Physiological Mechanisms and Behavior •Nervous System •Hormones
• Organised behaviour is a result of sensory and motor integration in an organism – nervous system (NS) – How has behavioural needs shaped the anatomy and organiszation of NS? – Are there specific centres and pathways for the control of particular behaviours? – Are adaptive behaviours “hard-wired”? – How is important information sifted from “noise”? – How do hormones affect behaviour and how do they work alongside the NS?
• Behaviour is the tool with which an animal manuvres itself in an organised and directed way, and manipulates objects in its environment to suit itself.• This chapter deals with accounts of behaviour in terms of physiological mechanisms (“nuts and bolts” explanation) (see Fig 3.1)
• Multicellular animals have developed a complex systems of cells • Detect, transmit, integrate, store information supplied by an animals internal and external environment for decision making processes – sensory cells – detects changes in environment – Nerve cells – transmits and integrates information – Chemical messengers – transmits information around the body (slower than NS) – Muscle cells – translate information into action• The scope and sophistication of behaviour in the animal kingdom is linked with the evolution of neural complexity• Vertebrate nervous systems are not always more complex than those of invertebrates• What then are the properties of NS that makes complex behaviour possible?
• We will not discuss the neural anatomy and physiology• Read up on: – Nerve cell structure (Fig 3.2) – Nerve cell function – Communication between neurones
Evolutionary trends in nervous systems and behaviourAs we move from unicellular organisms to vertebratesthe NS changes in 2 ways:•Greater differentiation•Greater centralization
• In unicellular organisms (NO ns) there is spatial differentiation between sensory and motor function – Paramecium is covered in motile cilia – for helical propulsion in water – When it collides with obstacle, mechanoreceptors at anterior end are stimulated and cilia beat in reverse and organism move off in different direction (Fig 3.3a) – If hit from rear posterior mechanoreceptors triggers forward thrust – How do the mechanoreceptors communicate in absence of nerve fibre? • Caused by changes in electrical potential of the cell membrane and picked up by each cilia – entire organism acts like a nerve cell! (See Fig 3.3b) – The control linking the receptor to effector is unrefined
NS and behaviour in Invertebrates• In multicellular animals the linking is taken over by axons having dendrites and synapses• In an advanced NS: • Sensory receptor – cell or group of cells • Afferent or sensory neuron – carrying impulse from sensor • Efferent or motor neuron – carry impulse to effector • Internuncial neuron or interneuron linking sensory and motor neuron • Effector organ (performs the motor task)• Nerve Nets • Cnidarians (sea anemone, jelly fish, etc) and echinoderms (starfish, urchin, etc) (See Fig. 3.4a & b) – diffused nets • Lengthening and thickening of axons – fast action – withdrawal action of sea anemone (Actinaria and Metridium) • Behavior is stereotyped – exhibit reflex
• Nerve tracts and Centralization – In platyhelminthes the tracts are more pronounced and ns shows trend towards centralization (Fig 3.4c) – recognisable CNS and cephalisation (concentration of nerve tissue in the head region into an anterior ganglion or simple brain) – Nerve cord extends down the body (Fig 3.4c) from anterior ganglion – Nerve fibres extend from the cord to all regions of the body in a network arrangement – peripheral ns ( in most vertebrates and all vertebrates) – simplest in flatworms – Peripeheral contain the sensory cells and the CNS has the motor nerve cells – Sensory cells in Planaria respond to touch, temp, chemicals, light – Nerve cord allows for much rapid transmission – increase in speed and variety of behavioral responses to different environmental stimuli – The differentiation and centralisation of NS in flatworms allows for a degree in learning ability – which way to turn in a T-maze and to avoid noxious mechanical stimulus, assessment during mate choice
Nerve cords and Ganglia• In higher invertebrates (metamerically segmented) – annelids and arthropods and molluscs (non segmented) the ns is differentiated into ganglia linked by nerve cords (see Fig 3.3 c-e)• Increasing centralisation – neural switchboard: – Afferent fibres (sensory receptors) connect to interneurons and to motor neurons – Depending on input different motor neurons are brought into play – In leeches ganglia has 400 cells; in Aplysia a mollusc has 1500 cells• Function of ganglia – regulation of local reflex arc – local control – Long range coordinated control via long interneurons along nerve cords facilitating coordinated operation of different body parts
