2.85 Reflexes

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

In some cases the response to a stimulus is always the same and needs to be applied quickly in order to not damage the body. In order to reduce reaction time the brain is removed from the process such that the impulse travels up the sensory nerve through a small relay nerve in the spinal cord that connects directly to the motor nerves so the reflex response can take place.

Nevertheless it is important to note that the receptor does still relay the impulses to the brain so that we are aware that we have touched a hot/sharp object (for example) and this creates the response of us saying “ouch” but this is significantly later than the reflex response... The reason reflex responses work is because the decision and response is always the same.

2.84 Responses

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.

Impulses from our sensory organs are passed from the receptor to the coordinator (CNS) by sensory nerves. The impulse is then passed up the spinal cord to the brain where the brain generates a response and sends impulses back down the spinal cord through motor nerves which create the response.

For example if the eyes saw a red pen the co-ordinator (the brain) could decide “I like red; pick up the pen” this signal will be sent back down the spine through a motor nerve in the arm where the effectors (arm muscles) pick up the pen. It is important to note that all of this takes place reasonably fast but can be sped up by removing the interaction of the brain…[see 2.85]

2.83 Central Nervous System

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

The Diagram shows the Central Nervous System (CNS) consisting of the Spinal Cord and Brain. Linked to the spinal cord are nerves which carry impulses sent by the brain to the effector. These nerves are known as the Peripheral Nervous System.

2.82 Communication

2.82 Describe how responses can be controlled by nervous or by hormonal communications and understand the difference between the two systems.

There are two ways in which responses are controlled in the body: the first is by using nerves. The diagram below shows a motor nerve cell. The cell body would be embed into the spine and the Synaptic knob at the opposing end would be embedded into the effecter (usually a muscle). Electrical/nerve impulses travel down the axon from the cell body; in mammals the axon is surrounded by a second kind of cell called the Schwann cell and these form what is called a Myelin sheath to increase the speed of nerve conduction.

The second way of linking a receptor to an effector is known as the Endocrine system. This involves an endocrine gland which produces Hormones (Proteins or Steroids). The Hormone then enters the blood stream and travels through the blood to the target tissue/organ where it will have an effect. Because of this it is possible for hormones to have multiple targets and effects.

Bill's Coffee

Bill wakes up to the smell of fresh coffee (Chemical Stimuli). He leaps out of bed and makes his way downstairs intent on gulping down a fresh brew.

Identify the:

• Stimuli: Smell of coffee (chemical)
• Receptor: Nose
• Coordinator: Central Nervous System (CNS)
• Effector: Muscles
• Response: running downstairs, or gulping the coffee

Other examples of Stimuli: Chemical, Light, Pressure, Temperature

2.77 Homeostasis

2.77 understand that homeostasis is the maintenance of constant internal environment and that body water content and body temperature are both examples of homeostasis

Homeo~ = ‘same’/’constant’
~stasis = ‘conditions’
Homoeothermic = constant temperature

Mammals maintain a constant body temperature no matter what the environmental temperature; these are homoeothermic organisms and carry out Thermoregulation.
Mammals must contain this constant body temperature in order to maintain optimum temperatures for enzyme catalysed reactions.

Thermoregulation:

A negative feedback loop is used to regulate body temperature in the human body about a constant condition. For human body temp, this constant condition (the co-ordinator) is roughly 37°C.

In thermoregulation the receptor is the Hypothalamus, a region of the brain that responds to a stimulus: the temperature of Blood. The hypothalamus compares the body temperature of blood to the theoretical level (37°C). If the body temperature needs to be increased or decreased effectors such as the skin are used to create a responsive increase or decrease in body temperature. This temperature feeds back to the hypothalamus to produce a new output.

Using Skin to Regulate body temperature:

There are several important features of the skin used to regulate body temperature: these include sweat glands, hair, and blood vessels.

On a hot day when body temperature needs to be lowered blood flow to the surface of the skin increases through dilated blood vessels and the body is cooled through the evaporation of sweat and radiation of heat from the blood.

On a colder day less blood is pumped towards the surface of the skin as the blood vessels constrict allowing less heat to radiate from the body. The hairs also stand upright in order to trap air as an insulation layer.

2.76 Sensitivity

New Topic: Coordination and Response

2.76 understand that organisms are able to respond to changes in their environment

MRS GREN: S = Sensitivity

Changes in environment (stimuli):

• Light levels
• Temperature
• Pressure levels
• Chemical

In order to detect these changes, organisms must have receptors, and in order to respond to these changes in the environment organisms have effectors such as muscles and glands.