Sensation, Movement, and Behaviour

Sensation as Input for Behaviour

All behaviour begins with sensory input. Animals need to detect what is happening in their environment before they can respond. The basic flow is: input (sensing) → integration (brain processing, memory, internal state) → output (motor response/behaviour). Sensing evolved alongside movement – as animals became mobile, they needed increasingly refined senses to respond to a rapidly changing environment.

Four Stages of Sensing

Reception: detection of a stimulus by specialised receptor proteins. Transduction: converting external energy into an internal electrical signal (change in membrane potential). Transmission: encoding the signal as action potentials and sending it to the brain via afferent neurons. Perception: the brain filtering and interpreting sensory information so the animal can respond appropriately. Not all incoming sensory data reaches conscious perception – irrelevant signals are filtered out (e.g., you stop noticing the feeling of your shoes until attention is drawn to it).

Primary Sensory Neurons vs Secondary Sensory Cells

Primary sensory neurons directly detect the stimulus and generate action potentials themselves (e.g., touch, pain, temperature, smell receptors). Secondary sensory cells detect the stimulus but do not fire action potentials – instead they release neurotransmitter onto an afferent neuron (e.g., photoreceptors in vision, hair cells in hearing and balance, taste receptor cells). General pattern: distributed receptors (spread across the body, like skin) tend to be primary; localised sensory epithelia (concentrated in one organ, like the retina or cochlea) tend to use secondary cells.

Receptor Proteins: Channels vs Amplifiers

Channel-type receptors (ionotropic): the receptor protein is itself an ion channel that opens directly when stimulated (e.g., mechanoreceptors for touch). Fast, direct response. Amplifier-type receptors (metabotropic/G-protein coupled): the receptor activates a G-protein signalling cascade that eventually opens channels. Slower but amplifies weak signals. Small stimuli (e.g., a single photon) need amplification; large stimuli (e.g., touch) can use direct channels.

Major Classes of Sensory Receptors

Mechanoreceptors: detect membrane stretch/pressure (touch, hearing, proprioception). Chemoreceptors: detect chemicals (smell, taste, blood O₂/glucose). Photoreceptors: detect electromagnetic radiation/light (vision, circadian rhythms). Thermoreceptors: detect temperature changes. Nociceptors: detect tissue damage (leading to pain perception). Electroreceptors: detect electric fields (electrolocation in fish). Magnetoreceptors: detect magnetic fields (navigation in birds, turtles). Hygroreceptors: detect humidity (used by insects like mosquitoes).

Afferent vs Efferent Neurons

Afferent neurons carry sensory information from the periphery into the CNS (input). Efferent neurons carry motor commands from the CNS to muscles and organs (output). A nerve is a bundle of axons and can contain both afferent and efferent fibres.

Specialisation of Touch Receptors

Skin contains multiple types of primary sensory neurons with specialised structures. Free nerve endings detect temperature and pain (close to skin surface). Pacinian corpuscles detect deep pressure and vibration (deep in dermis, with layered supportive cells). Meissner’s corpuscles detect light touch. Ruffini endings detect skin stretch. Hair follicle receptors detect hair deflection. Each type is specialised for a different aspect of the touch stimulus.

Stimulus

Any form of energy in the environment that can be detected by a sensory receptor (e.g., light, sound, pressure, chemicals).

Reception

The initial detection of a stimulus by specialised receptor proteins in sensory cells.

Transduction

The conversion of external stimulus energy into an internal electrical signal (change in membrane potential) by sensory cells.

Transmission

The encoding of sensory information as action potentials and sending it along afferent neurons to the brain.

Perception

The brain’s filtering and interpretation of sensory signals, resulting in conscious awareness or an appropriate behavioural response.

Primary sensory neuron

A neuron that directly detects a stimulus via receptor proteins and generates its own action potentials.

Secondary sensory cell

A non-neuronal cell that detects a stimulus and releases neurotransmitter onto an afferent neuron, but does not itself fire action potentials.

Afferent neuron

A sensory neuron carrying information from the periphery into the CNS.

Efferent neuron

A motor neuron carrying commands from the CNS out to muscles or glands.

Mechanoreceptor

A receptor protein or cell that detects mechanical force such as pressure, stretch, vibration, or sound.

Chemoreceptor

A receptor that detects the presence of specific chemicals (taste, smell, blood chemistry).

Photoreceptor

A receptor that detects light/electromagnetic radiation (vision, circadian light detection).

