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Biology 12
12.3 The Central Nervous System
The central nervous system is composed of the spinal cord and the brain.
• Brain: controls breathing, heart rate, body temperature, blood pressure, emotions,
reasoning, memory, and creativity
• Spinal cord: a means of communication between the brain and the peripheral nerves
that leave the cord
• The brain and spinal cord are wrapped in protective membranes called meninges
• The spaces between meninges are filled with cerebrospinal fluid, which cushions and
protects the CNS
• This fluid is produced and stored in the brain’s ventricles (hollow cavities) and
the spinal cord’s central canal
• If the fluid accumulates in the brain and does
not properly drain, the brain can push against
the skull, causing brain damage
The Spinal Cord
Structure of the Spinal Cord
• Individual vertebra protect the spinal cord
• Spinal nerves project from the cord between the
vertebrae in the vertebral column
• Fluid-filled intervertebral disks cushion and separate the
• Central canal: contains the cerebrospinal fluid
• Grey matter: centrally located, shaped like the letter H
• Contains parts of sensory neurons, motor
neurons, and interneurons
• Dorsal root: contains sensory fibres entering grey
• Ventral root: contains motor fibres exiting grey matter
• Spinal nerves: part of PNS
• White matter: surrounds grey matter
o Contains ascending tracts taking information to
the brain and descending tracts taking
information from the brain
o Tracts cross each other after entering and exiting
− Left side of brain: controls right side of
− Right side of brain: controls left side of
Biology 12
Functions of the Spinal Cord
The spinal cord sends sensory information to the brain, receives motor input from the brain,
and carries out reflex actions.
• Example: Sensation
o When someone touches your hand, sensory receptors generate nerve impulses
that pass through sensory fibres to the spinal cord and up ascending tracts to the
• Example: Voluntary movement
o When we move our limbs, motor impulses in the brain pass down descending
tracts to the spinal cord and out to our muscles through motor fibres
The Brain
The brain has four major parts:
• Cerebrum (two lateral ventricles)
• Diencephalon (third ventricle)
• Cerebellum (fourth ventricle)
• Brain stem (fourth ventricle)
The Cerebrum
The cerebrum is the largest part of the brain in
• Communicates with and coordinates activities of other parts of
the brain
Structure and Function of the Cerebrum
The cerebrum has two halves (cerebral hemispheres) that
communicate via the corpus callosum, a bridge of nerve tracts.
• The cerebral cortex is a thin outer layer of grey matter that
covers the cerebral hemispheres
• Grooves called sulci divide the hemisphere into four lobes:
frontal, parietal, occipital, temporal
Frontal Lobe
• Primary motor area: involved in voluntary movement
• Premotor area: involved in organizing motor functions
• Prefrontal area: processing centre involved in reasoning
and planning
• Broca’s area: involved in speech musculature (lips,
tongue, larynx)
Biology 12
Parietal Lobe
• Primary somatosensory area: involved in somatic sensing
• Primary taste area: involved in taste
• Somatosensory association area: processes and analyzes sensory information from skin
and muscles
Temporal Lobe
• Primary auditory area: involved in hearing
• Auditory association area: associates new
audio information with previous audio
• Wernicke’s area: helps us understand written
and spoken words
Occipital Lobe
• Primary visual area: involved in vision
• Visual association area: associates new visual information with previous visual
information (e.g., facial recognition)
The Diencephalon
The diencephalon is a region that encircles the third ventricle.
Structure and Function of the Diencephalon
• Integrating centre that helps maintain homeostasis
• Regulates hunger, sleep, thirst, body temperature, and water balance
• Controls the pituitary gland and serves as a link between the nervous and endocrine
• Consists of grey matter that receives all sensory input
except smell
• Integrates visual, auditory, taste, and somatosensory
information and sends it to the appropriate area in the
• Involved in higher mental functions (memory,
Pineal gland
• Secretes the hormone melatonin, which is involved in
maintaining a normal sleep-wake cycle
Biology 12
The Cerebellum
The cerebellum is located under the occipital lobe
of the cerebrum.
Structure and Function of the Cerebellum
• Has two portions that are primarily
composed of white matter (a thin layer of
grey matter overlays the white matter)
• Involved in maintaining posture and balance
o Receives sensory input from the
joints, muscles, and other sensory
pathways about the position of body parts
o Receives motor output from the cerebral cortex about where body parts should
be located
• Involved in producing smooth, coordinated voluntary movements (e.g., playing piano,
hitting a baseball)
The Brain Stem
The brain stem contains the midbrain, the pons, and the medulla oblongata.
Structure and Function of the Brain Stem
• A relay station for tracts passing between the cerebrum and spinal cord or cerebellum
• Has reflex centres for visual, auditory, and tactile responses
• Contains bundles of axons travelling between the cerebellum and the rest of the CNS
• Functions with medulla oblongata to regulate breathing rate
Medulla oblongata
• Regulates heartbeat, breathing, and blood pressure
• Contains reflex centres for vomiting, coughing, sneezing, hiccupping, and swallowing
Reticular activating system (RAS)
• Relay centre that is involved in alertness
• Also involved in filtering out unnecessary sensory stimuli (e.g., studying while the TV is