The Nervous System

Functions, Structures and the Classification of Neurons

http://www.alz.org/brain/01.asp

FEATURES

• Composed of unique tissue which is both responsive and conductive (messages can travel at least 200 mph)

• Pinpoint control (targets specific tissues in body)

• The most intricately organized matter on Earth (1cm3

Brain = Several million neurons)

Organs of the Nervous System• Central Nervous System (CNS)

– Brain & Spinal Cord

• Peripheral Nervous System (PNS)– Cranial Nerves & Spinal Nerves

CNS PNS

Sensory

(afferent neurons)

*incoming messages to CNS

*senses external and internal environment

Motor (efferent neurons)

*messages from CNS to muscle or glands

AutonomicSomatic

SympatheticParasympathetic

Nervous System

Effectors: skeletal muscles: conscious control

Effectors: smooth muscles, cardiac, glands: unconscious control

*Maintains homeostasis: self-maintenance fxns (calming affect)

*NT: AcH

*fight or flight

•Arousal or energy generation

•Increase heart &respiratory rate

*NT: norepinephrine

Structural Classification1. Multipolar

a. Many processes/extensions but only one is an AXON

b. Abundant in CNS

2. Bipolara. ? Extensions; one is still the axon

b. Abundant in eyes, nose and ears

3. Unipolara. ? Extension; forms central and peripheral processes

General Functions of the Nervous System

1. Sensory

2. Integrative

3. Motor

FUNCTIONS

• Sensory Input - Conduction of signals from receptors to integration center (eyes)

• Integration - Process by which info. from environment is interpreted & associated with appropriate response of the body– Most occurs in CNS (brain & spinal cord)

• Motor Output - Conduction of signals from CNS to effector cells (muscles or glands) that actually carry out response to stimulus

Integrative Function• CNS

• Type of neuron it occurs in: interneuron

• Stimulated by: NT-neurtotransmitter– Released by a sensory neuron

• Receptive portion: dendrites

• Involves interpretation of an incoming sensory impulse

• Begins a motor impulse from dendrites down axon into terminal end of the axon

Sensory Input Function• PNS• Type of neuron: sensory neuron (afferent)

• Sensory Receptors(receptive portion) detect changes occurring in their surroundings at end of dendrites

• Stimulated by: light, temp change, etc– Once stimulated, sensory receptors transmits a sensory

impulse to the CNS

• Sensory impulse is carried on a sensory neuron from dendrites through cell body down axon to synaptic knobs

Motor Function• PNS• Type of neuron it occurs in: motor neuron (efferent)

– involves the response of a body part

• Motor impulses are carried from CNS to responsive body parts called effectors

• Stimulated by: NT released by interneurons

• Effectors– Muscles (contract)

– Glands (secrete a hormone)

Cells of the Nervous System

• There are 2 main classes of cells:

• A. Neurons - conduct messages• Common Features

– Cell body (contains nucleus & other organelles…no centrioles???)

– Dendrites (conveys signal to cell body)

– Axons (conduct messages away from cell body)

– Myelin Sheath (insulation layer composed of Schwann Cells)

– Synaptic Terminal (relays signals to other cells by releasing neurotransmitters)

– Synapse (site of contact between terminal & target cell)

– Nodes of Ranvier (gaps in myelin, voltage channels)

• B. Supporting Cells - protects, insulates, and assists neurons (Outnumber neurons 10-50 times)

• Do not conduct impulses, but are essential for the integrity of nervous system

• Supporting Cells are called “Glial Cells” (glue)

• Several Types1. Astrocytes - encircles capillaries in brain, restricts passage of substances

into brain

2. Oligodendrocytes - form insulating myelin sheaths around axons in neurons of CNS

3. Schwann Cells - form insulating myelin sheaths around axons in neurons of Peripheral nervous system– Myelin Sheaths - jelly roll composed of lipid (poor conductor)– Multiple Sclerosis = degenerative disease of myelin sheaths (What symptoms

would you expect to see?)

Histology of the Neuron1. Cell Body

2. Dentrites

3. Axona. One per neuron

b. Long, thin processes with uniform diameter

c. carry impulses away from cell body

d. Many fine extensions at end called collaterals

e. Axon ending= at ends of collaterals contain synaptic knob (comes in contact with receptive surface of another cell)

Histology of the NeuronDifferent types of Axons

1. Axons in PNSa. Large and surrounded by myelin sheath (made of

lipids)b. Contains Schwann Cells

i. Myelinated nerve fibers/ produce the myelinii. Between each Schwann cell is a space called Nodes of

Ranvier

2. Small axonsa. Lack myelin sheathb. However all axons are associated with Schwann

cells

Histology of the Neuron

3. Axons in CNSa. Myelin is produced by oligodendrocyte NOT

schwann cells

b. Myelinated nerve bundles are referred to as white matter

c. Gray mattera. Unmyelinated nerve fibers

Histology of the Neuron

Regeneration of Nerve Fibers

1. Injury to cell body: dead neuron

2. Injury to axon: possible regeneration

Histology of the NeuronNeuroglial Cells

accessory cells of the nervous system form the supporting network for neurons (nerve glue)

1 PNS= Schwann2 CNS= 4 types

• Oligodendrocyteso Looks like an eye balls; produces myeline

• Astrocyteso Nourishes neurons

• Microgliao Looks like spider; phagoctosis

• Ependyal cellso Epithelial like layero Functions to line spaces in CNS

Classification of the Neuron

• Neurons of classified based on – Function– Structure

Functional Classification1. Sensory neurons

a. PNSb. Afferent neurons: carry sensory impulses from sensory receptors

to CNSc. Location: skin and sense organs

2. Interneuronsa. CNSb. Links other neurons togetherc. “Association”

3. Motora. PNSb. Efferent neurons; carry motor impulses away fron CNS and to

effectorsc. Effectors: muscles and glands (involuntary and voluntary

controls)