia (body) or a cranial ganglia (head)
2nd order has cell body in dorsal horn of spinal cord or brainstem nuclei
3rd has cell body in thalamic nuclei
This neuron synapses with the somatosensory cortex
What acts as a receptor in the somatosensory pathway
The peripheral terminal of the first order neurons
How is a receptor potential created
The stimulus (mechanical, thermal, or chemical) opens cation-selective ion channels in peripheral terminal of primary sensory afferent Causes a depolarizing response
What is the amplitude of a potential dependant upon
It is proportional to the intensity of the stimulus
It is graded
What is the frequency of a potential dependant upon
Proportional to the amplitude of the stimulus
The greater the amplitude the higher the frequency
This is known as frequency coding
What is meant by modality of a neuron
The neurons are tuned to respond to a specific type of energy/stimulus to excite them
Can be touch/pressure, proprioception, temperature, pain, itch
What is meant by the threshold of a neuron
The intensity of a stimulus needed to excite the sensory unit
What are low threshold receptors responsible for
Low intentisty ‘normal’ sensations
Fine discriminatory touch (can never cause pain)
Cold through to hot temperatures (not extremes)
What are high threshold receptors responsible for
Noxious stimuli
Pain
Extreme temperatures
Inflammation in response to chemical insult
Describe adaption of a sensory unit
Some units can change their firing rate if the stimuli changes strength
This is called a fast adapting response
Slower adapting responses will not change and rate remains constant for the duration of stimulus
How do rapidly adapting sensory units work
An abrupt change in stimulus will cause an AP to be fired
Gradual change has no effect
Used to detect vibration
How does axon diameter affect conduction velocity
The bigger the diameter the faster the conduction
Largest are the A-alpha fibres
How does degree of myelination affect conduction velocity
The greater the myelination, the faster the conduction
Again A-alpha have the greater velocity
List the classes of axons
A-alpha (proprioceptors) A-beta (mechanoreceptors) A-delta (pain and temp) C fibres (temp, pain, itch)
Listed in order of conduction velocity (therefore size and myelination)
what is the receptive field
The area of the skin from which a particular sensory unit can be excited
It varies across the body and some may overlap
Related to the density of innervation inversely
How does sensory acuity relate to receptive field
It correlates inversely
A small RF gives high acuity as there is a greater density of innervation
How and why do we test 2 point discrimination
It measures somatosensory function - checks if their RF are average or not
Apply 2 sharp point stimuli simultaneously and ask patient if they feel one or 2
Describe free nerve endings
type of sensory receptor found in the skin
Found in all areas
Sensitive to pain and temperature
Describe Meissner’s corpuscles
Highly sensitive to touch
Found superficially and in areas where discrimination is highest (e.g. hands)
Small RF
Not found in hairy skin
Describe Merkel’s discs
Type of cutaneous receptor Consist of the terminals of a sensory axon and a Merkle cell
Similar to Meissner’s
Sensitive to touch and found in hairy skin as well as normal
Describe Ruffini endings
Cutaneous receptors found deep in the dermis and in joint capsules
Respond to deep pressure
Describe Pacinian corpuscles
Found in the dermis and fascia Surrounded by a capsule of non-neurological tissue Anatomically they are large receptor Sensitive to pressure Larger RF
Describe Krause end bulbs
Receptors that are found at the border of dry skin and mucous membrane
Sensitive to touch
How can low threshold mechanoreceptors be classified
By rate of adaption (fast or slow)
By size of receptive field (small=1 wide=2)
Combined to give the following groups
Fast acting small field – FA1
Fast acting wide field – FA2
Slow acting small field – SA1
Slow acting wide field – SA2
Which type of receptors are A-beta fibres associated with
Follicular nerve endings Merkel cells Meissner corpuscles Ruffini endings Pacinian corpuscles
Which type of receptors are A-delta fibres associated with
Free nerve endings
Follicular nerve endings
Which type of receptors are C fibres associated with
Free nerve endings
What forms a dermatome
The area of skin innervated by the left and right dorsal roots of a spinal segment
List the location of the sensory afferent terminals in the grey matter dorsal horn
Horn is divided into 10 laminae of Rexed
Nociceptors terminate in 1 and 2
LTM - terminate in 3-6