• A number of behavioral advances are associated with these developments – Elaboration of appendages and musculature – Emergence of fluid filled body (coelom) • allows for subtle movements and complex manipulative tasks eg. In the web building • Elaborate courtship songs and ornamented nest construction of some insect species • Stimulus discrimination and learning but learning is short lived (small capacity of the ganglia) just like the shortlive span of species• Among invertebrates there is trend towards enlargement of brain – amalgamation of somatic ganglia • Somatic ganglia still retain considerable independence of control • If cerebral ganglia is removed earthworms still can crawl, feed, copulate but are hyperactive • Nereids are able to learn certain tasks even after removal of cerebral ganglia from CNS
Evolutionary trends in invertebrate brains• Vary considerably is structure. At lower end, flatworms brains have 2000 cells, insects with 34,000 cells while cephalopods have 170 mil cells (a tenth of humans)• The slow moving molluscs (gastropods - snails, slugs; and lamellibranch -bivalve) have losse string of ganglia (50,000 cells) as compared to the cephalopods• The tendency towards fusion of ganglia in ns with evolution of more sophisticated sensory system and behaviour (See Fig 3.5)
NS and Behaviour in Vertebrates• Ns develops from dorsal tissues and as a tube rather than as a solid structure• Traces of ancestral segmented patterns are also present – distribution of sensory and motor neurons• Centralisation, cephalisation and functional differentiation reaches its peak in vertebrates• Structure and function of CNS (See Fig 3.6): – Brain and spinal cord – Brain has 3 regions and they can be further sub-divided: forebrain prosencephalon), midbrain (mesencephalon), hindbrain (rhombecephalon)
Evolutionary trends in vertebrate brains• 2 main evolutionary trends: – First, elaboration of the midbrain (shown in fish) – optic tectum thickens and stratified – a integration center for information from other parts of brain – Second, elaboration of the cerebral hemisphere (shown in mammals) shown in the forebrain – major association centres (See Fig 3.7a & b)• Forebrain of lower invertebrates remain in the form of hippocampus but the neocortex in humans extends to the whole of the brain• Plot of brain and body size (See Fig 3.7c). The discontinuity is a result of the elaboration of the forebrain cortex (neocortex) in higher vertebrates• In lower vertebrates brain is <0.1% of body mass, in birds and mammals > 0.5%, in humans it is 2.1% (See Fig. 3.8)
• Neocortex has many visible divisions which reflect different functional areas (See Fig. 3.8a)• In advanced mammals voluntary motor control are in front of somatic sensory functions and separated by deep fissures• Part of the motor cortex communicates with the spinal cord via the pyrimidal tract (see Fig. 3.7b)• The 2 halves of the cortex are connected by the corpus collosum – Severence of corpus collosum results in loss of speech control and verbal comprehension• Further motor control is at the corpus striatum (subcortical); in birds it is important for sterotype bahaviour• The limbic system (subcortical) – hippocampus, cingulate gyrus, septum and amygdala – control arousal, learning, agnostic behaviour and decision making (See Fig. 3.8b)• The thalamus – relay information from retina, ear, cerebellum, and tectum and function in appreciation of temp, pain and pleasure; production of hormones in hypothalamus – emotional arousal, sleep, feeding, aggression, osmoreceptors (drinking orintation behaviour)
NS and the Adaptive Organisation of Behaviour• There is association between organisation of ns and the range and complexity of behaviour of different taxonomic groups• Behaviour results from coordinated neural control of the effector system• Variation in organisation reflects adaptive specialisation between and within species – Does gross anatomy of CNS reflect different adaptive behaviour patterns? – Are adaptive behaviours “hard wired” into nervous systems – do they have neural circuits dedicated to their control?• Can we infer about behaviour specialisations from anatomy of nervous system from evolutionary trends towards centralisation and cephalisation?
