The Nervous SystemThe Nervous SystemChapter 8 – Overview and Chapter 8 – Overview and
Neural TissueNeural Tissue
The Nervous SystemThe Nervous SystemChapter 8 – Overview and Chapter 8 – Overview and
Neural TissueNeural Tissue
The Nervous system has three major functions:
Sensory – monitors internal & external environment through presence of __________
Integration – interpretation of sensory information (information processing); complex (higher order) functions
Motor – response to information processed through stimulation of ____________
_________________
_______________
Two Anatomical Divisions Central nervous system (CNS)
__________ __________
Peripheral nervous system (PNS)All neural tissue outside CNS – includes cranial nerves
and spinal nerves __________ division (sensory input) __________ division (motor output)
Somatic nervous system Autonomic nervous system
General Organization of the nervous system
General Organization of the nervous system
Brain & spinal cord
Histology of neural tissue
Two types of neural cells in the nervous system:
Neurons - For processing, transfer, and storage of information
Neuroglia – For support, regulation & protection of neurons
Neuroglia (glial cells)
CNS neuroglia:
• astrocytes
• oligodendrocytes
• microglia
• ependymal cells
PNS neuroglia:
• Schwann cells (neurolemmocytes)
Astrocytes • create supportive framework for neurons• create “blood-brain barrier”• monitor & regulate interstitial fluid surrounding neurons• secrete chemicals for embryological neuron formation• stimulate the formation of scar tissue secondary to CNS injury
Oligodendrocytes
• create myelin sheath around axons of neurons in the CNS. Myelinated axons transmit impulses faster than unmyelinated axons
Microglia
• “brain macrophages”
• phagocytize cellular wastes & pathogens
Ependymal cells• line ventricles of brain & central canal of spinal cord• produce, monitor & help
circulate __________
Schwann cells
• surround all axons of neurons in the PNS creating a neurilemma around them. Neurilemma allows for potential regeneration of damaged axons
• creates myelin sheath around most axons of PNS
Neuron structure
•Most axons of the nervous system are surrounded by a myelin sheath (myelinated axons)
•The presence of myelin speeds up the transmission of action potentials along the axon
•Myelin will get laid down in segments (internodes) along the axon, leaving unmyelinated gaps known as “__________________”
•Regions of the nervous system containing groupings of myelinated axons make up the “_____________”
•“gray matter” is mainly comprised of groups of neuron cell bodies, dendrites & synapses (connections between neurons)
of Ranvier
Anatomical organization of neuronsNeurons of the nervous system tend to group together into organized bundles
The axons of neurons are bundled together to form nerves in the PNS & tracts, columns & pathways in the CNS.
The cell bodies of neurons are clustered together into ganglia in the PNS & nuclei/centers in the CNS.
Classification of neuronsStructural classification based on number of processes coming off of the cell body:
Multipolar neuron
• multiple dendrites & single axon
• most common type
Bipolar neuron
• two processes coming off cell body – one dendrite & one axon
• only found in eye, ear & nose
Unipolar neuron
• single process coming off cell body, giving rise to dendrites (at one end) & axon (making up rest of process)
Classification of neuronsFunctional classification based on type of information & direction of information transmission:
• Sensory (afferent) neurons –
• transmit: ________________________________________
• most sensory neurons are unipolar, a few are bipolar
• Motor (efferent) neurons –
• transmit: ________________________________________
(muscles/glands/adipose tissue) in the periphery of the body
• all are multipolar
• Association (interneurons) –
• transmit information between neurons, located entirely within the CNS; analyze inputs, store information, coordinate outputs
• are the most common type of neuron (20 billion)
• are all multipolar
Reflex arc (p. 283-286)
Reflex – a quick, unconscious, automatic response to a stimulus to protect or maintain homeostasis. e.g. stretch reflex, withdrawal reflex, pupillary light reflex, etc.
