Chapter 12 – Introduction to the Nervous System

• Organization

• Cell Types

Review

What 3 parts make up the nervous system?

1. Brain

2. Spinal cord

3. Nerves

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Functions of the Nervous System

• Detect changes (stimuli) in the internal or external environment

• Evaluate the information• Initiate a change in muscles or glands

Goal – maintain homeostasis

What does this remind you of??

Organization of the Nervous System• Central nervous system (CNS)

– Brain and spinal cord

• Peripheral nervous system (PNS)– Nervous tissue in the outer regions of the

nervous system– Cranial nerves: originates in the brain– Spinal nerves : originates from the spinal cord– Central fibers: extend from cell body towards

the CNS– Peripheral fibers: extend from cell body away

from CNS

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Afferent vs Efferent

Nervous pathways are organized into division based on the direction they carry information

• Afferent division: incoming information (sensory)

• Efferent division: outgoing information (motor)

(Efferent = Exit)

Somatic & Autonomic Nervous Systems

Nervous pathways are also organized according to the type of effectors (organs) they regulate

• Somatic nervous system (SNS)– Somatic sensory division (afferent)– Somatic motor division (efferent)

Somatic & Autonomic Nervous Systems cont…

• Autonomic nervous system (ANS): Carry information to the autonomic or visceral effectors (smooth & cardiac muscles and glands)– Visceral sensory division (afferent)– Efferent pathways

• Sympathetic division – “fight or flight”• Parasympathic division – “rest and repair”

Figure 12-2

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Review

What are the two main cell types in the nervous system?

(Hint: we talked about this when we covered tissue types)

Answer: neurons and glia

Cells of the Nervous System

Neurons: excitable cells that conduct information

Glia (also neuroglia or glial cells): support cells, do not conduct information– Most numerous– Glia = glue

Types of Glia

Five major types:

1. Astrocytes

2. Microglia

3. Ependymal cells

4. Oligodendrocytes

5. Schwann cells

Astrocytes (12-3A)• Star-shaped, largest, most numerous

• Cell extension connect neurons and capillaries– Transfer nutrients from blood to neuron– Help form blood-brain barrier (BBB)

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Blood-Brain Barrier

• Helps maintain stable environment for normal brain function

• “feet” of astrocytes wrap around capillaries in brain

• Regulates passage of ions• Water, oxygen, CO2, glucose and alcohol

pass freely• Important for drug research

– Parkinson’s Disease

Microglia (12-3B)

• Engulf and destroy cellular debris (phagocytosis)

• Enlarge during times of inflammation and degeneration

Ependymal cells (12-3C)• Similar to epithelial cells

• Forms thin sheets that line the fluid-filled cavities of the brain and spinal cord

• Some cells help produce the fluid that fills these cavities (cerebral spinal fluid - CSF)

• Cilia may be present to help circulate fluid

http://www.lab.anhb.uwa.edu.au/mb140/corepages/nervous/Images/epen100he.jpg

Oligodendrocytes (12-3D)• Hold nerve fibers together

• Produce myelin sheaths in CNS

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Multiple Sclerosis (MS)• Most common myelin disorder• Characterized by:

– myelin loss and destruction injury and death plaque like lesions

– Impaired nerve conduction weakness, loss of coordination, vision and speech problems

– Remissions & relapses

• Autoimmune or viral infection• Women 20-40 yrs• No known cure

Multiple Sclerosis (MS)

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Schwann cells (12-3E)

• Only in PNS

• Support nerve fibers & form myelin sheaths

• Satellite cells (12-3G)– Types of schwann cell that covers a neuron’s

cell body

http://legacy.owensboro.kctcs.edu/gcaplan/anat/images/Image425.gif

Neurons

All neurons have 3 parts:

1. Cell body (soma)

2. Axon

3. One or more dendrites

Neuron Anatomy• Soma resembles other cells• Nissl bodies – part of rough ER; contain

proteins necessary for nerve signal transmission & nerve regeneration

• Dendrites – branch out from soma; receptors; conduct impulse towards soma

• Axon – process that extends from the soma at a tapered portion called the axon hillock– Axon collaterals: side branches– Telodendria: distal branches of axon– Synaptic knob: ends of telodendria

http://academic.kellogg.edu/herbrandsonc/bio201_mckinley/f14-3a_structures_in_a__c.jpg

Neuron Anatomy

• Myelin sheaths: areas of insulation produced by Schwann cells; increases speed of nerve impulse– Myelinated = white matter– Unmyelinated = gray matter

• Nodes of Ranvier: breaks in myelin sheath btwn Schwann cells

• Synapse: junction btwn two neurons or btwn a neuron and an effector

http://academic.kellogg.edu/herbrandsonc/bio201_mckinley/f14-3a_structures_in_a__c.jpg

Structural Classification of Neurons• Multipolar

– One axon, several dendrites– Most numerous

• Bipolar– One axon, one dendrite– Least numerous– Retina, inner ear, olfactory pathway

• Unipolar– Axon is a single process that branches into a central

process (towards CNS) and a peripheral process (towards PNS)

– Dendrites at distal end of peripheral process– Always sensory neurons

http://www.google.com/imgres?imgurl=http://psyweb.com/Physiological/Neurons/NImages/Unipolar

http://www.google.com/imgres?imgurl=http://psyweb.com/Physiological/Neurons/NImages/bipolar

http://www.google.com/imgres?imgurl=http://psyweb.com/Physiological/Neurons/NImages/multipolar

Functional Classification of Neurons

• Afferent– Sensory– Towards CNS

• Efferent – Motor– Towards muscles & glands

• Interneurons– Connect afferent & efferent neurons– Lie within CNS

Reflex Arc

Examples of Reflex Arcs

• Ipsilateral• Contralateral• intersegmental

Nerves vs Tracts

• Nerves – bundles of parallel neurons held together by fibrous CT in the PNS

• Tracts – bundles of parallel neurons in the CNS

Nerve Fibers

• Remember the difference between nerves and tracts?– Endoneurium: surrounds each nerve fiber– Perineurium: surrounds fascicles (bundles of

nerve fibers– Epineurium: surrounds a complete nerve

(PNS) or tract (CNS)

Review: Gray vs White Matter

• White matter – myelinated nerve fibers– Myelin sheaths help increase the speed of an

action potential

• Gray matter – unmyelinated nerve fibers & cell bodies– Ganglia: regions of gray matter in PNS

Nerve Fiber Repair

• Nervous tissue has a limited repair capacity b/c mature neurons are incapable of cell division

• Repair can take place if soma and neurilemma remain intact

Steps of Nerve Fiber Repair

1. Injury2. Distal axon and myelin sheaths

degenerates3. Remaining neurilemma & endoneurium

forms a “tunnel” from the injury to the effector

4. Proteins produced in the nissl bodies help extend a new axon down the tunnel to the effector

Nerve Impulses

• Neurons are specialized to initiate and conduct signals nerve impulses– Exhibit excitability & conductivity– Nerve impulse wave of electrical fluctuation

that travels along the plasma membrane

Membrane Potentials

• Difference in charges across the plasma membrane– Inside slightly negative – Outside slightly positive

• Result in a difference in electrical charges membrane potential – Stored potential energy– Analogy = water behind a dam

Membrane Potentials• Membrane potential creates a polarized membrane

– Membrane has – pole & + pole

• Potential difference of a polarized membrane is measured in millivolts (mV)– The sign indicates the charge of the inside of a polarized

membrane

Resting Membrane Potential (RMP)

• When not conducting electrical signals, a membrane is “resting”– -70mV

• RMP maintained by ionic imbalance across membrane– Sodium-Potassium Pump

• Pumps 3 Na+ out for every 2 K+ pumps in• Creates an electrical gradient (more positive on

outside)

Resting Membrane Potential (RMP)

