Summary Notes - Topic 14 Coordination and Responses - CAIE Biology IGCSE

Coordination and Response

The nervous system is crucial for both controlled movement and autonomic reflexes within the body. It regulates these body functions by transmitting electrical signals called nerve impulses via specialized nerve cells known as neurones. This system maintains coordinated movement and homeostasis, ensuring a constant internal environment. The nervous system is divided into two primary sections: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS comprises the brain and spinal cord, whereas the PNS includes the nerves that exist outside the CNS, facilitating the communication of impulses to and from the CNS.

Types of Neurones

• Sensory Neurones: These carry impulses from receptors to the spinal cord and brain.

• Relay (Connector) Neurones: These transfer impulses between different areas of the CNS.

• Motor Neurones (Effectors): These transmit impulses from the CNS to effectors, such as muscles.

Reflex Actions

Reflex actions are involuntary movements that organisms exhibit to evade danger, such as fire or sharp objects. These rapid responses occur without the involvement of the brain to ensure an almost instantaneous reaction. Voluntary movements, on the other hand, are controlled by the brain.

Reflex Arc

The reflex arc is the pathway taken by reflex impulses and consists of the following steps:

1. A stimulus, like heat, is detected by a receptor.

2. The receptor transmits an impulse via the sensory neurone to the spinal cord.

3. The relay neurone in the CNS passes the impulse to the motor neurone.

4. The motor neurone carries the impulse to an effector (e.g., a muscle) that reacts to move the organism away from danger.

Synapses

A synapse is the junction between two neurones, featuring a gap called the synaptic cleft between the presynaptic and postsynaptic neurones. When a nerve impulse reaches the presynaptic neurone, vesicles in the neurone fuse with its membrane to release neurotransmitters into the synaptic cleft. These neurotransmitters diffuse across the synapse and bind to receptors on the postsynaptic neurone, thereby triggering a new nervous impulse that can be transmitted further. Synapses are designed for unidirectionality, as vesicles containing neurotransmitters are exclusive to the presynaptic neurone, while receptors are exclusively present in the postsynaptic neurone, preventing backward impulse travel.

Sense Organs

Sense organs are aggregates of receptor cells that react to specific stimuli. For instance, the eye responds to light, while other organs respond to temperature, touch, sound, and chemicals.

Eye Structure

• Cornea: A transparent layer that covers the iris and refracts light entering the eye.

• Iris: The colored part of the eye that adjusts light entry by contracting and dilating the pupil.

• Pupil: The opening that allows light to enter the eye.

• Lens: Located behind the iris, it changes shape to focus light onto the retina.

• Retina: Contains photoreceptors (rods and cones) sensitive to different light colors and includes blood vessels for nourishment.

• Fovea: The region in the retina with a high concentration of cone cells providing the clearest image.

• Optic Nerve: A collection of neurones that transmits impulses from photoreceptors to the brain.

Pupil Reflex

The eye's pupil can dilate or constrict to manage the light entering it. This response involves two sets of muscles working antagonistically: circular and radial muscles. In low light, the pupil dilates by relaxing circular muscles and contracting radial muscles, while in bright light, the opposite occurs to safeguard the eye from damage.

Accommodation

The eye accommodates by altering the lens shape to focus on objects at various distances, controlled by ciliary muscles and suspensory ligaments. For nearby objects, the ciliary muscles contract while the ligaments relax, making the lens fatter. For distant objects, the ciliary muscles relax and the ligaments contract, leading to a thinner lens.

Types of Photoreceptors

• Rods: Rod-shaped cells for night vision, sensitive to low light levels, located primarily at the retina's periphery.

• Cones: Cone-shaped cells for color vision in bright light, concentrated in the fovea.

Hormones

The endocrine system produces and releases hormones, which are chemical substances that travel through the bloodstream to signal various body functions. Hormones are produced in glands such as the pituitary and adrenal glands.

Endocrine Glands

These glands form a network that aids in controlling growth, metabolism, and homeostasis.

Example Glands and Functions

Endocrine System vs Nervous System

• Transportation: Nervous impulses travel along neurones; hormones travel in the blood.

• Speed: Nervous impulses are faster than hormone responses.

• Duration: Nervous impulses are short-lived; hormonal responses can last longer.

• Method: The endocrine system functions using hormones, while the nervous system relies on electrical signals.

Homeostasis

Homeostasis is the process of maintaining a stable internal environment in organisms despite external changes. This stability allows cells to function optimally. Internal conditions must be kept within specific limits, and when these are exceeded, negative feedback mechanisms act to restore balance.

Regulating Blood-Glucose Concentration

Maintaining blood-glucose levels is vital for homeostasis. High glucose levels can cause cells to shrink and die, while low levels can lead to cell bursting. The pancreas regulates blood-sugar levels through insulin and glucagon:

• High blood glucose triggers insulin secretion to convert glucose to glycogen, reducing blood-glucose levels.

• Low blood glucose causes glucagon release, promoting glucose release into the blood.

Diabetes occurs when insulin production fails, with Type 1 being an autoimmune response and Type 2 involving inadequate insulin production or response.

Regulating Temperature

Maintaining an internal temperature of 37°C is crucial for optimal enzyme function. The hypothalamus in the brain monitors temperature and triggers responses when it deviates:

• Low Temperature Responses: Shivering increases heat; vasoconstriction reduces heat loss by narrowing blood vessels.

• High Temperature Responses: Sweating cools the body through evaporation; vasodilation increases blood flow to surface for heat loss.

Tropic Responses

Tropisms are plant growth movements in response to external stimuli, which can be positive or negative.

Gravitropism and Phototropism

• Gravitropism: Shoots grow upward (negatively gravitropic), while roots grow downward (positively gravitropic) in response to gravity.

• Phototropism: Shoots grow towards light (positively phototropic) for better photosynthesis, while roots grow away from it (negatively phototropic).

Phototropic Response Process

1. Auxins produced in shoot tips are transported downward.

2. Light causes auxin to accumulate on the shaded side.

3. Auxin promotes cell elongation on the shaded side, causing bending toward light.