Proprioceptors in 7-9
Describe the dorsal column lemniscal pathway
Carries touch, pressure, vibration and proprioception
Uses A-a/b fibres
First order carries from sensory receptor to medulla
Second order decussates and then synapses in thalamus
3rd order carries to cortex
Describe the spinothalamic tract
Carries pain, temp, itch and crude touch
Uses A-delta and C fibres
First order carries from receptors to spinal cord
Synapses and decussates in cord (second order)
2nd carries up to thalamus and synapses
3rd to cortex
How is the dorsal column of the spinal cord organised
Split into the medial gracile tract and the more lateral cuneate tract
Sensory info from T6 and below (inc. lower limb) travels in gracilis (runs whole length of cord)
Input above T6 travels in cuneate tract
Frome lateral to medial: cervical, thoracic, lumbar, sacral
What does the DCML pathway allow us to do
Stereognosis - recognise objects by feeling them
Detect vibration - activates Pacinian and Meissner’s corpuscles
Fine touch - inc. 2 point discrimination
Conscious proprioception
Which tuning fork is used for testing vibration detection on the body surface
128Hz
What is lateral inhibition
The process by which an activated neurone can inhibit the activity of its neighbours
This is done via inhibitory interneurons
It sharpens the perception of the important stimuli - gets rid of ‘background noise’
Which nerves are responsible for the sensation to the anterior head
The 2 trigeminal nerves - each with 3 divisions
Describe the path of the trigeminal nerves
Receptors are found in the skin of the head and face
The cell bodies are found in the trigeminal sensory ganglion
The 1st order synapse with 2nd in the principle sensory or spinal nucleus
2nd order then decussate and project to the thalmus (VPM nucleus)
3rd order relay info to the cortex
Where is the somatosensory cortex found
The post central gyrus of the parietal cortex
Immediately behind central sulcus
How is the somatosensory cortex divided
Into Brodmann areas
3a, 3b, 1 and 2
What type of information is received in Brodmann area 3a
Proprioception
What type of information is received in Brodmann area 3b
Cutaneous sensation from Merkel cells or Meissner’s
What type of information is received in Brodmann area 1
Cutaneous sensation from fast acting mechanoreceptors or area 3b
What type of information is received in Brodmann area 2
Deep pressure and joint position
Comes from joint afferents and tendons
Describe the somatotopic map of the body
Each area of the somatosensory cortex correlates to an area of the body
Represented by the homunculus
Toes are at the top of the brain and tongue at the lower end
How is the somatosensory cortex organised
Into 6 layers of cells
Input from thalamus mainly comes in at level 4
Also split into vertical columns which extend across all layers
Can the somatosensory map of the body change
YES
Change with life, activity and injury
For example if you lose a digit, the somatosensory area from can change and become responsible for other digits
This is also responsible for phantom pain
What is the function of the posterior parietal cortex
It receives and integrates information from the somatosensory areas and others such as visual, auditory and thalamus
Allows us to determine deeper meaning of the info
what can damage to the posterior parietal cortex lead to
Bizarre neurological defects
Agnosia - unable to recognise sensory info such as objects, smells, people
Hemispatial neglect - patients cannot perceive the existence of one side of their body/world
Where are the upper motor neurons found
Within the brain
Where are the lower motor neurons found
Cell bodies are in the brain stem and ventral horn of the spinal cord
Axons exit the cord via the ventral roots or by cranial nerves
Joins with the posterior/dorsal root to form a mixed spinal nerve
Carries signals to effector cells
What is the function of an upper motor neuron
They supply input to the lower motor neurons to modulate their activity
What is the function of a lower motor neuron
Command muscle contraction
Form the final common pathway
Receive input from the UMN, proprioceptors and interneurons
What do alpha motor neurons do
Innervate the bulk of fibres within a muscle that generate force
Found in LMN
What do gamma motor neurons do
Innervate a sensory organ within a muscle = muscle spindle
Found in LMN
What is meant by synergistic muscles
Muscles that work together to produce a movement
e.g. biceps brachii and the brachialis
What is meant by antagonist muscles
Those which have opposite effects - flexors vs extensors
What do axial muscles do
