What are the divisions of the nervous system?

Central nervous system (CNS) and Peripheral nervous system (PNS)

What is the function of the CNS and what does it consist of (body parts)?

integration and command center. Consists of the brain and spinal cord

What does the PNS consist of and function?

Paired spinal and cranial nerves. Carry messages to and from the CNS

What are the two functional divisions of the PNS?

Sensory and Motor

what are the functions of the sensory division?

afferent, "carrying towards". Visceral afferent fibers- convey impulses from visceral organs.

What is the functions of the somatic afferent fibers?

convey impulses from the body (skin, skeletal muscles, and joints).

What is the function of visceral afferent fibers?

convey impulses from visceral organs.

Describe motor division

efferent, "carrying away"

Transmits impulses from the CNS to effector organs

What are the motor divisions of PNS

Somatic nervous system and autonomic nervous system

Define somatic nervous system

voluntary, conscious control of skeletal muscle

Define autonomic nervous system

involuntary, visceral motor nervous fibers. Regulates smooth muscle and glands

What are the functional subdivisions of the autonomic nervous system ?

Sympathetic and Parasympathetic

What is a neuron?

excitable cells that transmit electrical signals

What are neuroglia?

supporting cells.

What are astrocytes?

most abundant versatile, and highly branched glial cells

What are the functions of astrocytes?

Cling to neurons, synaptic endings, and capillaries.

Support and brace neurons. Help determine capillary permeability.

Guide migration of young neurons.

Control the chemical environment

Participate in information processing in the brain

What are the functions of the microglia?

Small, ovoid cells with thorny processes that migrate toward injured neurons and phagocytize microorganisms and neuronal debris.

Describe the structure and function of Ependymal cells

range in shape from squamous to columnar. May be ciliated.

Line the central cavities of the brain and spinal column,

Seperate the CNS interstitial fluid from the cerebrospinal fluids in the cavities.

Oligodendrocytes

branched cells. Processes wrap CNS nerve fibers, forming insulating myelin sheathe.

what are satellite cells?

surround neuron cell bodies in the PNS

what are Schwann cells

also known as neurolemmocytes.

Surround peripheral nerve fibers and form myelin sheaths

Vital to regeneration of damaged peripheral nerve fibers

What are the special characteristics of neurons

Long-lived (up to 100 years or more)

Amitotic- w/few exceptions

High metabolism rate- depends on continuos supply of glucose and oxygen

Plasma membrane functions in electrical signaling and cell to cell interactions during development

Describe the cell body

aka Perikaryon or Soma

Biosynthetic center of a neuron

Spherical nucleus with nucleolus

Well- developed Golgi appartua

Rough ER is called Nissl bodies

Network of neurofibrils (neurofilmaents)

Axon hillock

What does chromatophilic mean?

color loviong

Define axon hillock

cone-shaped area from wich axons arises

What are clusters of cell bodies called in the CNS?

nucliei

What are clusters of cell bodies called in the PNS?

ganglia

Dendrites

short, tapering and diffusely branched

receptive (input) region of a neuron

convey electrical signals toward the cell body as graded potentials

Axon

one axon per cell arising from the axon hillock

long axons (nerve fibers)

occasional branches (axon collaterals)

numerous terminal branches (telodenria)

Telodenria

numerous terminal branches of axons

What are the functions of axon

conducting region of a neuron

generates and transmits nerve impulses (action potentials) away from the cell body

Molecules and organelles are moved along axons by motor molecules in 2 directions. What are those 2 directions?

Anterograde and Retrograde

Define anterograde

molecules and organelles are moved along axons by motor molecules toward axonal terminal

Define retrograde

molecules and organelles are moved along axons by motor molecules towards the cell body

Myelin Sheath in the PNS

Schwann cells wraps many times around the axon and is the concentric layers of Schwann cell membrane

Neurilemma- peripheral bulge of Schwann cell cytoplasm

Nodes of Ranvier- myelin sheath gaps btw adjacent Schwann cells, sites where axon collaterals can emerge

