Chapter 4 Notes
The brain contains anywhere between 10 billion and 100 billion nerve cells and about as many helper cells
that take care of support and housekeeping functions.
Donald Hebb considered how individual nerve cells are organized into larger units. He proposed specific
principles of organization by which these units were structurally organized and suggested how they could
generate higher processes of the brain such as memory, thought, and decision making.
Since Hebb’s work, neuroscientists have learned that the nerve cells are actually organized in modules i.e.
clusters of nerve cells that communicate with each other. Particular modules have particular functions just
as transistors, resistors and capacitors in a computer chip do.
The brain controls behaviour, processes and retains information we receive from the environment and
regulates the body’s physiological processes.
The nervous system consists of 2 divisions:
Central nervous system – The brain and the spinal cord
-Spinal cord – A long, thin collection of nerve cells attached to the base of the brain
and running length of the spinal column. It contains a circuit of nerve cells that
control simple reflexes such as pulling away from hot objects.
-Nerve – A bundle of nerve fibres that transmit information between the central
nervous system and the body’s sense organs, muscles, and glands.
Peripheral nervous system – The cranial and spinal nerves; that part of the nervous system
peripheral to the brain and spinal cord.
The human brain has 3 major parts:
Brain stem – The “stem” of the brain, including the medulla, pons, and mid-brain. It is one of the
most primitive regions of the brain and its functions are basic ones such as primarily control of
physiological functions and automatic behaviours. Usually amphibians’ brain only have brain
stem and the cerebellum.
Cerebral hemisphere – The largest part of the brain; covered by the cerebral cortex and
containing parts of the brain that evolved most recently.
Cerebellum – A pair of hemispheres resembling the cerebral hemispheres but much smaller and
lying beneath and in back of them; controls posture and movements, especially rapid ones.
Vertebra – One of the bones that encase the spinal cord and constitute the vertebral column.
Meninges – The 3 layered set of membranes that enclose the brain and spinal cord.
Chapter 4 Notes
Cerebrospinal fluid (CSF) – The liquid in which the brain and spinal cord float; provides a shock-
Cerebral cortex – The outer layer of the cerebral hemispheres of the brain, approximately 3 mm thick.
Grey matter – The portions of the central nervous system that are abundant in cell bodies of neurons
rather than axons. The colour appears grey relative to white matter. It contains billions of nerve cells. This
is where perceptions take place, memories are stored and plans are formulated and executed.
White matter – The portions of the central nervous system that are abundant in axons rather than cell
bodies of neurons. The colour derives from the presence of the axons’ myelin sheaths.
The human cerebral cortex is very wrinkled; full of bulges separated by grooves. The bulges are called
gyri and the large grooves are called fissures. Fissures and gyri expand the amount of surface area of the
cortex and greatly increase the number of nerve cells it can contain.
Nerves are bundles of many thousands of individual fibres, all wrapped in a tough, protective membrane.
Spinal nerve – A bundle of nerve fibres attached to the spinal cord; conveys sensory information from the
body and carries messages to muscles and glands.
Cranial nerve – A bundle of nerve fibres attached to the base of the brain; conveys sensory information
from the face and head and carries messages to muscle and glands. There 12 pairs of it attached to the
Neuron – A nerve cell; consists of a cell body with dendrites and an axon whose branches end in terminal
buttons that synapse with muscle fibres, gland cells, or other neurons. Elements of the nervous system
that bring sensory information to the brain, store memories, reach decisions and control the activity of the
Glial cell – A cell of the central nervous system that provides support for neurons and supplies them with
some essential chemicals. During development of the brain, some types of glial cells form long fibres that
guide developing neurons from their place of birth to their final resting place. Others form protective
insulating sheaths around nerve fibres. Others serve as the brain’s immune system.
3 basic parts of a neuron:
Soma – A cell body; the largest part of a neuron. It contains mechanisms that control the
metabolism and maintenance of the cell. It also receives messages from other neurons.
Dendrite – A treelike part of a neuron on which other neurons form synapses.
Axon – A long, thin part of a neuron attached to the soma; divides into a few or many branches,
ending in terminal buttons. It carries messages away from the soma to the cells with which
Chapter 4 Notes
neurons communicate. These messages are called action potentials and it consists of brief changes
in the electrical charge of the axon. It is usually referred to as the firing of an axon.
There are 2 complex structures seen in neurons:
Dendritic spine – A small bud-like protuberance on the surface of a neuron’s dendrite.
Terminal button – The rounded swelling at the end of the axon of a neuron; releases transmitter
Terminal substance – A chemical released by the terminal buttons that causes the postsynaptic neuron to
be excited or inhibited. These chemicals are referred to as neurotransmitters.
Many axons, especially long ones, are insulated with a substance called myelin. The white matter located
beneath the cerebral cortex gets its colour from the myelin sheaths around the axons that travel through
Myelin sheath – The insulating material that encases most large axions.
Myelin is part protein and part fat and it is produced by the glial cells. The function of the myelin is to
insulate axons from each other and thus to prevent the scrambling of messages. It also increases the speed
of the action potential.
When the axon is resting, the inside is charged at -70 millivolts with respect to the outside.
Action potential – A brief electrochemical event that is carried by an axon from the soma of the neuron
to its terminal buttons; causes the release of a transmitter substance.
Ion – A positively or negatively charged particle; produced when many substances dissolve in water.
Ion channel – A special protein molecule located in the membrane of a cell; controls the entry or exit of
Ion transporter – A special protein molecule located in the membrane of a cell; actively transports ions
into or out of the cell. It pumps sodium ions out of the axon and pump potassium ions back in, restoring
the normal balance.
Synapse – The junction between the terminal button of one neuron and the membrane of a muscle fibre, a
gland, or another neuron.
Presynaptic neuron – A neuron whose terminal buttons form synapses with and excite or inhibit another
Postsynaptic neuron – A neuron with which the terminal buttons of another neuron form synapses and
that is excited or inhibited by that neuron.