Sensitivity

Nervous Control

A nerve impulse is an electrical signal that passes along nerve cells called neurons.

The Human Nervous System

The nervous system can be split into 2 parts:

• Central: brain and spinal cord
• Peripheral: all the other nerves that connect to the central nervous system

Reflex Arc

There are 3 different types of neurons involved in the relfex arc.

• Sensory: long and have a cell body branching off the middle of the axon
• Relay: short and have a small cell body at one end with many dendrites branching off it
• Motor: long and have a large cell body at one end with long dendrites branching off it

Relay neurons are located in the central nervous system and do not have myelin sheaths on their axons. Motor neurons are also located in the central nervous system.

Reflex arcs are a way of responding to stimuli using effectors such as muscles and glands.

Voluntary and Involuntary Actions

Voluntary actions require conscious thought whereas involuntary actions are automatic.

Sense Organs

Eye

The eye is made up of many parts.

• Cornea: refracts light
• Iris: controls how much light enters pupil
• Lens: focuses light onto retina
• Retina: contains light receptors that are sensitive to light from different colours
• Optic Nerve: carries impulses to the brain

The pupil reflex is used to control the amount of light entering the eye. The action of the radial and circular muscles is described as antagonistic because they work opposite to each other.

• In bright light:
  • circular muscles contract
  • radial muscles relax
  • pupil size decreases
• In dim light:
  • circular muscles relax
  • radial muscles contract
  • pupil size increases

Hormones

A hormone is a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.

Homeostasis

Homeostasis is the maintenance of a constant internal environment.

Tropic Responses