Organization of the Nervous System A.Central nervous system (CNS) 1.Components a) brain b) spinal cord 2.Function a) integrating and command center of.

Organization of the Nervous System

A. Central nervous system (CNS)

1. Components

a) brain

b) spinal cord

2. Function

a) integrating and command center of the nervous system

dorsal body cavitydorsal body cavity

Three overlapping functionsThree overlapping functions

Marieb; Fig 11.1

Receptor

Central Nervous System

Peripheral Nervous System

Afferent neuron

Efferent neuron

Inter-neuron

Axon - peripheral processAxon - central process

Marieb; Fig 11.2

Marieb; Fig 11.2b

B. Peripheral nervous system (PNS)

1. Outside the central nervous system

2. a) functional subdivisions

(1) sensory, or afferent, division

(a) carries impulses toward the CNS from sensory receptors located throughout the body

Ascending Pathways

1. first-order neuron

2. second-order neuron

3. third-order neuron

pages 475pages 475 and 476and 476

Ascending Pathways

1. first-order neuron

2. second-order neuron

3. third-order neuron

sensory unipolar

sensory multipolar interneuron

sensory multipolar interneuron

Marieb; Fig 12.34

Marieb; Fig. 12.8Marieb; Fig 12.8

a) functional subdivisions

(2) motor, or efferent, division

(a) function

(i) carries impulses from the CNS to effector organs

(a) muscles

(b) glands

Marieb; Fig 12. 35

Somatic motor pathways

involve two neurons,

referred to as the upper and lower motor

neurons (anterior horn

neurons).

(page 478)

1. All neurons whose cell bodies are in the spinal cord gray matter are what structural type? ____________________

2. The posterior horns consist entirely of what functional type of neuron? ____________________

3. The anterior horns have some interneurons but mainly house the cell bodies of what functional type of neuron? ____________________

4. Within the spinal cord, where are the anterior horns the largest?

____________________

5. What are the names of the two motor divisions of the peripheral nervous system?

____________________ and ____________________

6. The cell bodies of the sensory neurons are found where? _________________

7. After entering the spinal cord, what route(s) does the axon of a sensory neuron take?

multipolar

interneurons

somatic motor neurons

limb-innervating cervical and lumbar regions of the cord

somatic autonomic

dorsal root (or cranial) ganglion

Some enter the posterior white matter of the cord directly and travel to synapse at higher cord or brain levels. Others synapse with interneurons in the posterior horns of the spinal cord gray matter at their entry level.

page 472page 472

(2) motor, or efferent, division

(b) two main parts

(i) somatic nervous system

(a) consists of somatic nerve fibers

(b) conduct impulses from the CNS to skeletal muscles, and allows conscious control of motor activities

(ii) autonomic nervous system

(a) involuntary system

(b) consists of visceral motor nerve fibers

(c) regulate the activity of smooth muscle, cardiac muscle, and glands

Histology of Nervous Tissue

A. Neuroglia, or glial cells, are closely associated with neurons, providing a protective and supportive network (pp. 389–391; Fig. 11.3)

1. Neuroglia of CNS

a) astrocytes – most abundant and versatile

(1) regulate the chemical environment around neurons

(2) exchange between neurons and capillaries

b) microglia

(1) monitor health and perform defense functions for neurons

Marieb; Fig 11.3

astrocytomaastrocytoma

1. Neuroglia of CNS

c) ependymal cells

(1) line the central cavities of the brain and spinal cord

(2) help circulate cerebrospinal fluid

d)oligodendrocytes

(1) wrap around neuron fibers

(a) form myelin sheaths Marieb; Fig 11.3

Marieb; Fig 11.3

2. Neuroglia of the PNS

a) satellite cells

(1) function is largely unknown

(2) found surrounding neuron cell bodies within ganglia

b) Schwann cells, or neurolemmocytes

(1) surround nerve fibers

(a) form the myelin sheath

Marieb; Fig 11.3

B. Neurons1. specialized cells that conduct messages in the form of electrical impulses throughout the body

Marieb; Fig 11.4

1. neurons function optimally for a lifetime, are mostly amitotic, and have an exceptionally high metabolic rate requiring oxygen and glucose.

a) “typical” neuron components

(1) cell body

(a) also called the perikaryon or soma

(b) major biosynthetic center containing the usual organelles except for centrioles

