Nervous System Evolution, Sensation and Behavior in Invertebrates• Evolution of the central nervous system• Sensory perception• BehaviorTrends in Nervous System Evolution• Nerve nets• Paired nerve cords (3-4 pairs)• Reduction in number of nerve cords• Dominance of ventral pair of nerve cords• Increasing size of cerebral ganglia and formation of a “brain”• Dorsal nerve cord of chordates • Nerve nets (Cnidaria, Ctenophora, Echinodermata)Nervous systems show diverse patterns of organizationPlatyhelminthes (flatworm) Nervous systemsFrom multiple pairs of nerve cords to a single pair located ventrally• The insect nervous system consists of a pair of ventral nerve cords with several segmental ganglia.– The two chords meet in the head, where the ganglia from several anterior segments are fused into a cerebral ganglion (brain).– This structure is close to the antennae, eyes, and other sense organs concentrated on the head. Cephalization - concentration of sensory apparatus and nervous system anteriorly)Nervous System in Sea SquirtsPhylum Chordata – Subphylum Urochordata – Class AscidiaceaSensory receptors transducestimulus energy and transmit signals to the nervous system• Sensory reception begins with the detection of stimulus energy by sensory receptors.– Exteroreceptors detect stimuli originating outside the body.– Interoreceptors detect stimuli originating inside the body.– Sensory receptors convey the energy of stimuli into membrane potentials and the transmit signals to the nervous system.• This involves: sensory transduction, amplification, transmission, and integration.• Sensory Transduction– The conversion of stimulus energy into a change in membrane potential.– Receptor potential: a sensory receptor’s version of a graded potential.• Amplification– The strengthening of stimulus energy that is can be detected by the nervous system.• May be a part of, or occur apart from, sensory transduction.• Transmission.– The conduction of sensory impulses to the CNS.– Some sensory receptors must transmit chemical signals to sensory neurons.– Some sensory receptors are sensory neurons.•Integration–The processing of sensory information.•Begins at the sensory receptor.–For example, sensory adaptation is a decrease in responsiveness to continued stimulation.–For example, the sensitivity of a receptor to a stimulus will vary with environmental conditions.Sensory receptors are categorized by the type of energy they transduce• Mechanoreceptors respond to mechanical energy.– For example, muscle spindles is an interoreceptor that responds to the stretching of skeletal muscle.– For example, cilia detect motion.• Thermoreceptors respond to heat or cold.• Electromagnetic receptors respond to electromagnetic energy.– Photoreceptors respond to the radiation we know as visible light and UV radiation.• Chemoreceptors respond to chemical stimuli.– General chemoreceptors transmit information about total solute concentration.– Specific chemoreceptors respond to specific types of molecules.– Internal chemoreceptors respond to glucose, O2, CO2, amino acids, etc.– External chemoreceptors are gustatory receptors and olfactory receptors.Chemo/mechano-sensitive hair in a spider (Arthropoda: Arachnida)From Foelix (1996)• Statocysts are mechanoreceptors that function in an invertebrates sense of equilibrium.– Statocysts function is similar to that of the mammalian utricle and saccule.Many invertebrates have gravity sensorsSensory apparatus in Cnidaria• Ocellus or pigment spot in groups other than Hydroids• Complex eyes with a lens in the box jellies• Statocysts that function to allow animal to maintain orientationAurelia sp. (Cnidaria: Scyphozoa)Details of Rhopalium in ScyphozoaSensory Apparatus in Ctenophora• Statocyst that functions to coordinate beat of cilia on comb rows to coordinate locomotion and feeding• A planarian has a head with a pair of eyespots to detect light and lateral flaps that function mainly for smell.• The planarian nervous system is more complex and centralized than the nerve net of cnidarians.– Planarians can learn to modify their responses to stimuli.A diversity of photoreceptors has evolved among invertebrates• Eye cups are among the simplest photoreceptors– Detect light intensity and direction — no image formation.– The movement of a planarian is integrated with photoreception.• Image-forming eyes.– Compound eyes of insects and crustaceans.• Each eye consists of ommatidia, each with its own light-focusing lens.• This type of eye is very good at detecting movement.• Single-lens eyes of invertebrates such as jellies, polychaetes, spiders, and mollusks.– The eye of an octopus works much like a camera and is similar to the vertebrate eye.• Cephalopods use rapid movements to dart toward their prey which they capture with several long tentacles. – Squids and octopuses have a well developed single-lens eye, are nearsighted, and have color visionVertebrates have single-lens eyes• Is structurally analogous to the invertebrate single-lens eye.• Arthropods have well-developed sense organs, including eyes for vision, olfactory receptors for smell, and antennae for touch and smell.– Most sense organs are located at the anterior end of the animal, showing extensive cephalization.Some Butterflies can detect UV Radiation reflected by plants and other insects• Sound sensitivity in insects depends on body hairs that vibrate in response to sound waves.– Different hairs respond to different frequencies.• Many insects have a tympanic membrane stretched over a hollow chamber.Ear of a Noctuid moth (Lepidoptera: Noctuidae)• Taste receptors in insects are located on their feet.Perceptions of taste and smell are usually interrelatedBehavior stimulated by Sensory Information often requires CNS Integration• Feeding responses in insects are not reflex, CNS processes chemosensory data prior to behavior• Removal of particular chemosensory cells does not necessarily alter feeding responses• CNS integrates chemosensory information from a variety of cells that respond to stimulate or inhibit feedingProcessing and Integration of sensory information provides a basis for complex behavior•Foraging• Predator Avoidance• Mate location• Mate attraction• Mate choice• Kin recognition• Relocation of nests sites• Migration• Deception• Territoriality •