Reflex arc – neural pathway involved in the production of a reflex. Structures include:
• receptor
• sensory neuron
• integrating center (brain or spinal cord; may or may not involve association neurons (interneurons))
• motor neuron
• effector
Stretch reflex- simplest type of reflex
- no association neuron involved
Figure 8-29
Simplified Withdrawal reflex
Figure 8-28
Neuron FunctionNeurons at rest have an unequal distribution of charged ions inside/outside the cell, which are kept separate by the plasma membrane
The sum of charges makes the outside of the membrane positive, & the inside of the membrane negative
• more Na+ ions outside• more K+ ions inside• large negatively charged proteins & phosphate ions inside
Because of the difference of ionic charges inside/outside the cell, the membrane of the resting neuron is “polarized”
The difference in charges creates a potential electrical current across the membrane known as the “membrane potential (transmembrane potential)”
At rest, the transmembrane potential can also be referred to as the “_______________________” (RMP)
The RMP of a neuron = -70mV
For ions to cross a cell membrane, they must go through transmembrane channels
“leakage channels” – open all the time, allow for diffusion
“gated channels” – open & close under specific circumstances (e.g. voltage changes)
When a stimulus is applied to a resting neuron, gated ion channels can open
This change from the cell’s resting membrane potential is known as hyperpolarization
If a stimulus opens gated K+ channels in a resting neuron, positive charges leave cell membrane potential becomes more negative (-70mV -90mV)
When a stimulus causes Na+ gates open, Na+ diffuses into the cell
This changes the electrical charge inside the cell membrane, bringing it away from its RMP of -70mV toward 0mV
This change in membrane potential is known as depolarization
If a stimulus only affects a few Na+ gates at a specific site of the axon, the depolarization is small & localized only to that region of the cell. This is known as a graded potential
But if the stimulus reaches a certain level (threshold level), voltage controlled Na+ gates will begin to open in sequence along the length of the axon. The depolarization will propagate (spread) along the entire surface of the cell membrane
The propagated change in the membrane potential is known as an action potential (nerve impulse)
Action potentials• Propagated change in transmembrane potential with two phases:
• Depolarization - movement of Na+ ions into the cell through voltage controlled Na+ gates; followed immediately by
• Repolarization - movement of K+ ions out of the cell through voltage controlled K+ gates
• Only nerve cells & muscle cells are excitable, i.e. can generate APs.
• Once an AP begins, it will propagate down the entire cell at a constant & maximum rate. This is known as the “__________” principle
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
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REFRACTORY PERIOD
DEPOLARIZATION REPOLARIZATION
Localcurrent
Depolarization to threshold
Sodium ions
Activation of voltage-regulated sodium channels
and rapid depolarization
Potassium ions
Inactivation of sodiumchannels and activation of
voltage-regulatedpotassium channels
The return to normalpermeability and resting state
Time (msec)0 1 2 3
Action Potential Conduction
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
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DEPOLARIZATION
Localcurrent
Depolarization to threshold
Sodium ions
Time (msec)0 1 2 3
• Nerve cell at rest (RMP= -70mV)
• Stimulus applied to cell
• Na+ gates at axon hillock cause localized depolarization (graded potential)
• If stimulus is strong enough, flow of Na+ ions into cell reach threshold level triggering opening of voltage gated Na+ channels & formation of an action potential (nerve impulse)
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
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DEPOLARIZATION
Localcurrent
Depolarization to threshold
Sodium ions
Activation of voltage-regulated sodium channels
and rapid depolarization
Potassium ions
Time (msec)0 1 2 3
• Once threshold is reached, Na+ will quickly diffuse into the cell causing a rapid depolarization of the membrane (- 70 mV 0 mV +30 mV)
• this depolarization will spread to adjacent parts of the membrane, activating more voltage controlled Na+ gates in succession
