Physiology Flashcards
Give examples of some of the viruses that exploit retrograde transport to infect neurones
Herpes, polio, rabies
What are the 4 functional regions of a neuron?
Input
Integrative
Conductile
Output
What ions are responsible for the AP in neurones?
Na is responsible for the upstroke
K is responsible for the downstroke
How can passive current spread be increased?
By decreasing the axon resistence- possible by increasing axon diameter
Increasing membrane resistence- done by adding an insulating material (myelin)
What is Saltatory conduction?
The propagation of action potentials along myelinated axons from one node of ranvier to the next, this increases the conduction velocity of APs
How might synapses be classified?
Morphologically- by the location of the presynaptic terminal upon the postsynaptic cell
Functionally- excitatory or inhibitory
What are some of the morphological classifications of synapses?
- Axodendritic
- Axosomatic
- Axoaxonic
What are the major amino acid neurotransmitters in the CNS?
Glutamate, GABA and Glycine
What is excitatory post-synaptic potential EPSP?
An excitatory synapse e.g. Glutamate, causes the activation of poststynaptic receptors which then generates a local, depolarizing response.
Glutamate synapses leading to influx of Na which brings the membrane to threshold making an AP more likely
What is an inhibitory synapse?
Most commonly in the CNS are GABA or Glycine
The activate postsynaptic receptors which generate a graded inhibitory (hyperpolarizing) response.
- Cl- ions influx causing the neurone to move further from threshold
What is spatial summation?
The effect of triggering an AP in a neurone from one or more presynaptic neurones
Many inputs converge upon a neuron to determine its output
Excitation occurs when more than 1 ESPS originate simultaenously at different points of the neurone
What is temporal summation?
A single input may modulate output by variation in AP frequency of that input
What a metatropic receptors?
Subtype of membrane receptors that do not form an ion channel pore but use signal transduction mechanisms- often G proteins
What are some dymylinating disorders and what do they do?
Examples include- MS and Guillain-Barre
Cause slowing or even the cessation of nerve conduction
What is Pruriception?
Itch
What is mechanosensation?
Fine discrimatory senasation- light touch, pressure, vibration, flutter and stretch
What is proprioception?
Joint and muscle position sense
Describe a somatosensory pathway
Composed of 3 neurons in a sequence- 1st, 2nd and 3rd order neuron
- 1st- the primary sensory afferent. Picks up the sense then transports it to 2nd
- 2nd- projection neuron. Usually in the dorsal horn of the spinal cord or brainstem nuclei.
- 3rd- found in the thalmic nuclei and then project to the somatosensory cortex
What is sensory adaptation?
The change in sensitivity of your perception of a sensation
What are Tonic receptors?
Slow adapting receptors
Respond to the stimulus for as long as it persists and will produce a continous, high frequency of APs
What are phasic receptors?
Rapidly adapting receptors
Quickly respond to stimuli but stop responding upon continual stimulation. AP frequncy decreases during prolonged stimulation
Receptor still remains sensitive to a change in stilulus energy or removal of the stimulus
Describe conduction velocity
The rate at which an AP is conducted along an axon
There are 2 important contributing factors to conduction velocity
- Diamter of axon
- Insulating myelin
Large diameters and myelin favour conduction
What is a receptive field?
The target territory from which a sensory unit can be excited
RF size varies greatly and is inverseyly related to innervation density
Sensory acuity correlates inversley with RF size
How is 2 point discrimination tested?
Applying simultaenously 2 sharp point stimuli separated by a variable distance at different sites on the body.
Patient reports whether 1 point or 2 are sensed and a threshold distance between the 2 is established
What cutaneous receptors sense pressure?
Pacinian Corpuscles
What are Laminae of Rexed?
The grey matter of the spinal cord is subdivided into dorsal and ventral horns as well as 10 distinct Laminae of Rexed
The laminae are layers of neurons that perform specific functions
Describe the somatotopic organisation of the dorsal columns
The columns are made up of the medial gracile tract (fasciculus gracilis) and the lateral cuneate tract (fasciculus cuneatus)
Input to T6 and below (legs and lower trunk) travel in the gracile tract
Input from above T6 (arms and upper trunk) travel in the cuneate tract
What is the major route by which touch and concious proprioceptive information are carried to the cerebral cortex?
The Dorsal Column Medial Lemniscal Pathway
What is stereognosis?
The ability to recognise an object by feeling it
To test the patient is asked to identify a familiar object by touch alone
What is contrast enhancement?
As information is conveyed from one neurone to the next in a sensory pathway, differences in the activity of adjacent neurones is amplified
This produced contrast enhancement
Describe lateral inhibition
When one neurone is active it inhibits the activity of its neighbouring neurones via inhibitory interneurones
This works to sharpen stimulus perception
How is sensory information from the anterior head relayed to the brain?