Thermoreceptor

A receptor detecting changes in temperature.

Nociceptor

A receptor detecting tissue damage; the initial signal leading to pain perception.

Electroreceptor

A receptor detecting electric fields; used for electrolocation in some fish.

Magnetoreceptor

A receptor detecting magnetic fields; used for navigation by birds, turtles, and some fish.

Hygroreceptor

A receptor detecting humidity; found primarily in insects.

Signal cascade

A series of intracellular molecular events triggered by a G-protein coupled receptor that amplifies the initial signal.

Pacinian corpuscle

A deep touch receptor with layered supportive cells; specialised for detecting vibration and deep pressure.

Intensity Coding with Action Potentials

Sensory neurons encode stimulus intensity by changing their firing rate: stronger stimulus = higher frequency of action potentials (rate coding). Some neurons have a baseline firing rate and encode stimulation by pausing (decreasing rate). Secondary sensory cells encode intensity by releasing more or less neurotransmitter. The magnitude of individual action potentials does not change – it is always all-or-nothing.

Sensory Adaptation

Sensory receptors change their responsiveness to a constant stimulus over time. Rapidly adapting receptors (e.g., Pacinian corpuscle) respond only at the onset and offset of a stimulus – they detect changes. Slowly adapting receptors (e.g., Ruffini endings) maintain their response for the duration of a stimulus – they encode sustained contact. Adaptation conserves energy and draws attention to new or changing stimuli rather than constant ones. Example: we stop feeling a watch on our wrist after minutes (rapid adaptation of touch receptors).

Receptive Fields

Each sensory neuron responds to stimuli from a specific, limited area of the stimulus space (e.g., a small patch of skin, a spot in the visual field). Smaller receptive fields = finer spatial resolution (e.g., fingertips have very small receptive fields for precise touch). Larger receptive fields = lower precision but broader detection (e.g., back skin, peripheral vision). Receptive fields allow the brain to localise where a stimulus occurred.

Magnetoreception and Navigation

Many animals (birds, sea turtles, some fish) detect the Earth’s magnetic field for navigation. The magnetic field has a location-specific signature (not just north/south) based on field strength and direction. Bar-tailed godwits use magnetoreception to migrate over 10,000 km from Alaska to New Zealand without landing. Loggerhead turtles can learn to associate magnetic signatures with reward – shown experimentally via ‘turtle dancing’ when the rewarded magnetic field is applied.

Vision: Eye Structures

Vertebrate camera-type eye: lens focuses light onto the retina (sheet of photoreceptors at the back of the eye). Muscles change lens shape to focus at different distances. Pupil controls how much light enters. Optic nerve carries signals from retina to brain; creates a blind spot where there are no photoreceptors. Compound eye (arthropods): many ommatidia, each with its own lens and 6–8 photoreceptors. Different structure but same function of detecting spatial patterns of light. Convergent evolution: mammalian eyes and octopus eyes are structurally similar despite evolving independently.

Photoreceptors: Rods and Cones

Rods: larger outer segments, more photoreceptor proteins (rhodopsin), more sensitive to low light – specialised for night/dim-light vision. Cannot distinguish colour. Cones: three types in primates (short/blue, medium/green, long/red wavelength), each containing a different opsin protein sensitive to different wavelengths. Less sensitive but enable colour vision in bright light. Colour perception arises from comparing activity across all three cone types (triangulation). Rods are saturated in daylight and ignored; cones are inactive in very dim light (we see only in greyscale at night).

Rate coding

Encoding stimulus intensity by changing the frequency of action potentials; more intense stimulus = higher firing rate.

Sensory adaptation

The decrease (or change) in a sensory neuron’s response to a constant, unchanging stimulus over time.

Rapidly adapting

A sensory neuron that responds mainly to the onset/offset of a stimulus and quickly stops responding to a sustained stimulus (e.g., Pacinian corpuscle).

Slowly adapting

A sensory neuron that maintains its firing throughout a sustained stimulus (e.g., Ruffini endings for skin stretch).

Receptive field

The specific region of stimulus space (e.g., area of skin, spot in visual field) that a single sensory neuron responds to.

Magnetoreception

The ability to detect the Earth’s magnetic field; used for navigation by birds, sea turtles, and some fish.

Bar-tailed godwit (kuaka)

A migratory bird that flies over 10,000 km from Alaska to New Zealand using magnetoreception; a NZ example of long-distance navigation.