Comparative studies of Invertebrates• Lifestyle – Early development are nerve nets – cnidarians – rapid through conduction – quick response to noxious stimuli. This fast through conduction is also in annelids and arthropods in the form of large axons – giant fibres. The annelids and arthropods have wide range of adaptive behavioural specialisations. Are these reflected in the anaotomical arrangement of the fibres? – There is difference in the oligochaetes (earthworms) and polychaetes (ragworms) – anatomy and conduction of giant fibres in terms of lifestyle • In Lumbricus (burrowing oligochaete) (See Fig 3.9a) the primary function of giant fibres is for fast contraction of longitudinal muscles and forward and reversal withdrawal in response to mechanical stimuli – emergency responses in burrowing organism • The giant fibres makes up 10% of cross section area of the nerve cord
• Among polychaetes the giant fibres make up 25-70% of the cross section of nerve cord (Fig. 3.9b) – In sedentary fan worms (Myxicola, Sabella, Branchiomma) the function is for withdrawing the feeding mechanisms into burrow – Errant polychaete (Neries) – are active surface predators – have complex eyes and sensaory tentacles and have a large and well differentiated brain, greater locomotory actions (withdrawal and creeping), side-side swimming and rotary motion of the parapodia • The giant fibres in Nereis are same as in Lumbricus but there is thickening of the lateral fibres (Fig 3.9c) – have extensive connections and closer associatin with motor axons • The 3 central fibres (median and paramedian) have control over parapodia while control of longitudinal muscles Is by lateral fibres • The division of labour has allowed for rapid but diverse locomotory control – important to a mobile predator• The brains of annelids and arthropods share broadly similar glomeruli – The relative sizes of glomeruli is related to differences in behavior
• Hanstrom (1928) compared glomeruli between actively hunting lycosids (wolf spiders) and web spinning agelenids (Fig. 3.10) – Lycosids – more extensive developed optic centres and corpora pedunculata – glomeruli for sensory association and visual memory – Agelenids – larger central body – associated with integration of preprogrammed behaviour• The anatomy thus reflects the diferent lifestyles of the two groups and their different demands on sensory integration and motor skill• A comparison across the insects shows a general assocaition between the size and structure of the corpora pedunculata and behaviour particularly to – Social organisation – Spatial complexity of foraging behaviour
Comparative Studies in Vertebrates• Lifestyles – Broad behavioural characteristics is correlated with brain – the relative sizes of the different parts of the brain (see Fig 3.8) • Otter example (See Fig 3.11) – as a group, otters have a range of foraging skills and manipulating food with fore paw – these skills vary with species – Sea otter (Enhydra lutris) – clawless – breaks shellfish against stone placed on its chest – Clawed relatives (Lutra canadensis – river otter; Pteroneura braziliensis – South American giant otter) use their fore paws in a much less specialised way – more emphasis on sensory information face and vibrissae • Radinsky (1968) showed differences in the cortex of the two groups (See Fig 3.11) – forelimb projection in cortex for handlers larger while sensory projections in cortex larger for face and vibrissae information
• Songs in birds range from simple notes to complexity and melody – song control system is due to discrete nuclei in the forebrain that is projected to the syrinx (vocal organ)• The nuclei are of 2 groups (See Fig 3.12a): – Higher vocal centre (HVC) and robustus archistrialis (RA) – role in song production – Area X and lateral magnocellular nucleus (l-MAN) – song acquisition• DeVoogd et al. (1993) – compared HVC and Area X in 45 songbird that differed in song complexity – repertoire size and number of syllable per song – using DNA phylogeny – Positive correlation between size of song repertoire and volume of HVC (Fig. 3.12b) – Species with larger repertoires had larger nuclei associated with control of song production
• Sex Differences in Brain Structure – Ns differs between males and females in relation to reproductive roles, ecology – many differences are also driven by hormones – mediated by sex differences of hormone receptors in the brain – In human there are 3 differences in structural architecture of the brains of males and females resulting in different sexual behaviour • First, Dimorphism in size of nucleus INAH-3 – 2 to 3X larger in males – packed with androgen sensitive cells – male typical sexual behaviour – High levels of androgens in women – male like assertive sexual behaviour, small breasts, low vocal pitch, hirsuteness – Controversially INAH-3 has been linked to male homosexuality (Levay, 1991 – studied males who died of AIDS and men who did not die of AIDS) » INAH-3 nucleus in homosexuals smaller than in heterosexual males and roughly the same size as that of women – this study is not conclusive 36
• Second, connection between the cerebral hemispheres – the corpus collosum, anterior commisure, and nerve fibre bundles are larger in women – greater empathy and emotional sensitivity. Women have greater connection between the 2 halves of the thalamus – important relay for sensory information to cortex• Third, ageing – men lose more brain tissue as they age and earlier in life – Tissue loss is in frontal and tempral lobe in men; hippocampus and parietal area for women – Accounts for personality and behaviour changes with age – Increased irritability for men and reduced memory and visual skills for women
• Ecology and Sexual Dimorphism in Brain Structure (long term selection pressure) – Sexes differ in general features and brain anatomy but specific differences can arise due to ecological selection pressure – eg. spatial memory and navigational skills (greater in males) – Males have larger home range or territories – greater spatial learning tasks. Males would have greater development of the brain related to spatial memory – the hippocampus (spatial awareness and navigation) – Studies show that males do have larger hippocampus (for birds and mammals) – there are exceptions however! 38
– Studies by Sherry et al. (1993) on brown headed cowbirds (Molothrus ater) – brood parasites • The females search for host nests and lay single egg in each nest – up to 40 eggs in each breeding season. Male cowbirds play no part. • On this basis Sherry et al. predicted that spatial abilities of females (locate and return to host nest) would show larger hippocampus than males – females have larger hippocampi than males (Fig. 3.13) • Sherry also compared with closely related non parasitic birds (control species – Red winged blackbird and the Grackle) • Sex differences in brain structures do not always reflect differences in associated behaviour.