Via the trigeminal system. Impulses originate at the terminals of CNV
The soma of the sensory neurones are located in the trigemenial sensory ganglion
The central terminals of CNV then synapse onto 2nd order neurones in the cheif sensory nucleus or spinal nucleus
These then descussate and project to the VPM nucleus of the thalamus
Describe the somatosensory cortex
Located in the post central gyrus of the parietal cortex, immeadiatley posterior to the central sulcus (SI) and adjacent to the posterior parietal cortex (SII)
SI contains Brodmann Areas (BA) 1, 2, 3a and 3b
How many layers does the somatosensory cortex have?
6 layers
Thalmic inputs to SI terminate mainly on neurones in level IV which in turn project to cells towards the surface of the cortex and deeper layers
What is the role of the posterior parietal cortex (SII)?
It recieves and integrates the information from SI and other cortical areas (visual, auditory) and sub-cortical areas (thalamus)
It deciphers the deeper meaning of the information of SI
(A key has a defined shape, size, weight, texture etc. when we feel for one in our pocket we immediately recognise it as a key not e.g. a coin without analysing it individual characteristics)
What are LMNs composed of?
- Alpha-MNs= innervate the bulk of fibres within a muscle that generate force
- Gamma-MNs= innervate a sensory organ within the muscle known as a muscle spindle
True or False?
Motor Neurones are distributed evenly within the spinal cord
False
There is a greater number in the cervical enlargement (C3-T1) that supplies the arm and in the lumbar enlargement (L1-L3) supplying the leg
What is a motor unit?
The alpha-MN and all the skeletal muscle fibres that it innervates forms a motor unit- the smallest functional component of the motor system
What is a motor neurone pool?
The collection of alpha-MNs that innervate a single muscle
What are the 3 sources of input to alpha-MN that regulate its activity?
- Central terminals of dorsal root ganglion cells whose axons innervate the muscle spindles
- UMNs in the motor cortex and brain stem
- Spinal interneurones
What does muscle strength depend on?
- Activation of the muscle fibres
- The firing rate of LMNs involved
- Number of LMNs simulatenously active
- Co-ordination of the movement
- Force production by innervated muscle fibres
- Fibre size (hypertrophy)
- Fibre phenotype (fast or slow twitch)
Describe slow-oxidative (Type 1) muscle fibres?
The ATP is derived from oxidative phosphorylation
Slow relaxation and contraction- fatigue resistant
What are the subtypes of Fast (type II) muscle fibres?
- Type IIa- ATP mostly from oxidative phosphorylation. Fast contraction and relaxation. Fatigue resistent
- Type IIb, IIx- ATP from glycolysis. Fast contraction but not fatigue resistent. (Type IIb isnt in mammals)
What is the Henneman Size Principle?
The susceptability of an alpha-MN to discharge APs is a function of its size.
Smaller alpha-MNs (part of slow motor units) have a lower threshold than larger ones. Slow motor units are more easily activated and ‘trained’ by any training that activates the muscle.
What is the myotatic reflex?
Also called the stretch reflex- a muscle contraction in response to stretching within the muscle.
The change in length is registered by a sensory organ within the muscle- the muscle spindle.
What is the jendrassik manouver?
Reinforces the myotatic reflex
Patient interlocks fingers and trys to pull their hands apart strongly when instructed. Tap patellar tendon immediately- reflex usually will be exagerated
May be employed when the reflex is apparently absent
True or False?
The intrafusal muscle fibres are innervated by alpha-MN?
False
Gamma-MN innervate the intrafusal fibres (motor)
Also contain afferent neurons
The alpha-MN innervate the extrafusal fibres
What are the subdivisions of muscle spindles?
- Nuclear Bag fibres
- Bag 1/Dynamic- very sensitive to the rate of change of muscle length. Innervated by dynamic Gamma-MNs
- Bag 2/Static- more sensitive to the absolute length of the muscle. Innervated by static gamma-MNs
- Chain fibres
- Sensitive to the absolute length of the muscle. Innervated by static gamma-MNs
What are the afferent fibres that innervate the intrafusal fibres?
- Ia fibres- primary annulospiral nerve ending, winding around the centre of all intrafusal fibres
- II fibres- more slowly conducting, form flowerspray endings on the intrafusal fibres except bag 1 dynamic types
Both sets respond to stretch but Ia are more sensitive to rate of change of stretch whereas II are more sensitve to absolute length
Where are golgi tendon organs located?
At the junction of the muscle and the tendon
What is the role of the golgi tendon organs?
Monitor changes in muscle tension, working in series with the extrafusal muscle fibres. Act to regulate muscle tension to;
- Protect the muscle from overload (in extreme circumstances like weight lifting)
- More generally regulate muscle tension to an optimal range
What is the innervation of the golgi tendon organs?
