Chapter 6 Responding to the environment – humans PDF

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These are the life science chapter 6 lecture notes...

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Chapter 6: Responding to the environment – humans (paper 1) 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

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

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.

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.

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

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.

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.

6.4.3 Visual defects

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.

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.

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

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...