Chapter 10 Nervous System I. Divisions of the Nervous System The organs of the nervous system are...

Chapter 10

Nervous System I

Divisions of the Nervous System• The organs of the

nervous system are divided into two major groups:– Central Nervous System

(CNS) = brain & spinal cord

– Peripheral Nervous System (PNS) = nerves that extend from the brain (cranial nerves) and spinal cord (spinal nerves)

Divisions of PNS• Sensory Division

– picks up sensory information and delivers it to the CNS

• Motor Division– carries information to muscles and glands

• Divisions of the Motor Division– Somatic – carries information to skeletal muscle– Autonomic – carries information to smooth

muscle, cardiac muscle, and glands

Three Major Functions of the Nervous System

Functions of the Nervous System• Sensory Function

– sensory receptors (located at the ends of peripheral neurons) gather information

– information is carried to the CNS– A sensory impulse is carried on a sensory

neuron

Functions of the Nervous System• Motor Function

– decisions are acted upon – impulses are carried to effectors– Motor impulses are carried from CNS to

responsive body parts called effectors– A motor impulse is carried on a motor neuron– Effectors = 2 types:

• muscles (that contract)• glands (that secrete a hormone)

Functions of the Nervous System• Integrative Function

– Can involve CNS or PNS– sensory information used to create

• sensations• memory• thoughts• decisions

Neuron Structure• Each neuron is composed of a cell body and

many extensions from the cell body called neuron processes or nerve fibers

• Cell Body = central portion of neuron – contains usual organelles, except centrioles

• identify: nucleus, prominent nucleolus, and many Nissl bodies = RER

• Neuron Processes/ Nerve Fibers = extensions from cell body

Neuron Structure cont.• Dendrites:

– many per neuron– short and branched– receptive portion of a neuron– carry impulses toward cell body

• Axons:– one per neuron– long, thin process– carry impulses away from cell body

Figure 10.01

Neuron Structure cont.• Note terminations of axon branch = axonal

terminals (synaptic knobs)• Axons in PNS:• Large axons are surrounded by a myelin sheath

produced by many layers of Schwann Cells (neuroglial cell)– "myelinated nerve fiber"– myelin = lipoprotein– Interruptions in the myelin sheath between Schwann cells

= Nodes of Ranvier• Small axons do not have a myelin sheath

– "unmyelinated nerve fibers"– however all axons (in PNS) are associated with Schwann

cells

Figure 10.04c

Neuron Structure cont.• Neuron = the structural & functional unit of the

nervous system– a nerve cell

• Neuron Structure– Nerve Fibers – Axons (continued)– Axons in CNS (i.e. in brain & spinal cord)– Myelin is produced by an oligodendrocyte rather than

Schwann Cells• A bundle of myelinated nerve fibers = "White

Matter"• This is in contrast to CNS "Gray Matter" = A bundle

of cell bodies (or unmyelinated nerve fibers)

Structural Classification• Bipolar Neurons

– two extensions– one fused dendrite leads toward cell body, one axon leads

away from cell body• Unipolar Neurons

– one process from cell body– forms central and peripheral processes– only distal ends are dendrites

• Multipolar Neurons– many extensions– many dendrites lead toward cell body, one axon leads

away from cell body

Functional Classification• Sensory neurons

– afferent neurons– carry sensory impulses from sensory receptors to

CNS– input information to CNS– Location of receptors = skin & sense organs

• Interneurons (Association)– CNS– link other neurons together (i.e. sensory neuron

to interneuron to motor neuron)

Functional Classification cont.

• Motor Neurons– efferent neurons– carry motor impulses away from CNS and to

effectors– output information from CNS– Effectors = muscles & glands

Neuroglial Cells• Neuroglial Cells = accessory cells of nervous

system form supporting network for neuronsPNS = 2 Types• Schwann cells

– produces myelin (in the PNS)• Satellite Cells

– support clusters of neuron cell bodies (ganglia)

Neuroglial Cells cont.CNS =4 types:

– provide bulk of brain and spinal cord tissue

• Astrocyte – scar tissue– star-shapedFunction: – mop up excess ions, etc– induce synapse formation– connect neurons to blood

vessels

• Oligodendrocyte – looks like eyeball– Function: produces

myelin• Microglia

– looks like spider– Function: phagocytosis

• Ependyma– epithelial-like layer– Function: lines spaces in

CNS– brain = ventricles

Nerve Repair• Regeneration of Nerve Axons

– Cell body injury = death of neuron– Damage to an axon may allow for regeneration

The Synapse

•Nerve impulses pass from neuron to neuron at synapses

Synaptic Transmission

•Neurotransmitters are released when impulse reaches synaptic knob

Distribution of Ions

Potential Difference• A resting neuron's cell membrane is said to be

polarized = electrically charged:– Consequently, a potential difference (PD) exists

across this resting cell membrane• Potential Difference (PD) = the difference in

electrical charge between 2 points (i.e. across a cell membrane)

Resting Membrane Potential• The resting membrane potential (RMP) of a

neuron results from the distribution of ions across the cell membrane – K+= high inside– Na+= high outside– Cl-= high outside– Negatively charged proteins or Anions-; high

inside• Recall that these ion concentrations are

maintained by active transport mechanisms – mainly the Na+/K+ pump

Resting Membrane Potential cont.