(2) dendrites

(a) cell processes that are the receptive regions of the cell

Clusters of cell bodiesCNS - nuclei

PNS - ganglia

Clusters of cell bodiesCNS - nuclei

PNS - ganglia

Dendrites transmit graded potentials

Dendrites transmit graded potentials

a) “typical” neuron components

(3) single axon

(a) generates and conducts nerve impulses away from the cell body to the axon terminals

(b) myelin sheath

(i) whitish, fatty, segmented covering

(ii) protects, insulates, and increases conduction velocity of axons

Axons do Axons do notnot have Nissl bodies have Nissl bodies or Golgi apparatior Golgi apparati

Axons do Axons do notnot have Nissl bodies have Nissl bodies or Golgi apparatior Golgi apparati

Axonal TransportAxonal Transport

SlowSlow - axoplasmic or cytoplasmic flow that moves from the cell body to the axon

FastFast - energy requiring process goes in both directions to deliver synaptic and secretory vesicles and mitochondria to the axon terminal (anterograde) or to return old membrane components to the cell body for recycling (retrograde)

Silverthorn; Fig. 8-4

No Nissl bodies or Golgi No Nissl bodies or Golgi apparati in cytoplasm of apparati in cytoplasm of axonaxon

Slow axonal transport – 2 mm/dayFast axonal transport – 400 mm/day

Slow axonal transport – 2 mm/dayFast axonal transport – 400 mm/day

Marieb; Fig 11.5

Adjacent Schwann cells do nottouch one another –

separated by nodes of Ranvier

Silverthorn; Fig. 8-6a

2. Structural classes of neurons

a) multipolar neurons

(1) three or more processes

2. Structural classes of neurons

b) bipolar neurons

(1) single axon and dendrite

2. Structural classes of neurons

c) unipolar neurons

(1)single process extending from the cell body that is associated with receptors at the distal end

3. Functional classes of neurons

a) sensory, or afferent, neurons

(1) conduct impulses toward the CNS from receptors

Cell bodies outside the CNS

3. Functional classes of neurons

b)motor, or efferent, neurons (multipolar)

(1) conduct impulses from the CNS to effectors

c) interneurons

(1) association neurons

(2) conduct impulses between sensory and motor neurons, or in CNS integration pathways

Cell bodies inside the CNS

Neurophysiology

A. Basic Principles of Electricity

1. voltage

a) measure of the amount of difference in electrical charge between two points, called the potential difference

2. current

a) flow of electrical charge from point to point

b) dependent on

(1) voltage

(2) resistance (hindrance to current flow)

c) in the body, electrical currents are due to the movement of ions across cellular membranes

Ohm’s Law

Current (I) = voltage (V)

resistance (R)

Ohm’s Law

Current (I) = voltage (V)

resistance (R)

1. current is directly proportional to voltage – the greater the potential difference, the greater the current (or flow of ions)

2. current is inversely proportional to resistance

1. current is directly proportional to voltage – the greater the potential difference, the greater the current (or flow of ions)

2. current is inversely proportional to resistance

B. Role of Membrane Ion Channels

1. ion channels of plasma membrane

a) some are always open

(1) called leakage channels

b) some have a protein “gate” that changes shape or opens in response to the proper signal

Marieb; Fig 3.7

(c) open (leak) channels: spend most time in open configuration

(d) gated channels: spend most time in closed state

(i) chemically gated: specific ligands open or close the channel

(ii) voltage gated: electrical state of cell opens or closes

(iii) mechanically gated: physical force opens or closes

regulate movement

of molecules through

them

Silverthorn; Fig. 5-8

Information from Chapter 3

Marieb; Fig 11.6

Ion movement follows an

electrochemical gradient

Electro – move toward an area

of opposite charge

Chemical – diffuse

passively from an area of high concentration to an area of

low concentration

Marieb; Fig 11.6

C. Resting Membrane Potential

1. Polarized neuron cell membrane

a) more negatively charged inside than outside

b) degree of this difference in electrical charge is the resting membrane potential (RMP)