Local Potential• Local potential - The slight shift away from the

RMP– Isolated to a particular region of the plasma

membrane

• Stimulus-gated Na+ channels open Na+ enters membrane potential to moves closer to zero (depolarization)

• Stimulus-gated K+ channels open K+ exits membrane potential away from zero (hyperpolarization)

• **Local potentials do not spread to the end of the axon**

Local Potentials

Action Potentials

Definitions:

• Membrane potential of an active neuron (one that is conducting an impulse

• Action potential = nerve impulse

• An electrical fluctuation that travels along the plasma membrane

Steps of Producing an Action Potential (table 12-1)

1. A stimulus triggers stimulus-gated Na+ channels to open Na+ diffuses inside the cell depolarization

2. Threshold potential is reached (-59mV) voltage-gated Na+ channels open depolarization continues

3. Action potential peaks at +30mV, voltage-gated Na+ channels close

4. Voltage-gated K+ channels open K+ diffuses outward repolarization

5. Brief period of hyperpolarization (below -70mV) RMP is restored by Na+/K+ pump

Refractory Period

Refractory Period

• Period of time where the neuron resists restimulation (AP cannot fire)– Absolute refractory period: half a millisecond

after membrane reaches threshold potential • Will not respond to ANY stimulus

– Relative refractory period: few milliseconds after absolute refractory period (during repolarization)

• Only respond to VERY strong stimulus

Refractory Period – What does this mean?

• Greater stimulus = quicker another action potential can take place

• The magnitude of the stimulus does not affect the magnitude of the AP– b/c APs are “all or nothing”– Does cause proportional increase in

frequencies of impulses

Conduction of an Action Potential

• During the peak of an AP, the polarity reverses– Negative outside, positive inside– Causes impulse to travel from site of AP to

adjacent plasma membrane– No fluctuation in AP due to “all or nothing”

principle – AP cannot travel backwards on axon due to

refractory periods

Conduction of an Action Potential

How does myelin sheaths affect the speed of an action potential?

• Sheaths prevent movement of ions• Electrical changes can only take place at

Nodes of Ranvier• APs “leap” from node to node (current

flows under sheaths)• Saltatory conduction

Random Facts

• In nerve fibers that innervate skeletal muscle, impulses travel up to 130 m/s (300 mph)

• Sensory pathways from skin 0.5 m/s (<1 mph)

• Many anesthetics block the sensation of pain by inhibiting opening of Na+ channels

Types of Synapses

Electrical synapses: two cells joined end to end by gap junctions

Ex: btwn cardiac muscle cells, smooth muscles cells

Types of Synapses

Chemical synapses: use neurotransmitter to send a signal from a presynaptic cell to postsynaptic cell

3 Parts:

1. Synaptic knob

2. Synaptic cleft

3. Plasma membrane of postsynaptic neuron

Mechanisms of Synaptic Transmission

1. AP depolarizes synaptic knob

2. Voltage-gated Ca2+ channels open Ca2+ diffuses inside the cell

3. Ca2+ triggers exocytosis of neurotransmitter vesicles

4. NTs diffuses across synaptic cleft bind w/ receptors on postsynaptic cell

Postsynaptic Potentials (Fig 12-22)

• Excitatory NTs cause Na+ and K+ channels to open depolarization excitatory postsynaptic potential (EPSP)

• Inhibitory NTs cause K+ and Cl- channels to open hyperpolarization inhibitory postsynaptic potential (IPSP)

Summation

• For every postsynaptic cell there are usually 1K-100K synaptic knobs

• Both excitatory & inhibitory NTs are released– Summation of local potentials (EPSP & IPSP)

occur at axon hillock• EPSP > IPSP reach threshold action

potential• EPSP < IPSP threshold not reached no AP

NeurotransmittersSmall-Molecule Transmitters:1. Acetylcholine2. Amines

• Serotonin• Dopamine• Epinephrine• Norepinephrine

3. Amino Acids• Glutamate• GABA• Glycine

Large-Molecule Transmitters:1. Neuropeptide

• Endorphins