Control movement of the trunk - maintain posture
What do proximal muscles do
Also called girdle muscles
Found in the shoulder, elbow, pelvis and knee
They mediate locomotion
What do distal muscles do
Move the hands, feet and digits
Allows fine movement and manipulation of objects
Where are the greatest number of spinal nerves found
At the cervical and lumbar enlargements of the spinal cord
These areas supply the arms and legs so need more nerves
What is a motor unit
The smallest functional component of the motor system
The group of muscle fibres innervates by one alpha-motor neuron
What name is given to the group of alpha motor neurons that innervate a signal muscle
Motor neuron pool
What decides the force of a muscle contraction
Frequency of action potential discharge of the α-MN - more AP’s = stronger
The number of LMN that are simultaneously active - more motor units = stronger
The coordination of the movement - recruiting synergistic muscles
The fibre size and fibre phenotype
Describe the somatotopic organisation of LMN
The LMN innervating axial muscles are found medial to those innervating distal ones
The LMN innervating flexors are found posterior to those innervating extensors
A single AP does what to a muscle fibre
Causes it to twitch - sudden contraction then relaxation
Need several AP’s to generate a continuous contraction
What are the differences between small and large motor units
Small - few fibres, used for fine movement, innervated by small aMN
Large - many fibres, used in large antigravity muscles and innervated by large aMN
Motor units contain only one ‘speed type’ of fibre- true or false
True
Only contain either fast or slow twitch fibres
Describe Slow (type 1) fibres
Slow contraction and relaxation Fatigue resistant Get ATP from oxidative phosphorylation Red fibres - due to high myoglobin Lots of capillary
Describe fast (type 2) fibres
Type 2a - fast contraction and relaxation, fatigue resistant
Get ATP mostly from oxidative phosphorylation
Red fibres - well vascularised
Type 2b - fast contraction and relaxation, not resistant to fatigue
Get ATP from glycolysis
Poorly vascularised - white fibres
What activities are type 2b fast fibres used for
Sudden bursts of high energy movement
e.g. weightlifting
What activities are type 2a fast fibres used for
Sustained locomotion
Need some resistance to fatigue
Longer runs
What activities are type 1 slow fibres used for
Antigravity/posture
Sustained movement
What is the Henneman Size principle
The susceptibility of a aMN to discharge an AP is related to its size
Smaller ones have a lower threshold
Slow fibres are more easy to activate
In which order are motor units activated
In order of their size as the smaller ones haver a lower threshold
This allows for finer control of muscle force
Slow ones also recruit first and build up towards the fast ones to build up to the maximal force
What is the myotatic reflex
When a skeletal muscle is pulled, it pulls back - e.g. knee jerk
The change in length is registered by the muscle spindle (sensory organ) and triggers reflex
Non-conscious proprioception
What forms a muscle spindle
A fibrous capsule
Intrafusal muscle fibres
Sensory afferents that innervate these fibres
Gamma motor neurones
For each of the main myotactic reflexes, state the spinal level being assessed
Biceps - C5-6 Supinator - C5-6 Triceps - C7 Knee - L3-4 Ankle - S1
How can you reinforce a reflex
Get the patient to interlock their fingers and pull apart as you hit the tendon
Or get them to hold something and squeeze it
Jendrassik manoeuvre
Stimulation of gamma motor neurons does what
Causes the intrafusal fibres of the muscle spindle to contract
They are activated by higher centres in the CNS
Intrafusal and extrafusal contract simultaneously - true or false
True
This prevents the spindle from going slack when the extrafusal ones contract
If it was to go slack the neurons wouldn’t be stimulated and the contraction wouldn’t be maintained
When are dynamic gamma motor neurons activated
When there is a lot of active movement that means the muscle lengths will be changing rapidly
A low levels of activity there is no dynamic activity
What are the Golgi tendon organs
Organs which are found at junction of muscle and tendons
Monitor changes in muscle tension and regulate it to prevent overloading the muscle
They also help with fine grip – prevent it being too hard
What innervates the Golgi tendon organs
Group Ib sensory afferents
What are the functions of proprioceptive axons within joints
Found in the connective tissue of a joint
Respond to changes in angle, direction and velocity of movement of a joint.
