The survival of an organism is dependent on its response to the environment. All internal and external changes to a biological system that can affect its function must be resolved.
The process of maintaining an organisms’ internal properties such as blood glucose levels, core temperature or pH relatively stable is known as homeostasis. Factors such as temperature and pH are critical to support cellular function and must remain unaffected by any change or pressure. Other external inputs that may constitute a threat, like a fast object moving towards you or touching something sharp, must also elicit a corrective action that avoids or mitigates harm to an organism.
Coordination systems
Homeostasis is achieved through regulatory mechanisms that provide stability when changes introduce an imbalance to a biological system. Receptors first detect any meaningful change through stimuli, its meaning is assessed in a control centre and a response, usually a corrective action, is generated and sent to an effector.
Complex multicellular organisms, like animals or plants, require extensive coordination and signalling systems that can support communication between receptors and effectors and enable complex regulatory responses between different parts of the body.
In animals, two coordination systems transmit information between receptors and effectors. Both of these systems play a role in responding to stimuli but in different ways:
- Nervous system – information is transmitted quickly and in a targeted way through electrical impulses that travel through nerves spread across the body, connecting receptors to control centres and effectors.
- Endocrine system - information is transmitted more slowly and in a less specific manner through hormones released to the bloodstream. Hormones are chemical messengers produced by glands once a stimulus is received.
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 neurone
- Intermediate 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
A stimulus occurs when a specific factor like pressure/movement, chemicals, heat, light, or other changes and is detected by a sensory receptor.
There are different types of receptors present throughout the sense organs responsible for picking up on different stimuli, but they are roughly divided into four main categories:
- Mechanoreceptors – triggered by changes in pressure and movement and can be found, for example, in the skin or ears.
- Chemoreceptors – detect chemical presence in smells or foods and can be found in our nose or tongue.
- Thermoreceptors – detect significant heat changes found in our skin or tongue.
- Photoreceptors – detect light and can be found in our eyes.
Despite reacting to different stimuli, all sensory receptors share two main features:
- They detect one specific type of stimulus (e.g., light)
- They act as transducers
Transducers convert energy from one form to another. In this case, different energy inputs, in the form of heat or light among others, are converted by all sensory receptors into electric energy in the form of an electric nervous impulse.
The electric impulse is generated because upon activation by a stimulus, receptors create an action potential in a neighbouring sensory neurone that propagates through nerves in the PNS until it reaches the CNS. The frequency and quantity of similar electric impulses that reach the CNS inform the control centre about the nature and threat of the change. The CNS then forwards an adequate response to an organ effector in the form of an electric impulse. The effector takes appropriate, normally corrective action that counters the effects of the change in the biological system.
The Pacinian corpuscle is a mechanoreceptor found deep in our skin that detects mechanical pressure.
Most sensory receptors are specialised cells with sensory zones that pick up on environmental cues like chemoreceptors in our taste buds that distinguish different tastes by detecting different chemical compounds and relay the information to sensory neurones. However, there are receptors like some touch receptors which are the ends of sensory neurones themselves.
Eye receptor cell
Two specialised cells function as our vision sensory receptors. Rod cells and cone cells are both photoreceptors found in the retina, the innermost layer of the mammalian eye. As photoreceptors, rod and cone cells are responsible for converting light energy into electrical energy in the form of an electric impulse propagated through the optic nerve up to the CNS. This allows visual stimuli to be assessed and inform behaviour.
Light stimuli cause the degradation of the pigment rhodopsin (rod cells) or iodopsin (cone cells), triggering a generator potential in neighbouring bipolar cells (neurones) initiating the electrical impulse.
A generator potential describes a change in the voltage difference (membrane potential) of a cell membrane. When the membrane potential exceeds a threshold value, an action potential is created.
Rod and cone cells complement each other in responding to different light stimuli. Rod cells are able to detect light of very low intensity, allowing us to see in low light intensity environments, such as at night. However, rod cells can only provide black and white vision and low visual acuity. Cone cells, on the opposite, can distinguish between different light wavelengths, which is why we can see in full colour. Cone cells also only respond to high light intensities and give very accurate vision (vision acuity).
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, the last element in the stimuli-response chain. Once activated, a response usually entails a muscle contracting or a gland releasing a hormone that can enact the desired outcome.
Reflex action
A reflex action is a particular type of involuntary response to sensory stimuli. Specific and potentially harmful stimuli can elicit a very fast automatic response from the nervous system without involving conscious thought.
When you touch something hot, your hand quickly recalls away to avoid potential further damage to your body. Even before the brain realises that your hand is hot, it is already being pulled away by muscle contraction in your arm.
The involuntary nature of such action minimises the potential harm of threats by reducing reaction time. The pathway of neurones responsible for reflex actions is also called the reflex arc.
Reflex actions diagram
The diagram in Fig. 2 illustrates the mentioned reflex arc (also known as withdrawal reflex) and follows a typical stimulus-response nervous system chain.
Fig. 2 - The withdrawal reflex
Thermoreceptors in the skin detect the temperature change and a muscle effector is contracted to pull the hand away and annul the change that triggered the stimulus. In this case, the control centre is located in the spinal cord. The entire pathway only involves two or three neurons (sometimes there is no intermediate neurone involvement) with one or two synapses (communication mechanism between neurones) which is why the action is so fast. The impulse also travels up to the brain from the spinal cord, but a reaction is immediately triggered from the spinal cord to accelerate response time.
Synapses describe the junctions between two neurones or a neurone and an effector cell.
There are several different reflex arcs for different dangerous stimuli, but each specific stimulus’s response is always the same because it is unconscious. Besides the withdrawal reflex, other reflex arcs include the knee-jerk reflex (spinal reflex resulting from stretching of the patellar tendon) or the blinking reflex (involuntary blinking of the eyelids).
Survival and Response - Key takeaways
- Sensory receptors are found in our sensory organs (skin, tongue, nose, eyes, ears) and pick up on external stimuli. They are responsible for our five senses (touch, taste, smell, vision, hearing).
- The sensory receptors are divided into four main categories: mechanoreceptors, chemoreceptors, thermoreceptors, and photoreceptors.
- Each sensory receptor only detects one type of stimulus and acts as a transducer, converting energy in different forms to electrical energy. Cone and rod cells in the mammalian eye retina for example are photoreceptors, responsible for converting light energy into an electrical impulse that allows us to see our surroundings.
- Effectors are cells or organs, usually muscles or glands, that carry out a response coordinated by the nervous system in reaction to a stimulus.
- Reflex actions are fast involuntary responses to potentially harmful stimuli that happen without conscious thought. The pathway of neurones involved in a reflex action is called a reflex arc.
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