RESPONDING TO THE ENVIRONMENT- HUMANS 
LIFE SCIENCES
STUDY GUIDES AND NOTES
GRADE 12

• The brain
  • Structure and functions of the brain
  • Activity 1
• Neurons
• Reflex arc
  • Activity 2
• The human eye
  • Accommodation
  • Pupillary mechanism
  • Visual defects
  • Activity 3
• The human ear
  • Structure of the ear
  • Hearing
  • Balance
  • Hearing defects
  • Activity 4

CHAPTER 6: RESPONDING TO THE ENVIRONMENT - HUMANS

The human nervous system

The nervous system is responsible for processing and transmitting information throughout the body:

• It tells the body how to react to stimuli (changes in the environment to which the body responds). For example, it regulates body temperature on a hot or cold day. It is also responsible for the reflex action, for example, when you step on a pin or touch a hot surface.
• The nervous system also coordinates the various activities of the body, such as walking, hearing, seeing, and so on. 

The central nervous system consists of the brain and the spinal cord.

6.1 The brain
6.1.1 Structure and functions of the brain

Figure 6.1 below shows the different parts of the brain and their functions.
Figure 6.1 The structure and functions of the brain

Activity 1

Questions
Write down the name of the part which:

1. Controls heartbeat (1)
2. Contains the centres that control balance, muscle tone and equilibrium (1)
3. Has centres that interpret what you see (1)
4. Coordinates voluntary muscle movements (1)
5. Controls body temperature (1) [5]

Answers to activity 1

1. Medulla oblongata✔(1)
2. Cerebellum✔ (1)
3. Cerebrum✔ (1)
4. Cerebellum✔ (1)
5. Hypothalamus✔ (1) [5]

6.2 Neurons

Neurons are specialised cells which connect the brain and spinal cord to all other parts of the body.
Figure 6.2 A neuron

There are three types of neurons, namely sensory (afferent) neurons, motor (efferent) neurons and interneurons (or connectors). Table 6.1 below shows the structure and function of these neurons.

Type of neuron	Function	Structure
Sensory (afferent) neuron   Senses the stimulus	Transmits impulses from the sense organs  or receptors to the spinal cord and brain.
Motor (efferent) neuron Response to the stimulus	Transmits impulses from the brain and spinal cord to the effectors (muscles and glands). The effectors bring about the response.
Interneuron (connector) Found in the brain and the spinal cord.	Links the sensory neuron to the motor neuron

Table 6.1 Sensory, motor and interneurons

A synapse is the functional connection between the axon of one neuron, and the dendrites of another neuron.

6.3 Reflex arc

A reflex action is a quick, automatic action that involves the spinal cord and does not involve the brain. It is an important function to protect the body from harm. Examples are blinking the eye, coughing, sneezing, dilation and constriction of the pupil of the eye, and quickly withdrawing your hand when it touches a hot surface.
The reflex arc is the path along which an impulse is transmitted to bring about a response to a stimulus during a reflex action.
Figure 6.6 below shows what happens when you hold your finger close to a flame. The grey arrows represent the reflex arc.

Figure 6.6 The reflex action of withdrawing a finger when placed in a flame

Activity 2

Questions
Use the diagram of the reflex arc in Figure 6.6 on page 44 to answer the following questions.

1. Part B indicates the …
   1. dendrite of the motor neuron.
   2. axon of the motor neuron.
   3. dendrite of the sensory neuron.
   4. axon of the sensory neuron. (2)
2. The correct sequence in which impulses move from the receptor to the effector in the reflex arc in Figure 6.6 is ...
   1. A → B → C → D→ E
   2. C → A → B → E→ D
   3. C → B → E → D→ A
   4. A → D → E → B→ C (2)
3. Give the correct term for the following definitions:
   1.  A structure which receives a stimulus and converts it into a impulse
   2. A structure which responds to a stimulus, e.g. a muscle or gland
   3. A neuron that carries impulses from the central nervous system to the effectors
   4. A neuron that carries impulses from the receptors to the central nervous system
   5.  A neuron that carries impulses from a sensory neuron to a motor neuron in the spinal cord
   6. A very quick, automatic action that involves the spinal cord and not the brain
   7. The pathway along which an impulse is transmitted to bring about a response to a stimulus during a reflex action 7 × 1 = (7) [11]

Answers to activity 2

1. C✔✔(2)
2.  A✔✔ (2)
3.            
   1. Receptor✔
   2. Effector✔
   3. Motor/efferent neuron✔
   4. Sensory/afferent neuron✔
   5. Interneuron✔/ connector
   6. Reflex action✔
   7. Reflex arc✔                 7 × 1 = (7)   [11]

6.4 The human eye

Figure 6.7 below shows the different parts of the eye and their functions.

