The Nervous System
•
Consists of groups of neurons
and neuroglia found in two
major systems
1.
Central Nervous System
•
•
Brain
Spinal Cord
2. Peripheral Nervous System
•
•
•
Motor Nerves
Sensory Nerves
Interneurons
Two Major Cell Types
Neurons
• Neurons are electrically excitable cells in the nervous system that process
and transmit information
Neuroglia (Glial Cells)
• Glia are estimated to out-number neurons by about 10 to 1 in the brain
• The main functions of glial cells are to:
– surround neurons and hold them in place (known as the « glue » of the
nervous system
– form myelin
– participate in signal transmission
– supply nutrients and oxygen to neurons,
– insulate one neuron from another,
– and to destroy pathogens and remove dead neurons
A Generalized Neuron
Nucleus
Axon terminals
Cell body
Myelin sheath
Nodes
Axon
Dendrites
The Neuron
• Although there are several types of neurons, they all have certain
features in common
Cell body – contains most of the cytoplasm, nucleus, most
metabolic activity occurs here.
Dendrites – carry impulses from the environment or from other
neurons toward the cell body
Axon – carries impulses away from the cell body towards the axon
terminals (axon buds)
Myelin sheath – accelerates the transmission of electric signal across
the axon
Generalized Nerve Cell Detailed
Neurons in Culture
Schwann
Cells and
the Myelin
Sheath
•
Schwann cells provide
•
They also have
phagocytotic activity
and clear cellular debris
that allows for regrowth
of PNS neurons.
myelination to axons in
the peripheral nervous
system (PNS).
Myelinization of the Axon by
an Oligodendrocyte
Oligodendrocyte
Axon
Myelin
Sheath
Myelin Sheath
Astrocytes (type of glial cell),
Blood vessels and Neuron
Blood
vessels
Astrocyte
Branches
Cell body of
neuron
Astrocyte
foot
Cell body of
astrocyte
Microgliocyte (Macrophagic)
The Nervous Impulse
• The transmission of an electrical impulse across
the axon is called an Action Potential
• An action potential occurs when the normally
negatively charged environment within the axon
becomes momentarily positively charged
• How does this happen?
Sodium-Potassium
Pump at Resting Potential
The electrical
potential of a
neuron is 70
millivolts (1/20
the voltage of a
1.5 V battery)
Neuron at Resting Potential
Resting to Action Potential
• The neuron moves from a resting
potential (axon is –vely charged) to an
action potential (axon becomes
momentarily +vely charged) when a
stimulus causes Sodium ion
gates/channels to open and allow Na+
ions to flow into the axon and thus make
it positively charged
Nerve Impulse & Action Potential
Propagation of Action
Potential
• As the Na+ gates open along the axon, K+ gates
also open allowing K+ ions to flow out of the
axon and thus return the axon to resting
potential
• The nervous impulse is self-propagating. That is
that, an impulse at any point on the membrane
causes an impulse at the next point along the
membrane by opening the Na+ channels
Propagation
of Action
Potential
The All-or-Nothing Principle
• A nervous impulse/action potential is only
caused when a stimulus from the environment is
strong enough reach a neuron’s Threshold
• A threshold is the minimum level of stimulus
need to cause an action potential
• The nervous impulse, however, is the same no
matter how strong the stimulus is as long as it
has reached the neuron’s threshold.
Passing on the Message
• When the action potential reaches the
axon terminal, the neuron may pass it’s
impulse on to the next cell
• The next cell may be another neuron, or it
may be a muscle cell, if the neuron is a
motor neuron
The Synapse
• A synapse is the location where a neuron can transfer an impulse to
another cell
• The synapse is the cleft, or gap between the axon terminal of one
neuron and the dendrite of another neuron or muscle cell
• The axon terminals contain vessicles that carry neurotransmitters
• They release the neurotransmitters into the synaptic cleft which
bind to receptor proteins on the adjacent cell
• Here, the neurotransmitter either propagates another action
potential in the adjacent neuron or causes a muscle to contract
The Synapse
Direction of Impulse
Dendrite of
adjacent neuron
Axon
Vesicle
Receptor
Axon
terminal
Synaptic cleft
Neurotransmitter
Nervous Propogation
Cell
Body
Axon
Axon
Terminal
Synaptic
Vessicle
Ionic Gates/Channels
Neural Interactions
The Nervous System
CNS & PNS
The Central
Nervous System
Central Nervous System
• Control centre of the body
• Processes, analyses information and relays
messages
• Consists of
• Brain
• Spinal cord
• Very well protected by the skull, vertebrae and
meninges
Meninges
• Three layers of connective tissue that surround
both brain and spinal cord
• Between the meninges we find the cerebrospinal
fluid which bathes the CNS and acts as a shock
absorber
• Cerebrospinal fluid also allows the exchange of
nutrients and waste products between blood and
nervous tissue
Gray vs. White Matter
• Cerebrum consists of two surfaces:
– Cerebral Cortex
• Gray matter (~1 inch thick)
• Densely packed nerve cell bodies
– Inner layer
• White matter
• Axons covered by myelin sheaths which give it its
white colour (cholesterol)
Gray vs. White Matter
The Brain – Parts
• Major parts:
• Cerebrum
• Cerebellum
• Brain stem (pons & medulla oblongata)
• Thalamus
• Hypothalamus
