Nervous System Functions

Organization of Nervous System

• Control • Communication

CENTRAL NERVOUS SYSTEM CNS Brain and Spinal Cord

– Sensory – Integration – Motor

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PERIPHERAL NERVOUS SYSTEM PNS Cranial Nerves and Spinal Nerves (Nerve tissue outside of CNS) S. Davenport ©

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PNS and Sensory Division

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Sensory Division (afferent PNS)

CENTRAL NERVOUS SYSTEM CNS Integration and Control

Special senses (such as for sight, smell, and hearing)

PERIPHERAL NERVOUS SYSTEM PNS

Visceral receptors (such as baroreceptors, chemoreceptors, mechanoreceptors)

SENSORY (AFFERENT) DIVISION Somatic and Visceral

Somatic receptors (such as skeletal muscle stretch receptors)

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Brain

CNS

Spinal cord

Cranial nerves

PNS

SOMATIC RECEPTORS Special senses, Proprioceptors, etc. 10/26/2004

VISCERAL RECEPTORS Chemoreceptors, Baroreceptors, Mechanoreceptors S. Davenport ©

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Spinal nerves

Somatic and visceral (have some receptors in common such as pressure receptors) 10/26/2004

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PNS and Motor Division

Brain

Motor Division (efferent PNS)

CENTRAL NERVOUS SYSTEM CNS Integration and Control

Parasympathetic (autonomic, visceral)

PERIPHERAL NERVOUS SYSTEM PNS

SOMATIC NERVOUS SYSTEM To skeletal muscles Voluntary

AUTONOMIC NERVOUS SYSTEM Cardiac muscle, smooth muscle, and glands (some) Involuntary

Sympathetic Fight or flight response

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Spinal cord

Sympathetic (autonomic, visceral)

MOTOR (efferent) DIVISION Somatic and Autonomic

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Cranial nerves

PNS

Somatic (skeletal muscle, voluntary)

Parasympathetic Rest and repose response

CNS

Spinal nerves

Sympathetic (sweat glands, involuntary) 10/26/2004

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Nerve Tissue • Nervous tissue forms the nervous system – consists mostly of the (1) brain, (2) spinal cord, and (3) nerves.

Neurons

• Two fundamental types of cells form the basis of nervous tissue: • (1) neurons (nerve cells) • (2) neuroglia (supporting cells) 10/26/2004

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• Structure • Functions

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Neurons

• There are many types of neurons • Most neurons have – cell body (soma) – cell processes (axon and dendrites)

Typical Neurons

• Functions include – Generation and transmission of electrical events resulting in the inhibition or excitation of postsynaptic neuron (or effector)

• Neuroglial cells – Mostly function in supporting and insulating the nervous tissue. There are several different types of neuroglial cells. 10/26/2004

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• Must live (amitotic) and function for the life of the individual. • Function is the generation and conduction of an electrical event, the nerve impulse. • Result is the excitation or inhibition of the associated (synapsed) neuron, muscle, or gland. 10/26/2004

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Structure of the Neuron Histology of “Nerve Tissue”

• Soma (body)

Label: • • • • • •

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– The portion of the cell with houses abundant cytoplasmic organelles and is associated with the cell processes – Cell bodies are either located within the gray matter of the CNS (most) or in structures called ganglia of the PNS

Axon Collaterals Dendrites Neuroglia Nucleus Soma (body)

• Nucleus – Control center of the cell

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Structure of the Neuron

Structure of the Neuron

• Axon

• Dendrites – The cell processes which function as the receptive portion of the neuron. – May be modified and described as receptors such as corpuscles, spindles, etc. – Conduct toward the cell body – Generate electrical information as graded potentials (not nerve impulses, or action potentials)

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– Originate from cell body at region called hillock – Typically axons are called nerve fibers (are thin and may be several feet in length) – May branch to form collaterals – Usually ends in terminal branches each with knoblike ending called synaptic knobs, axonal terminals, or boutons.

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Structure of the Neuron • Axon

Synapse

– Can be functionally divided into three regions (1) generator, (2) conductor, (3) and secretor. • Generation occurs at its most proximal region called the axon hillock • Conduction proceeds along the length of the fiber • Secretion (the release of neurotransmitter) occurs upon the arrival of the nerve impulse at the axon terminals

– Axons are organized into regions called tracts (or columns) of the CNS or into the nerves of the PNS.

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• Anatomical relationship between neurons, or neurons and an effector organ, and at which a nerve impulse is transmitted through the action of a neurotransmitter.

