Survival is all about how organisms respond to their surroundings. Any changes, both inside and out, that can affect how a body works need to be dealt with. Homeostasis is the process of keeping things like blood sugar levels, temperature, and pH stable. These are all important for cells to work properly, and any changes can be dangerous. Even things like touching something sharp or seeing a fast-moving object need to be responded to, so an organism doesn't get hurt. To keep everything in check, there are control systems that help maintain balance. Receptors detect changes, control centers assess what they mean, and effectors take action to fix things. Animals and plants have complex systems that help them communicate between different parts of their bodies. Animals have two systems for this, the nervous and endocrine systems. The nervous system uses electrical impulses to send messages quickly and directly, while the endocrine system uses hormones to send messages more slowly and broadly throughout the body.

Electrical impulses are also known as action potentials.

In plants, the response to stimuli is made exclusively using chemical communication systems such as plant hormones rather than electrical impulses.

Nervous system

There are two components of the nervous system:

The central nervous system (CNS)The peripheral nervous system (PNS)The CNS acts as the control centre in all mammals and is composed of the brain and spinal cord, while the PNS comprises nerves that connect the CNS to sensory receptors/organs and effector organs. Nerve cells are called neurones and are the functional unit of the nervous system. Stimuli-induced electric impulse propagation through neurones allows effectors to be activated and a response to be generated.According to their function in propagating information within the nervous system, there are three different types of neurones.Sensory neurones – transmit information (electric impulse) from receptors to the CNS.Intermediate neurones – part of the CNS (control centre), transmit the impulses from sensory to motor neurones connecting receptors to effectors.Motor neurones - transmit information (electric impulse) from the CNS to organ effectors. Intermediate neurones are also known as relay neurones.Most external stimuli are processed and resolved through nervous system coordination. From the moment a change that threatens homeostasis is detected in a biological system, triggering a stimulus to the moment, a response is needed to restore balance. An electrical impulse travels across the nervous system via the following the pathway:Receptor Sensory neuroneIntermediate neurone Motor neurone Effector cell/organ

The sensory organs

The sensory organs are a group of specialised cells or sensory receptors responsible for perceiving our surroundings and signal towards changes around us. The sensory organs/receptors are the first element in the stimuli-response chain that enables all organisms to adjust to external pressures and survive in an ever-changing environment. Our sensory or sense organs are responsible for our five senses:

Touch – skin Taste – tongue Smell – nose Hearing – ears Vision – eyes

Types  of sensory receptors

Sensory receptors are specialized cells that detect specific stimuli like pressure, movement, chemicals, heat, or light. They are found in sense organs and are roughly divided into four main categories: mechanoreceptors, chemoreceptors, thermoreceptors, and photoreceptors. These receptors act as transducers, converting energy from one form to another, and generate an electric impulse upon activation by a stimulus. This impulse travels through the PNS until it reaches the CNS, where it informs the control centre about the nature and threat of the change. The CNS then sends an appropriate response to an organ effector in the form of an electric impulse, which takes corrective action. The Pacinian corpuscle is an example of a mechanoreceptor that detects mechanical pressure. While most sensory receptors are specialized cells, some, like touch receptors, are the ends of sensory neurones themselves.

Eye receptor cell

Our vision is dependent on two specialized cells called rod cells and cone cells, which are both photoreceptors found in the retina of the mammalian eye. These cells are responsible for converting light energy into electrical energy in the form of an electric impulse that travels up to the CNS through the optic nerve. Light stimuli cause the degradation of the pigment rhodopsin (rod cells) or iodopsin (cone cells), triggering a generator potential in neighbouring bipolar cells (neurones) that initiates the electrical impulse. Rod and cone cells complement each other in responding to different light stimuli. Rod cells are better at detecting light of very low intensity, allowing us to see in low-light environments, such as at night, but they only provide black and white vision and low visual acuity. Cone cells, on the other hand, can distinguish between different light wavelengths and provide full-colour vision with high visual acuity, but they only respond to high light intensities.

Effectors definition

Effectors are cells or organs that carry out a response in reaction to a stimulus. They are usually muscles or glands activated by motor neurones, which are the last element in the stimuli-response chain. When activated, an effector typically involves a muscle contracting or a gland releasing a hormone that can enact the desired outcome. A reflex action is a particular type of involuntary response to sensory stimuli that involves a very fast automatic response from the nervous system without involving conscious thought. This type of action can occur to minimize the potential harm of threats by reducing reaction time. For example, when you touch something hot, your hand quickly pulls away from the heat to avoid further damage, even before the brain realizes that your hand is hot. This involuntary action is carried out by muscle contraction in your arm and is initiated by the reflex arc pathway of neurones responsible for reflex actions.

Reflex actions diagram

In summary, sensory receptors found in our sensory organs pick up on external stimuli and are responsible for our five senses. These receptors are divided into four main categories and act as transducers, converting energy in different forms to electrical energy. Effectors are cells or organs that respond to a stimulus and carry out a response coordinated by the nervous system. Reflex actions are fast, involuntary responses to potentially harmful stimuli that happen without conscious thought and involve a reflex arc pathway of neurones. Different reflex arcs exist for different stimuli, and each specific stimulus's response is always the same because it is unconscious.

Survival and Response

What are the sensory organs?

The sensory organs are the skin, tongue, eyes, ears, nose. They carry the necessary sensory receptors that pick up on external stimuli thus making up our five senses: touch, taste, vision, hearing,  smell.

What are the effectors in our body?

The effectors are organs, usually muscles or glands, that produce a protective response in our body in response to a change in the internal or external environment. Muscles contract and glands release hormones carrying out responses.

What is the common reflex action in the eye usually called?

The reflex action in the eye is called the blinking reflex.

What is a quick response to stimuli called?

A  quick response to stimuli is called reflex action.

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