Matheson Flashcards
What are the proximate explanations of Tinbergen’s 4 Qs?
- How is the behaviour achieved? Mechanism / Causation
- How does the behaviour develop? Development / plasticity
What are the ultimate explanations of Tinbergen’s 4 Qs?
- What is the behaviour for? Function / adaptation
- Where has the behaviour come from? Evolution
Name 6 types of nervous systems w/ eg.
- Paramecium - no nervous system
- Hydra - nerve net
- Jellyfish - nerve rings
- Flatworm - ladder like
- Arthropod - condensed ganglia
- Vertebrate - extreme cephalisation
Describe neurons
- Neurones are electrically excitable cells that typically have one or more neurites (processes or extensions) from their soma (cell body).
- The processes arborise (branch) in characteristic patterns, and are usually categorised as:
axons (predominantly regions of output) or
dendrites (predominantly regions of input). - Most neurones communicate with each other at specialised junctions called synapses.
What other cells make up the nervous system
Glial cells, muscle cells and sensory neurons
What are glial cells
- Oligodendrocytes, schwann (myelin) & astrocytes
- Glial cells sit amongst neurones, and may form specialised wrappings around axons
How are muscle cells involved in the nervous system
Although strictly speaking not part of the nervous system - are electrically excitable non-neuronal cells that receive signals from neurons.
Describe sensory neuron
Sensory neurones are usually associated intimately with non-neuronal cells that help in signal detection through mechanical means (e.g. hairs).
Name the ways that we study nervous system
- Physiology (record electrical activity).
Surface, extracellular, or intracellular electrodes.
Optical imaging (voltage or calcium-sensitive dye).
‘Brain scans’ – NMR (anatomy), PET (activity), fMRI (activity). - Anatomy (stain the neurones).
Golgi stain, backfilling, intracellular filling. - Immunocytochemistry (look for specific biochemical components).
E.g. use monoclonal antibodies raised against neurotransmitters. - Molecular biology (examine or manipulate patterns of gene expression).
Determine when and where specific components of neurones are formed (e.g. ion channels, transmitters). - Behaviour, computational and theoretical approaches.
Make deductions from behaviour, make predictions from computational models or from theory (physics, chemistry).
Define membrane potential
Voltage difference across a membrane
Define equilibrium potential
In a single-ion system, the voltage at which there is no net flow of that species of ion across the membrane.
Define reversal potential
- The voltage at which there is no net flow of ions (of all species) across the membrane. (If there is just one ion, it is the same thing as the equilibrium potential.)
- Can be calculated using the Nernst Equation
- If one type of ion channel dominates (I.e. has high permeability), the membrane potential tends towards the reversal potential for that ion.
Describe a chemical synapse
- Depolarisation of the presynaptic cell triggers Ca2+ entry through voltage-sensitive Ca2+ channels.
- Elevated intracellular Ca2+ triggers release of vesicles of neurotransmitter into the synaptic cleft.
3,4 Depending on the type of postsynaptic receptor, the transmitter can cause either a depolarising (excitatory) or hyperpolarising (inhibitory) effect in the postsynaptic cell.
The PSP reversal potential depends on the ion channels affected.
Define EPSP
Synaptic potential with a reversal potential positive to the spike threshold potential.
Define IPSP
Synaptic potential with a reversal potential negative to the spike threshold potential.
Define the terms NT, neuromodulator and neurohomone
Neurotransmitter: released at a synapse, with direct effects on the postsynaptic cell.
Neuromodulator: released from a neurone in the vicinity of a target cell. No direct synaptic contact.
Neurohormone: released from a neurone into the blood circulation, to act on a distant target.
Describe electrical synapses
- They look like flowers
- Electrical synapses do not use neurotransmitter, but instead allow electrical signals to pass directly between cells.
- Electrical synapses are therefore very rapid – and are usually found where speed is important (e.g. in escape circuits).
Describe passive electric sense
Passive sense (detection of external fields)
- Animate (‘bioelectricity’: gills, muscle, heart)
- Inanimate (electrochemical, geomagnetic)
- Sharks, skates, rays, catfish & all electric fish
8,600 species of total 32,000 species (25% of all species)
Describe active electric sense
Active sense (detection of perturbations in the fish’s own electric field)
- Animate (conspecifics, predators, prey)
- Inanimate (anything with an electrical conductivity different to that of water)
- All weakly electric fish (the subjects of the next two lectures)
Some strongly electric fish (electric eel)
218 species (0.7% of all species)
Name the two orders of weakly electric fish
Gymnotiformes
Mormyriformes
Describe the two orders of weakly electric fish
Electric senses very similar but:
Orders diverged 140MYA.
