Sensory Systems Flashcards
Central nervous system in vertebrates
> 95% of 116 genes involved in brain or neural Morphogenesis were commonly shared among all vertebrates
30% of planarian nervous system-related genes are homologous sequences in Arabidopsis and yeast- which do not posses a nervous system
Dendrites
Conduct electric excitation in a directed way
Axon
A long, slender projection of a neuron that conducts electrical impulses away from the neuron’s cell body- transmit information to different neurons, muscles and glands
Myelinated axons = nerve fibres
Brain
A cluster of specialised groups of neurons
Most prominent anterior condensation of neurons
Nerve cord
Cluster of neurons
Most prominent longitudinally extending condensed part of the nervous system
How does the nerve cord run in invertebrates
Ventrally
How does the nerve cord run in vertebrates
Dorsally
Ganglion
Group of specialised neurons
Parts of CNS
Neuronal somata concentrated at the surface - forming a cell cortex
Neurites are concentrated in the centre of the ganglion to form the neuropil
Distinct unit
Which animals do not have a centralised nerve system
Animals without bilateral symmetry eg Cnidarian
Cephalisation
The process by which nervous tissue, over many generations , becomes concentrated towards one end of the organism
Variation of CNS in chelicerata (arachnids)
Exhibit maximum concentration of the nervous system
Whole series of ganglia are aggregated together and fused (into one great central brain), from where nerves radiate to all parts of the body
Touch (tactile) receptors in Cnidarians
A simple nervous system , without brain, controls homeostasis
Eg nematocyst mechanism - If touched the hair triggers the cell explosion, a harpoon-like structure which attaches to organisms that trigger it and injects a dose of venom
PNS. Chordotonal organ- insects and crustaceans
Stretched neurons that detect different stimuli
Detection of vibration, touch receptors, chemoreceptors
Each unit consists of a sensory neuron, glial cells, scolopidal cells
Subcuticular mechanireceptors
Specialised sensory organs that receive vibrations in arthropods
Important for ground-dwelling species, especially nocturnal species
Subgenal organ
Complex ciliated mechanoreceptor below the knee in insects
Tricoid sensilla
Touch receptors on bodies of anthropods
Johnston organ
Largest mechanoreceptor organ of fruit fly
Gravity and sound detection
In invertebrates- where do tympanic ears occur
Insects
Vertebrate senses
Vision
Chemoreceptors (smell and taste)
Mechanoreceptirs (sound and other vibrations)
Electroreception
Magnetoreception
Temperature sensing
How do eyes vary
Acuity
Range of wavelengths they can detect
Sensitivity in low light levels
Ability to detect motion
Whether they can discriminate colour
Bird eyes
are able to perceived a wider range of light wavelengths than we can – in effect they can see ultra-violet light.
As mammals we tend to see eyes as being spherical but avian eyes vary in shape from being rather flattened to being bowed.
They also have a blood-rich pecten that protrude from the retina and is considered as a means of maximising nutrition to the eye.
Field of vision in prey
Wide possible view
Much of the angle is only viewed by one eye - monocular vision
Field of vision in predators
Stereopsis and depth perception
Binocular vision
Blind spot
Rods
Sensitive to low light
Rhodopsin
Cones
Need brighter light
Sensitive to various wavelengths
Chromophore
Colour vision
4 cone types - tetrachromatic eg birds
2 cone types - dichromatic eg macaque
1 cone type - monochromatic - colour blind eg dolphin
3 cone types - trichromatic eg humans
Adaption to low light levels
High density of rods
A ‘tapetum lucidum’ biological reflector system that is a common feature in the eyes of vertebrates.
Functions to provide the light-sensitive retinal cells with a second opportunity for photon-photoreceptor stimulation,
Enhances visual sensitivity at low light levels.
Chemoreception
Vertebrate chemoreception consists of taste, olfaction and the vomeronasal (or Jacobson’s) organ
Olfaction involves detection of an airborne (or waterborne) molecule into a specific receptor on the surface of an olfactory sensory cell
Taste involves detection of a waterborne molecule into a specific receptor on the surface of a taste bud
Vomeronasal organ
Detects non-volatile chemical cues - Flehman’s response
Linked to hypothalamus
Used in the detection of pheromones (eg major urinary proteins in mice)
Removal of the organ impairs sexual and social behaviour in rodents
Snakes have a well developed paired Jacobson’s organs in the top of the buccal cavity. Molecules are collected by the wet tongue and then intorducted into the organ when the tongue is retracted
Mechanoreception
detects physical perturbations of the environment – the commonest are sound waves in the air (or water).