Unmyelinated axons

thin nerve fibers

One Schwann cell may incompletely enclose 15 or more unmyelinated axons

Myelin Sheath in the CNS

formed by processes of oligodentrocytes, not the whole cells

nodes of Ranvier are present

no neurilemma

thinnest fibers are unmyelinated

What are the types of structural classifications of neurons

mulitpolar- 1 axon and several dendrites. Most abundant. Motor neurons and interneurons

biplolar- 1 axon and 1 dendrite

unipolar (pseudounipolar)- single, short process that has 2 branches: peripheral process: more distal branch, often associated with a sensory receptor. Central process: branch entering the CNS

What are the functional classifications of neurons

Sensory (afferent), motor (efferent) and interneurons (association neurons)

Function of sensory (afferent) neurons

transmit impulses from sensory receptpors toward the CNS

Function of motor (efferent) neurons

carry impulses from the CNS to effectors

Function of interneurons (association neurons)

shuttle signals through CNS pathways; mostly are entirely w/in the CNS

Neuron functions

highly irritable, respond to adequate stimulus by generating an action potential (nerve impulses), impulse is always the same regardless of stimlus

Principles of electricity

opposite charges attract each other

energy is required to separate opposite charges across a membrane

energy is liberated when the charges move toward one another

if opposite charges are separated, the system has potential energy

define Voltage (V)

measure of potential energy generated by separated charge

define potential difference

voltage measure between two points

define Current (I)

the flow of electrical charge(ions) btw 2 points

Define resistance (R)

hindrance to charge flow (provide by the plasma membrane)

define insulator

substance w/high electrical resistance

define conductor

substance w/low electrical resistance

Roles of membrane in ion channels

proteins serve as membrane ion channels

2 main types of channels- leakage (nongated) channels and channels (3 types)

What are the 3 types of gated channels

chemical gated(ligand-gated) channels- open w/binding of a specific neurotransmitter

voltage-gated channels- open and close in repsonse to changes in membrane potential

Mechanically gated channels- open and close in response to physical deformation of receptors

What happens when gated channels are open?

ions diffuse quickly across the membrane along their electochemical gradients. Along chemical concentration gradients from higher concentration to lower concentration. Along the electrical gradients toward opposite electrical charge/

ion flow creates an electrical current and voltage changes across the membrane

describe resting membrane potential (Vr)

potential difference across the membrane of a resting cell

approximately -70 mV in neurons (cytoplasmic side of membrane is negatively charged relative to outside)

negative interior of the cell is due to much greater diffusion of K⁺ out of the cell than Na+ diffusion into the cell

Sodium-potassium pump stabilizes the Vr by maintaining the concentration gradients for Na⁺ and K⁺

What is the resting membrane potential generated by?

differences in ionic makeup of ICF than ECF. Differential permeability of the plasma membrane

what are the differences in ionic makeup of Vr?

ICF has lower concentration of Na⁺ and Cl^- than ECF

ICF has higher concentration of K⁺ and negatively charged proteins (A^-) then ECF

Differential permeability of membrane

Impermeable of A^-(protein)

slightly permeable to Na⁺ (through leakage channels)

75 times more permeable to K⁺ (more leakage channels)

freely permeable to Cl^-

Membrane potential changes when:

concentrations of ions across the membrane change and permeabilty of membrane to ions change

signals used to receive, integrate and send information

what are the 2 types of signals

graded- incoming short-distance

action- long-distance of axons

what are the the 2 types of changes of membrane potential?

Depolarization

Hyperpolarization

Depolarization

• A reduction in membrane potential (toward zero)

• Inside of the membrane becomes less negative than the resting potential

• Increases the probability of producing a nerve impulse

Hyperpolarization

• An increase in membrane potential (away from zero)

• Inside of the membrane becomes more negative than the resting potential

• Reduces the probability of producing a nerve impulse

Graded Potential

• Short-lived, localized changes in membrane potential

• Depolarizations or hyperpolarizations

• Graded potential spreads as local currents change the membrane potential of adjacent regions

• Occur when a stimulus causes gated ion channels to open

• E.g., receptor potentials, generator potentials, postsynaptic potentials

• Magnitude varies directly (graded) with stimulus strength

• Decrease in magnitude with distance as ions flow and diffuse through leakage channels

