INTRODUCTION TO PSYCHOLOGY ESSAY I, TITLE I: "Outline the path the neural signals take when you see a cup of coffee on the table and pick it up. What pathway (s) is (are) involved when you drop it because it's too hot?" J.D. GRIFFITHS UNIVERSITY OF WARWICK ESSAY I, TITLE I: "Outline the path the neural signals take when you see a cup of coffee on the table and pick it up. What pathway (s) is (are) involved when you drop it because it's too hot? " This situation describes two distinct types of neural response to a stimulus. The first involves the faculties of i) sense (specifically visual) perception; ."..

See a cup of coffee... ". and ii) information processing, or cognition. ".. pick it up... ". , - which necessarily implies an aspect of decision making following the perceptual event - resulting in voluntary movement through the formulation and execution of an suitable motor programme for the task. The second type of process mentioned is a reflex action, .".. drop it because it's too hot... ". ; an involuntary muscular contraction, acting as a natural defensive reaction to an immediately harmful situation (in this case extreme heat). Chronological Analysis of events: Seeing and picking up the cup The image of the coffee cup is translated into coded action potentials and directed to the dorsal and ventral streams in the brain like so: Photons, units of light radiation, are reflected by object surfaces into the eye to create the visual field. These collide with photosensitive rhodopsin and ops in pigment molecules in rod and cone cells respectively in the retina (the groundbreaking study of which earned George Wald the 1967 Nobel Prize in physiology or medicine) (Rozenweig et al, 1999).

Excitation and subsequent molecular alteration of the pigments causes an opening of membrane protein channels, allowing an influx of sodium ions. This depolarization establishes a generator potential sufficient to initiate an action potential in synapsing bipolar cell (s), which in turn synapse with retinal ganglia. From thereon, the coded image of the coffee cup to is transmitted throughout the brain via the geniculostriate system (primary visual pathway) or the tectopulvinar system (secondary visual pathway) (Churchland, 1989). The 4 million rods and 100 million cone cells respond to a constant barrage of particles (just 3 W of visible light equates to 8 quintillion photons per second) (Rozenweig, 1999) - which amass on the retina, eliciting neural activity sufficient to imprint a detailed, streaming visual image in the brain. Ganglionic axons from the left and right temporal hemiretinae are kept separate as they converge (around 1 million in total - organized into 800-1200 fascicles) on the optic disk, penetrate the sclera at the cribriform plate, and leave the eye bound in a pal sheath; collectively termed the optic nerve (Nobak, 1967). Sensory inputs from like temporal hemispheres of each eye are united as the optic nerves cross within the middle cranial fossa at the optic chiasma.

They leave here as the optic tracts - each accounting completely and solely for one side of the visual field, and directing the information the (known) visual pathways: Secondary visual pathway Those retinal axons not following the primary visual pathway are routed, via the optic tract, to the superior col loculus in the midbrain as part of the Tec to Pulvinar System. Cells in the deepest layers of this area form visual, somatosensory and auditory maps, which help create an 'orientating response' to input from visual and other sense modalities. Axons then head to an area of the thalamus known as the pulvinar, and also to motor systems within the brain. The function of this system is to sustain a stable visual picture of the coffee cup on the table.

Primary visual pathway In this, the geniculostriate system, the majority (some 70 to 80 percent) of retinal axons are carried in the optic tracts to the thalamus, terminating in the ipsilateral geniculate bodies. Information is then directed down the highly interconnected dorsal and ventral streams to temporal and parietal lobes respectively: Some axons of postsynaptic cells ('optic radiations') in the lateral geniculate nuclei (LG) take information to the striate / primary visual cortex in the occipital region, where information is passed through the ventral stream, inferior temporal lobe, and on to the ex striate / higher visual cortex (Nobak, 1967). Information reaching here is integrated into the indivudal's conscious perception, allowing them to relate this to their immediate observations, and their general past experiences of the world. Axons comprising the dorsal stream pass to the parietal lobe, where qualities of the image are determined such as location, depth, movement and spatial features. This stream directs information to the prefrontal cortex, where sequences of movements are planned, and later organized in the pre motor cortex. The actions needed to pick up the cup are now being assembled, and the serial order of these sequences are planned in the primary motor cortex.

These fully coded motor programmes are now delivered, via the corticospinal tract, into the spinal cord. Major movements (e.g. extension of the arm) and subtle ones (such as finger muscles contractions) are directed to their respective effector muscles, and the action is performed - the coffee cup is picked up. The visual image, and information from other sense modalities is in a constant state of evaluation and adaptation to the situation (Winters & Crago, 2000), following pathways described above. Dropping the cup The instant withdrawal action that occurs when the unfortunate individual in question drops the coffee cup, recoiling from the intense heat is a flexor reflex response. This is initiated through the actions of the di synaptic, ipsilateral and intersegmental (describing neurons whose axons bifurcate into ascending or descending branches, traversing the white matter of many spinal segments before arborizing and terminating in the grey matter) reflex arc that governs the response. This structure has (in simple terms) five components: i) sensory receptor (free nerve endings in the skin detect heat in this case), ii) afferent neuron, ) spinal intersegmental neuron, iv) alpha motor neuron and v) effector organ (i.e. Biceps brachialis) (Nobak, 1967).

The arc will in fact be composed of bundles, rather than singular nerve cells, and the branching interneurons within the spinal cord will relay the information to other neural pathways around the body. The afferent neurons form a heterogenous group known as flexor reflex afferents (FR As), reacting to stimuli at receptor endings in skin, joints, muscles and viscera - and activating circuits throughout the spinal cord with their numerous poly synaptic connections. The resultant contractile effects of the reflex arc and any dispersive stimulation are excitation of the agonist muscle, and inhibitory signals being sent to the antagonist. Depending on the strength of the stimulus in these situations, and the excitability of circuits in the spinal cord, further muscular responses may be triggered via contralateral afferents, as stimulation spreads across excitable nerve pathways to the other side of the body.

This accounts for 'recoiling' movements as the cup of coffee is released, as parallel contractions serve to support and balance the body. The action of the flexor reflex arc pathway and its expanding dispersion of action potentials thus accounts for the withdrawal and recoil response to the hot cup of coffee. Further consequences of the action will inevitably involve perception and evaluation of the action (involving cognition, memory and emotion), any and all reactions - such as voluntary muscle contractions (exclamations and expletives for instance), and the instant or gradual experience of other sensations, such as wetness and coffee smells.

Bibliography

Nobak, 1967 - The human nervous system, basic principles of neurobiology Churchland, 1989 - Neurophilosophy Rozenweig et al.
1997 - Biological Psychology Winter & Crago, 2000 - Biomechanics and Neural control of posture and movement Bibliography Nobak, 1967 - The human nervous system, basic principles of neurobiology Churchland, 1989 - Neurophilosophy Rozenweig et al.
1997 - Biological Psychology Winter & Crago, 2000 - Biomechanics and Neural control of posture and movement.