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STRUCTURES & PROCESSES OF
THE NERVOUS SYSTEM
Miss Richardson
SBI4U
THE HUMAN NERVOUS SYSTEM

Components:

Central nervous system (CNS)
Brain
 Spinal cord
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
Peripheral nervous system (PNS)
Somatic nervous system (conscious control)
 Autonomic nervous system (no conscious control)


Messages are communicated through three types
of nerve cells (neurons)


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Sensory (afferent)neurons
Interneurons
Motor (efferent) neurons
THE HUMAN
NERVOUS SYSTEM
THE REFLEX ARC

A simple reflex is an immediate response to a
stimulus that doesn’t require the brain – only the
spinal cord
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6.
Sensory receptors pick up the
stimulus – the sensation of the tap on
the knee.
This information travels along the
afferent (sensory) neuron to the
spinal cord.
The information is transmitted from
the afferent neuron in the spine to an
interneuron in the spine
The interneuron relays the
information to an efferent (motor)
neuron in the spine.
The efferent neuron sends the
information to the muscles (the
effector) in the thigh.
These muscles contract and the knee
jerks.
NEURONS
Neurons are responsible for the reflex response
 All neurons contain:

Cell body
 Dendrites
 Axon

NEURONS
NEURONS
The dendrites receive information and send it out
to the cell body
 The cell body is where the nucleus is found

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Axons project out from the body

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Determines if information will be passed on
Carry the nerve impulse from the cell body to the
axon terminal and synapse
When nerve impulse reaches the axon terminal,
the messages are relayed to the effector using
chemicals called neurotransmitters
RESTING MEMBRANE POTENTIAL
If a neuron is not sending a signal or impulse it is
said to be at rest
 While at rest, potassium ions (K+) are found
mainly inside the membrane, and sodium ions
(Na+) are found mainly outside the membrane
 Resting potential - difference in electric charge
across the membrane of a neuron, measured in
millivolts (-70mV)

MEMBRANE PERMEABILITY
While at rest the neuronal membrane is more
permeable to K+ than to Na+
 Therefore K+ diffuses out of the nerve cell (down
its concentration gradient) faster than Na+
diffuses in.

Na+
K+
THE NERVE IMPULSE
Neurons communicate with each other by
sending nerve impulses down the length of the
axon and across a gap (called a synapse) to
another neuron.
 As a nerve cell is stimulated there is a rapid
influx of Na+
 A nerve impulse occurs as there is a rapid
reversal of the electrical potential across the
neurons membrane due to this influx

 ACTION POTENTIAL
TERMINOLOGY


Depolarization: Diffusion of Na+ ions into the
nerve cell resulting in an ACTION POTENTIAL
Repolarization: Na+/K+ pump restores the
original resting potential of the neuron
THE ACTION POTENTIAL
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At rest, the outside of the neuron is more
positive than the inside
Nerve stimulation causes membrane to become
more permeable to Na+
Depolarization: Na+ rushes in making the
inside more permeable and resulting in an
ACTION POTENTIAL
Repolarization: K+ flows out of the
membrane restoring the resting potential
(outside more +ve)
Na+/K+ pump uses ATP to pump Na+ out and
K+ into neuron restoring ion concentrations
ACTION POTENTIAL
THE ACTION POTENTIAL

Action potentials are often represented in a
graph
“ALL OR NOTHING”
Neurons either fire maximally or not at all, this
is referred to as the “all or none” response
 Increasing neuronal stimulation beyond a critical
level will not result in an increased response
 Neurons response to increased stimulation by
increasing the frequency of firing, not the
intensity at which they fire.
 The Threshold level is the minimum strength
of stimulation required to produce an action
potential
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COMMUNICATION

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When an AP reaches the end of the axon the impulse
must be transmitted to another adjacent neuron.
This communication occurs between neurons across
spaces called synapses.
The message is not transmitted electrically as it was
down the axon, but rather by the release of chemical
messengers.
SYNAPSE