Detecting and responding to stimuli (8.2, 8.5, 8.6, 8.7)

The mammalian nervous system coordinates responses to internal and external stimuli. It is organised in the following way:

• The Central Nervous System (CNS) consists of the brain and spinal cord. It is responsible for processing, integrating, and coordinating responses.
• The Peripheral Nervous System (PNS) is made up of all neurones outside the CNS (sensory and motor neurones). It connects the CNS to the rest of the body and can be divided into the sensory (afferent) and motor (efferent) pathways.

Most nerve pathways involve multiple neurones in the CNS (central nervous system).

An individual sensory neurone connects to multiple CNS neurones and the brain must coordinate a response.

Motor neurones cause the response to the stimulus by connecting the CNS with effectors, such as muscles.

A reflex is an involuntary response to a sensory stimulus (e.g., blinking).

Reflexive actions are faster than actions done under conscious thought and increase the chances of survival.

Nerve pathways responsible for reflexes are called reflex arcs.

The pupil reflex controls light entering the eye; this is part of the autonomic nervous system.
Antagonistic muscles in the iris control pupil size:

• When circular muscles relax, dilating the pupil. This is controlled by a reflex associated with the parasympathetic nervous system.
• When the radial muscles relax, the pupil constricts; this is mediated by a sympathetic nervous system reflex.

The pupil reflex is a rapid, involuntary response to changes in light intensity.

Light–sensitive cells in the retina send impulses along the optic nerve to the CNS. The brain then sends impulses along motor neurones to the muscles in the iris, causing them to change the pupil size and therefore the amount of light entering the eye.

In intense light the parasympathetic motor neurones make the circular muscles contract and the sympathetic motor neurones make the radial muscles relax. This causes pupil constriction.

This reflex protects the retina from excessive light and ensures optimal vision.

The retina is made up of three layers of cells:

• Rod and cone cells: photoreceptors that detect light.
• Bipolar neurone cells: help transmit visual information to the brain.
• Ganglion neurones: which bundle together to form the optic nerve.

Each layer has a synapse with the next.

Photoreceptors detect light.

Rod cells are found outside the centre of the retina; they are able to distinguish between low and bright light. They detect light intensity, but not colour.

Cone cells are concentrated in the centre of the retina; they can distinguish colour and are only effective in bright light.

Rhodopsin is found in rod cells and is a photochemical pigment that detects light.

Pigment molecules are found in the membranes of flattened vesicles in the outer segment of the rod cells.

In dark conditions, the following occurs in rod cells:

• Non-specific cation channels are open, allowing to enter the outer segment of rod cells. They flow down a concentration gradient into the inner segment.
• Glutamate, a neurotransmitter, is released from rod cells due to the potential difference being slightly less negative, leading to slight depolarisation.
• At an inhibitory synapse, the glutamate stops the bipolar cell from depolarising.

In the light, the following occurs in rod cells:

• Rhodopsin is broken down into opsin and retinal.
• Opsin triggers a set of reactions that break down a second messenger, which closes cation channels.
• Hyperpolarisation occurs as the inner segment pumps out sodium ions, causing the potential difference to become negative.
• The release of glutamate stops, bipolar cells and the optic nerve become depolarised. An action potential is triggered.

After rhodopsin is broken down, it is important that it is reformed.

A higher light intensity causes more rhodopsin to be broken down, meaning that it takes longer for rhodopsin to reform

The period where rhodopsin is reformed is known as dark adaptation.

Animals use two main systems to respond to internal and external stimuli:

• The endocrine system uses hormones as chemical messengers transported in the blood, and they are usually slower but longer-lasting.

• The nervous system uses electrical impulses to produce rapid, short-term responses (e.g., muscle movement).

These systems work together to maintain homeostasis and allow the body to respond effectively to stimuli.

Plants must respond to changing environmental conditions to optimise growth.

• A positive tropic response is when a plant grows towards a stimulus.

• A negative tropic response is when a plant grows away from a stimulus.

There are many types of tropism each resulting from a different environmental stimulus:

• Light triggers phototropism; shoots grow towards light.

• Gravity triggers gravitropism; roots grow downward.

Some plants respond to touch by folding or curling their leaves, which may help reduce damage or deter herbivores.

This response to mechanical stimulation occurs when cells lose potassium ions, causing water to leave the cells by osmosis. The cells become flaccid, leading to movement of the leaf.

Auxins are plant hormones (e.g., IAA) that play many roles. They are involved in regulating plant growth and are responsible for phototropism, where plant shoots grow towards a light source. Auxins are produced mainly in the shoot tip and move down the shoot.

Auxins bind to protein receptors on target cells, triggering second messengers that activate transcription factors. These switch on auxin-regulated genes, producing proteins that drive cell expansion, differentiation and division.

Auxins also promote cell elongation by triggering the acidification of cell walls. This increases ion uptake, followed by water uptake via osmosis, leading to cell swelling and elongation.

Plants respond to changes in light conditions (intensity, direction and wavelength) using photoreceptors such as phytochromes. These receptors exist in two forms:

• P_r – absorbs red light (daylight)

• P_fr – absorbs far-red light (dawn/dusk, or in shade)

Red light converts P_r into P_fr , while far-red light converts P_fr into P_r . Sunlight contains more red light than far-red light, so P_fr accumulates during the day. During darkness, P_fr is slowly converted back into P_r .

Many seeds require light to germinate, and this response is controlled by phytochromes.

Red light stimulates germination, and far-red light inhibits germination. Given that the two forms are interconvertible, the effects of red and far-red light on germination are reversible.

Plants flower at particular times of year in response to photoperiods – the relative lengths of day and night. Phytochromes enable plants to detect day length.

• During winter (long nights), more P_fr is converted into P_r, so P_fr levels are low by morning. This prompts flowering in short-day plants.

• During summer (short nights), less P_fr is converted, so more P_fr remains by morning. This prompts flowering in long-day plants.

Phytochromes detect when a shoot emerges from the soil and is exposed to light and trigger plant greening. This refers to changes such as leaf expansion and the production of chlorophyll.

Phytochromes also influence plant growth by inhibiting internode elongation in bright light, preventing excessive stem elongation.

Phytochromes change form when they are exposed to different wavelengths of light. Activated phytochromes interact with proteins by either binding or affecting the binding of protein complexes.

These proteins act as or activate transcription factors which bind to DNA to enable transcription of light-regulated genes. The resultant proteins facilitate the plant’s responses to light.