Ch 12-Nervous System
3% of your body weight
2 divisions
• Central Nervous System (CNS)- brain & spinal cord
• Peripheral Nervous System (PNS)- nerves in rest of body
Afferent- brings sensory information to CNS from peripheral
tissues & organs
Efferent- carries motor from CNS to muscles, glands, & adipose
1. Somatic NS- sensory receptors for senses, motor from skeletal
2. Autonomic NS- sensory for visceral, motor CNS to smooth &
cardiac muscle & glands
1. Sympathethic- fight or flight
2. Parasympathetic- rest & digest
Nervous Tissue
• Neurons- structural & functional units
- react to physical & chemical changes
- send nerve impulses along nerve fibers
• Neuroglia- supporting cells (protect, nourish, cleanup, regulate)
Parts: Pg 376
1. cell body- nucleus surrounded by
cytoplasm, rough ER called Nissl
bodies
2. dendrites- receiving portions of
neurons, usually not myelinated
3. axon- moves nerve impulses toward
another neuron, muscle fiber, or
gland
Others:
- joins cell body at axon hillock
- first part called initial segment
- axoplasm- cytoplasm
- axolemma- plasma membrane
- axon collateral- branch & axon
terminal- end
- Trigger zone- initial segment meets
axon hillock, where impulse arises
Synapse
• site of communication b/w two neurons, must have at least 2 neurons for
a reflex
• Presynaptic cell-sends a message
• Synaptic end bulb- tips of axon terminal
• contain synaptic vesicles that store neurotransmitters
*Separated by synaptic cleft*
• Postsynaptic cell-receives the message
• If receiving cell is a:
muscle = Neuromuscular junction
Gland = neuroglandular junction
Structural Classes of Neurons
• 1. Anaxonic- not distinguishing dendrite from axon
• 2. Bipolar- 2 distinct processes(1 dendrite & 1 axon)
• Rare- in special sense organs for sight, smell, hearing
• small
• 3. Unipolar- dendrites & axons are continuous, cell body inbetween
• Peripheral nervous system, long
• 4. Multipolar- 2 or more dendrites & an axon
• Most common in CNS, control skeletal muscles, long
Functions
*SAME*
1.Sensory- “afferent neurons” (to brain), at the ends of neurons
-gather info if something changes & convert to impulses
• Interoceptors- inside
•
•
systems (digestive, respiratory, cardiovascular)
senses (deep pressure, pain)
• Exteroceptors- outside
•
touch, temp, pressure, sight, smell, hearing
• Proprioceptors- body position from muscles & joints
2. Integrate- “interneurons” (in brain), signals are sent to brain
- create sensation, memory, produce thoughts
3. Motor- “efferent neurons” (away from brain), act upon impulses
effectors- response structures (muscles & glands)
Myelination
• Myelin Sheath- multilayered lipid & protein covering, insulate the
axon & increases speed
- more myelin = faster impulse
- Neurolemma- outer layer of Schwann cell, aids in regeneration of
injury
- Internodes-where myelin occurs
- Nodes of Ranvier- gaps in myelin sheath
Conduction- how it travels
1. Continuous- straight line
2. Saltatory- myelinated fibers, jumps, fastest
Disorders of Myelin
-all can lead to paralysis
• Chronic exposure to heavy-metals (lead, arsenic,
mercury) leads to demyelination
• Diphtheria- from bacterial infection, toxin damages
myelin, we have a vaccine
• Multiple Sclerosis- affects axons in the optic nerve,
brain, & SC
• Loss of vision, problems with speech, balance, coordination
• Can be progressive, 30-40yrs of age, more in women
• Guillain-Barre Syndrome- autoimmune,
demyelination of PNS
• Weakness & tingling > paralysis
• Triggered by virus
• Most fully recover
MShttp://www.youtube.com/watch?v=qgySDmRR
zxY
What’s the Matter?