(1) generation of RMP

(a) differences in ionic makeup of intracellular and extracellular fluids

(b) differential membrane permeability to solutes

Marieb; Fig 11.7

Marieb; Fig 11.7

Marieb; Fig 11.8K+ plays the most importantrole in generating the RMP

At rest, the membrane is 75X more permeable to K+ than Na+

D. Membrane Potentials That Act as Signals

1. neurons use changes in membrane potential as communication signals

a) changes in membrane permeability to any ion

b) alteration of ion concentrations on the two sides of the membrane

2. changes in membrane potential relative to resting membrane potential

a) depolarizations

(1) interior of the cell becomes less negative

b) hyperpolarizations

(1) interior of the cell becomes more negatively charged

Marieb; Fig 11.9

Increases probabilityof producing

nerve impulses

Increases probabilityof producing

nerve impulses Decreases probabilityof producing

nerve impulses

Decreases probabilityof producing

nerve impulses

3. Graded potentials

a) short-lived

b) local changes in membrane potentials

c) can either be

(1)depolarizations

(2)hyperpolarizations

d) critical to the generation of APs

Marieb; Fig 11.10

Graded – magnitude varieswith stimulus strength

Graded – magnitude varieswith stimulus strength

Names for Graded Potentials

(if located at)

sensory receptor

(if the result of a)

neurotransmitter stimulus

generator generator potentialpotential

postsynaptic postsynaptic potentialpotential

Marieb; Fig 11.11

Graded potentials are decremental –

they will die out quickly

Graded potentials are decremental –

they will die out quickly

4. Action potentials (AP), or nerve impulses, occur on axons and are the principle way neurons communicate

Marieb; Fig 11.4

Site where graded potential transitions to AP

4. Action potentials (AP), or nerve impulses, occur on axons and are the principle way neurons communicate

Marieb; Fig 11.12

Marieb; Fig 11.12

Marieb; Fig 11.12

Ion Movement During the

Action Potential

Ion Movement During the

Action Potential

Marieb; Fig 11.12

Ion Movement During the

Action Potential

Marieb; Fig 11.12

4. Action potentials (AP), or nerve impulses, occur on axons and are the principle way neurons communicate

Marieb; Fig 11.12

4. Action potentials (AP), or nerve impulses, occur on axons and are the principle way neurons communicate

a) generation of an action potential

(1) involves a transient increase in Na+ permeability

(2) followed by restoration of Na+ impermeability

(3) then a short-lived increase in K+ permeability

Depolar-ization

Repolarization and hyperpolarization

Marieb; Fig 11.12

http://bio.winona.msus.edu/berg/ANIMTNS/NaKpump2.htm

Repolarization restores Repolarization restores electricalelectrical conditions conditions

BUT BUT

sodium-potassium pump restores sodium-potassium pump restores ionicionic conditions conditions

Repolarization restores Repolarization restores electricalelectrical conditions conditions

BUT BUT

sodium-potassium pump restores sodium-potassium pump restores ionicionic conditions conditions

Marieb; Fig 11.13

b) propagation, or transmission, of an action potential

(1) local currents of an area undergo depolarization, causing depolarization of the forward adjacent area

4. Action potentials

c) repolarization

(1) restores RMP

(2) follows depolarization along the membrane

5. A critical minimum, or threshold, depolarization is defined by the amount of influx of Na+ that at least equals the amount of efflux of K+

Marieb; Fig 11.12

6. Action potentials

a) all-or-none phenomena

(1) either happen completely, in the case of a threshold stimulus

(2) or not at all, in the event of a subthreshold stimulus

7. Stimulus intensity

a) coded in the frequency of action potentials

Remember – all APs look alike!

Marieb; Fig 11.14

8. Refractory period of an axon

a) related to the period of time required so that a neuron can generate another action potentialMarieb; Fig 11.15

The absolute refractory period

ensures that a second action

potential will not occur before the first has finished.

The refractory period is a key characteristic

that distinguishesaction potentials from

graded potentials.

The refractory period is a key characteristic

that distinguishesaction potentials from

graded potentials.