Also prevent excessive flexion, or extension
What forms a proprioceptive axon
Mixture of different units - both fast and slow acting
Lots of different receptor types are there as well to carry all types of information (free endings, Golgi, paciniform, ruffini)
e.g. pain, acceleration, position
Where do spinal interneurons receive their input from
Primary sensory axons
Descending axons from the brain
Branches of the lower motor neurons
Other interneurons
What is the function of the interneurons
They can send excitatory or inhibitory signals to muscles
Involved in the inverse myotatic response - cause the muscle opposite the one contracting to relax
Reciprocal inhibition between extensors and flexors - ensures they are peforming opposite actions
What is reciprocal inhibition
When a muscle contracts, inhibitory interneurons cause the opposing ones to relax so that the movement is unopposed
Initiated by the motor cortex
Occurs between extensors and flexors
Which reflexes do excitatory interneurons mediate
The flexor reflex
The crossed extensor reflex
What is the flexor reflex
When noxious stimuli cause a limb to flex - to move it further away from source
Excitatory interneurons cause contraction of the muscle
Inhibitory ones cause the extensors to relax
What is the crossed extensor reflex
Noxious stimulus causes the limb to extends - this prevents us falling over when we withdraw (flexor reflex in other limb)
Excitatory interneurons cause contraction of the extensor muscle
Inhibitory ones cause the flexors to relax
How can interneurons help us walk
excitatory ones need to fire rhythmically to cause repeated flexion then extension when they are inhibited
Fire, then inhibited over and over
what are the 3 levels of motor coordination and control
Strategy - what’s the aim of the move (controlled by basal ganglia etc)
Tactics - what sequence of muscle movements are needed (motor cortex and cerebellum)
Execution - activation of the motor pools (brain stem and cord)
How are signals passed through the retina
Vertically - photoreceptors receives signal then pass back up to bipolar cells then the ganglion cells
Some horizontal pathways exist to spread signals to other cells
What is the function of the horizontal cells in the retina
Receive input from photoreceptors and project to other photoreceptors and bipolar cells
The greater the signal, the further they project by releasing more GABA
What is the function of the amacrine cells in the retina
Receive input from bipolar cells and project to ganglion cells, bipolar cells, and other amacrine cells
Describe the dark current
There is a flow of sodium in cells that keeps potential steady
When exposed to light the cell is hyperpolarised and a signal can be created
What is the basis of colour vision
Different opsins present in the cone cells are sensitive to different wavelengths of light
Which type of retinal cell can see in dim light
The rods
Cones see in normal daylight
What facilitates high acuity vision
The distribution of rods and cones
Higher density of cone in the fovea increases the acuity
In the periphery there is larger spacing between rods and more cells acting on one ganglion - increased sensitivity but less acuity
Describe the rod cells of the eye
Don't see colour Found in the peripheral retina High convergence High light sensitivity Low acuity
Describe the cone cells
See colour Found in the central retina (fovea) Low convergence Low light sensitivity High acuity
What is the function of lateral inhibition in the visual pathway
Inhibits signals in neighbouring neurons to exaggerate the difference in stimulus intensity
Helps with localisation
What signals are sent from the retina to the brain
Simultaneous input from two eyes Depth and distance Light vs dark - centre ganglion cells Movement Form and colour
Do the visual fields of the left and right eyes overlap
YES
Forms the binocular visual field
Peripheral vison is monocular
How is the visual field mapped in the visual cortex
Different regions of the retina are mapped into the cortex - each region has an area in the cortex
Larger than proportional area is dedicated to the fovea - this is why it has high acuity
Which nucleus does visual information pass through to reach the visual cortex
Lateral geniculate nucleus
How are the hair cells responsible for generating AP’s
They have cation channels at the top of them