Evolved their electric senses completely independently: common ancestor did not possess electric sense.
Describe electric organs
- Electric organs usually consist of modified muscle cells (electrocytes).
Excitation-contraction coupling is disabled so that the modified fibres don’t contract!
Which muscles have become specialised in weak and strong electric fish (E.g)
Weakly electric skates: tail muscles.
Strongly electric rays: branchial muscles.
What is the electric organ formed from in the Gymnotid family (Apteronotidae)
The motor endplates of spinal motor neurones, not from muscle cells.
Describe generation of electric discharge (EOD)
Stacks of flattened muscle cells called electrocytes form ‘Electroplaques’ or ‘electroplax’.
Whole thing surrounded by an insulating connective sheath.
Channelled to the body surface so return current flow must be through the surrounding medium.
FW conductivity 100uS/cm cf. body tissues 5000uS/cm.
Timing of pulses controlled by a pacemaker centre in the medulla (brainstem).
The strongly electric ray Torpedo generates 1kW peak power !!
What kind of charge do strongly electric fish have
Strongly electric fish all have a monopolar discharge, which seems to be best for stunning prey.
What kind of charge do weakly electric fish have
Weakly electric fish generally have bipolar or even more complex waveforms.
What polarity is the first pulse
The polarity of the first pulse can be positive or negative depending on species (measured relative to the body long axis).
What maintains timing and shape
The timing comes from the pacemaker, the shape from the electroplaques.
What are the two electroreceptor categories
Ampullary
Tuberous
What are all electroreceptors?
Hair cells - v. similar to mechanoreceptors
What part of the sensory system do electroreceptors belong to
They form part of the octavo-lateral sensory system (‘lateral line’), which includes the receptors for hearing, equilibrium, gravity / rotation and water currents.
What are ampullary receptors?
- They have a jelly-filled canal through the epidermis to the outside.
- They’re found in many fish including weakly electric fish, and in elasmobranchs (sharks & rays).
What are tuberous receptors
- Are covered by the outer layers of the epidermis – but the epidermal cells are loosely attached to each other forming a low-resistance ‘plug’.
- Are only found in electric fish.
Describe Ampullary receptors
- Exceptionally sensitive to weak electric field gradients:
5nV / cm in marine fish (1V in 2000km !!)
1µV / cm in fresh water (1V in 10km)
(Behavioural threshold 10-100 fold higher). - Differ in detail between fresh water and sea water fish because of differences in the electrical conductivity of FW and SW.
- The sensory neurones are spontaneously active.
- Respond only to low frequency (0 – 30 Hz) signals.
-Those of elasmobranchs are known as ampullae of Lorenzini.
These are important in navigation.
Describe tuberous electroreceptors
- Found only in Gymnotids and Mormyrids – which produce electric signals.
- Respond to the discharges of electric organs (therefore respond to high frequency signals).
- Fall into two types:
Time markers with high sensitivity and fixed latency.
Amplitude coders with low overall sensitivity but very high sensitivity to tiny changes in the amplitude of a fish’s own EOD.
Describe time markers and amplitude coders of tuberous electroreceptors (what they detect)
- Time markers:
Detect the timing of a fish’s own EOD or that of a conspecific. - Amplitude coders:
Detect the amplitude of a fish’s own EOD. - These receptors together permit:
Detection of EODs generated by conspecifics (electrocommunication).
The presence of objects in the environment (electrolocation).
What are the two types of tuberous electroreceptors in Mormyrids fish
Knollenorgane
Mormyromasts
Describe knollenorgane
- time coders, K receptors
- 1-35 receptor cells but only a single neurone.
- Receptor cells fire a single action potential per EOD.
- Electrical synapses to the neurone (high fidelity) .
Describe Mormyromasts
- amplitude coders, D receptors
- The most complex type.
- Two types of receptor cell (A,B), innervated separately.
- Additional structural specialisations of the receptor organ.
- Respond with bursts of spikes in which the latency of the first spike signals amplitude (higher amplitude = shorter latency).
What are the time markers and amplitude coders in gymnotid fish?
- Not so easy to distinguish
- Different in pulse and wave fish
- Pulse:
M receptors (basically like Knollenorgane).
B (burst) receptors (like mormyromasts). - Wave fish:
T (time) receptors (basically like Knollenorgane).
P (probability) receptors (like mormyromasts).
What are ampullary receptors used for
- Used for passive electrolocation (e.g. detection of electric fields emitted by prey).