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
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3DEPOLARIZATION REPOLARIZATION
Localcurrent
Depolarization to threshold
Sodium ions
Activation of voltage-regulated sodium channels
and rapid depolarization
Potassium ions
Inactivation of sodiumchannels and activation of
voltage-regulatedpotassium channels
Time (msec)0 1 2 3
• When the transmembrane potential reaches +30mV, Na+ gates will close & K+ gates will open• K+ will quickly exit cell resulting in repolarization of membrane & return to resting state
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
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DEPOLARIZATION REPOLARIZATION
Localcurrent
Depolarization to threshold
Sodium ions
Activation of voltage-regulated sodium channels
and rapid depolarization
Potassium ions
Inactivation of sodiumchannels and activation of
voltage-regulatedpotassium channels
The return to normalpermeability and resting state
Time (msec)0 1 2 3
Continuous propagation
(continuous conduction) Involves entire membrane
surface Proceeds in series of
small steps (slower) Occurs in unmyelinated
axons (& in muscle cells)
Propagation of an Action Potential
Figure 8-9(a)
Propagation of an Action Potential
Saltatory propagation(saltatory conduction)
Involves patches of membrane exposed at nodes of Ranvier Proceeds in series of large steps (faster) Occurs in myelinated axons
Figure 8-9(b)
“Information” travels within the nervous system primarily in the form of propagated electrical signals known as action potentials. An action potential occurs due to a rapid change in membrane polarity (depolarization followed by repolarization) Depolarization is due to: ____________________; repolarization is due to: ________________________
Conduction across synapses
Most synapses within the nervous system are chemical synapses, & involve the release of a neurotransmitter
In order for neural control to occur, “information” must not only be conducted along nerve cells (via action potentials), but must also be transferred from one nerve cell to another across a synapse
The Structure of a Typical Synapse
Figure 8-10
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 8-112 of 5
An action potential arrives anddepolarizes the synaptic knob
Action potential
EXTRACELLULARFLUID
Synapticknob
PRESYNAPTICNEURON
Synaptic vesicles
ER
AChE
POSTSYNAPTICNEURONCYTOSOL
• An action potential arrives & depolarizes the synaptic knob (end bulb)
• Before repolarization can occur, _____ channels open & diffuses _______ end bulb
• Repolarization occurs
Events at a Typical Synapse
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 8-113 of 5
An action potential arrives anddepolarizes the synaptic knob
Action potential
Extracellular Ca2+ enters the synapticcleft triggering the exocytosis of ACh
EXTRACELLULARFLUID
Synapticknob
PRESYNAPTICNEURON
Synaptic vesicles
ER
AChE
POSTSYNAPTICNEURONCYTOSOL
ACh
Ca2+
Ca2+Synaptic
cleft
Chemically regulatedsodium channels
• Ca+2 causes the synaptic vessicles to fuse with the end bulb membrane causing ___________ of the neurotransmitter
Figure 8-114 of 5
An action potential arrives anddepolarizes the synaptic knob
Action potential
Extracellular Ca2+ enters the synapticcleft triggering the exocytosis of ACh
ACh binds to receptors and depolarizesthe postsynaptic membrane
EXTRACELLULARFLUID
Synapticknob
PRESYNAPTICNEURON
Synaptic vesicles
ER
AChE
POSTSYNAPTICNEURONCYTOSOL
ACh
Ca2+
Ca2+Synaptic
cleft
Chemically regulatedsodium channels
Initiation ofaction potential
if thresholdis reached
ReceptorNa2+
Na2+Na2+Na2+Na2+
• The neurotransmitter diffuses across the synaptic cleft & binds to its receptors on the post synaptic membrane, causing an effect on the post synaptic cell
The effect on the post synaptic neuron will depend on whether the neurotransmitter released is
Excitatory (e.g. Ach, norepinephrine (NE))
Inhibitory (e.g. seratonin, dopamine, GABA)
Excitatory neurotransmitters cause Na+ gates to open in the post synaptic membrane ________________ impulse conduction
Inhibitory neurotransmitters cause K+ or Cl- gates to open in the post synaptic cell _______________ no impulse conduction
The effects of neurotransmitters on the post synaptic neurons are usually short lived because most neurotransmitters are rapidly removed from the synaptic cleft by enzymes or reuptake
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