Open when tilted one way and closed in the other
Sound moves the hair cells and changes the channel
The hair cells release glutamate in response which is passed to nearby nerve cells creates the AP
What is sound
A change in the pressure/density in the air
Often created by an object vibrating
Can measure the frequency and amplitude
Describe how we hear sound (pathway through the ear)
Vibration in the air travels up canal and causes drum to vibration
This spreads through the ossicles to the cochlea(via oval window)
This is caught by the hair cells in the cochlea
They transduce the vibration into an AP
Travels to the auditory cortex via cohlear nerve
Where and why is sound amplified in the ear
It is amplified by the bones in the ear - ossicles
This is because the inner ear is fluid filled which is denser than air - needs to be louder to be received
Describe the structure of the cochlea
Made up of 3 fluid filled cavities
Scala vestibuli, scala media and scala tympani
Separated by membranes
Which cavity of the cochlea meets the oval window
Scala tympani
Which cavity of the cochlea meets the round window
Scala vestibuli
Which cavity of the cochlea contains the organ of corti
Scala media
How is the basilar membrane in the cochlea affected by pitch
It differs in thickness/rigidity along its length - allows us to identify pitch
Higher pitch sounds have more energy so is able to move the membrane easier
Lower pitch will have to travel further along the membrane before it is able to move it
How do hair cells allow us to determine pitch
Different frequencies of sound waves activate hair cells maximally at different locations
Wherever they bend the most will fire the most nerve signals - we hear this frequency
What is the importance of K+ channels in hair cells
K+ moves into the cells
Mutations in these channels can lead to deafness
How are hair cells innervated
The inner hair cells are innervated by many ganglions (more fibres)
The outer hair cells can be innervated by just one ganglion
Inner cells send signals out via multiple afferents whilst multiple outer HC will send via same single afferent
What effect can furosemide have on hair cells
It inactivates the motor component of the outer hair cells
No longer get their amplifying effect
Is tonotopy present in the auditory cortex
YES
Areas of the brain will also respond to particular pitch
Describe the path of an auditory signal to the brain
Signal arises in the organ of corti in the cochlea
This travels to the spiral ganglion (cochlea) before going either to the dorsal or ventral cochlear nucleus (brainstem)
The ventral one passes to the superior olivary nucleus and then to the inferior colliculus via the lateral lemniscus
The dorsal one goes straight to the inferior colliculus via the lateral lemniscus
Both paths pass to the medial geniculate nucleus of the thalamus before finally reaching the auditory cortex
How do we localise sound
The superior olivary nucleus helps us do this
If the sound is louder on one side, this area will generate the signal but also an inhibitory signal to block/dampen the other side
What are the functions of the vestibular system
Integrates information about body position and movement - gravity, rotation and acceleration
Communicates with somatosensory and visual systems
Allows for stability and orientation
How does the vestibular system detect position and movement
Head angular acceleration and rotation - semi-circular canal
Head linear acceleration (moving forward and back) - saccule and utricle
What are the different functional areas of the vestibular system
Peripheral sensory apparatus - found in the vestibular organ and detects and relays info about head postion/movement
Central processing system
Motor output - generates the compensating eye and body movement
What are the otolith organs
The saccule and utricle
They sense linear acceleration and gravity
How do the otolith organs detect movement
Have small crystals called otoconia which move within the organ
This moves the hair cells leading on to NT release
Detects the movement and leads to the change in head position relative to the movement
What are the 3 vestibular reflexes
Vestibulo-ocular – keep the eyes still when the head moves.
Vestibulo-colic – keeps the head still or at least level when you walk
Vestibular-spinal – adjusts posture for rapid changes in position.
