9/30/2014

1

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

Chapter 10Functional

Organization of

Nervous Tissue

Neuron Network

Functions of the Nervous System

• master controlling & communicating system of body

• Functions

1. Sensory input: detects external & internal stimuli

2. Integration: processes & responds to sensory input

3. Control of Muscles and Glands

4. Homeostasis maintained by regulating other systems

5. Center for Mental Activities

Fig. 10.1

Parts of the Nervous System

• Two anatomical divisions

– Central Nervous System (CNS)• Brain & spinal cord

• Encased in bone

– Peripheral Nervous System (PNS)• Nervous tissue outside of CNS

• sensory receptors & nerves

• anatomical divisions perform different functions– PNS detects stimuli & transmits information to CNS

& receives information from CNS

– CNS processes, integrates, stores, & responds to information from PNS

Parts of the Nervous System

• PNS has 2 divisions

– Sensory division transmits action potentials from

sensory receptors to CNS

– Motor division carries action potentials away from

CNS in cranial or spinal nerves (2 subdivisions)

• Somatic nervous system innervates skeletal muscle

• Autonomic nervous system (ANS) innervates cardiac

muscle, smooth muscle, and glands (3 subdivisions)

– Sympathetic division is most active during physical activity

(fight or flight division)

– Parasympathetic division regulates resting functions (rest and

digest division)

– Enteric nervous system controls digestive system

Central Nervous System

(CNS)

•Brain and Spinal Cord

Peripheral Nervous System

(PNS)•Nervous tissue outside the CNS

•Sensory receptors and nerves

Sensory Division•Transmits action potentials from

sensory receptors to the CNS

Motor Division•Carries action potentials away

from the CNS in cranial nerves or

spinal nerves

Sympathetic DivisionMost active during physical activity

Somatic

Nervous

System• Innervates

skeletal muscle

Autonomic

Nervous

System (ANS)• Innervates cardiac

muscle, smooth

muscle, and glands

Parasympathetic DivisionRegulates resting functions

Enteric Nervous SystemControls the Digestive System

Parts of the Nervous System

9/30/2014

2

Fig. 10.2

Cells of the Nervous System

• 2 principal cell types of nervous system

– Neurons: excitable cells that transmit

electrical signals

– Non-neural cells (Glial cells): cells that

surround neurons. >1/2 of brain’s weight

• < 20% is extracellular space

Neurons

• Receive stimuli and transmit action potentials

• 3 components:

– cell body (soma) is primary site of protein synthesis

– Dendrites - short, branched cytoplasmic extensions of cell body that usually conduct electric signals toward cell body

– axon - cytoplasmic extension of cell body that transmits action potentials to other cells

Fig. 10.3

Neuron Structure

• Cell Body (Soma)– Contains nucleus & nucleolus

– Nissl substance is aggregate of rough ER & free ribosomes• Primary site of protein synthesis

– Golgi apparatus, mitochondria, other organelles present

– no centrioles (hence its amitotic nature)

– Clusters of cell bodies in CNS are called nucleiand in PNS ganglia

Fig. 10.3

Neuron Structure

– Usually only 1 unbranched axon per neuron• Rare branches are called collateral axons

– Presynaptic terminal: branched terminus of an axon (10,000 or more)

– Synapse: junction between a nerve cell & another cell

– Bundles of processes are called nerve tractsin CNS and nerves in PNS

Fig. 10.3

• Axons (Nerve Fibers)

– Trigger zone is part of neuron where axon originates

• Action potential generated from here

– Slender processes of uniform diameter & may vary in length from a

few mm to >1m

9/30/2014

3

Types of Neurons

• Multipolar neurons have several dendrites

& single axon

– Interneurons & motor neurons

• Bipolar neurons have a single axon &

dendrite

– Components of sensory organs

• Unipolar neurons have a single axon

– Most sensory neurons

Fig. 10.4

Glial Cells

• Glial Cells of CNS

– Astrocytes

– Microglial

– Ependymal cells

– Oligodendrocytes

• Glial Cells of PNS

– Satellite cells

– Schwann cells

• Glial Cells (Supporting Cells):

– Provide a supportive scaffolding for neurons

– Segregate and insulate neurons

– Guide young neurons to the proper connections

– Promote health and growth

Glial Cells of CNS

• Astrocytes

– Most abundant, versatile, highly branched

– cling to neurons and their synaptic endings, cover

capillaries

– Functions:

• Support and brace neurons and blood vessels

– Anchor neurons to their nutrient supplies

• Influence functioning of blood-brain barrier

• Guide migration of young neurons

• Process substances

– mopping up leaked potassium ions

– recycling neurotransmitters

• Isolate damaged tissue and limit spread of inflammation

Fig. 10.5

Astrocytes Glial Cells of the CNS

• Ependymal cells: range in shape from squamous to columnar and many are ciliated

– line ventricles of brain and central canal of spinal cord

– Some are specialized (choroid plexuses) to produce cerebrospinal fluid (CSF)

– Help to circulate CSF using their cilia

Fig. 10.6

9/30/2014

4

Glial Cells of the CNS

• Microglia

– Small, ovoid cells with

spiny processes

– Phagocytes that

monitor health of

neurons

Fig. 10.7

Glial Cells of the CNS

• Oligodendrocytes: form myelin sheaths

around axons of several CNS neurons

Fig. 10.8

Glial Cells of the PNS

• Schwann cells: form a myelin sheath around part of axon of a PNS neuron

• Satellite cells: support and nourish neuron cell bodies within ganglia

Fig. 10.9

Myelinated and Unmyelinated Axons

• Myelinated axons

– Plasma membrane of Schwann cells or

Oligodendrocytes repeatedly wraps around a

segment of an axon to form the myelin sheath

– Myelin is a whitish, fatty (protein-lipid), segmented

sheath around most long axons

– It functions to:

• Protect axon

• Electrically insulate

fibers from one another

• Increase speed of

nerve impulse transmission

• Node of Ranvier

– Gaps in the myelin sheathFig.

10.10

Myelinated and Unmyelinated Axons

• Unmyelinated axons

– Rest in invaginations of Schwann cell (PNS)

or Oligodendrocytes (CNS)

– Conduct action potentials slowly

Fig.

10.10

Fig.

10.10

9/30/2014

5

Organization of Nervous Tissue

• Nervous tissue can be grouped into white matterand gray matter

– White matter • Consists of myelinated axons

• Propagates action potentials

• Forms nerve tracts in CNS and nerves in PNS

– Gray Matter• Collections of neuron cell bodies or unmyelinated axons

• Forms cortex and nuclei in CNS and ganglia in PNS

• Axons synapse with neuron cell bodies, which are functionally site of integration in nervous system

Electric Signals

• Electric signals produced by cells are called action potentials– When action potentials are received from sensory cells it can

result in sensations of sight, hearing, and touch

– Complex mental activities, such as conscious thought, memory, and emotions, result from action potentials

– Contraction of muscles and secretion of certain glands occur in response to action potentials

• Electrical properties of cells result from– Ionic concentration differences across the plasma

membrane

– Permeability characteristics of the plasma membrane

Electric Signals

• Concentration Differences Across Plasma

Membrane

– Sodium ions (Na+), calcium ions (Ca2+), and

chloride ions (Cl-) are in much greater concentration

outside cell than inside

– Potassium ions (K+) and negatively charged

molecules, such as proteins, are in much greater

concentration inside cell than outside

• Negatively charged proteins are synthesized inside cell

and cannot diffuse out of it

Electric Signals

• Concentration gradients of ions result mainly from

1. Na+-K+ pump

• Moves ions by active transport

• K+ moved into cell, Na+ are moved out of it

2. Permeability characteristics of plasma membrane are

determined by

• Leak channels (always open)

K+ leak channels more numerous than Na+ leak channels; thus,

plasma membrane more permeable to K+ than to Na+ when at rest

• Gated ion channels

– Include ligand-gated ion channels, voltage-gated ion channels, and

other gated ion channels

Fig.