(only mammals have true pinnae that focus sound waves down the ear canal).
Sound waves going down the ear canal cause the ear drum to vibrate, which causes the ossicle bones to vibrate and transfer the sound waves to the cochlea. As the waves move through the fluid within the cochlea tiny hairs in the organ of Corti are disturbed, which generates nerve impulses which are perceived as sound.
Sound detection
The cochlea is filled with perilymph, - moves in response to the vibrations coming from the middle ear via the oval window
As the fluid moves, the cochlear partition (basilar membrane and organ of Corti) moves
Thousands of hair cells sense motion via their stereocilia, and convert that motion to electrical signals communicated via neurotransmitters to many thousands of nerve cells
Hair cells in the organ of Corti are tuned to certain sound frequencies by way of their location in the cochlea
Fish mechanorecpetion system
In fish there is an otilith which resists movement from sound waves whilst the endolymph surrounding it moves – this causes hairs to be moved and triggers a nerve response. Fish lack any outer ear structure as their whole body can absorb sound waves.
Amphibian mechanorecpetion system
In amphibians there is a tympanic membrane on the side of the head (behind the eye) and this resonates and moves the columella – a bone that connects the tympanum to the otic capsule. Amphibians lack the semi-circular canals used in perceiving position as seen in mammals, crocodilians and birds.
Evolution of sound detection
Fish
No bony structure to transmit vibrations
No Tympanum
No middle ear
Otoliths vibrate
Frogs, Reptiles, Birds
Single columella
Straight cochlea
Mammals
External pinna
Three middle ear ossicles
Coiled Cochlea (except monotremes)
Vibration detection - the lateral line
System ofmechanosensory organs found in aquatic vertebrates, mainlyfish but mostamphibianlarvaeand some fully aquatic adult amphibians posses mechanosensitive systems comparable to the lateral line
Provides information about flow patterns around animal and nearby objects Used to detect movement and vibration
Provides spatial awareness and the ability to navigate in space
Plays an essential role in orientation, foraging behaviour, andshoaling
The major unit of functionality of the lateral line is the neuromast
Superficial on surface of skin – respond to water motion
Within canals – respond to pressure variations and gradients in adjacent cells
Electroreception
Electroreception is used inelectro-location(detecting objects) and forelectro-communication
Passive – Animal senses the weakbioelectric fieldsgenerated by other animals and uses it to locate them (e.g. foraging)
Active – Animal senses its surrounding environment by generatingelectric fieldsand detecting distortions in these fields using electroreceptor organs (e.g. communication)
Ampullary receptors
located in skin
sensitive to electrical fields of low frequency - <0.1 – 25 Hz)
Found in many types of fish
Tuberous receptors
in depressions in epidermis and covered by epidermal cells
Sensitive to electrical fields of high frequency – 50 Hz – 2 kHz)
Found in species that produce their own electrical fields and most sensitive to the frequencies produced by the fish
Passive Electroreception
Animal senses the weakbioelectric fieldsgenerated by other animals and uses it to locate them (e.g. foraging)
Active Electroreception
Animal senses its surrounding environment by generatingelectric fieldsand detecting distortions in these fields using electroreceptor organs (e.g. communication)
Ampullae of Lorenzini
Electric field sensors of sharks
Detect voltage differences
Use electroreception to forage and possibly navigate
Weakly electric fish
Use electroreception for mate attraction, territorial and agonisitic displays and mimicry
Active electroreception has a range of about one body length
Magnetoreception
Allows an animal to detect amagnetic fieldto perceive direction, altitude or location.
