Transcript Structures and Processes of the Nervous System Chapter 8.2 McGraw-Hill Ryerson Biology 12 (2011)

Structures and Processes of the
Nervous System
Chapter 8.2
McGraw-Hill Ryerson
Biology 12 (2011)
Overview of Nervous System
Looks overwhelming, doesn’t it?
Here’s a chart that’s a little simpler
Central Nervous System: brain & spinal cord integrates and
processes information sent by nerves from peripheral nervous
system
Peripheral Nervous System: network of nerves that carry
messages to CNS and send commands from CNS to the
muscles and glands
Nerve Cells (Neurons)
Neurons are similar to other cells in the body
– Surrounded by cell membrane
– Have a nucleus that contains genes
– Contain cytoplasm, mitochondria and other
organelles
– Carry out basic cellular processes such as
protein synthesis and ATP production
Neurons are different by
- Specialized extensions called
dendrites and axons
- Communicate with each
other by electrochemical
process
- Contain some specialized
structures (synapses) and
chemicals (neurotransmitters)
Neurons
• Most neurons consist of a cell
body and extensions called
dendrites and axons.
• Dendrites carry impulses
towards cell body
• Axons carry impulses
away from the cell body
• Cell Body contains the
nucleus
Neurons
• Axons enclosed in fatty,
insulating layer called myelin
sheath
– Myelin sheath protects
neurons and speeds up
rate of nerve impulse
transmission
• Schwann cells form myelin
by wrapping themselves
around the axon
• Nodes of Ranvier are gaps
in myelin sheath
– Capable of electrical activity
Types of Neurons
• Sensory neurons
– Carry nerve impulses from a receptor to the CNS
– Have long dendrites and short axons
• Motor neurons
– Carry nerve impulses from the CNS to an effector
• (ex. muscle or gland)
– Have short dendrites and long axons
• Interneurons
– Found completely within the CNS
– Provide a link within the CNS between sensory neurons
and motor neurons
– Have short dendrites and long or short axons
Reflex Arc
• Simplest nerve pathway
• Occurs without brain coordination
• Five components
– Receptor
– Sensory neuron
– Interneuron in spinal cord
– Motor neuron
– Effector
Animation
Electrical Nature of Nerves
• Neurons use electrical signals to communicate
with other neurons, muscles, and glands
• Signals = nerve impulses
Caused by changes in the amount of electric charge across a
cell’s plasma membrane
Resting Membrane Potential
• Uneven concentrations
of Na+ (outside) and K+
(inside) on either side
of neuron membrane
– results in the inside of
the neuron being -70
mV
– Electrical charge inside
of the cell is negative
relative to outside of
the cell
Resting Membrane Potential
• 3 factors contribute to maintaining resting
membrane potential
1) Large, negatively charged protein
molecules present inside the cell
2) Ion-specific protein channels on
cell membrane allow passive
movement of Na+ and K+
• K+ channels open at resting
allowing K+ to leave cell, making
inside cell negative relative to
exterior
3) Sodium-Potassium pump actively
transports Na+ and K+ in ratios
that leave the inside of the cell
negatively charged
Resting Membrane Potential
• Sodium-potassium pump
– Most important contributor to separation of charge
– Every 3 Na+ transported out of cell, 2 K+ brought in
• Excess positive charge accumulates outside cell
Resting Membrane Potential
Polarization: The process of generating a resting
membrane potential of -70mV
Action Potential
• A nerve cell is polarized because of the
difference in charge across the membrane
– More negative inside the cell than outside
• Depolarization occurs when the cell becomes
less polarized (membrane potential is reduced
to less than resting potential of -70mV)
• Action Potential causes depolarization to
occur
Action Potential
• Nerve signals are transmitted by
action potentials
• Abrupt, pulse-like changes in membrane
potential (few ten thousandths of a second)
• Can be divided into three phases
• Resting/polarized state
• Depolarization
• Repolarization
• The amplitude is nearly constant
and is not related to the size of the
stimulus. Therefore action
potentials are all-or-nothing events.
Threshold potential: a certain level in membrane potential. Once it’s crossed
action potential occurs (point of no return)
Action Potential timeline
Saltatory Conduction
Saltatory conductions
• Nodes of Ranvier contain
many voltage-gated Na
channels
– Na moves into cell, charge
moves quickly through
cytoplasm to next node
– Causes next node’s
membrane to dpolarize to
threshold
• Previous node’s membrane
in refractory state
(prevents action potential
going backward)
– Depolarization initiated to
conduct action potential
– This continues until pulse
reaches end of neuron
Terminology
• Synapse
– Region at which neurons come
nearly together to communicate.
(neuron or effector organ)
• Synaptic Cleft
– Gap between neurons (at a
synapse)
– Impulses can not propagate
across a cleft
• Synaptic Vesicle
– Packets of neurotransmitter in
presynaptic neuron
• Presynaptic Neuron
– Neuron sending a signal (before
the synapse)
• Postsynaptic Neuron
– Neuron receiving a signal (after
the synapse)
Neurotransmitters
Classical transmitters are small molecules (often amino acid based)
Non-classical transmitters can be peptides or even gasses
• 5 general criteria:
1) synthesized and released by
neurons
2) released at the nerve terminal in
a 'chemically identifiable' form
3) the chemical should reproduce
the activity of the presynaptic
neuron
4) can be blocked by competitive
antagonist based on
concentration
5) active mechanisms to stop the
function of the neurotransmitter
Types of Neurotransmitters
Acetylcholine
+ muscles, learning, memory
Serotonin (a derivative
of tryptophan)
Norepinephrine (aka
noradrenaline)
Dopamine
Endorphins
+ sleep, relaxation, self esteem,
too little = depression, perception
+ stress and fight/flight response,
sympathetic NS:+BP & heart rate
+ prolactin (milk production),
involved in pleasure, movement
(-) pain, involved in pleasure
GABA (gamma
aminobutyric acid)
Glutamate
(-) anxiety, too little in parts of
brain can lead to epilepsy
Most common NT, memory, toxic
Homework
• Pg 362 #1, 2, 3, 6, 7, 9, 10