– Clayton et al. (1997) however showed that sex difference in size of cowbirds hippocampus (female and male) was related to breeding season when birds are actively looking for host– The size of forebrain nuclei (controlling song) in canaries also show cyclical variation with season and thus song production– This suggest adaptive plasticity in brain development – the cowbirds and canaries are not alone!
• Brain Development and Experience – Some species of birds store food for later retreival – spatial memory required to retrieve items • Black capped chikadees (Parus atricapillus) scatter-hoard seeds and insects in bark and moss clumps over a wide area • Items are hidden individually and each location used only once and can retrieve the food items even after a month! (Hitchcock & Sherry, 1990) • Birds remember specific sites – the Clarks’s nutcracker (Nucifraga columbiana) can hide 9000 items and can find them 9 months later – birds really do remember specific sites - not using approximates of visual or odour cues to detect food about the area to search for food • Birds not only can remember what items they hide in different places but also can time the recovery of the items according to how fast they decay – episodic memory
– The region of the brain involved in the episodic memory is the hippocampus – so food hoarding species would have larger hippocampus than closely related species that that do not – this is the case • Size of hippocampus appears to be affected by the hoarding experience of individual birds • Clayton & Krebs (1994) experiment on marsh tits (Parus palustris) – divided birds into 2 groups – first group allowed to cache sunflower seeds and second not able to • They then examine brain of both groups – second group had smaller hippocampi (Fig 3.14 b) – experience of hoarding had profound effect on hippocampus development
• Macguire et al. (2000) used MRI to study the spatial learning in taxi drivers – Taxi drivers who had completed training had larger posterior hippocampus as compared to controls and increased with the length of time the drivers had been operating (Fig 3.14c)
Is Adaptive Behaviour “hard wired”?• In previous section – adaptive behaviour patterns can be linked to particular areas of the brain• Size of these areas reflect the relative importance of the behaviour in different species and individuals• Is there a particular component of the ns as the mechanism controlling a behaviour?• Do adaptive behaviours have neural circuits?• Answer is YES and it depends on the definition and complexity and on the nature of information processing within the ns
• Local versus distributed processing – Before 1990 – behaviour was thought to be mapped to simple neuronal circuits – local and self contained – Now it is thought as neural networks (different ways and different times) and distributed processing system (at immediate site and away – Connectionist view – even simple tasks may require 100 million cells or more (John et al., 1986)
• Reflexes – A simple form of behaviour – an automatic, quick, stereotype unit of behaviour in response to a stimulus (internal and external) – Can examine the relationship between behaviour and the functioning of specific neural circuits – eg., knee-jerk response and limb withdrawal from a painful stimulus (Reflex arc, Fig 3.15) – Limb withdrawal reflex involves • Simultaneously contract flexor muscles and relax the antagonistic extensor muscles so that limb can be pulled towards body • 1st – flexion reflex – as affected limb is flexed, has to use other muscles to steady itself (cross-extension reflexes) – in combination the 2 allow for emergency actions and locomotion • Stretch reflexes – to grade flexion and extension in antagonistic muscles so that limb is controlled in stages and not in one violent action
• There is no elaborate CNS in invertebrates but sensory and effector organs still communicate through the central nerve cord and ganglia. Eg.in the gill withdrawal response of sea hare Aplysia (a mollusc)• The gills are retracted into the mantle cavity in response to weak mechanical stimulation of the siphon• The reflex arc involes – excitatory and inhibitory interneurons relaying sensoty information from siphon to motor neurons in the abdominal ganglia which causes withdrawal of gill (Fig 3.16)