Innervated by group Ib sensory afferents- myelinated, slightly slower conducting than Ia
They enter the spinal cord and synapse upon inhibitory interneurons which in turn synapse on the alpha motor neurones of the homonymous muscle, forming the basis of the reverse motonic reflex
What does proprioceptive information arise from? (3)
Muscle spindles
Golgi tendon organs
Joint receptors
What are joint receptors?
Proprioceptive axons found in the connective tissue of joints. Mix of fast or slow acting with either high or low thresholds for activation.
Give examples of joint receptors (4)
- Golgi type endings- found in ligaments. High threshold and slow acting. Protective role?
- Free nerve endings- capsule and connective tissue. Most numerous. HT, SA. Noicereceptive function
- Paciniform endings- periosteum near articular attachments and fibrous part of joint capsule. LT, SA, acceleration detectors
- Ruffini endings- joint capsule. LT, SA, static position and speed of movements
What do inhibitory interneurons mediate?
The inverse myotatic response
Reciprocal inhibition between extensor and flexor muscles
What do excitatory interneurones mediate?
The flexor reflex
The crossed extensor reflex
What is the flexor reflex?
Noxious stimuli cause the limb to flex by;
- contraction of the flexor muscles via excitatory interneurones
- Relaxation of the extensor muscles via excitatory and inhibitory interneurones
What is the crossed extensor reflex?
Noxious stimuli causes the limb to extend by;
- Contraction of the extensor muscles via excitatory interneurones
- Relaxation of the flexor muscles via excitatory and inhibitory interneurones
This reflex enhances postural support during withdrawl of foot from a stimulus- you do not fall over
What is a central pattern generator (CPG)?
A neuronal network in which interconencted inhibitory and excitatory neurones produce an oscilitating, rhythmic output in the absense of sensory feedback.
What are the 3 levels of the motor control heirarchy?
-
Strategy- What is the aim of the movement? How is it best achived?
- Neurocortical association areas, basal ganglia
-
Tactics- What sequence of muscle contractions and relaxations will fulfil the aim?
- Motor cortex, cerebellum
-
Execution- Activation of motor pools and interneuron pools that command the desired movement and makes postural adjustments
- Brain stem, spinal cord
What are the pathway divisions of the descending spinal tract?
-
Lateral Pathways
- Controlled by cerebral cortex
- Voluntary control of distal musculature, particularly discreete, skilled movements
-
Venteromedial Pathways
- Controlled by brainstem
- Controls posture and locomotion
What is the major lateral pathway?
Corticospinal (pyramidal) tract
Cell nodies are located in the motor cortex and somatosensory areas of the parietal cortex
Where do the axons of the corticospinal tract terminate?
In the dorsolateral region of the ventral horn and intermediate grey, the location of the LMNs and interneurons controlling the distal muscles, particularly flexors
What does the rubrospinal tract control?
Limb flexor muscles, exciting the LMNs of those muscles
What are lesions to the lateral pathways associated with?
- Loss of fractioned movement (shoulders, elbow, wrist and fingers cannot be moved independently)
- Slowing and impairment of accuracy of voluntary movements
- Little effect on normal posture (e.g. standing, sitting)
What occurs if only the corticospinal tract is damaged?
Initially the patient will have deficits just as profound as a lesion to the lateral columns. Over time, major recovery can occur - although weakness of distal flexors and inability to independently move fingers persist.
However, such recovery is reversed if the rubospinal tract is also lesioned.
The rubospinal tract is capable of compensating significantly if the corticospinal tract is damaged
What do the axons of the vestibulopsinal tract do?
- Axons from the lateral vestibular nucleus descend ipsilaterally as the lateral vestibulospinal tract as far as the lumbar spinal cord
- Helps to hold upright and balanced posture by facilitating extensor MNs of antigrav muscles
- Axons from the medial vestibular nucleus descend as the medial vestibulospinal tract as far as the cervical spinal cord
- Activate cervical spinal circuits that control neck and back muscles guiding head movements
What does the tectospinal tract do?
Originated in the superior collicus which acts as a map of the external world guiding, e.g. the head and eyes to a new, important visual stimulus
Describe the reticulospinal tracts?
Made up of the Pontine (medial) and medullary (lateral) reticulospinal tracts
Both arise from the reticular formation, a diffuse mesh of neurones located along the length and at the core of the brainstem
Describe the Pontine (medial) reticulospinal tract
- Descends ipsilaterally
- Enhances the antigravity reflexes of the spinal cord
- Helps manintain standing posture by facilitating contraction of the extensors of the lower limb
Describe the medullary (lateral) reticulospinal tract
- Descends bilaterally
- Opposes the action of the medial tract
- Releases antigravity muscles from reflex control
What are the 3 organs of the inner ear that detect different sensory modalities?