• Pineal gland
• Pituitary gland
Cerebrum
• Largest structure in human brain
• Voluntary activities, intelligence, learning, judgement
• Deep groove running from front to back separates brain
into left & right hemispheres
• Each hemisphere is divided into four lobes named after
the bones that cover them
• Left half controls right side of the body and right half
the left side
• Right side is associated with creativity and artistic talent
• Left side is associated with analytical & math skills
• Folds & grooves serve to increase surface area of the
brain
Cerebellum
• 2nd largest structure
• Although the commands to make muscles
contract comes from the cerebral cortex,
the cerebellum coordinates & balances the
actions of the muscles so that they move
efficiently
Brain Stem
• Pons & medulla oblongata
• Acts as the brain’s switchboard by regulating the
flow of info between the brain and the rest of
the body
• « Primitive » brain that controls the vitals
•
•
•
•
Blood pressure
Heart rate
Breathing
Swallowing
Thalamus & Hypothalamus
• Thalamus
• Receives info from sensory organs and relays the
proper info to the cerebrum
• Hypothalamus
• Control centre for the recognition & analysis of
hungar, thirst, fatigue, anger, and body
temperature
Pineal & Pituitary Glands
• Pineal gland (non-nervous tissue)
• Releases melatonin which controls sleep-wake
cycles
• Pituitary gland (non-nervous tissue)
• Produces hormones that regulate many other
endocrine glands
Evolution of the Vertebrate
Brain
Penfield & Rasmussen
The Spinal Cord
• Main link between the
brain and the rest of the
body
• Reflexes (ex. sneezing,
blinking) are processed
directly by spinal cord
instead of being first sent
to the brain in order to
make the movement
more rapid
The Spine
The Spine – Whole
Peripheral
Nervous
System
The Peripheral Nervous System
• Divided into two major divisions:
– Sensory (transmits impulses from sensory organs to CNS)
– Motor (transmists impulses from CNS to muscles or glands)
• Consists of:
– Cranial nerves
– Spinal nerves
– Ganglia (collections of nerve cell bodies)
Motor Division of the PNS
• Divided into:
– Somatic Nervous System (voluntary movement such
as the movement of skeletal muscles & arc reflexes)
– Autonomic Nervous System (involuntary movement
such as heartbeat, peristalsis in digestive tract)
• Divided into:
– Sympathetic NS – like the accelerator
– Parasympathetic NS - like the brakes
Arc Reflex – Monosynaptic vs.
Polysynaptic reflexes
Autonomic
Nervous System
The Nervous
System
is divided into
Central nervous
system
Peripheral nervous
system
which consists of
Motor nerves
that make up
Somatic nervous
system
Autonomic nervous
system
which is divided into
Sympathetic nervous
system
Parasympathetic
nervous system
Sensory nerves
Drugs & the
Nervous
System
Pharmaceutical Drugs
Drug Type
Medical Use
Examples
Effects on the body
Stimulants
Used to increase alertness,
relieve fatigue
Amphetamines
Increase heart and respiratory rates;
elevate blood pressure; dilate pupils;
decrease appetite
Depressants
Used to relieve anxiety,
irritability, tension
Barbiturates
Tranquilizers
Slow down the actions of the central
nervous system; small amounts cause
calmness and relaxation; larger
amounts cause slurred speech and
impaired judgement
Opiates
Used to relieve pain
Morphine
Codeine
Act as a depressant; cause
drowsiness, restlessness, nausea
Drugs & the Brain
• One of the reasons that drugs of abuse can
exert such powerful control over our
behaviour is that they act directly on the
more evolutionarily primitive brain-stem
and limbic system structures, which can
override the cerebral cortex in controlling
our behaviour.
Drugs Affect
Neurotransmission
•
Almost all drugs work by affecting
neurotransmission in one of four ways:
1.
Mimic natural neurotransmitters (ex. LSD)
2. Block receptors and thus block neurotransmission
(ex; PCP)
3.
Block the reabsorption of a neurotransmitter back
into the axon terminal after nervous impluse (ex.
cocaine)
4. Cause neurotransmitters to be released in greater
than normal quantities (ex. methamphetamine)
Cocaine
• Causes the release of dopamine in the pleasure centre of
the brain and then blocks its reabsorption back into the
axon terminal after the nervous impulse.
• This causes the adjacent neurons to keep firing and
produce even more pleasure
• Problem: dopamine stores get used up and the user feels
depressed without the drug
• Causes addicts to commit serious crimes and abandon
their families and children to get another « hit »
• Crack users may become addicted even after the first try
Opiates
• Mimic endorphins (natural transmitters) which are used
to overcome pain
• Produce pleasure and a feeling of security, but then the
body gets used to the higher concentrations and then
can deal without this new concentration.
• Users who stop suffer from uncontrollable pain and
sickness because the body cannot produce enough
natural endorphins
• Include opium, heroin, codeine and morphine
Marijuana
• Active ingredient is THC
(tetrahydrocannabinol)
• Binds to cannabinoid receptors in the brain
• Alters the messages that affect sensory
perception and coordination
• Receptors are found in the parts of the brain
that control memory, thought, concentration,
time and depth perception and coordinated
movement