May underlie long-distance navigation in several animal species
A method for animals to develop regional maps, e.g. of the Earth’s magnetic field
Infrared sensing
Evolved independently in several different families ofsnakes
It allows them to ‘see’ radiant heat at wavelengthsbetween 5 and 30μm
Infrared light heats up parts of a thin membrane inside the pit organ, and TRPA1 proteins embedded on that membrane detect the temperature change
Directional and sensitive
Warm-blooded animals emit heat as infrared radiation
Pit vipers can detect potential prey a meter away
Accuracy enough to target vulnerable body parts of the prey
Not only used for prey detection
Evolution of eyes
So the simplest ‘eye’ would be some photoreceptors in the epidermis (a) that sense the absence or presence of light. If the photoreceptors are then located within a depression in the epidermis (b) then the animal can determine whether light hitting the photoreceptors can come from a particular direction. If the gap through which light can travel is reduced to a ‘pin-hole’ (c) then a basic image can be projected on to the photoreceptors and offers finer directional perception. If the hole is covered by a transparent surface (d) then the space can be filled with a transparent humour. If a lens then develops next to the hole (e) then there is an ability to focus an image on the photo receptors. The final stage (f) sees the development of an iris that can actively regulate the amount of light entering the eye.
How does environment affect what colours of light you can see
ocean light penetration diminished quickly as you descend down the water column and the light wavelengths are filtered. Blue light extends furthest down and a full colour spectrum is only observable within a few metres of the surface.
Tapetum lucidum
A reflector system that provides retinal cells with a second change at light detection- increasing light sensitivity in low light level
Sound threshold
Lower thresholds = greater sensitivity to sound
Sound detection in alligators
In air- more sensitive to low frequencies but less sensitive to high frequencies
In water - more sensitive to all frequencies
Mechanoreception in fish
No bony structure to transmit vibrations
No tympanum
No kiddle ear
Otoliths vibrate
Mechanoreception in fish, reptiles and birds
Single columella
Straight cochlea
Mechanoreception in mammals
External Pinna
Three middle ear ossicles
Coiled cochlea (except monotremes)
Major unit of functionality of the lateral line
Neuromast -
- superficial on surface of skin = respond to water motion
- within canals = respond to pressure vibrations and gradients in adjacent cells
How do birds carry out Magnetoreception
Through eyes
Via iron deposits in the beak
Through ears
What is important for magnetoreception
Light
Colour of light
Sensory transduction
Sensory receptor cells convert stimulus energy into an electrical signal
Ionotropic transduction
Receptor binds to a neurotransmitter and opens an ion channel across a cell membrane
Ligand-gated channels
Enlarged cell membrane of receptor cells
Increase surface area
Increase receptor number
2 types of transduction
Ionotropic
Metabotropic
Metabotropic transduction
Receptor molecule acts like a neurotransmitter to activate a metabotropic cascade
Receptor protein activates a G protein, which activates a second effector molecule in the cell that then affects ion permeability of the membrane
Ionotropic transduction - response
Rapid
10-50ms
Ionotropic transduction - latency
Short latency action
Ionotropic transduction - mechanism
Binding site and channel combined
Ionotropic transduction - location
Postsynaptic in general
Metabotropic transduction - response
Slow
Metabotropic transduction - response
Slow
Metabotropic transduction - latency
Longer
Metabotropic transduction - mechanism
Binding site not associated with channel
G-protein or 2nd messenger involvement
Metabotropic transduction - location
Pre or postsynaptic
What are commonly metabotropic systems
Some taste receptors
Photoreceptors
Which mechanism of transduction are most senses
Ionotropic
2 roles of sensory receptor
Transduction
Encoding information about the stimulus
Labelled lines principle
Even though all receptors use action potentials to encode stimuli the physical destination of the sensory axons deals with each different type of sensory receptor
Sensory receptor function- the insect bristle (sensillum)
Hollow shaft protrudes through exoskeleton and is attached to the end of a dendrite (nerve cell), which is supported by sheath and socket cells
Move the bristle and the distal tip of the dendrite is stretched and deformed
Deformation opens stretch-activated channels to allow cations (e.g. Na+ or K +) to move through the cell membrane generating a receptor potential across the dendrite’s plasmalemma
If a threshold for membrane depolarisation is reached then an action potential is propagated
Insect bristle (sensillum)
Small bristle movement does not cause enough depolarisation to reach a threshold – bristle movement is not detected
More displacement cross the threshold and generates a train of action potentials