10.11

9/30/2014

6

Electric Signals

• Gated Ion Channels– Open and close in response to stimuli

• Ligand-gated ion channels– Open/close with binding of specific ligand (neurotransmitter)

» Ligand - molecule that binds to receptor

» Receptor - protein or glycoprotein with receptor site to which a ligand can bind

– Common nervous and muscle tissue, glands

• Voltage-gated ion channels– Open/close in response to small voltage changes across

plasma membrane

– Common in nervous and muscle tissues

• Other gated ion channels– Open/close in response to physical deformation of receptors

– Touch receptors (mechanical stimulation) and temperature receptors (temperature changes) of skin

http://highered.mcgraw-

hill.com/olcweb/cgi/pluginp

op.cgi?it=swf::535::535::/si

tes/dl/free/0072437316/12

0107/anim0013.swf::Volta

ge%20Gated%20Channel

s%20and%20the%20Actio

n%20Potential

Electric Signals

• Establishing the Resting Membrane

Potential

– Resting membrane potential

• Charge difference across plasma membrane when

cell not being stimulated

• Inside of cell is negatively charged, compared with

outside of cell

– Due mainly to tendency of positively charged K+ to

diffuse out of cell

– Opposed by negative charge that develops inside

plasma membrane

Fig.

10.12

Fig.

10.13

9/30/2014

7

Electric Signals

• Changing the Resting Membrane Potential

– Depolarization is a decrease in the resting membrane potential caused by

• decrease in K+ concentration gradient

• decrease in membrane permeability to K+

• increase in membrane permeability to Na+ or Ca2+

• decrease in extracellular Ca2+ concentrations

– Hyperpolarization is an increase in the resting membrane potential caused by

• increase in the K+ concentration gradient

• increase in membrane permeability to K+

• increase in membrane permeability to Cl-

• decrease in membrane permeability to Na+

• increase in extracellular Ca2+ concentrations

Electric Signals

Fig. 10.14

Graded Potentials

• small changes in resting membrane potential

• Confined to a small area of plasma membrane

– increase in membrane permeability to Na+ can cause graded depolarization

– increase in membrane permeability to K+ or Cl- can result in graded hyperpolarization

• Decreases in magnitude as distance from stimulation increases

Fig. 10.15

• The term graded potential is used because a stronger stimulus produces a greater potential change than a weaker stimulus

• Graded potentials can summate, or add together

Action Potentials

• larger changes in resting membrane potential that

spread over entire surface of cell

– graded potential causes depolarization of plasma

membrane to a level called threshold

– an all-or-none fashion and are of same magnitude, no

matter how strong the stimulus

– Occurs in 3 phases

• Depolarization phase

• Repolarization phase

• Afterpotential

9/30/2014

8

Action Potentials

Fig. 10.16

• Depolarization Phase– Inside of membrane becomes

more positive

– Na+ diffuses into cell through voltage-gated ion channels

• Repolarization Phase– Return of membrane potential

toward resting membrane potential

– Voltage-gated Na+ channels close

– Voltage-gated K+ channels open and K+ diffuses out of cell

• Afterpotential– Brief period of hyperpolarization

following repolarization Tab.

10.3

Fig.

10.17

Refractory Periods

• Absolute refractory period

– Time during an action potential when a second stimulus (no matter how strong) cannot initiate another action potential

• Relative refractory period

– Time during which a stronger-than-threshold stimulus can evoke another action potential

Action Potential Frequency

• number of action potentials produced per

unit of time in response to stimuli

– is directly proportional to stimulus strength

and to size of graded potential

• Subthreshold stimulus: graded potential

• Threshold stimulus: a single action potential

• Submaximal stimulus: action potential frequency

increases as the strength of stimulus increases

• Maximal or a supramaximal stimulus: produces a

maximum frequency of action potentials

Fig.

10.19

9/30/2014

9

Propagation of Action Potentials

• action potential generates ionic currents

– Currents stimulate voltage-gated Na+ channels in

adjacent regions of plasma membrane to open

– Producing new action potentials

• Reversal of direction of action potential

propagation is prevented by absolute refractory

period

• Occurs most rapidly in myelinated, large-

diameter axons

Fig.

