AP 2

• -Most abundant,versatile and highly branched glial cells
• -cling to neurons, synaptic endings and capilaries
• -Support and brace neurons
• -Help determine capillary permeability
• -guide migration of young neurons
• -Control the chemical environment
• -Participate in information processing in the brain

small, ovoid cells with thorny processes

Migrate toward injured neurons

Phagocytize microortanisms and neuronal debris.

Range in shape from squamous to columnar

May be cilated

Line the central cabities of the brain and spinal column

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

Branched cells

Processes wrap CNS nerve fibers, forming insulating myelin sheaths

-Satellite cells

Surrond neuron cell bodies in the PNS

Schwann cells (newrolemmocytes)

Surrond peripheral nerve fibers and form myelin sheaths

Vital to regeneration of damaged peripheral nerve fibers

- Short, tapering, and diffusely branched

- Receptice (input region of a neuron

- Convey electrical sgnals toward the cell body as graded potentials

- One axon per cell arising from the axon hillock

- Long axons (nerve fibers)

- Occasional brances (axon collaterals)

- Numerous terminal branchess (teledendria)

• - Knoblike axon terminals (symaptic knobs or boutons)
• Secretory region of neuron
• Release neurotransmitters to excite or inhibit other cells

- conducting region of a neuron

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

- Segmented protein-lipoid sheath around most long or large-diameter axons

• -It functions to:
• Protect and electrically insulate the axon

Increase speed of nerve impuls transmission

- Schwann cells wraps many times around the axon

- Myelin sheath-concentric layers of Schwann cell membrane

- Neurilemma-peripheral bulge of Schwann cell cytoplasm

-Nodes of Ranvier

Mylein sheath gaps between adjacent Schwann cells

Sites where axon collateral can emerge

Three types:

• 1. Sensory (afferent)
• Transmit impulses from sensory receptors toward the CNS

• 2. Motor (efferent)
• Cary impulses from the CNS to effectors

• 3. Interneurons (association neurons)
• Shuttle signals through CNS pathways; Most are entirely within the CNS

- Voltage (V): measure of potiential enery generated by seperated charge

- Potential difference: Voltage measured between two points

- Current (I): the low of electircal charge (ions) between two points

- Resistance (R): hindrance to charge flow (provided by the plasma membrane)

- Insulator: subtance with high electrical resistance

- Condutor: subtance with low electrical resistance

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

- Depolarization

- A reduction in membrane potential (toward zero)

- inside of the membrane becomes less negative than the resting potential


- Increases the probablitlity of producing a nerve impulse

Hyperpolarization

- An increase in membrane potential (away from zero)

- Inside of the membrane be omes more negative than the resting potential

- Reduces the probability of producing a nerve impulse

- 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 short0circuiting of nerve impulses occurs

- Impulse conduction slows and eventually ceases

Treatment

- Some immune system-modifying drugs including interferons and copazone

-Hold symptoms at bay

-reduce complications

- Reduce disability

• - 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

- Axodendritic--between the axon of one neuron and dendrite of another

- Axosomatic--between the axon of one neuron and the soma of another

less common types;

- Axoaxonic (axon to axon)

- Dedrodendritic (dentrite to dendrite)

- Dendorsomatic (dendrite to soma)

- A single EPSP cannot induce an action portential

- EPSPs can summate to reach thresholod

- IPSPs can also summate with EPSPs canceling each other out

Temporal summation

One or more presynaptic neurons transmit impulses in rapid-fire order

Spatial summation

Postynaptic neuron is stimulated by a large number of terminals at the same time

50 or more neutotransmitters have been identified

One to know is ACH acetylcholine

Catecholamines

--Dopamine, norepinephrine (NE) and epinephrine

Indolamines

--Serotonin and histamine

Diverging circuit

-One incoming fiber stimulates an ever-increasing number of fibers, often amplifying circuits

- May affect a single pathway or several

- Common in both sensory and motor systems

Converging circuit

- Oppostie of diverging circuits, resulting in either strong stimulation or inhibition

- also common in sensory and motor systems

Reverberating (oscillating) circuit

- Chain of neurons containg collateral synapses with previous neurons in the chain

Parallel after-discharge circuit

- Incoming fiber sstimulates several neurons in parallel arrays to stimulate a common output cell.

Serial Processing

- Example: reflexes--rapid, automatic responses to stimuli that always cause the same response

- Reglex arcs (pathways) have five essential components: receptor, sensory neuron, CNS integration center, motor neuron and effector

1. Receptor

2. Sensory neuron

3. INtegration center

4. Motor neuron

5. Effector

RESPONSE