2.85 describe the structure and functioning of a simple reflex arc illustrated by the withdrawalof a finger from a hot object

Notes


Let us observe the flowchart of co-ordination in a reflex system:

Stimulus -> Receptor -> Sensory neurone ->  relay neurone in CNS -> motor neurone -> effector -> Response

Here we see that the brain has no function in the reflex system, thereby eliminating a stage in the response system in order to reduce time (i.e. no conscious decision).

Example of hot surface reaction:
Sensory organs contain receptors which, when stimulated by the intense heat, fire electrical impulses down through the spinal nerve. These impulses enter the CNS and are directly connected to the motor nerve by means of a relay nerve (this dramatically reduces response time and always produces the same response from the effector organs -> these are predetermined from DNA). This electrical impulse stimulates a response from the effector organ which in this case, would be to remove the sensory organ from the hot surface.

Note that reflexes are unconscious, always produce the same response, and are very rapid so as to reduce likelihood of organ or tissue damage.

2.84 understand that stimulation of receptors in the sense organs sends electrical impulses along nerves into and out of the central nervous system, resulting in rapid responses

Notes


Receptor organs (i.e. eyes) contain many neurones. These neurones, when acted upon by a stimuli, send electrical impulses across synapses and to other neurones which conduct this electrical information. This chain of neurones are known as nerves.

Once a receptor is stimulated, this electrical impulse is conducted down the spinal nerves to the spine. This is where it enters the CNS, from here, the electrical signal is interpreted by the brain, which sends another impulse to the effector organs, causing a response.

This all happens very rapidly due to the fast speed at which electrical energy can be transferred, which is the reason for our 'rapid responses'.

2.83 recall that the central nervous system consists of the brain and spinal cord and is linked to sense organs by nerves

Notes


The CNS consists of:
a) the brain
b) the spinal cord

The sensory organs (receptors) send nerve impulses through the CNS to the effector organs (effectors) by means of nerves. A nerve consists of neurones (nerve cells, essentially) amongst other things.

2.82 describe how responses can be controlled by nervous or by hormonal communication and understand the differences between the two systems

Notes


Nervous system refers to how the body responds to changes in the external (environment) and internal (organism) environment -> electrical impulses sent along the neurones to the CNS, which then decides activates the effectors and response.

The hormonal system refers to hormones as a means of communication between stimulus and response. Hormones are made in the endocrine glands which are then released directly into the blood and are carried by the circulatory system to the targeted effector muscles and organs.

The difference between the two is that since the endocrine gland is typically far away from the effector, it takes longer for the hormonal system to co-ordinate a response than the electrical impulse dependent nervous system. Also, since hormones stay in the circulatory system for a while after being secreted, the reactions they cause tend to be long-lasting.

2.77a Thermoregulation

Notes


Homeostasis = "conditions are kept constant"
-> Homeothermic = "maintaining constant temperature (internal)"
     -> e.g. Mammals are homeothermic organisms, carry out thermoregulation

Why do mammals want to preserve homeothermic conditions?
Due to enzyme reactions, attempt to keep temperature at 'optimum temperature' for most efficient rates of reactions - therefore body functions
-> Optimum temperature = homeothermic temperature of that organism

2.76 Sensitivity

Notes


Sensitivity = how organisms respond to changes in environment

Types of stimulus (change in environment):
-Light
-Temperature
-Pressure
-Chemicals

Detecting changes = receptors
Responding to changes = effectors (muscles/glands)