The Nervous System
Neurons: specialized cells of the
nervous system.

3 major regions:
–
1) Dendrites: receive
signals from other
neurons
–
2) Cell Body:
contains the nucleus
and organelles
–
3) Axon: Carries
impulse signal to
other
neurons/muscles
–
4) Myelin sheeth –
surrounds portions
of the axon to protect
and insulate.
Types of neurons:

Sensory: Send impulses from receptors in the sense
organs to the spinal cord and brain.

Motor: Carry a signal from brain and spinal cord to
glands/muscles.

Interneuron: Found in the spinal cord and brain.
Sends messages between sensory and motor
neurons.
Reflex arc

A reflex is an preprogrammed
response to a certain stimulus.

In a reflex a sensory neuron
sends a message to the spinal
cord which immediately relays a
response to a motor neuron.

The brain does not interpret the
response until after it has
happened!
Nerve Impulse – charge traveling
through a neuron.



1. A stimulus is received by
a sensory neuron.
2. The impulse is sent
towards the spinal cord and
then up to the brain to
receive the message.
3. The brain sends a
response to a motor neuron
with reaction instructions.
Action-Potential Nerves: All or nothing!



In order for nerves to send an impulse, they rely
upon the concentration of ions to function properly.
Specifically Sodium and Potassium ions control
impulses.
At rest: Slightly negative inside the cell, slightly
positive outside.
Stimulus causes pumps to activate, dumping positive
sodium ions outside the cell and pulling in less
potassium – reversing the charges on either side of
the membrane.
Sodium-Potassium Pumps



To allow for action
potentials, a protein
pump is used.
The one protein dumps
3 positive charges out of
the cell (Na+) and pulls
in 2 K+ ions.
Each time the pump
makes this switch, ATP
is used.
Impulses

The impulse travels the length of the
nerve cell’s axon until it reaches the
end.

Between 2 neurons there is a gap
called a synapse.

The impulse causes the axon to send
neurotransmitters to the dendrites of the
next neuron to continue the message.
Organization of the Nervous System: 2
Divisions


The nervous system has 2 divisions:
Central Nervous System (CNS)
–
–

Brain and spinal cord
Mostly interneurons
Peripheral Nervous System (PNS)
–
Sensory and motor neurons that carry info to and
from CNS.
CNS - Brain

Cerebrum: Largest portion.
–

Cerebellum: Found in the back.
–

Involved with learning, memory,
speech/language, senses and
voluntary movement.
Controls balance and coordination,
smooth muscle (involuntary
movement).
Medulla Oblongata: Relays signals
between brain and spinal cord.
–
Controls breathing rate, heart rate,
and blood pressure.
CNS – Spinal Cord



Extends from the brain
to the lower back
Protected by the
spinal column or
vertebrae.
Reflexes are
processed by the
spinal cord.
PNS 
Subdivided into Somatic and
Autonomic.

Somatic Nervous System
relay info from sensors to
CNS.
–

comprises voluntary actions
Autonomic carries impulses
from CNS to heart and other
internal organs.
–
Comprise involuntary actions
(out of your control).
Autonomic Divisions

Sympathetic and Parasympathetic divisions.
–
–
–
Sympathetic Nervous System act in times of
stress: High heart rate and breathing.
Parasympathetic Nervous System active when
the body is relaxed.
Both relay messages to the same organs, but the
over responses depends on which division is in
control.
Drug Effects


A drug is a substance that alters some function of the body.
Certain drugs:
–
–
–

change the amount of neurotransmitters released.
Block receptors of neurotransmitters.
Imitate neurotransmitters.
Types of drugs:
–
Stimulants: Increase activity


–
Caffine
Nicotine
Depressants: Decrease CNS, blood pressure and heart rate. Often raise
levels of dopamine  creating a happy feelings.

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Alcohol
Inhalants
Illegal drugs
Disorders


Meningitis – and inflammation
of the meninges.
This can be life threatening
and lead to a number of other
disorders if not treated.
Multiple Sclerosis – Breakdown of the
myelin protective coverings around
neurons.
Can cause paralysis, loss of function,
cognitive issues and death if untreated.