Compound eye

An eye composed of many ommatidia, each with its own lens and photoreceptors; found in arthropods.

Ommatidium (pl. ommatidia)

A single optical unit of a compound eye, containing a lens (facet) and a small cluster of photoreceptors.

Rod

A photoreceptor with high sensitivity to light (large outer segment, many rhodopsin molecules); specialised for dim-light vision; cannot distinguish colour.

Cone

A photoreceptor with lower sensitivity but colour discrimination; three types in primates (short, medium, long wavelength opsins).

Opsin / Rhodopsin

Photoreceptor proteins embedded in the membranes of rods and cones; change conformation when a photon is absorbed, initiating a signal cascade.

Dark current

The depolarised, neurotransmitter-releasing state of photoreceptors in the dark; light causes hyperpolarisation and a decrease in neurotransmitter release.

Trichromatic vision

Colour vision based on three types of cone photoreceptors; characteristic of primates.

Colour blindness

Loss or mutation of one cone type (usually medium/green, X-linked); causes confusion between certain wavelengths. More common in XY individuals.

Convergent evolution

Independent evolution of similar structures in unrelated lineages (e.g., mammalian eye and octopus eye).

Fovea and Primate Vision

Primates have a specialised region of the retina called the fovea with very high cone density, providing maximal visual acuity (sharpness) at the centre of gaze. Rods dominate the peripheral retina – periphery is better for detecting motion and has superior night vision but lower acuity. Primates constantly make eye movements (saccades) to place objects of interest on the fovea. Other animals have analogous structures: artiodactyls (sheep, deer) have a horizontal visual streak matching their flat grassland environments and wide-set eyes (270° field of view).

Ascending Sensory Pathway

Sensory signals ascend through a series of relay neurons to reach the cortex. For touch: (1) primary sensory neuron (in skin) → (2) second-order neuron in the spinal cord (crosses to the contralateral side) → (3) third-order neuron in the thalamus → (4) primary somatosensory cortex. Almost all senses pass through the thalamus before reaching cortex. Signals cross to the opposite side: the left brain processes sensory information from the right side of the body and vice versa (contralateral processing).

Primary Sensory Cortices

Each sense has its own primary sensory cortex – the first cortical area that receives that sensory information. Primary somatosensory cortex: touch, pain, temperature, proprioception. Primary visual cortex (V1): at the back of the brain (occipital lobe). Primary auditory cortex: near the temporal lobe. Primary olfactory cortex: smell. The relative size of each primary sensory cortex varies across species depending on how important that sense is (e.g., rodents have large somatosensory cortex; primates have large visual cortex).

Cortical Magnification

Brain areas expand in size when a sense becomes more important to a species. Duck-billed platypus: huge cortical area devoted to the bill (electroreception and mechanoreception). Star-nosed mole: expanded nose representation. Raccoons: expanded hand area. Rats: expanded whisker barrel cortex. This is a direct relationship between the ecological importance of a sense and the neural resources allocated to it.

Association Cortex and Higher-Order Processing

Beyond primary sensory cortex, information is passed to higher-order (association) cortex where it becomes increasingly abstract. In vision: the dorsal stream (‘where/how’ pathway) processes location, motion, and how to interact with objects. The ventral stream (‘what’ pathway) processes object identity and recognition. Humans have proportionally more association cortex than other mammals, enabling more complex perception and behaviour.

The Sensory-Motor Hierarchy

Sensory information ascends through progressively more abstract cortical areas (primary → association → prefrontal cortex). The prefrontal cortex integrates sensory information with context and internal state to form a plan. The motor system then descends: prefrontal cortex (abstract plan) → premotor/supplementary motor areas (organise the plan) → primary motor cortex (specific muscle commands) → spinal cord → muscles. The sensory and motor hierarchies are interconnected at multiple levels, including attention (motor areas directing sensory focus).

Perception and the Prefrontal Cortex

Perception occurs when processed sensory signals reach the prefrontal cortex and enter awareness. Neural recordings show that the same stimulus can activate primary sensory cortex without being perceived – it is only perceived when there is strong corresponding activity in the prefrontal cortex. This is an important distinction: sensation ≠ perception.

Fovea

A small, cone-dense region at the centre of the primate retina providing maximum visual acuity.

Saccade

A rapid, voluntary eye movement that repositions the fovea onto an object of interest.

Visual streak

A horizontal band of high photoreceptor density in artiodactyl retinas; an adaptation for scanning flat, open environments.