Further deformation increases the frequency of the action potentials
Transduction illustrated by the resting potential triggering an action potential
Encoding illustrated by the frequency of action potentials
Rods structure
Lamellae form discs detached from the plasmalemma
Cones structure
Lamellae are continuous with the plasmalemma
In the dark - vertebrate eye
In the dark Na+ channels are relatively open and there is a flow of ‘dark’ current across a relatively depolarised (-30mV) cell membrane in the outer part of the cell
Light stimulation
Metabotropic transduction
Rods contain rhodopsin- activated by light
Activated rhodopsin stimulates transducin, a G-protein to activate a phosodiesterase enzyme
Enzyme decreases cyclic guanosjne monophosphate (cGMP) concentration in the receptor cytoplasm
Decrease in cGMP closes cyclic-nucleotide-gated channels for Na+ and reduces its influx into the cytoplasm
Response to light
Hyperpolarisation
Causes ion channels to close reducing the dark current
Neuron
long, thin cell surrounded with plasmalemma and Schwann Cells (neurolemmocyte) and a Myelin Sheath
Resting membrane potential
Sodium (Na+) and potassium (K+) ions continually diffuse across the plasmalemma
Cl- ions and large negatively charged molecules cannot cross the plasmalemma
Na+ and K+ concentrations on each side of the plasmalemma remain constant because sodium-potassium ATPase actively moves 3x Na+ out and 2x K+ in to the cytoplasm
Produces a resting potential of –70mV
Depolarisation
A neuron transmits a signal as an action potential in response to a stimulus
The stimulus must of a sufficient size to cross a threshold that triggers the localised movement of ions across the plasmalemma
The threshold stimulus increases permeability to Na+ ions which all rush into the cytoplasm and neutralises the resting potential, i.e. –70 mV → 0 mV = depolarisation
The influx of Na+ ions temporarily confer a positive charge to the cytoplasm
Repolarisation
Almost immediately voltage-gated Na+ channels close and voltage-gated K+ channels open and K+ ions leave the cytoplasm to build up a positive change again
This sequence of events triggers the same molecular changes in adjacent areas of the plasmalemma and the action potential moves along the axon
Hyperpolarisation - refractory period
After the action potential has moved on the membrane becomes hyperpolarised (more than –70mV) because of uncontrolled movement of K+
Sodium-potassium ATPase restores the resting potential
During hyperpolarisation the membrane is insensitive to triggering another action potential
This whole process takes ~ 3 milliseconds
Total length of action potential
3 ms
All or nothing
exceeding the stimulus threshold does not increase the size of the action potential
Speed of transmission
increased by increasing axon diameter - most vertebrate axons < 10μm
Some fish and amphibians have unmyelinated axons ~ 50μm (involved in rapid escapes)
Myelination increases transmission by Saltatory conduction from node of Ranviers
Squid axons
1 mm wide and conduct an action potential at 36 m per second
Cell bodies cluster
nuclei” in CNS and “ganglia” in PNS
Types of synapses
Axosecretory
Axoaxonic
Axodendritic
Axoextracellular
Axosomatic
Axosynaptic
Axosectretory
Axon terminal secretes directly into bloodstream
Electrical synapses
between two neurons
Positively charged ions from the presynaptic end bulb directly depolarise the adjacent neuron
Chemical synapse
between neuron and a range of cell types
Requires chemical agent called a neurotransmitter (e.g. acetylcholine or norepinephrine) to diffuse across the synaptic cleft
Chemical synapse mechanism
An arriving action potential stimulates the free diffusion of calcium ions into the presynaptic terminal
Increased calcium ion concentration stimulates the merging of vesicles containing neurotransmitter molecules with the plasmalemma
The neurotransmitters are released into, and diffuse across, the synaptic cleft
Binding of the neurotransmitter molecules to the post-synaptic plasmalemma triggers depolarisation of the post-synaptic cell to generate an action potential
Neurotransmitter is quickly de-activated by the post-synaptic cell
Failure to deactivate would lead to continual stimulation of the cell
Cnidarians neural networks
Interconnected neurons that form complex 2D nerve nets
Polyp neural networks
Entire nervous system is comprised of 2 nets
Invertebrate neural networks
One net lies at the base of the epidermis and the other at the base of the gastrodermis
Neurons bridge the mesoglea
Nerve impulses can move in any direction though the nets
Diffusion conduction means that a stimulus at one point on the net radiates outwards like ripples on a pond
No central control for coordinated responses
Neural networks in swimming medusa
In swimming medusa, first indications of coordinated control are seen as nerve rings and ganglia are found around the margin of the bell
Associated with sensory organs – e.g. ocelli, and the swimming musculature
Ganglia are concentrations of neurons that serve as simple integration centres.
