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 CordStructure 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

vertebrae• 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 matter

• 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 CNS− Left side of brain: controls right side of

body− Right side of brain: controls left side of

body

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Functions of the Spinal CordThe 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 brain

• Example: Voluntary movemento 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 BrainThe brain has four major parts:• Cerebrum (two lateral ventricles)• Diencephalon (third ventricle)• Cerebellum (fourth ventricle)• Brain stem (fourth ventricle)

The CerebrumThe cerebrum is the largest part of the brain in humans• Communicates with and coordinates activities of other parts of

the brain

Structure and Function of the CerebrumThe 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)

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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 information

• 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 DiencephalonThe diencephalon is a region that encircles the third ventricle.

Structure and Function of the DiencephalonHypothalamus• 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

systems

Thalamus• 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 cerebrum

• Involved in higher mental functions (memory, emotions)

Pineal gland• Secretes the hormone melatonin, which is

involved in maintaining a normal sleep-wake cycle

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The CerebellumThe 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 StemThe brain stem contains the midbrain, the pons, and the medulla oblongata.

Structure and Function of the Brain StemMidbrain• A relay station for tracts passing between the cerebrum and spinal cord or cerebellum• Has reflex centres for visual, auditory, and tactile responses

Pons• 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

on)