The Nervous System Chapter 8 – Overview and Neural Tissue.

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

Figure 8-81 of 5

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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


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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)


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Localcurrent


Sodium ions



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


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3DEPOLARIZATION REPOLARIZATION

Localcurrent


Sodium ions



Potassium ions



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


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Threshold

Restingpotential

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REFRACTORY PERIOD

DEPOLARIZATION REPOLARIZATION

Localcurrent


Sodium ions



Potassium ions



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


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


Figure 8-113 of 5


Action potential

Extracellular Ca2+ enters the synapticcleft triggering the exocytosis of ACh

EXTRACELLULARFLUID

Synapticknob

PRESYNAPTICNEURON

Synaptic vesicles

ER

AChE


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


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


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

The Nervous System Chapter 8 – Overview and Neural Tissue.

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