Each ganglion is associated with a sensory organ and can generate motor output
Greatest output from ganglia controls muscular activity
Contraction of the musculature must be simultaneous so neurons in the nerve ring couple with muscle cells electrically with gap junctions
Nematoda
Intraepithelial nervous system
Brain is collar-like and circumpharyngeal nerve ring
2 longitudinal nerve cords
Annelids
Anterior brain consisting of 2 dorsal suprapharyngeal ganglia
2 ventral longitudinal nerve cords and paired segmental ganglia
Insecta
Dorsal supra-oesophageal ganglia (brain`0 in the head with 2 connectives that circle the gut linking to 2 ventral longitudinal nerve cords
Paired segmental ganglia with sensory and motor neurons
Brain has 3 parts = protocerebrum, deutocerebrum and triocerebrum
Arachnida
Highly cephalised central nervous system
All segmental ganglia have coalesced in the supra-oesophageal and sub-oesophageal ganglia
2 abdominal nerves extend longitudinally along the body
Gastropoda
Brain and 2 large, longitudinal nerve cords
Brain-ring of four anterior ganglia wrapped around the anterior alimentary tract
Trend towards Cephalisation and fusion of the ganglia into a brain-liege structure
Twisted configuration
Cephalopoda
CNS is highly cephalised, strongly concentrated and bilaterally symmetrical, enclosed in a cartilaginous cranium
Supra-oesophageal part associated with sensory function
Sub-oesophageal part associated with motor function
Each appendage has a brachial nerve
Squid axons
Squid have giant axons – 1 mm in diameter, which allow for rapid transmission of action potentials
Diameter of axons depend on length – short ones are narrow – allows for coordinated motor responses because action potentials all arrive at the same time
Echinodermata
Radial symmetry- - posses nerve ring around the gut and 5 radial nerves (1 per ambulacra)
2 connected neural nets - sensory ectoneural = epidermis
Motor hyponeural = coelomic lining
Evolution of invertebrate nervous system
Neural nets increasingly integrated with a localised central nervous system with different sensory and motor networks
Segmented ganglia
Increasing coalescence of nerve tissue associated with the head and often ringing the anterior gut linked to ventral longitudinal nerve cords
Nerve rings that coordinate responses to stimuli
Generalised nerve nets
Chordata - amphioxus
Rudimentary hollow brain
Dorsal hollow nerve cord almost to the end of the tail
Segmental sensory nerves
Anterior neuropore that contain cilia that maintain a flow of water over and into the pore
Brain no longer wrapped around the anterior part of the alimentary tract
Sensory (afferent) nerves
Visceral
Somatic
Motor (efferent) nerves
Somatic nervous system
Autonomic - sympathetic, parasympathetic and enteric
Parasympathetic nervous system
During relaxation
Nerves arise from the brain and sacral region of spinal cord
Long efferent nerve fibres that synapse with ganglia in vicinity of organs and short efferent neurons that extend from the ganglia to the organs
Stimulates salivary gland secretions, contracts pupils and relaxes sphincter muscles
Sympathetic nervous system
division functions during “fight or flight” response
Nerve fibres originate from the thoracic and lumbar regions of spinal cord to ganglia near the spine and long efferent neurons that extend to the organs
Inhibits salivary gland secretions, dilates pupils and tightens sphincter muscles
Enteric nervous system
Networks of neurons in the pancreas, gall bladder and digestive tract that control gut function
Normally regulated by the sympathetic and parasympathetic systems
Development of nervous tissue in vertebrates
Nervous tissue is derived from ectoderm (outer cell layers)
All vertebrates have a mesodermal notochord during development – forms basis for vertebral column
Associated with a dorsal neural tube created by folding of ectoderm tissues
3 layers enclosing the CNS
Dura matter – tough fibrous
Arachoid – delicate
Pia mater – contains small blood vessels that nourish the tissue
Spinal cord
Central neural canal filled with cerebrospinal fluid
Composed of grey and white matter
Spinal nerves – number depends on degree of segmentation
Grey matter
Cell bodies and dendrites (associated with reflex connections)
Where do spinal nerves extend from
Grey matter
White matter
Axons surrounded by myelin sheaths
Lamprey - fish
No myelinated axons (no white matter)
Shape helps facilitate diffusion of nutrients and oxygen
In spinal cord
Comparative morphology of spinal column
The distribution of white and grey matter differs between the various vertebrate groups.