- Whether ampullary receptors are also involved in active electrolocation is unclear – but they could be involved.
What are tuberous receptors used for
- Used for active electrolocation (the detection of distortions of own EOD caused by objects in the water).
SUMMARY ELECTROLOCATION
- Found only in Mormyrids and Gymnotids
- Requires an electric organ and tuberous electroreceptors!
- Is accompanied by substantial modifications of some brain areas to control the EOD and process the sensory signals.
A pacemaker nucleus in the medulla of the brainstem.
A somatotopic map of the electrosensitive body surface.
- Can detect the following properties: Location (& therefore movement) Conductance Capacitance Distance
- Severely limited by the spread of current through the water.
- The behavioural detection limit for an object of 2-4mm diameter was 2-4cm.
In Mormyrids, it is the mormyromasts that are involved in active object location. - We will see later that the responsiveness of the Knollenorgane is shut off during a fish’s own EOD so they cannot participate.
-In Gymnotids, probably all types (T, M, P & B receptors) are important.
- Different parameters of the ‘electric image’ provide different type of information about the object:
The location of receptors detecting a distortion indicates the location of the object relative to the body.
The sign of the distortion indicates conductance.
Changes to the waveform or timing indicate the capacitance.
-Object size, shape and distance must be computed from this information – which is a complex process that is not yet fully understood.
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Describe the Gerhard von de Emde experiment
- 1999
- By placing recording electrodes near the body of the fish von der Emde could monitor what different objects ‘looked like’ to the fish’s sensory receptors when the objects were placed at different distances.
- Far object = large image, low contrast
- Near object = small image, high contrast
- The ratio of slope/amplitude was related to object distance irrespective of object shape, size or material. (Perfect spheres do lead to an overestimate, but are unlikely to occur in nature).
- You could think of this as being ‘fuzzy faint objects are further away than sharply defined dark objects’
Describe the proposed mechanism for depth perception
- The proposed mechanism for depth perception is rather different to:
Stereoscopic vision in toads (triangulation)
Accommodation in chameleons (eyes angle in)
Auditory depth perception (sound spectra)
Bat echolocation (time delay) - It uses a single stationary array of receptors without resort to temporal or spectral measurements.
Describe capacitance inrelation with electrolocation
- Capacitance is an electrical property of some materials to store an electrical charge.
Living organisms generally have a high capacitance.
Inanimate objects generally have a low capacitance. - So: capacitance may be useful in distinguishing living organisms from the environment.
- Capacitance modifies the shape, but not timing of EOD pulses in pulse-type mormyrids.
- In wave-type gymnotids, capacitance modifies the timing (phase) of the field relative to the emission.
Describe mormyrid, capacitance and electrolocation
- The mormyromast receptors of Mormyrids detect waveform distortion. The responses of A and B cells within a single mormyromast are compared to permit the fish to distinguish between resistive and capacitative objects.
Describe gymnotid, capacitance and electrolocation
The T type receptors detect timing distortions, and by comparing the distortions at different receptors scattered across the body the fish can detect and localise capacitative objects.
What are problems with the active sense of electrocommunication
- How to maintain sensitivity of receptors when the fish is itself producing an EOD?
- Corollary discharge inhibition of Knollenorgane in Mormyrids (pulse fish).
- How to avoid interference from nearby conspecifics (this affects both communication and electrolocation)
- Use sparse signals (Mormyrid pulse type fish).
- In wave type fish (Gymnotids), individuals within a species have characteristic frequencies within a wide range from 300 – 600 Hz.
- In Gymnotids, use jamming avoidance response (JAR).
Describe the jamming avoidance response (JAR) in gymnotids
- Problem: Summation of similar frequency EODs distorts detected EOD - “Jamming”
- Solution: Both fish shift the frequency of their EODs within 10-30s.
~ Fish with highest frequency shifts higher.
~ Fish with lower frequency shifts lower. - Initial hypothesis: fish monitors its own centrally generated EOD pacemaker pattern and compares it to the conspecific pattern detected on the body surface.
- NO: Fish detects its own and conspecific pattern together on the body surface, and computes the frequency difference.
What can electrocommunication determine?
- Individuals can determine small differences in the EOD waveform of conspecifics
- Can probably determine:
species,
sex,
age,
(even individual identity?)
What i need to know
- Active versus passive sense.
- Different groups of ‘weakly electric fish’
- Gymnotids, Mormyrids.
- Production of the EOD.
- Types and characteristics of the different receptors.
- Mechanisms for detection of location, conductance, distance & capacitance
- Corollary discharge inhibition
- Jamming avoidance response