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Termination of Neurotransmitter • Enzymes associated with postsynaptic membrane or present in cleft • Reuptake by astrocytes into presynaptic terminal where degraded by enzymes • Neurotransmitter diffuses away from synapse

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Structural Classification of Neurons • Unipolar • Bipolar • Multipolar

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Classification of Neurons

Structural Classification • Unipolar neuron

Neurons

– Has a single process associated with the cell body. – Functions as sensory (afferent) neuron

• Bipolar neuron – Has two processes associated with the cell body – Functions as sensory neuron (locations include retina and olfactory mucosa)

Structural Classification (based upon number of processes associated with cell body)

Unipolar

Bipolar

Functional Classification (based upon direction of conduction in reference to CNS)

Multipolar

Sensory (afferent)

Motor (efferent)

Association

• Multipolar neuron – Has more than two processes (usually many) associated with the cell body. – Functions as motor (efferent) neuron 10/26/2004

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Sensory Neurons • Transmit impulses generated at their receptors toward the central nervous system are sensory, or afferent, neurons. They constitute the sensory (afferent) division of the peripheral nervous system. • Consist of somatic and visceral neurons and are mostly unipolar

Functional Classification of Neurons • Sensory neurons • Association neurons • Motor neurons

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Sensory neuron

Spinal cord

Golgi tendon organs (stretch receptors)

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Motor Neurons Types of Receptors

• Motor, or efferent, neurons transmit impulses from the central nervous system to effectors (glands and muscles). They constitute the motor (efferent) division of the peripheral nervous system. • Consist of somatic and visceral neurons and are mostly multipolar

• Exteroceptors – External environment in form of touch, temperature, pressure, sight, smell, and hearing.

• Proprioceptors – Monitor position and movement of skeletal muscles and joints

Motor neuron

Spinal cord

• Interoceptors – Monitor systems such as urinary, digestive, respiratory, cardiovascular – provide pressure, pain, and input into autonomic pathways. 10/26/2004

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Neuromuscular junctions 10/26/2004

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Interneurons (association) • Transmit impulses from one neuron to another. They are located in the central nervous system.

Neuroglia of the CNS • • • •

Association neuron Spinal cord

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Astrocytes Microglia Ependymal Oligodendrocytes

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Astrocytes (as-tro-cytes) Function in exchange –

Neuroglia of the PNS

Star-shaped cells which associates neurons and their surrounding capillaries.

Microglia (mi-krog-le-ah) • Satellite cells • Schwann cells

Phagocytosis – neural debris and microorganisms\

Ependymal (ep-en-di-mal) Form the lining of ventricles of the brain and central canal of spinal cord.

Oligodendrocytes (ol-i-go-den-dro-site) Form insulating covering called myelin sheath around the axons of the CNS 10/26/2004

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Myelination of Axon

Schwann and Satellite Cells • Schwann cells (neurolemmocytes) are associated with axons of PNS. – May produce myelinated or unmyelinated fibers

• Satellite cells surround neuron cell bodies which are located within ganglia

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Schwann Cells – Myelinated fibers in PNS – The sequential wrapping of each Schwann cell membrane around the axon produces a myelinated fiber. – The space between adjacent Schwann cells is called the node of Ranvier. – Neurilemma is nucleus and cytoplasm outside of myelin sheath S. Davenport ©

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Schwann Cells Unmyelinated fibers of PNS

• Myelinated fiber

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• All axons of the PNS are associated with Schwann cells • All axons of the CNS are associated with oligodendrocytes • Myelin (my-e-lin) is a lipoid substance located in the membranes of Schwann cells (neurilemmocytes) and oligodendrocytes • Axons are either MYELINATED OR UNMYELINATED depending upon whether the axon is wrapped or loosely associated with its associated cells

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– The loose association of each Schwann cell membrane around the axon produces an unmyelinated fiber.

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Myelinated Fiber

Histology of PNS Axons Identify: Axons Myelin sheath Node of Ranvier Neurilemma Schwann cell

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CNS Axons Oligodendrocytes (ol-i-go-den-dro-site)

Neurophysiology: Ions and Electrical Signals

• Form insulating covering called myelin sheath around CNS axons • Single oligodendrocyte associates with many axons • Lack neurilemma

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Electrical Terminology

Membrane Potentials

• Potential energy

• Resting membrane potential

– State of electrical energy as measured by the potential to produce electrical effects

– Potential maintained by neuron.

• Graded potential

• Voltage (potential)

– Temporary, localized change in resting membrane potential. Diminishes with distance

• Action potential – Electrical impulse that travels along an axon. Propagated and does not diminish with distance.

• Synaptic activity – Typically involves release of neurotransmitter from axon. Binding to postsynaptic membrane causes graded potential. – Usually the result of an overall change due to the influence of many graded potentials (many synapses).

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• Current – Flow of electrical charge and is due to the electrical difference (voltage) between two points

• Resistance

• Postsynaptic cell response

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– Electrical measurement used to describe electrical potential between two points.