The lamprey, being a very primitive fish, lacks any white matter.
Sharks have dorsal and ventral horns of grey matter surrounded by white matter and a sheath.
Amphibians have ventral and dorsal horns of grey matter – the latter extend to the edge of the spinal cord – within white matter.
Reptiles have grey matter surrounded by white matter
but in the spinal cords of mammals and birds the dorsal grey matter extends to the edge of the spinal cord.
Formation of the brain
During early development anterior neural tube expands by accumulation of cerebrospinal fluid to form different parts that become folded and lie on top of each other
Brains have three main parts – forebrain (telencephalon and diencephalon), midbrain (mesencephalon) and hindbrain (rhombencephalon) each with different roles
Telencephalon
Forebrain
Cerebrum – two hemispheres and related to sensory and motor integration
Expanded tissue and increased complexity reflects importance of the brain region
Cerebral cortex – primary sensory and motor areas
Other areas deal with processing visual or auditory perception
Diencephalon
Forebrain
Thalamus – relays sensory information to higher brain centres
Hypothalamus – controls many bodily functions – body temperature, sexual drive, metabolism, hunger and thirst
Pineal gland – controls bodily rhythms
Fish – processes sensory information
Cranial nerves
Fish & amphibians – 10 pairs; Reptiles, birds and mammals – 12 pairs
Sensory-only and mixed nerves – e.g. vagus has sensory neurons leading to the brain and motor neurons leading to the heart and visceral organs
Midbrain
Centre for coordinating responses to visual stimuli
Evolution of mid brain – also coordinates touch and auditory processing
Hindbrain
Medulla oblongata connects brain to spinal column – reflex centre for breathing, swallowing, cardiovascular function and gastric secretion
Cerebellum – outgrowth of medulla oblongata and coordinates motor activity (posture, movement, spatial orientation)
Pons connects forebrain cerebrum to cerebellum and is important in the regulation of breathing
Medulla oblongata
Reflex centre for breathing, swallowing, cardiovascular function and gastric secretion
Cerebellum
Coordinates motor activity (posture, movement, spatial orientation)
Pons
Regulation of breathing
Pineal gland
Controls bodily rhythms
Secretes melatonin
Hypothalamus
Controls bodily functions- body temperature, sexual drive, metabolism, hunger and thirst
Thalamus
Relays sensory information to higher brain centres
Number of cranial nerves in fish and amphibians
10 pairs
Number of cranial nerves in birds and mammals
12 pairs
Somatosensory systems
Proprioception
Touch
Temperature
Pain
Which senses detect particulate matter not energy
Olfaction
Gustation
Olfactory system
Olfactory epithelium
Olfactory nerves - olfactory receptors
Olfactory bulbs - glomeruli and mitral cells
Rest of CNS
Accessory olfactory bulb
What is found in the nasal cavity
Olfactory epithelia
Olfactory nerves and receptors
How does olfaction aid in survival
Finding food
Finding mates
Mother and young bond
Detecting predators
Chain of olfaction
Air passes through the nares into the nasal cavity
Pushed with turbulence over the olfactory epithelium
Particles bind to olfactory receptors
Transducer into signals via olfactory nerve to olfactory bulb
Glomeruli activated
Information passed via olfactory tract to a range of brain areas
Decision are made - motor outputs
What is an odour
A mixture of molecules that is different from its surroundings and that trigger a sensation in the animal detecting it
Defined by the nature and concentration of the odorants that are present in it
Form a plume in air or carried by water
Olfactory neurons
Cilia
Mucous
Protects from toxins/oathogens
Helps some odorants bind
Contains enzymes which degrade odours once bound
Which senses are present at birth in cats and dogs
Warmth/touch
Olfaction
Importance of glomerular mapping
Glomeruli can be in or off so this allows for a vast array of odour maps 2^N
N - number of odour receptors
Vast number of odours can be distinguished
Learning and activation of olfactory bulb
Odour + reward = increased activation
Odour + danger = increased activation
2 olfactory pathways
One associated with innate odour properties