Thalamus

A midbrain relay station through which almost all sensory information passes before reaching the cortex.

Contralateral processing

The principle that sensory information from one side of the body is processed by the opposite cerebral hemisphere.

Primary sensory cortex

The first cortical area receiving input for a given sense (e.g., V1 for vision, S1 for somatosensory).

Association cortex

Higher-order cortical areas that process sensory information beyond initial detection; involved in recognition, interpretation, and integration.

Cortical magnification

The expansion of a cortical area in proportion to the ecological importance of the sense it processes.

Dorsal stream

The ‘where/how’ visual pathway; processes spatial location, motion, and interaction with objects.

Ventral stream

The ‘what’ visual pathway; processes object identity and recognition.

Sensory-motor hierarchy

The ascending sensory and descending motor processing chain from receptors through cortex to muscles, connected at the prefrontal cortex.

Prefrontal cortex

The anterior-most cortical area; integrates sensory information with context and internal state for planning and conscious perception.

Cortical column

A vertical unit of cortex containing neurons that process the same region of stimulus space; the basic processing module of cortex.

Striated vs Smooth Muscle

Striated (skeletal) muscle: contracts in one dimension along a single axis; faster and stronger; attached to bones via tendons; under voluntary (conscious) control; responsible for body movement. Smooth muscle: contracts multi-directionally (the whole cell shrinks); slower; lines internal tubes (GI tract, blood vessels, airways); mostly involuntary; responsible for peristalsis, blood flow regulation, and sphincter control. Cardiac muscle: a specialised type of striated muscle with branching fibres and gap junctions; involuntary; heart only.

Muscle Organisation (Zooming In)

Whole muscle → fascicles (bundles of muscle fibres wrapped in connective tissue) → muscle fibres (individual cells, very long, multinucleated from fused precursor cells) → myofibrils (organelles filling the cell, the contractile machinery) → sarcomeres (the fundamental contractile units, arranged in series between Z-lines) → thick filaments (myosin) and thin filaments (actin).

The Sarcomere and Contraction:

Sarcomeres are bounded by Z-lines. Thick filaments (myosin) extend from the centre; thin filaments (actin) extend from the Z-lines. Contraction occurs by the sliding filament mechanism: myosin heads bind to actin, perform a power stroke pulling the actin inward, then detach and reset. Each cycle requires ATP: one ATP to detach the myosin head, one to cock it for the next power stroke. Contraction is initiated by Ca²⁺ influx (triggered by an action potential from the motor neuron).

Motor Units

A motor unit is a single motor neuron plus all the muscle fibres it innervates. When the motor neuron fires, all muscle fibres in that unit contract together (all-or-none at the motor unit level). Small motor units (1–50 fibres): found in muscles requiring fine control (e.g., eye muscles, hand muscles). Large motor units (hundreds of fibres): found in muscles requiring gross force (e.g., quadriceps). More motor units recruited = stronger contraction.

Antagonistic Muscle Pairs

Every skeletal movement requires at least two muscle groups working in opposition across a joint (fulcrum). Flexor: decreases the joint angle (e.g., bicep). Extensor: increases the joint angle (e.g., tricep). In any movement, the agonist is the muscle dominating the movement and the antagonist is the opposing muscle providing controlled resistance. These roles switch depending on the direction of movement (e.g., bicep is agonist during flexion, antagonist during extension). Both muscles are always active to some degree – the antagonist prevents uncontrolled movement.

Insect Flight Muscles

Direct flight muscles: muscles attach directly to the wings and pull them up and down (e.g., dragonflies). Analogous to vertebrate endoskeletal muscle. Indirect flight muscles: muscles do not attach to wings – instead they change the shape of the thorax (exoskeleton), which moves the wings. Dorsoventral muscles pull the thorax top down (wings go up); longitudinal muscles scrunch the thorax front-to-back (wings go down). More energy-efficient; exploits exoskeleton elasticity.

Smooth Muscle Functions

Lines internal tubes and organs. Peristalsis: wave-like contractions moving contents along tubes (GI tract, blood vessels, reproductive tract, urinary system). Segmentation: rhythmic contractions mixing contents without propulsion (digestive system). Sphincters: rings of smooth muscle controlling passage between compartments (e.g., capillary bed sphincters directing blood flow).

Striated / Skeletal muscle

Voluntary muscle attached to bones; contracts in one axis; composed of muscle fibres, myofibrils, and sarcomeres.