10.20

Propagation of Action Potentials

• In an unmyelinated axon, action potentials are generated immediately adjacent to previous action potentials

• In a myelinated axon, action potentials are generated at successive Nodes of Ranvier

Fig. 10.21

http://highered.m

cgraw-

hill.com/sites/007

2943696/student

_view0/chapter8/

animation__actio

n_potential_prop

agation_in_an_u

nmyelinated_axo

n__quiz_2_.html

http://high

ered.mcgr

aw-hill.com/sit

es/007294

3696/stud

ent_view0/

chapter8/animation_

_action_p

otential_pr

opagation

_in_an_unmyelinated

_axon__q

uiz_2_.htm

l

The Synapse

• synapse is junction between two cells

where communication takes place

– Presynaptic cell: transmits signal towards a

synapse

– Postsynaptic cell: receives the signal

• Two types of synapses

– Electrical synapse

– Chemical synapse

Electrical Synapses

• Gap junctions in which tubular proteins called connexons allow ionic currents to move between cells

• action potential in one cell generates an ionic current that causes action potential in adjacent cell

• Action potentials conducted rapidly between cells allowing for synchronized activity

• Common in cardiac muscle and in many types of smooth muscle where coordinated contractions are essential

9/30/2014

10

Chemical Synapses

• Have three anatomical

components

– enlarged ends of axon are

presynaptic terminals

containing synaptic

vesicles

– postsynaptic membranes

contain receptors for

neurotransmitter

– synaptic cleft, space,

separates presynaptic and

postsynaptic membrane

Fig. 10.22

Chemical Synapse Activity

1. Action potentials arriving at presynaptic terminal cause voltage-gated Ca2+ channels to open

Fig. 10.22

Chemical Synapse Activity

1. Action potentials arriving at the presynaptic terminal cause voltage-gated Ca2+ channels to open

2. Calcium ions diffuse into cell and cause synaptic vesicles to release neurotransmitters

Fig. 10.22

Chemical Synapse Activity

1. Action potentials arriving at the presynaptic terminal cause voltage-gated Ca2+ channels to open

2. Calcium ions diffuse into the cell and cause synaptic vesicles to release neurotransmitters

3. Neurotransmitters diffuse from presynaptic terminal across synaptic cleft

Fig. 10.22

Chemical Synapse Activity

1. Action potentials arriving at the presynaptic terminal cause voltage-gated Ca2+ channels to open

2. Calcium ions diffuse into the cell and cause synaptic vesicles to release neurotransmitters

3. Neurotransmitters diffuse from the presynaptic terminal across the synaptic cleft

4. Neurotransmitters combine with receptor sites and cause ligand-gated ion channels to open. Ions diffuse into cell (shown) or out of cell (not shown) and cause a change in membrane potential

Fig. 10.22

http://highered.mcgr

aw-

hill.com/sites/0072495855/student_vie

w0/chapter14/anim

ation__chemical_sy

napse__quiz_1_.ht

ml

9/30/2014

11

Chemical Synapse Activity

• effect of neurotransmitter on postsynaptic

membrane is stopped in two ways

– neurotransmitter is broken down by an enzyme

– neurotransmitter is taken up by presynaptic terminal

• list of neurotransmitters and neuromodulators in

Table 10.4

– Neuromodulators are substances released from

neurons that can presynaptically or postsynaptically

influence likelihood that an action potential will be

generated

http://highere

d.mcgraw-

hill.com/sites/0072495855/

student_view

0/chapter14/

animation__t

ransmission_across_a_sy

napse.html

9/30/2014

12

Chemical Synapse Activity

• Excitatory and inhibitory postsynaptic potentials

– excitatory postsynaptic potential (EPSP) is a

depolarizing graded potential of postsynaptic

membrane

– inhibitory postsynaptic potential (IPSP) is a

hyperpolarizing graded potential of postsynaptic

membrane

– Presynaptic inhibition decreases neurotransmitter

release

– Presynaptic facilitation increases neurotransmitter

release

Fig.

10.23

Fig.

10.24

Spatial and Temporal Summation

• Presynaptic Aps through neurotransmitters produce graded potentials in postsynaptic neurons. The graded potential can summate to produce an AP at trigger zone– Spatial summation occurs when two or more presynaptic

terminals simultaneously stimulate a postsynaptic neuron

– Temporal summation occurs when two or more action potentials arrive in succession at a single presynaptic terminal

• IPSPs and EPSPs can converge on a postsynaptic neuron– AP is produced at trigger zone when graded potential is

produced as a result of sum of EPSPs and IPSPs reaching threshold

Fig.

10.25

Neuronal Pathways and Circuits

• Convergent pathways have many

neurons synapsing with a few neurons

• Divergent pathways have a few

neurons synapsing with many neurons

• Oscillating circuits have collateral

branches of postsynaptic neurons

synapsing with presynaptic neurons

9/30/2014

13

Fig.

10.26