- Cochlea
- Otolith organs= Saccule and Utricle (tilt and acceleration)
- Semicircular canals (rotational movements)
What are the 3 mechanisms for impedence matching in the middle ear?
- Area ratio of the ear drum to staped footplate (20:1)
- Lever action of the osiciles
- Buckling of the ear drum
What are the membranes of the inner ear?
Reissner’s membrane- separates the scala vestibuli from the scala media
Basilar membrane- separated the scala media from the scala tympani
Organ of corti sits on basilar membrane
True or False?
The perilymph in the scala tympani and scala vestibuli is continous
True
ST and SV connect at the helicotrema and so the perilymph is contious
ST meets oval window whilst the SV meets the round window
What is unusual about the K+ current of the hair cell?
The K+ current is inwards
What is Tonotopy?
The ability of the brain to recognise sounds of different frequency (pitch)
What in the ear processes tonotopy?
The Basilar membrane- it is flexable and vibrates in sync with fluid motion
The width and flexability of the basilar membrane determines the distance a particular freqeuncy travels
What lights do rods and cones see in?
Rods- see in dim light
Cones- see in normal daylight
What is centre-surround organisation in the retina?
Also called lateral inhibition
Serves to emphasise areas of difference (contrast) - it sharpens the boundary between objects of different luminence
Define pain
An unpleasent sensory and emotional experience, associated with actual tissue damage or described in terms of such damage
What are the 3 forms that pain can be classified in?
- Nociceptive pain- adaptive, an immediate protective response that is short lived
- Inflammatory pain- adaptive, assists in healing and persists over days/weeks
- Pathological pain- maladaptive, no physiological purpose and persists for months/years/lifetime
What are some drug classes used to treat chronic pain?
Antidepressents, anticonvulsants, local anesthetics
How does Nociceptive pain begin?
Nociceptors are specific peripheral primary sensory afferent neurones that are normally activated preferentially by an intense stimuli (e.g. thermal, mechanical, chemical) that are noxious
First order neurones that relay information to 2nd order neurones in the CNS
What are the subtypes of nociceptor?
Adelta and C fibres
Describe Adelta nociceptors
Mechanical/thermal nociceptors that are thinly myelinated
Repsond to noxious mechanical or thermal stimuli
Mediate first/fast pain
Describe C-fibre Nociceptors
Unmyelinated nociceptors that collectively respond to all noxious stimuli (polymodal)
Mediate second or slow pain
Describe the Nociceptive pathway
- Noxious stimuli, stimulates nociceptor
- Synapses onto 2nd order neuron in the dorsal (posterior) horn of the spinal cord
- Travel to CNS via the Spinothalamic and Spinoreticulothalamic tracts
True or False?
Peptidergic Noiceptor C-fibres have only afferent fibres
False
Have both afferent and efferent fibres
The afferent fibres transmit noiceptive information to the CNS
Efferent fibres release pro-inflammatory mediators from the peripheral terminals which contributes to neurogenic inflammation
What is allodynia?
A condition where pain is caused by a stimulus that does not normally elicit pain
Breifly outline neurogenic inflammation
- Peptides (SP and CGRP) are released from the free nerve endings of peptidergic nociceptors due to tissue damage or inflammatory mediators
- SP and CGRP initiate an inflammatory response leading to vasodilation, release of mediators and sensitive surrounding nociceptors
- Leads to primary and secondary hyperalgesia and allodynia
Describe the character of visceral pain
Poorly localised with a dull, aching, throbbing character
Where does visceral pain originate and what is it caused by?
Originated from receptors that cover tissues (e.g. the peritoneum or the pleura) or the walls of hollow organs
Originates from twisting, stretching, inflammation and ischaemia- but not cutting or burning
Describe reffered pain
Visceral afferents from nociceptors follow sympathetic pathways before entering the dorsal horn
Some visceral and skin afferents converge upon the same spinothalamic neurones
The brain then interperets the nociceptive information arising from the viscera as originating from an area of skin, that may be distant to the internal organ
What might visceral pain be associated with?
Autonomic features such as nausea, vomiting, sweating, pallor
Describe the character of viscerosomatic pain
Sharp and well localised pain
What is the gate control theory of pain?
The theory suggest that the spinal cord has a neurological gate that either blocks pain signals or allows them to travel to the brain
It suggests that non-painful input closes the gate, preventing pain sensation travelling to the brain
Describes how non-painful sensations can override and reduce painful sensations
What are the major nociceptive tracts?
The spinothalamic tract (pain perception)
The spinoreticular tract (autonomic responses to pain, arousal, emotional responses, fear of pain)
Describe thermosensation at the skin
Thermoreceptors are neurones that are sepcialised to respond to small changes in termperature
Separate neurones and their associated receptors/channels must exist to encode between warm and cold