• Resting Potential– The RMP of a nerve cell is measured to be -70

mV or millivolts (inside / outside)– As long as the RMP in a nerve cell is undisturbed,

it remains polarized. – In order for a nerve impulse to be started or

propagated in a nerve cell, this resting potential must be disturbed

Local Potential Changes• caused by various stimuli

• temperature changes• light• pressure

• environmental changes affect the membrane potential by opening a gated ion channel

Development of resting membrane potentialSlide number: 1

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Local Potential Changes (Graded Potentials)

• The RMP of - 70 mV can be disrupted or changed in one of two directions:

• more negative = "hyperpolarization"• less negative (i.e. towards zero) =

"depolarization“• summation can lead to threshold stimulus

that starts an action potential

Figure 10.15

Action Potential• When the resting membrane potential (RMP)

of a neuron is depolarized to -55mV, threshold potential is reached– The threshold potential for a neuron is -55mV– Therefore, a threshold stimulus = +15 mV

• When threshold potential is reached, the rapid opening of Na+ channels results in rapid depolarization (and even reversal of the membrane potential [MP] to +30mV)– This event is called the action potential– The action potential represents the start of the

nerve impulse on a neuron.

Action Potential cont.• Then K+ channels open, (while Na+

channels close), and repolarization occurs = recovery of the RMP to -70mV

• This all occurs very quickly = 1/1000 sec• An action potential represents the start of a

nerve impulse in one small portion of the neuron's membrane

Figure 10.15

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Action potentialSlide number: 1


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Action Potential cont.• Nerve Impulse (NI) = the propagation of

action potentials (AP) along a nerve fiber; (i.e. the entire length of the neuron)– The NI is an electrical impulse

• An NI is similar to a row of dominos falling (i.e. once the first domino falls, the entire row will fall)

• A nerve impulse begins on a dendrite (or cell body of a neuron), runs toward the cell body, through the cell body, and then down the axon

Characteristics of a Nerve Impulse (NI)

• All or Nothing Response = if a nerve cell responds at all, it responds completely.– sub threshold stimulus (5mV) = no AP/no NI– threshold stimulus (15mV) = yes AP/yes NI

• > threshold stimulus (20mV) = yes AP– yes NI, but no greater intensity than above

• Refractory Period = the period following a NI when a threshold stimulus cannot produce another NI – The RMP has to be restored before it can be depolarized

again– (i.e. dominos must be set up in order to be knocked down

again)

Impulse Conduction Review

Characteristics NI cont.• Impulse Conduction = the manner in which

the NI runs down the neuron/nerve fiber• unmyelinated nerve fibers: NI must travel

the length of the nerve fiber– slow

• myelinated nerve fiber: "Saltatory Conduction"– NI jumps from node of Ranvier to node of Ranvier– Very fast transmission

THE SYNAPSE

Nerve impulses are transferred from one neuron to the next through synaptic

transmission.

The Synapse• Synapse = the junction between two neurons

where a nerve impulse is transmitted• occurs between the axon of one neuron and

dendrite or cell body of a second neuron• Note that the two neurons do not touch

There is a gap between them = synaptic cleft

Synaptic Transmission • NI reaches axonal terminal of pre-synaptic

neuron causing depolarization of synaptic knob– Ca++ channels open and calcium ions rush into

axonal terminal– synaptic vesicles (filled with neurotransmitter/NT)

to release NT via exocytosis into the synaptic cleft

– NT diffuses across synaptic cleft and depolarizes the post-synaptic neuron's membrane.

– An action potential (AP) is triggered and a NI begins in the post-synaptic neuron

Neurotransmitters

Synaptic PotentialsEPSP

• excitatory postsynaptic potential• graded• depolarizes membrane of postsynaptic neuron• action potential of postsynaptic neuron becomes more likely

IPSP• inhibitory postsynaptic potential• graded• hyperpolarizes membrane of postsynaptic neuron• action potential of postsynaptic neuron becomes less likely

Summation of EPSPs and IPSPs• EPSPs and IPSPs are added together in a process called summation• More EPSPs lead to greater probability of action potential

Neurotransmitters (NT) • at least 30 different produced by CNS• some neurons produce/release only one while

release many; • Most typical NT is Acetylcholine (ACh)

– ACh is released by all motor neurons (i.e. those that stimulate skeletal muscle)

– some CNS neurons

Other NTs• monoamines (modified

amino acids)• are widely distributed in

the brain where they play a role in: – emotional behavior and – circadian rhythm

• are present in some motor neurons of the ANS.

• include:– epinephrine– norepinephrine– dopamine– serotonin

• histamine• unmodified amino acids• glutamate• aspartate• GABA (gamma

aminobutyric acid) • glycine

Fate of Neurotransmitter in Synaptic Cleft

• Destruction of Neurotransmitter:– Enzymes that are present in the synaptic cleft

destroy NT– For example, acetylcholinesterase destroys ACh

• Reuptake of Neurotransmitter:– NT is transported back into pre-synaptic knob

• Both of the above processes prevent continual stimulation of the post-synaptic membrane!

Neuropeptides • synthesized by CNS neurons• act as NTs or neuromodulators that either:

– alter a neuron's response to a NT– block the release of a NT

• include enkephalins:– synthesis is increased during painful stress– bind to the same receptors in the brain as the narcotic

morphine– relieve pain

• include endorphines:– same as above, but with a more potent and longer lasting

effect

IMPULSE PROCESSING

Impulse Processing• Neuronal Pools – neurons that synapse and

work together– Working together results in facilitation – a

general excitation that makes stimulation easier to achieve

• Convergence– many neurons come together on fewer neurons

(summation occurs)– typical of motor pathways– many inputs from brain, but usually only one

motor response

Impulse Processing cont.• Divergence

– fewer neurons spread out to signal many neurons (signal amplifies)

– typical of sensory pathways– reason that a stimulus (i.e. odor) can cause many

responses

Chapter 10 Nervous System I. Divisions of the Nervous System The organs of the nervous system are...

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