Biology 12

Chapter 5 The Nervous System • MHR 143

number of major functions, each of which is related
to a particular structure. The cardiac centrecontrols
heart rate and the force of the heart’s contractions.
The vasomotor centreadjusts blood pressure by
controlling the diameter of blood vessels, and the
respiratory centrecontrols the rate and depth of
breathing. The medulla oblongata also contains
reflex centres for vomiting, coughing, hiccupping,
and swallowing. Any damage to this part of the
brain is usually fatal.
The cerebellum, shown in Figure 5.9, controls
muscle co-ordination. Although the cerebellum
makes up only 10 percent of the volume of the
brain, it contains 50 percent of the brain’s neurons.
If you stand in one place, specific muscles are
contracted while others are relaxed. As groups of
muscle fibres become fatigued, others are contracted
to compensate. The position of the head, the limbs,
and other parts of the body all affect decisions as to
which muscles should be involved. This series of
decisions is complex and, like other forms of
muscle co-ordination, develops over time. Have

you ever watched a young child just learning to

stand or walk? The child works hard to keep her

balance. However, older children and adults do not

even think about maintaining balance because the

cerebellum takes over. Although we consciously

decide to stand, walk, or run, we do not have

to consciously control all the separate muscle

actions involved.

Indeed, most of the physical skills that we

learn are slowly taken over by the cerebellum.

The difference between a beginner and a more

accomplished player of any game may be related

to the degree to which the basics have been

taken over by the cerebellum and are no longer

consciously controlled. For instance, do you think

an experienced hockey player concentrates on the

plays of the game or on his or her skating?

The thalamusis a sensory relay centre. It

receives sensations of touch, pain, heat, and cold,

as well as information from the muscles. If the

sensations are mild, the thalamus relays the

information to the appropriate part of the cerebrum

corpus callosum
(a series of nerve fibres that
connect the left and right
hemispheres of the brain)

skull meninges

(a series of three membranes that surround

and protect the central nervous system)

lateral ventricle

(ventricles are cavities

within the brain that

produce and store

cerebrospinal fluid)

third ventricle

pituitary gland

fourth ventricle

vertebra

spinal cord

CEREBRUM

(responsible for complex behaviour

and intelligence; interprets sensory

inputs and initiates motor impulses)

CEREBELLUM

(motor area of the brain)

THALAMUS

(sorts sensory

information)

HYPOTHALAMUS

(controls hunger, body

temperature,

aggression, and other

aspects of metabolism

and behaviour)

MIDBRAIN

(a short segment of the
brainstem between the cerebrum
and pons; particularly involved
in sight and hearing)

PONS
(contains bundles of axons travelling
between the cerebellum and the rest of
the CNS; functions with the medulla to
regulate breathing rate and has reflex
centres involved in head movement)
MEDULLA
OBLONGATA

Figure 5.9An overview of the structure and major functions of the brain