– Opposition to electrical flow • Insulators have high resistance • Conductors have low resistance 37

Electrical Terminology • How might the following terms apply to these two batteries?

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Electrical Terminology and the Cell Membrane • How might the following terms apply to the illustrated cell membrane?

– Potential energy • Are both the same?

– Potential energy – Voltage – Current – Resistance

– Voltage • Are both the same?

– Current • Do both produce the same?

Extracellular

– Resistance Size AAA 10/26/2004

Size D

• Does a battery contain a “resister?” S. Davenport ©

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Intracellular

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Electrical Terminology and the Cell Membrane • How might the following terms apply to the illustrated cell membrane?

Electrical Terminology and the Cell Membrane • How might the following terms apply to the illustrated cell membrane?

Extracellular

– Potential energy – Voltage – Current – Resistance 10/26/2004

– Potential energy – Voltage – Current – Resistance S. Davenport ©

Intracellular

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Transmembrane Potentials Extracellular

Extracellular

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Intracellular

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Resting Membrane Potential

• Ionic difference between intracellular and extracellular fluids – Extracellular higher concentration of Na+ (and Cl-) – Intracellular higher concentration of K+ and negative proteins.

Net result is potential difference between extracellular and intracellular. Intracellular 10/26/2004

– Extracellular is positive (Na+) – Intracellular is negative due to negative proteins. S. Davenport ©

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Mechanical Channels

Membrane Potential Changes • If the resting membrane potential is to change – must be a change in the distribution of positive and/or negative charges; a redistribution of ions –

• Sodium channels (typical) open when subjected to mechanical stimulus

• Movement of ions can result when ions move through channels which include – Mechanically-gated (regulated) channels • Open when subjected to a mechanical stimulus

– Voltage-gated (regulated) channels • Open when subjected to an electrical stimulus

– Chemically-gated (regulated) channels • Open when subjected to a specific chemical such as a neurotransmitter or hormone

– Passive (leakage) channels • Ions may leak through channels (or the phospholipid bilayer) 10/26/2004

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• • • •

• Identify regions which are – Mechanically gated – Electrically gated – Chemically gated

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Graded Potentials

Channels

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Local response (graded potential at stretch receptor) Sodium ions move across membrane Interior becomes less negative (more positive) Depolarization (changes toward less negative (positive) voltage – May not reach threshold, thus no effect (action potential) – May reach threshold and produce an action potential

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Threshold and Action Potentials

IPSP and EPSP

• Threshold

• EPSP • Excitatory postsynaptic potential results when interior becomes more positive • IPSP • Inhibitory post-synaptic potential results when interior becomes more negative 10/26/2004

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– Point of depolarization (stimulation) which initiates an effect (action potential) – In this case the electrically-gated Na+ channels open, (which are adjacent to the active mechanicallygated channels). – The mechanically gated Na+ channels become inactive

• Action potential

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– Not local; travels great distance – Involves electrically-gated channels – Propagated along fiber (axon) S. © –Davenport All-or-None Principle applies 50

Generation of Action Potential Resting Membrane Potential

1. Resting membrane potential is established 2. Depolarization phase – Increase in sodium ion permeability – Self propagating event

3. Repolarization phase – Decrease in sodium ion permeability – Increase in potassium ion permeability

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Undershoot or after-hyperpolarization occurs Redistribution of sodium and potassium by ATP driven sodium-potassium pump 10/26/2004

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Repolarization as K+ Moves Out

Depolarization as Na+ Moves Inward • Receptor’s Na+ channels become inactive • Local current opened adjacent electrically-gated Na+ channels (threshold) • These channels produce local current • Adjacent Na+ channels open 10/26/2004

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Na+ / K+ Pump

• Local current opens adjacent electrically-gated K+ channels • K+ moves outward and repolarization occurs • Local currents open adjacent Na+ channels • Action potential is propagated to adjacent forward 10/26/2004 section

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Propagation of Action Potential

• The Na+ / K+ re-establishes the extracellular and intracellular ionic gradients – Pump requires ATP – Na+ is pumped outward – K+ is pumped inward

• Propagation – Refers to the relay of the electrical event, the action potential, along the axon

• Continuous Propagation – Involves adjacent membrane proteins, typical of unmyelinated axons

• Saltatory Propagation – Involves propagation from node to node, typical of myelinated axons 10/26/2004

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Conduction Velocity

Synaptic Activity

• Myelinated fibers conduct faster than unmyelinated fibers

– Impulse passes from presynaptic membrane (typically the neuron’s axon) to postsynaptic membrane. Typical postsynaptic membranes are located on

– Continuous conduction – Saltatory conduction

• Large fibers conduct faster than small fibers

• Neurons (neuronal synapse) • Muscle cells (neuromuscular synapse) • Glands (neuroglandular synapse)