One associated with learning
Chief enervation
Olfactory cortex
Piniform cortex
Parts of amygdala
Mitral cells
May help in encoding information about concentration
Piriform cortex
Involved in:
Categorising odours with hedonic value
Memorising odours
Perceiving similarity
Integrating olfactory information with information from other sensory systems
Role of hippocampus in olfaction
Memory
Sniffing
Direct molecules to olfactory epithelium
Better odour detection
What is a sniff
Nostril movement
Respiratory muscles
Nostrils and sniffing
Nostrils sample separately
Lateralisation of sniffing
Ipsilateral information from nostril to hemisphere
Right hemisphere deals with arousing stimuli
Right nostril bias for arousing odours
Neutral stimuli - first right nostril sniff- novelty and then shift to left nostril sniff
Tracking
Bilateral sampling for odour localisation
How is the majority of odours sensed then processed
Ipsilateral
Advantage of sniffing
Back flow of air creates turbulence
Does not disrupt odour
Pressure sensitive receptors
Ruffini’s endings
Krause’s end bulbs
Pacinian corpuscle
Fine touch receptors
Meissner’s corpuscle
Merkel disks
Root hair plexus
Temperature and pain receptors
Free nerve endings
Warmth receptors
Ruffini’s endings
Cold receptors
Krause’s end bulbs
Where is the cell body in the somatosensory system
Dorsal root ganglion of a spinal nerve
Or
Ganglia of the trigeminal or cranial nerves
Where do the secondary somatosensory neurons terminate
Thalamus
Or
Cerebellum
Where do nociceptors or thermoreceptors decussate
Spinal cord
Where do mechanoreceptors or proprioceptors decussate
Medulla
Somatosensory cortex
Post central gyrus of the parietal lobe
Forms a sensory homunculus in the case of touch
Where are the tertiary somatosensory cell bodies
Thalamus
For posture- in the cerebellum
Nociception
Detection of noxious stimuli by nociceptors
Pain
Unpleasant emotional and sensory experience
Product of processing in the brain
International association for the study of pain
An unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage
Nociceptors
Free, branching, unmyelinated nerve endings
Sub population of peripheral nerve fibres
In skin, joints, viscera, bone and muscle
Detect thermal, mechanical or chemical stimuli suggestive of injury - ongoing inflammation
Different pain triggers
Thermal
Mechanical eg cut
Chemical
Mechanical stress
Chemical inflammation
Mechanical distension
Traction
Chemical irritants
Strenuous mechanical exertion
Chemical modalities
Biology of pain
Damaged tissue releases and produces factors activate nerve endings eg potassium, histamine and serotonin or bradykinin, prostaglandins or leukotrienes
What does noxious stimulus stimulate
Transducer channels, initiates receptor potentials and induces an action potential
2 classes of nociceptors
A-delta
C fibres
A-delta
Medium diameter
Myelinated
Convey an acute, well-localised fast pain
C fibres
Small diameter
Unmyelinated
Convey a poorly localised slow pain
Regions of brain associated with pain processing
Thalamus
Amygdala
Hypothalamus
Periaqueductal grey
Basal ganglia
Brain stem reticular formation
Pain modulation
Inputs from the frontal cortex and hypothalamus
Outflow is through the midbrain and medulla to the dorsal horn of the spinal cord
Inhibits pain-transmission cells, thereby reducing the intensity of perceived pain
Nociceptors withdrawal reflex
Action potential in sensory neuron sends an excitatory postsynaptic potential (EPSP) to a somatic motor neuron in the spinal cord —> activates multiple reflex pathways—> stimulate the flexor muscle of the ipsilateral limb causing withdrawal
Also activate an inhibitory postsynaptic potential to inhibit extensors of ipsilateral limb
Also activates an interneurons that decussates and crosses the spinal cord midline —> stimulates a contralateral extensor muscle —> crossed-extension reflex (postural support)
Opioid receptors
Regulate the neurotransmission of pain signals
Activation = reduction in neurotransmitter release and cell hyperpolarisation, reducing cell excitability and thus action potential generation
Endogenous opioids
Can modulate pain physiologically
Exogenous opioids
Eg morphine
Act on receptors
Analgesia
Where do the nociceptive fibres run in the spinal cord
Centre
