NERVOUS SYSTEM

NERVOUS TISSUE

Nervous System - General

Control SystemRegulator of HomeostasisElectrical Impulses Rapid & Transient Effects

Nervous System - Functions

Sensory - Monitors Internal & External Environments

Integrative- Analyzes Sensory Information- Stores- Makes Decisions Regarding Appropriate

Responses

Motor – Controls muscles & glands; responds to sensory information

Nervous System Divisions

Central Nervous System (CNS)- Brain, Spinal Cord- Dorsal Body Cavity- Integration & Command Center

Peripheral Nervous System (PNS)- All Neural Tissue outside CNS- Nerves (Cranial & Spinal)- Carries Info. Between CNS & Rest of Body- Sensory & motor

Peripheral Nervous System

Sensory- Afferent

- Conveys sensory information to CNS

Motor- Efferent

- Conveys motor commands to muscles & glands

PNS Motor Divisions

Somatic Nervous System (SNS)

- Voluntary Control of Skeletal MusclesAutonomic Nervous System (ANS)

- Involuntary Control of Smooth Muscle, Cardiac Muscle & Glands

- Sympathetic Division

- Parasympathetic Division

Cells of Nervous System

Neurons- Basic Unit of Nervous System- Most Specialized Cell in Body- Conduct Impulses

Neuroglia (“Nerve Glue”)- Support, Framework (fill spaces) &

Phagocytes- Most Numerous- Can Divide & Multiply

Neurons - Structure

Cell Body (Soma)- Nucleus & Various Organelles

* Nissl Bodies (Rough ER)

* Neurofibrils (Cytoskeletal)

Dendrites- Numerous, Short, Branched Processes- Receive Impulse from other Neurons or

Receptors- Carry Impulse Towards Cell Body (Afferent)

Fig. 8.3

Neurons – Structure continued

Axon- Long, Usually Singular Process- Many Mitochondria, Neurofibrils- Carries Impulse Away from Cell Body

(Efferent)- Carries Impulse Towards:

* ANOTHER NEURON* MUSCLE FIBER* GLAND CELL

Neurons – Axon continued

- Axon Hillock (Joins Cell Body & Axon)

- Collaterals (Axon Branches)

- Axon/Synaptic Terminals

*Numerous, Fine Processes at end of Axon & Axon Collaterals

*Some with Synaptic Knobs

Neurons – Axon continued

- Myelination

*Most Axons

*Enclosed in Schwann Cells (Neurolemmocyte)Myelin Sheath – Multilayered,

Inner, FattyNeurolemma – Outer Schwann

Cell Membrane & Cytoplasm

Fig. 8.6

Neurons – Axon Myelination continued

*Insulates & Increases Speed of Conduction

*Nodes of RanvierOccur Along Axon Between

Schwann CellsNo Myelin

Neurons – Functional Classification

Sensory

- Afferent

- Connect Receptors & CNSMotor

- Efferent

- Carry Commands from CNS to Effectors Interneurons (Association)

- CNS

- Integrate Sensory & Motor

- Most Numerous

Neurons – Structural Classification

Unipolar- One Process (Dendrites & Axon Fused)- Sensory

Bipolar- Two Processes: One Dendrite, One Axon- Rare (Special Senses)

Multipolar- Several Dendrites, One Axon- Common- Motor & Interneurons

Neurons - Terminology

Gray Matter – Unmyelinated Fibers & Cell Bodies

White Matter – Myelinated AxonsNerve – Bundle of Fibers (Axons) in PNSTract – Bundle of Fibers in CNSGanglia – Clusters of Neuron Cell Bodies

in PNSNuclei – Clusters of Neuron Cell Bodies

in CNS

Neuroglia

CNS- Astrocytes

* Large, Star-shaped* Link Neurons & Blood Vessels; Help form

Blood-brain Barrier - Oligodendrocytes

* Form Myelin Sheath- Microglia

* Derived from WBCs, Phagocytes

Fig. 8.5abc

Neuroglia continued

- Ependymal Cells* Epithelium* Line Ventricles & Central Canal* Produce & Help Circulate CSF

PNS- Schwann Cells (Neurolemmocytes)

* Form Myelin Sheath- Satellite Cells

* Support, Cushion Ganglia

Nerve Impulse Transmission

Two mechanisms involved

- Transmission along a neuron

*An electrical process

- Transmission between neurons

*A chemical process

*Occurs at synapse

Neuron Physiology

Transmission Requirements:

- Resting Membrane Potential (Cell Membrane is Polarized)

- Ion Channels in Cell Membrane (Allow Ions to Cross When Open)

- Delivery of Threshold Stimulus

Conduction Along Neuron

Resting Membrane Potential (+/Na+ outside, -/K+ inside

Appropriate Threshold Stimulus Opens Na+ Channels

Na+ Diffuses into Neuron, Results in Depolarization

Depolarization wave spreads from dendrite to axon

Fig. 8.11

Conduction Along Neuron continued

Na+ Channels Close, K+ Channels Open & K+ Diffuses Out of Neuron

Results In Repolarization Action Potential = Depolarization +

Repolarization (dendrite to axon)Repolarization Required before another

Action PotentialSodium-Potassium Pump moves Na+ out

& K+ in (Requires Energy)

Conduction continued

All-or-None Principle- Neurons respond to stimuli by generating an

impulse (action potential), or don’t respond at all

Refractory Period- Neurons must repolarize their cell membranes

before they respond to subsequent stimuli

Types of Conduction

Continuous- Typical of Unmyelinated Neurons (Slower)- Steps as Previously Described

Saltatory- Occurs along Myelinated Neurons- No Current where Myelin occurs- Action Potential Leaps from Node of Ranvier

to Node- Faster!

Fig. 8.12

Synaptic Transmission

Arriving Action Potential Depolarizes Synaptic Knob

Ca++ Enters Cytoplasm of Presynaptic NeuronExocytosis of Synaptic Vesicles, Releasing

NeurotransmitterNeurotransmitter Diffuses across Synaptic Cleft

& Binds to Receptors on Postsynaptic MembraneNa+ Channels Open, Postsynaptic Membrane

Depolarizes

Fig. 8.13

Neurotransmitters

Excitatory – Cause Depolarization/Na+ ions channels open (Dopamine)

Inhibitory – Raise the Threshold/ K+ or Cl- ion channels open (Serotonin & GABA)

Removed by Specific Enzymes