Biology 12
Unit 1:Maintaining Dynamic Equilibrium
Coordination and
Regulation in Humans
 The Nervous System
and Homeostasis
 Structure of a neuron
 Action Potential
 Synaptic
Transmission
 Structure of the Brain
 Organization of the
Nervous System
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Nervous System
Central Nervous
System
Brain
Peripheral Nervous
System
Spinal Cord
Somatic
Nerves
Sensory
Adapted from
Nelson Biology
Autonomic
Nerves
Motor
Sympathetic
Parasympathetic
Structure of a Neuron
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From Nelson Biology
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Types of Neurons
 Motor Neuron
 Connects the central nervous system to a
muscle or a gland
 Sensory Neuron
 Connects a sensory receptor to the central
nervous system
 Interneuron
 Connects two or more neurons
Action Potential
 In 1900 Bernstein hypothesized that
nerve impulses where electrochemical in
nature.
 Future experimentation proved this.
 Giant Squid Experiment:
 Cole and Curtis placed two tiny electrodes –
one inside the large axon of a squid and the
second across from the first outside the
axon.
Giant Squid Experiment
Squid Axon
•Cole and Curtis measured the electrical potential
across the membrane.
•The resting membrane potential was found to be
about –70mV.
 When stimulated, the action potential
jumped to about +40 mV.
 The action potential only lasted for a
few milliseconds before the nerve cell
returned to the resting potential.
+40
threshold
mV
-70
1
2 ms 3
4
Maintaining Resting
Potential
 Caused by an uneven distribution of
positively charged ions across the
membrane
 Set up and maintained by a SodiumPotassium pump.
 Potassium is pumped
into the cell, Sodium
out of the cell.
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 These positive ions want to move with
their concentration gradient by diffusion.
 Potassium diffuses out faster than
sodium diffuses in leaving a negative
charge inside the cell.
 The cell is polarized. At rest there are
more ion gates open for potassium.
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Action Potential
 When stimulated the ion gates for potassium
close and the ion gates for sodium open up.
 Positive ions flood into the cell making it
positive. This rapid inflow is referred to as
depolarization.
 After the impulse, the gates return to the
resting condition with extra potassium gates
open. The flow of potassium ions out of the
cell restores the resting potential.
 The Na+/K+ pump continues to pump the
sodium and potassium across the membrane
against the concentration gradient to restore
the separation of Na+ and K+.
Repolarization
 Repolarization takes about 0.001
seconds.
 Once stimulated, the membrane cannot
be polarized until after the refractory
period.
Summary of Impulse
1. At rest – Na+/K+ pump moving ions –
potassium gates open
2. Stimulation – potassium gates close – sodium
gates open
3. The flood of sodium into the cytoplasm
stimulate adjacent areas
4. Refractory – potassium gates open – sodium
gates close
5. At rest – Na+/K+ pump moving ions –
potassium gates open
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Reflex Arc
•Reflexes are autonomic responses to certain stimuli.
•They are not under conscious control, they are involuntary.
•The pathway that a nerve impulse takes is called a reflex arc.
•We need to identify the stimulus, receptor, sensory neuron,
motor neuron, effector, and the response.
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Synaptic Transmission
 Neurons are not directly
connected to each other.
 The electrochemical action
potential cannot jump the
synaptic cleft (or synapse).
 Synaptic transmission is
entirely chemical in nature.
 Not all neurons cause
depolarization in the post
synaptic membrane.
Some neurons are
inhibitory.
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Central Nervous System
Protection





Meninges: protective membranes
Dura mater: outer layer
Arachnoid: middle layer
Pia mater: inner layer
Cerebrospinal fluid: shock-absorbing
fluid that circulates in the inner meninges
and the vesicles of the CNS
 Skull: the hard bone "helmet"
The Spinal Cord and
Vertebrae Dorsal root: sensory
information to the CNS
Ventral root: motor
information to the PNS
White Matter: Myelinated
nerve fibres (sensory and
motor)
Grey Matter: Unmyelinated
interneurons
Gross Anatomy
 Hindbrain: oldest part of brain.
 Cerebellum: limb movements, balance, muscle tone.
 Medulla Oblongata: connects PNS and CNS, regulates
involuntary muscle movements.
 Pons: "bridge" between cerebellum and medulla.
 Midbrain: relay center for some eye and ear reflexes
 Reticular formation: controls respiration and circulation.
Filters sensory stimulus between conscious and
unconscious.
 Forebrain:
 Cerebrum: conscious thought
 Cerebral hemispheres: Left-logical; right-creative
 Corpus collosum: connects left and right
hemispheres
 Cerebral cortex: outer layer of cerebrum, sensory
processing area
 White matter: myelinated neurons
 Gray matter: unmyelinated neurons
 Thalamus: directs incoming sensory signals
 Hypothalamus: homeostasis
The Brain
Organization of
the Nervous System
The CNS:
 The brain and spinal cord
The PNS:
 Everything else
The PNS
 One division of the PNS is the
Autonomic Nervous System.
 The ANS controls the unconscious
activities of the body (heart rate etc.)
 The PNS is further subdivided into the
sympathetic (stress responses) and the
parasympathetic (relaxation) systems.
 Balance between these two systems
helps achieve homeostasis.
Sympathetic
Nervous System
Parasympathetic
Nervous System
Nervous System
Central Nervous
System
Brain
Peripheral Nervous
System
Spinal Cord
Somatic
Nerves
Sensory
Adapted from
Nelson Biology
Autonomic
Nerves
Motor
Sympathetic
Parasympathetic
Compare Endocrine and
Nervous System
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