• Short-distance signals

Action Potential (AP)

• Brief reversal of membrane potential with a total amplitude of ~100 mV

• Occurs in muscle cells and axons of neurons

• Does not decrease in magnitude over distance

• Principal means of long-distance neural communication

Generation of an AP

• Resting state

• Only leakage channels for Na + and K + are open

• All gated Na + and K + channels are closed

Properties of gated channels in AP

Each Na + channel has two voltage-sensitive gates

• Activation gates

• Inactivation gates

• Closed at rest; open with depolarization

• Open at rest; block channel once it is open

Depolarizing phase

• Depolarizing local currents open voltage-gated Na + channels

• Na + influx causes more depolarization

• At threshold (–55 to –50 mV) positive feedback leads to opening of all Na + channels, and a

reversal of membrane polarity to +30mV (spike of action potential)

Repolarizing phase:

• Na + channel slow inactivation gates close

• Membrane permeability to Na + declines to resting levels

• Slow voltage-sensitive K + gates open

• K + exits the cell and internal negativity is restored

Hyperpolarization:

• Some K + channels remain open, allowing excessive K + efflux

• This causes after-hyperpolarization of the membrane (undershoot)

Roles of sodium-potassium pump:

Repolarization

• Restores the resting electrical conditions of the neuron

• Does not restore the resting ionic conditions

• Ionic redistribution back to resting conditions is restored by the thousands of sodium-potassium

pumps

Propagation of AP:

• Na + influx causes a patch of the axonal membrane to depolarize

• Local currents occur

• Na + channels toward the point of origin are inactivated and not affected by the local currents

• Local currents affect adjacent areas in the forward direction

• Depolarization opens voltage-gated channels and triggers an AP

• Repolarization wave follows the depolarization wave

• (Fig. 11.12 shows the propagation process in unmyelinated axons.)

at Threshold:

• Membrane is depolarized by 15 to 20 mV

• Na + permeability increases

• Na influx exceeds K + efflux

• The positive feedback cycle begins

Threshold:

• Subthreshold stimulus—weak local depolarization that does not reach threshold

• Threshold stimulus—strong enough to push the membrane potential toward and beyond

threshold

• AP is an all-or- none phenomenon—action potentials either happen completely, or not at all

Coding for stimulus intensity:

• All action potentials are alike and are independent of stimulus intensity

• How does the CNS tell the difference between a weak stimulus and a strong one?

• Strong stimuli can generate action potentials more often than weaker stimuli

• The CNS determines stimulus intensity by the frequency of impulses

Refractory period: a time gap of setting and resetting

• Absolute refractory period

• Relative refractory period

Absolute refractory period:

• Time from the opening of the Na + channels until the resetting of the channels

• Ensures that each AP is an all-or- none event

• Enforces one-way transmission of nerve impulses

Relative refractory period:

• Follows the absolute refractory period

• Most Na + channels have returned to their resting state

• Some K + channels are still open

• Repolarization is occurring

• Threshold for AP generation is elevated

• Exceptionally strong stimulus may generate an AP

Effect of axon diameter

Larger diameter fibers have less resistance to local current flow and have faster impulse conduction

Effect of unmyelination

• Continuous conduction in unmyelinated axons is slower than saltatory conduction in myelinated axons

Effects of myelination

• Myelin sheaths insulate and prevent leakage of charge

• Saltatory conduction in myelinated axons is about 30 times faster

• Voltage-gated Na + channels are located at the nodes

• APs appear to jump rapidly from node to node

Multiple sclerosis (MS):

• An autoimmune disease that mainly affects young adults

• Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, and

urinary incontinence

• Myelin sheaths in the CNS become nonfunctional scleroses

• Shunting and short-circuiting of nerve impulses occurs

• Impulse conduction slows and eventually ceases

Treatment for MS

Some immune system–modifying drugs, including interferons and Copazone:

• Hold symptoms at bay

• Reduce complications

• Reduce disability

what are the Nerve fiber classifications:

• Group A fibers

• Large diameter, myelinated somatic sensory and motor fibers

• Group B fibers

• Intermediate diameter, lightly myelinated ANS fibers

• Group C fibers

• Smallest diameter, unmyelinated ANS fibers

How are nerve fibers classified

Diameter, Degree of myelination, and Speed of conduction

Synapse:

• A junction that mediates information transfer from one neuron:

• Postsynaptic neuron—transmits impulses away from the synapse

What are the types of Synapses

• Axodendritic—between the axon of one neuron and the dendrite of another

• Axosomatic—between the axon of one neuron and the soma of another

• Less common types:

• Axoaxonic (axon to axon)

• Dendrodendritic (dendrite to dendrite)

• Dendrosomatic (dendrite to soma)

Varieties of Synapses:

chemical and electrical synpases

Electrical synapses:

• Less common than chemical synapses

• Neurons are electrically coupled (joined by gap junctions)

• Communication is very rapid, and may be unidirectional or bidirectional

• Are important in:

• Embryonic nervous tissue

• Some brain regions

Chemical synapses:

• Specialized for the release and reception of neurotransmitters

• Typically composed of two parts

• Axon terminal of the presynaptic neuron, which contains synaptic vesicles

• Receptor region on the postsynaptic neuron

Synaptic cleft:

• Fluid-filled space separating the presynaptic and postsynaptic neurons

• Prevents nerve impulses from directly passing from one neuron to the next

• Transmission across the synaptic cleft:

• Is a chemical event (as opposed to an electrical one)

• Involves release, diffusion, and binding of neurotransmitters

• Ensures unidirectional communication between neurons

Information transfer:

• AP arrives at axon terminal of the presynaptic neuron and opens voltage-gated Ca 2+ channels

• Synaptotagmin protein binds Ca 2+ and promotes fusion of synaptic vesicles with axon membrane

• Exocytosis of neurotransmitter occurs

• Neurotransmitter diffuses and binds to receptors (often chemically gated ion channels) on the

postsynaptic neuron

• Ion channels are opened, causing an excitatory or inhibitory event (graded potential)

Termination of neurotransmitter effects:

• Within a few milliseconds, the neurotransmitter effect is terminated

1. Degradation by enzymes

2. Reuptake by astrocytes or axon terminal

3. Diffusion away from the synaptic cleft

Synaptic delay:

• Neurotransmitter must be released, diffuse across the synapse, and bind to receptors

• Synaptic delay—time needed to do this (0.3–5.0 ms)

• Synaptic delay is the rate-limiting step of neural transmission

Postsynaptic potential:

• Graded potentials

• Strength determined by:

• Amount of neurotransmitter released

• Time the neurotransmitter is in the area

Types of postsynaptic potentials

• EPSP—excitatory postsynaptic potentials

• IPSP—inhibitory postsynaptic potentials

EPSP = excitatory postsynaptic potential

• Neurotransmitter binds to and opens chemically gated channels that allow simultaneous flow of

Na + and K + in opposite directions

• Na + influx is greater that K + efflux, causing a net depolarization

• EPSP helps trigger AP at axon hillock if EPSP is of threshold strength and opens the voltage-gated

channels

PSP—inhibitory postsynaptic potentials

• Neurotransmitter binds to and opens channels for K + or Cl –

• Causes a hyperpolarization (the inner surface of membrane becomes more negative)

• Reduces the postsynaptic neuron’s ability to produce an action potential

Integration: synaptic potentiation:

• Repeated use increases the efficiency of neurotransmission

• Ca 2+ concentration increases in presynaptic terminal and ostsynaptic neuron

• Brief high-frequency stimulation partially depolarizes the postsynaptic neuron

• Chemically gated channels (NMDA receptors) allow Ca 2+ entry

• Ca 2+ activates kinase enzymes that promote more effective responses to subsequent

stimuli

Integration: presynaptic inhibition:

• Release of excitatory neurotransmitter by one neuron may be inhibited by the activity of

another neuron via an axoaxonic synapse

• Less neurotransmitter is released and smaller EPSPs are formed

What are Neurotransmitters:

• Most neurons make two or more neurotransmitters, which are released at different stimulation

frequencies

• 50 or more neurotransmitters have been identified

• Classified by chemical structure and by function

• Broadly distributed in the brain

• Play roles in emotional behaviors and the biological clock

Chemical classes of neurotransmitters:

Acetylcholine (Ach)

Biogenic amines- Catecholamines and Indolamines

Amino acids

Peptides (neuropeptides)

Purines such as ATP

Gases and lipids

Functional classification of neurotransmitters:

• Neurotransmitter effects may be excitatory (depolarizing) and/or inhibitory (hyperpolarizing)

• Determined by the receptor type of the postsynaptic neuron

• GABA and glycine are usually inhibitory

• Glutamate is usually excitatory

• Acetylcholine

• Excitatory at neuromuscular junctions in skeletal muscle

• Inhibitory in cardiac muscle

Neurotransmitter Action:

Neurotransmitter Action:

Direct Action and Indirect action

Direct action

• Neurotransmitter binds to channel-linked receptor and opens ion channels

• Promotes rapid responses

• Examples: ACh and amino acids

Indirect action

• Neurotransmitter binds to a G protein-linked receptor and acts through an intracellular

second messenger

• Promotes long-lasting effects

• Examples: biogenic amines, neuropeptides, and dissolved gases

Types of neurotransmitter receptors

1. Channel-linked receptors

2. G protein-linked receptors

Channel-linked (lonotropic) receptors:

• Ligand-gated ion channels

• Action is immediate and brief

• Excitatory receptors are channels for small cations

• Na + influx contributes most to depolarization

• Inhibitory receptors allow Cl – influx or K + efflux that causes hyperpolarization

G-protein- linked (metabotropic) receptors:

• Transmembrane protein complexes

• Responses are indirect, slow, complex, and often prolonged and widespread

• Examples: muscarinic ACh receptors and those that bind biogenic amines and neuropeptides

G-protein- linked receptors: mechanism:

• Neurotransmitter binds to G protein–linked receptor

• G protein is activated

• Activated G protein controls production of second messengers, e.g., cyclic AMP, cyclic GMP, diacylglycerol or Ca 2+

• Second messengers

• Open or close ion channels

• Activate kinase enzymes

• Phosphorylate channel proteins

• Activate genes and induce protein synthesis

Neural integration: neuronal pools:

functional groups and simple

Functional groups of neurons that:

• Integrate incoming information

• Forward the processed information to other destinations

Simple neuronal pool

• Single presynaptic fiber branches and synapses with several neurons in the pool

• Discharge zone—neurons most closely associated with the incoming fiber

• Facilitated zone—neurons farther away from incoming fiber

Types of circuits in neuronal pools:

Diverging circuit

Converging circuit

Reverberating circuit

Parallel after-discharge circuit

Patterns of neural processing

Serial processing and parallel processing

Serial processing

• Input travels along one pathway to a specific destination

• Works in an all-or- none manner to produce a specific response

• Example: reflexes—rapid, automatic responses to stimuli that always cause the same

response

• Reflex arcs (pathways) have five essential components: receptor, sensory neuron, CNS

integration center, motor neuron, and effector

Parallel processing

• Input travels along several pathways

• One stimulus promotes numerous responses

• Important for higher-level mental functioning

• Example: a smell may remind one of the odor and associated experiences

Developmental aspects of neurons:

• The nervous system originates from the neural tube and neural crest formed from ectoderm

• The neural tube becomes the CNS

• Neuroepithelial cells of the neural tube undergo differentiation to form cells needed for

• Cells (neuroblasts) become amitotic and migrate

• Neuroblasts sprout axons to connect with targets and become neurons

development

Axonal growth:

• Growth cone at tip of axon interacts with its environment via:

• Cell surface adhesion proteins (laminin, integrin, and nerve cell adhesion molecules or

• Neurotropins that attract or repel the growth cone

• Nerve growth factor (NGF), which keeps the neuroblast alive

• Astrocytes provide physical support and cholesterol essential for construction of synapses

N-CAMs)

Cell death:

• About 2/3 of neurons die before birth

• Death results in cells that fail to make functional synaptic contacts

• Many cells also die due to apoptosis (programmed cell death) during development