Figure 6.7 The structure of the eye

6.4.1 Accommodation

Accommodation is the adjustment of the shape of the lens to see objects clearly whether they are far away or close by. This is shown in Table 6.2 and Figures 6.8 and 6.9 below.

Table 6.2 Accommodation of the eye for distant and near vision

6.4.2 Pupillary mechanism

The pupillary mechanism (or pupil reflex) regulates the amount of light entering the eye by adjusting the size of the pupil. This is shown in Table 6.3 and Figures 6.10 and 6.11 below.

Table 6.3 Pupillary mechanism

6.4.3 Visual defects

	Visual defect	Nature of the defect	Corrective measures
	Short-sightedness Near objects can be seen clearly (myopia)	Inability of lens to becomemore flat/eyeball is longer thannormal Lens bends the light rays toomuch As a result it falls in front of theretina causing the image to be blurred Cannot see distant objectsclearly	Wearing glasses with converging (biconcave) lensFigure 6.12 A
	Long-sightedness Objects far-away can be seen clearly (hyperopia)	Inability of lens to become more convex/eyeball is shorter thannormal Lens does not bend light rays enough As a result it falls behind the retina causing the image to be blurred Cannot see near objects clearly	Wearing glasses with converging (biconvex) lensFigure 6.12 B
	Astigmatism	The curvature of the lens orcornea is uneven resulting indistorted images	Glasses with lenses shaped to correct the distortion
	Cataracts	Lens becomes cloudy and opaque	Surgery to replace thelens with a synthetic lens

Activity 3

Questions

1. Figure 6.13 shows a longitudinal section through the human eye. Study the diagram and answer the questions that follow.
   a) Label parts 2, 3, 4 and 5 respectively. (4)
   b) Name and describe the process that causes part 1 to dilate (become wider). (5)
2. Figure 6.14 is a longitudinal section through the human eye. The structures which enable the eye to focus on objects are missing in this diagram. Study the diagram and answer the questions that follow.
   Draw a longitudinal section through the missing parts of Figure 6.14 to indicate the appearance of these structures when you are ...
   a) reading a book. (6)
   b) looking at an object more than 6 metres away. (6) [21]

Answers to activity 3

1.                      
   a)     2 - Cornea✔
           3 - Lens✔
           4 - Suspensory ligaments✔
           5 - Ciliary muscles✔/ body   (4)
   b)  Pupillary mechanism✔/ pupil reflex
   The radial muscles3of the iris contract3and the circular muscles✔relax.✔
   The pupil dilates and more light enters the eye.✔ (5)
2.      [21]

6.5 The human ear
6.5.1 Structure of the ear

The human ear consists of three main parts:

• The outer ear
• The middle ear
• The inner ear

Figure 6.15 below shows the structure and function of each part of the human ear.

Figure 6.15 The structure of the ear

6.5.2 Hearing

Figure 6.16 below shows how the three parts of the ear work together to make it possible for us to hear. The grey arrows show the path of a sound wave.

Figure 6.16 How hearing takes plac

Look at Figure 6.16 above and read the information in Table 6.4 below to understand how hearing takes place.

Table 6.4 The hearing process

6.5.3 Balance

The human ear is responsible for balance in this way:

1.  The cristae in the semicircular canals are stimulated by changes in  the direction and speed of movement
2.  The maculae in the sacculus and utriculus are stimulated by changes in the position of the head

When stimulated, the cristae and maculae convert the stimuli received into nerve impulses.
The nerve impulses are transported along the auditory nerve to the cerebellum to be interpreted.
The cerebellum then sends impulses to the muscles to restore balance.

6.5.4 Hearing defects

Table 6.5 Hearing defects

Activity 4

Questions
Study Figure 6.17 below and answer the questions that follow.

1. Identify the parts labelled B, C and F. (3)
2. Give the function of the pinna. (2)
3. Write the letter of the part which:
a) contains receptors for balance. (1)
b) equalises the pressure on either side of part B. (1)
c) transmits impulses to the brain. (1)
4. Describe how hearing occurs. (8) [16]

Answers to activity 4
1.

B -Tympanic membrane✔
C - Malleus/hammer/an ossicle✔
F - Cochlea✔(3)

2. It directs sound waves✔ into the auditory canal✔. (2)
3.

a) D✔
b) G✔
c) E✔ (3)

4.

• Sound waves are directed into the auditory canal✔ by the pinna✔.
• The sound waves make the tympanic membrane vibrate✔ and the vibrations are passed on to the ossicles✔ in the middle ear.
• The ossicles make the oval window vibrate✔ and this causes pressure waves to be set up in the inner ear.
• These vibrations also cause the organ of Corti✔ to be stimulated and it generates impulses which are sent to the cerebrum✔ along the auditory nerve✔.
• The cerebrum interprets the impulses as sound✔. (8) [16]