What are the 3 forms of pain
Nociceptive - immediate and short lived
Inflammatory - days/weeks, assists healing
Pathological - no physiological purpose, long term
How does inflammatory pain aid healing
It forces us to protect the damaged area until it is healed – will get pain if you touch/used the damage tissue
Which drugs are used to treat pain
NSAIDs and paracetamol for mild
Opioids for mod/severe
Some antidepressants, anticonvulsants and local anaesthetics can be used
What are nociceptors
Specific peripheral primary sensory afferents that are activated by an intense and noxious stimuli (pain)
Includes thermal, mechanical and chemical
Where are nociceptors found
The have free nerve ending in a peripheral location
Central terminal is in the dorsal horn of the spinal cord
From here it can release glutamate to excite a 2nd order neurone
These travel in the spinothalamic or spinoreticulothalamic tracts
Describe A-delta nociceptive fibres
Respond to mechanical and thermal stimuli
Thinly myelinated so moderate speed conduction
Responsible for ‘first/fast;’ pain - stabbing
Describe C type nociceptive fibres
Unmyelinated fibres so slow to conduct
Respond to all types of noxious stimuli
Mediate slow pain - achy or inflammatory
What is allodynia
Pain arising when there is not a noxious stimuli
What is hyperalgesia
Where a painful stimulus becomes even more painful (enhanced pain response)
How can nociceptors affect their surrounding area
They can cause efferent affects by releasing pro-inflammatory mediators
Contributes to neurogenic inflammation in the area
How does neurogenic inflammation occur
Peptides are released from the nociceptors due to tissue damage or inflammation
Get local vasodilation, release of histamine etc and sensitization of nearby nociceptors = inflammation
What is the primary NT in the nociceptor pathway
Glutamate - causes an excitatory potential
What receptors does glutamate act on in the pain pathway
AMPA - activated first
NMDA - activated by intense stimuli
Where do nociceptors synapse In the spinal cord
In laminae I and II of the dorsal horn
They synapse with nociceptor specific cells that are only activated by noxious stimuli
Can also synapse with an interneuron which travels deeper and activates a wider reaching neuron
Where does visceral pain come from
From nociceptors in the coverings of internal organs - e.g. peritoneum
Caused by twisting/stretching sensations or inflammatory or ischemia
Tends to be dull/achy
Why is visceral pain sometimes referred to a distant area of skin
Some visceral and skin afferent converge upon the same spinothalamic neurones
Pain will be in one dermatome
Brain cannot differentiate where the pain is so you get referred pain
Visceral pain is often associated with autonomic symptoms - true or false
True
May get vomiting, nausea, sweating and pallor
What is viscerosomatic pain
Occurs when inflammation from a diseased organ reaches a somatic/body wall structure
Becomes a sharp, well localised pain
e.g. shifting pain in appendicitis
Are pain and nociception the same
NO
Pain involves the awareness of sufferings
Nociception can occur without pain
How can pain caused by activity in nociceptors be reduced
By simultaneous activity in low threshold mechanoreceptors
e.g. rubbing an injured area
Gate control theory
Describe the gate control theory
If the mechanical stimulus is stronger it ‘closes the gate’ by inhibiting the nociceptor and the pain is not passed on
If the pain fibres have a stronger signal it opens the gate and we feel it
What are the major nociceptive tracts
Spinothalamic tract - carries a-delta fibres, fast pain
Spinoreticular tract - carries C fibres, slow pain
What are thermoreceptors
Neurons that are specialised to respond to small changes in temperature
Is sensitivity to temperature uniform across the body
Nope
There are areas that are sensitive to heat and those that are sensitive to cold (wont respond to both)
What is the neuromuscular junction
The connection between the neurons and the muscle
What neurons innervate skeletal muscle
Motor neurons
Their cell bodies arise in the ventral horn of the spinal cord
The terminal portions give rise to fine projections which run along the muscle cell
What are the motor end plates
The synapses formed between motor neurons and muscle
A single motor neuron may control many muscle cells - true or false
True
However each muscle cell will only respond to one motor neuron
Describe the transmission of nerve impulses by acetylcholine
Action potential moves along the nerve
Voltage gated calcium channels open allowing influx of calcium
Vesicles of acetyl choline released into synaptic cleft
Acetyl choline diffuses across the synaptic cleft
The acetylcholine receptor opens and renders the membrane permeable to Na / K ions
The depolarisation starts an action potential at the motor end plate
Causes muscle to contract
What happens if you block acetylcholine receptors
You get no muscle contraction and therefore no respiration
This will kill you
Certain nerve toxins act this way
Describe how botulinum toxin works
Cleaves presynaptic proteins involved in vesicle formation and blocks vesicle docking with the presynaptic membrane
Leads to rapid onset weakness
It will eventually wear off but need to support breathing during the paralysis or it will be fatal
How can botulinum toxin be used clinically
Can temporarily paralyse over active muscles in spasticity
Can be used on overactive sweat glands
Some cosmetic uses - Botox
What surrounds a skeletal muscle fibre
A thin layer called the endomysium
What forms a fascicle
Groups of skeletal muscle fibres surrounded by a perimysium
Describe the structure of smooth muscle
Cells are not striated and have a single central nucleus
Gap junctions between cells
Lots of connective tissue around them
More actin than myosin