E. Influence of Axon Diameter and the Myelin Sheath on Conduction Velocity

1. Axons with larger diameters

a) conduct impulses faster than axons with smaller diameters

Marieb; Fig 11.16

Largest axons have the lowest thresholdsGiant squid axon

Mammalian axon

E. Influence of Axon Diameter and the Myelin Sheath on Conduction Velocity

2. Unmyelinated axons

a) conduct impulses relatively slowly

3. Myelinated axons (saltatory conduction)

a) high conduction velocity

Marieb; Fig 11.16

PERIPHERAL NERVE AXON CLASSIFICATION SCHEME

Scheme 1

Scheme

2

Diameter (m)

CV (m/sec)

Type of nerve fiber

A

Ia

12 - 20

72 - 120

Muscle spindle primary afferent

Ib 12 - 20 72 - 120 Golgi tendon organ afferent

12 - 20 72 - 120 Skeletal muscle efferent

A

II

6 – 12

36 – 72

Touch-pressure receptor afferent

5 – 12

20 – 72

Muscle spindle secondary afferent

A

2 – 8

12 – 48

Muscle spindle efferent

A

III

1 – 5

6 – 30

Pain-temperature afferent

B

<3

2 – 18

Preganglionic autonomic efferent

C

IV

<1

<2

Pain-temperature afferent

<1

<2

Postganglionic autonomic efferent

Scheme 1: all peripheral fibers Scheme 2: sensory fibers only

PERIPHERAL NERVE AXON CLASSIFICATION SCHEME

Scheme 1

Scheme

2

Diameter (m)

CV (m/sec)

Type of nerve fiber

A

Ia

12 - 20

72 - 120

Muscle spindle primary afferent

Ib 12 - 20 72 - 120 Golgi tendon organ afferent

12 - 20 72 - 120 Skeletal muscle efferent

A

II

6 – 12

36 – 72

Touch-pressure receptor afferent

5 – 12

20 – 72

Muscle spindle secondary afferent

A

2 – 8

12 – 48

Muscle spindle efferent

A

III

1 – 5

6 – 30

Pain-temperature afferent

B

<3

2 – 18

Preganglionic autonomic efferent

C

IV

<1

<2

Pain-temperature afferent

<1

<2

Postganglionic autonomic efferent

Scheme 1: all peripheral fibers Scheme 2: sensory fibers only

Robinson & Snyder-Mackler; Table 3.1

F. Synapse

Marieb; Fig 11.17

F. Synapse

1. junction that mediates information transfer between neurons or between a neuron and an effector cell

2. presynaptic cell

a) neurons conducting impulses toward the synapse

3. postsynaptic cell

a) cell carrying impulses away from the synapse

F. Synapse

4. electrical synapses

a) neurons that are electrically coupled via protein channels

b) allow direct exchange of ions from cell to cell

ELECTRICAL SYNAPSES CHEMICAL SYNAPSES

Cytoplasmic continuity between pre- and post-synaptic cells

No cytoplasmic continuity between pre- and post-synaptic cells

Communicating agent is ionic current

Communicating agent is a chemical transmitter

Essentially no synaptic delay Significant synaptic delay

Typically bidirectional Unidirectional

F. Synapse

5. Chemical synapses Marieb; Fig 11.18

F. Synapse

5. chemical synapses

a) specialized for release and reception of chemical neurotransmitters

(1) termination of neurotransmitter effects

(a) degradation by enzymes from the postsynaptic cell or within the synaptic cleft

(b) reuptake by astrocytes or the presynaptic cell

(c) diffusion away from the synapse

6. synaptic delay

a) related to the period of time required for release and binding of neurotransmitters

G. Postsynaptic Potentials and Synaptic Integration

1. neurotransmitters mediate graded potentials

a) may be excitatory

b)may be inhibitory

EPSP

IPSP

Marieb; Fig 11.19

Silverthorn; Fig. 8-25

Likelihood of many graded

potentials reaching the trigger zone

simultaneously is quite high

2. summation by the postsynaptic neuron

a) temporal summation

(1) occurs in response to several successive releases of neurotransmitter

b) spatial summation

(1) occurs when the postsynaptic cell is stimulated at the same time by multiple terminals

Marieb; Fig 11.20

G. Postsynaptic Potentials and Synaptic Integration

3. synaptic potentiation

a) presynaptic cell is stimulated repeatedly or continuously

b) enhanced release of neurotransmitter

Appears to be important in learning and memory

G. Postsynaptic Potentials and Synaptic Integration

4. presynaptic inhibition

a) another neuron inhibits the release of excitatory neurotransmitter from a presynaptic cell (axoaxonic synapse)

5. neuromodulation

a) neurotransmitter acts via slow changes in target cell metabolism

b) chemicals other than neurotransmitter modify neuronal activity

H. Neurotransmitters and Their Receptors

1. neurotransmitters

a) one of the ways neurons communicate

b) several chemical classes

(1) Acetylcholine (ACh)