• White Matter- has myelin, axons
• Gray Matter- little or no myelin
- cell bodies, dendrites, unmyelinated axons, axon terminals,
neuroglia
Neuroglia- Nourishes neurons
• Separate/protect neurons
• Provide framework for neural tissue
• Regulate composition of interstitial fluid
• Act as phagocytes
Classification of Neuroglia of CNS
1. Astrocytes- (star), most numerous
maintain blood-brain barrier, structural support, regulate ions, absorb & recycle
neurotransmitters, form scar tissue
2. Oligodendrocytes- form myelin in CNS, structural framework
3. Microglia- janitors/police (eat bacterial cells & debris), smallest, least numerous
4. Ependymal- lines central canal of sc and ventricles of brain, regulate the
composition of cerebrospinal fluid
Classification of Neuroglia of PNS
1. Schwann- produce myelin in the PNS
2. Satellite- regulate exchange of material b/w
neuronal cell bodies & interstitial fluid
Neural Response to Injury
Wallerian Degeneration- PNS
• 1. fragmentation of axon &
myelin occurs in distal stump
• 2. Schwann cells form cord, grow
into cut, & unite stumps
• Macrophages engulf debris
• 3. Axon sends buds into network
of Schwann cells and then starts
growing along cord of Schwann
cells
• 4. Axon grows
CNS- more complicated
• More axons are likely involved
• Astrocytes produce scar tissue that can prevent growth
• Astrocytes release chemicals that block growth
How does a nerve impulse happen?
Ion movements & electrical signals:
• All plasma membranes produce electrical signals
by ion movement
• Transmembrane potential is particularly important
to neurons
• Main membrane processes:
1. Resting Potential- work it may do, difference
in electrical charge
• Inside- negative (K+), Outside- positive (Na+) & (Cl-)
2. Graded potential- temporary, change in
resting potential caused by stimulus
3. Action potential- electrical impulse produced
by graded potential propagated along axon to
synapse
4. Synaptic activity- releases neurotransmitters
producing graded potentials in postsynaptic cell
membrane
5. Information processing- response of
postsynaptic cell
Electrochemical
GradientsSum of
chemical and
electrical forces
acting on ion
across
membrane
Why is the
inside -?
1. Negatively
charged
proteins inside
the cell are too
large to cross
2. it is easier for
K+ to diffuse
out than Na+ to
enter
Membrane channels
Passive/Leakage channels- always
open
Active/Gated channels- open or
close in response to stimuli
• Chemically/Ligand gatedresponse to chemicals
(hormones, neurotransmitters)
• Ex: Ach receptors at
neuromuscular junction
• Voltage gated- response to
changes in transmembrane
potential
• Mechanically gated- response to
physical distortion of membrane
surface by touch, pressure or
vibration
Action potential
when a nerve detects a change
1. Resting state- all channels closed
-Threshold- gates open
2. Depolarizing phase- Na channel
opens and Na comes in
- makes more positive
3. Repolarizing phase- Na channel
closes, K channel opens and K flows
out, makes more negative until
4. Resting state
*exception- can go from #1 to
Hyperpolarized = more negative
-happens if K+ opens right away
Refractory Period- short gap in time b/w impulses
All or None – if nerve responds, it is completely
http://www.youtube.com/watch?v=ifD1YG07fB8
Neurotransmitters
Excitatory-promote action potential
Inhibitory- suppress action potential
1. Acetylcholine (Ach)- skeletal muscles
2. Norepinephrine- consciousness & attention, body temp
3. Dopamine- emotions, addictions, pleasure, subconscious motor
function
4. Serotonin- senses, temperature, mood (lack of=depression), appetite
5. Glutamate & Aspartate- memory, learning, excitatory
6. GABA- inhibitory, anti-anxiety
Neuromodulators
• Other chemicals released by synaptic terminals
• Similar in function to neurotransmitters
• Alter rate of neurotransmitter release or change post-synaptic cell’s
response
• Opioids like endorphins- relieve pain
Drugs & Addiction
• Drugs can release two to 10 times the amount of dopamine that
natural rewards do, and they do it more quickly and more reliably
• brain responds by producing less dopamine naturally or eliminating
dopamine receptors
• Alcohol: increases GABA & decreases glutamate=increase in
dopamine
• Cocaine: inhibits removal of dopamine from synapses = “high”
• THC: stimulates release of dopamine = euphoria, drowsiness,
appetite
• Ecstasy: targets serotonin receptors > mood
Parkinson’s Disease
• damage/degeneration of dopamine producing neurons
• Usually after 50, more in men, can be genetic
Symptoms:
• Problems with balance and walking
• Rigid or stiff muscles
• Difficulty swallowing
• Drooling
• Slowed, quieter speech and monotone voice
• No expression in your face (like you are wearing a mask)
• Tremors
Epilepsy
• short, recurrent attacks of motor, sensory, or psychological malfunction
• http://www.youtube.com/watch?v=6NcqQkKjqTI