– Larger fibers offer less resistance

• What is the approximate range of conduction velocities? • What is multiple sclerosis (MS)? 10/26/2004

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2. Calcium ion channels open

• Involve a neurotransmitter – Excitatory neurotransmitters always produce an EPSP (excitatory postsynaptic potential) – Inhibitory neurotransmitters always produce an IPSP (inhibitory postsynaptic potential) – The production of an action potential depends upon “reaching threshold.” Thus, it is the property of the receptor not the neurotransmitter. The same neurotransmitter may be inhibitory at one location and excitatory at another location S. Davenport ©

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Synapse

Chemical Synapses

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1. Action potential arrives

3. Calcium ions promote exocytosis of neurotransmitter, calcium ions are quickly removed 4. Neurotransmitter binds to postsynaptic receptors

5. Receptors allow passage of specific type of ions

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6. Depending upon ion movement postsynaptic membrane is either 7. Neurotransmitter is deactivated depolarized (EPSP) or hyperpolarized (IPSP) by enzymatic action; some 10/26/2004 S. Davenport © 60 components may be reused

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Synapse

Synapse

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A

B

A

The result is “A” or “B”? Which is produced? A) action potential, B) IPSP, C) EPSP? S. Davenport ©

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B

The result is “A” or “B”? Which is produced? A) action potential, B) IPSP, C) EPSP? S. Davenport ©

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Cholinergic Synapses

How Neurotransmitters Work

• Release acetylcholine – At all neuromuscular juntions – At many CNS synapses – At all neuron-to-neuron synapses in PNS – At all effector sites (muscles and glands) of parasympathetic nervous system (ANS).

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• Compounds that have a direct effect on membrane potential • Compound that have an indirect effect on membrane potential or cell activity • Lipid-soluble gases that effect the inside of the cell

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How Neurotransmitters Work Mechanisms of Neurotransmitters

• Direct acting (effect) – Channel linked receptors result in the opening of ion channels – – Alter membrane potential of target – Can produce depolarization (sodium ions move inward) and hyperpolarization (potassium ions move outward)

• Indirect acting – Involves G-protein complex – Results in the production of a second messenger – Second messenger may influence enzymes to • Activate or inactivate proteins (translation) • Regulate gene activity (transcription) • Regulate membrane ion channels and potentials

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Postsynaptic Potentials

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Summation

• IPSPs (review) • EPSPs (review) • Summation

Temporal summation – Pertaining to time; the quick succession of SPs at a few synapses are summated

– The adding together of synaptic potentials (SPs). Could be EPSPs, IPSPs, or both EPSPs and IPSPs. – Temporal summation – Spatial summation

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Nerves

Summation

• Cordlike organ which conducts impulses from a part of the central nervous system and another region of the body.

• Spatial summation – Pertaining to space; many SPs occur over the postsynaptic membrane and are summated

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– Components of the peripheral nervous system (PNS) – Include the spinal nerves, the cranial nerves, and all of their branches.

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Nerve Structure

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Nerves

• Consisting of

• May contain

– parallel axons (fibers) and – their associated Schwann cells (neurilemmocytes)

– only myelinated fibers, – only unmyelinated fibers, – or a combination of both myelinated and unmyelinated fibers

• Classified as – enclosed in successive connective tissue wrappings (endoneurium, perineurium, epineurium).

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– Sensory (afferent) – Motor (efferent) – Mixed (combination of afferent and efferent)

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Nerve

Nerve

• Define and Identify: – Nerve – Epineurium – Perineurium – Fasicle What is the difference between a nerve and a tract?

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Sensory Nerves (fibers)

– Perineurium – Endoneurium – Schwann cell – Axon – Myelin sheath – Neurilemma

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• Includes

• Typically involve unipolar neurons • Impulse GENERATION begins at a dendrite (receptor) • Flow of information is into the CNS – Typically to an association neuron.

Sensory neuron

• Define and Identify

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Reflex

– Sensory (afferent) neurons – Association neurons – Motor (efferent) neurons

• Response is predictable

Spinal cord

Golgi tendon organs (stretch receptors)

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Brain

Motor Division (efferent PNS) Parasympathetic (autonomic, visceral)

Autonomic Regulation

CNS

Spinal cord

Sympathetic (autonomic, visceral)

Cranial nerves

PNS

Somatic (skeletal muscle, voluntary)

Spinal nerves

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Autonomic Systems • Sympathetic – “fight or flight response” – Terminal neurotransmitter is epinephrine (E) or norepinephrine (NE)

• Parasympathetic – “resting and digesting,” or “rest and repose” – Terminal neurotransmitter is acetylcholine (ACh)

• Organs – May have dual innervations, response is excitation by one system and inhibition by other system – May have single innervations, response is promoted or not promoted. 10/26/2004

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