(2) Biogenic amines

(a)catecholamines

(i) dopamine

(ii) norepinephrine (NE)

(iii) epinephrine

(b) indolamines

(i) serotonin

(ii) histamine

Marieb; Fig 11.21

1. Neurotransmitters

(c) amino acids

(i) GABA

(d) peptides

(i) substance P

(ii) endorphins

(e) other messengers

(i) nitric oxide (NO)

2. Functional classifications

a) effects

(1) excitatory (cause depolarization)

(2) inhibitory (cause hyperpolarization)

b) mechanism of action

(1) direct – open ion channels

(2) indirect – work through second messengers so generally will have longer lasting effects

some do both

Marieb; Fig 3.16

3. Receptor types

a) channel-linked receptors

(1) mediate direct transmitter action

(2) brief, localized changes

Excitatory – usually Na+ entry

which contributes to membrane depolarization

Inhibitory – usually allow K+ or Cl- to move which

contributes to membranehyperpolarization

Marieb; Fig 11.22

3. Receptor types

b) G protein-linked receptors

(1) mediate indirect transmitter action

(2) slow, persistent, and often diffuse changes

G proteins often control the production of second messengers,

such as cAMP and Ca2+

G protein-linked receptors

Marieb; Fig 11.23

Receptor is activated by neurotransmitter on extracellular

surface

Binding of G protein causes GTP to ‘displace’ GDP – this ‘activating’ the G

protein

Inactive G protein binds to receptor on intracellular surface

Activated G protein binds to another membrane protein – such as adenylate cyclase which results in

its ‘activation’

Adenylate cyclase catalyzes conversion of ATP to cAMP

cAMP then acts as a second messenger usually by activating other cellular proteins

G protein-linked

receptors

Basic Concepts of Neural Integration

A. Organization of Neurons: Neuronal Pools

1. Neuronal pools

a) functional groups of neurons

b) integrate incoming information from receptors or other neuronal pools

c) relay the information to other areas

Marieb; Fig 11.23

B. Types of Circuits

1. Diverging, or amplifying, circuits

a) common in sensory and motor pathways

b) characterized by

(1) incoming fiber that triggers responses in ever-increasing numbers of fibers along the circuit

Marieb; Fig 11.24

2. Converging circuits

a) common in sensory and motor pathways

b) characterized by

(1) reception of input from many sources

(2) funneling to a given circuit

(3) strong stimulation or inhibition

Marieb; Fig 11.24

3. Reverberating, or oscillating, circuits

a) characterized by feedback by axon collaterals to previous points in the pathway

b) ongoing stimulation of the pathway

Marieb; Fig 11.24

rhythmic activities

Notice the positive feedback

system

4. Parallel after-discharge circuits

a) complex activities

b) characterized by stimulation of several neurons arranged in parallel arrays by the stimulating neuron

Marieb; Fig 11.24

C. Patterns of Neural Processing

1. Serial processing

a) exemplified by spinal reflexes

b) involves sequential stimulation of the neurons in a circuit

Marieb; Fig 11.25

C. Patterns of Neural Processing

2. Parallel processing

a) inputs stimulating many pathways simultaneously

b) vital to higher level mental functioning

Developmental Aspects of Neurons

A. Nervous system originates from a dorsal neural tube and neural crest

1. begins as a layer of neuroepithelial cells that ultimately become the CNS

B. Differentiation of neuroepithelial cells

1. occurs largely in the second month of development

So, some cueing must

exist between nerves and

muscles

Developmental Aspects of Neurons

C. Growth of an axon toward its target

1. Appears to be guided by

a) older “pathfinding” neurons and glial cells

b) nerve growth factor

c) cholesterol from astrocytes

d) tropic chemicals from target cells

Developmental Aspects of Neurons

D. Growth cone

1. growing tip of an axon

2. takes up chemicals from the environment

a) used by the cell to evaluate the pathway taken for further growth and synapse formation

Lieber; Fig 1.3

After initial NMJ formation, several other motoneuronsform synapses at this point.

Developmental Aspects of Neurons

E. Unsuccessful synapse formation

1. cell death

2. certain amount of apoptosis occurs before the final population of neurons is complete