Smooth muscle

Involuntary muscle lining internal organs and tubes; contracts multi-directionally; responsible for peristalsis and sphincter control.

Cardiac muscle

Involuntary striated muscle of the heart; has branching fibres connected by gap junctions for coordinated contraction.

Fascicle

A bundle of muscle fibres wrapped in connective tissue within a whole muscle.

Muscle fibre

A single, very long, multinucleated muscle cell; the cellular unit of skeletal muscle.

Myofibril

A cylindrical organelle within a muscle fibre; composed of repeating sarcomeres; the contractile machinery.

Sarcomere

The fundamental contractile unit of striated muscle, bounded by Z-lines; contains overlapping thick (myosin) and thin (actin) filaments.

Z-line

The boundary structure between adjacent sarcomeres; the anchor point for thin (actin) filaments.

Myosin (thick filament)

A motor protein with heads that bind actin and perform the power stroke; the active component of contraction.

Actin (thin filament)

A structural filament that myosin heads pull on during contraction.

Power stroke

The conformational change of the myosin head that pulls the actin filament toward the centre of the sarcomere, shortening it.

Sliding filament mechanism

The model of muscle contraction in which myosin heads walk along actin filaments, pulling them inward without the filaments themselves shortening.

Motor unit

A single motor neuron and all the muscle fibres it innervates; the smallest controllable unit of muscle contraction.

Neuromuscular junction

The synapse between a motor neuron and a muscle fibre; functionally similar to a neuronal synapse.

Antagonistic pair

Two muscle groups that act in opposition across a joint to produce controlled movement (e.g., bicep and tricep).

Flexor

A muscle that decreases the angle at a joint.

Extensor

A muscle that increases the angle at a joint.

Agonist

The muscle producing the dominant force in a movement; determines the direction.

Antagonist

The opposing muscle in a movement; provides controlled resistance to the agonist.

Dermatome

A region of skin innervated by a single spinal nerve root; used clinically to localise nerve damage.

Innate (Instinctual) Behaviour Defined

Behaviour that is inherited, not learned. Four defining criteria (Tinbergen): (1) does not require prior experience or practice; (2) is released by specific external cues (sign stimuli); (3) is consistent – the same stimulus produces the same behaviour every time; (4) depends on an internal drive state that gates whether the behaviour is released. Example: male stickleback fish attack anything with a red underside (sign stimulus), even crude models, but only during breeding season (internal drive).

Fixed Action Patterns (FAPs)

A specific subtype of innate behaviour: a precise, stereotyped sequence of movements that is triggered by a sign stimulus and runs to completion once started. Examples: eared moths perform an evasive flight manoeuvre every time they detect bat echolocation chirps. Eigenmannia (knifefish) automatically shifts its electric organ discharge frequency to avoid jamming from a neighbouring fish’s similar frequency. Human newborns preferentially look at upright face-like configurations. FAPs are like a lock-and-key mechanism: a very specific stimulus unlocks a very specific behaviour.

Habituation and Sensitisation

Two fundamental forms of non-associative learning found in all animals. Habituation: decreasing response to a repeated, harmless stimulus. Conserves energy by preventing unnecessary reactions. Example: Aplysia (sea hare) gill-withdrawal reflex diminishes with repeated gentle touch. Sensitisation: increasing response to a stimulus after pairing with a negative/aversive event. Heightens awareness of potentially harmful stimuli. Example: Aplysia gill withdrawal increases after a mild tail shock is paired with touch. Both break animals out of potentially maladaptive behavioural loops.

Internal Drives and Homeostasis

Internal drives are motivational states that push an animal toward behaviours satisfying physiological needs. Each drive has a homeostatic set point: the ideal level (e.g., caloric intake, hydration, body temperature, rest, social interaction). The farther the animal’s current state is from the set point, the stronger the drive. Multiple drives compete simultaneously (hunger vs sleep vs social needs). Internal drives gate instinctual behaviours: a cheetah only chases prey when hungry enough; rams only fight during breeding season.

The Hypothalamus as Drive Controller

The hypothalamus contains distinct nuclei, each monitoring a specific physiological need and controlling the corresponding internal drive. Key nuclei control: sleep, thirst/osmoregulation, temperature, feeding/reward, stress, reproduction, circadian rhythms, social behaviour. Nuclei compete via mutual inhibition – the most urgent drive suppresses others. They also work together: lactating mammals upregulate both feeding drive and parental drive simultaneously.