NEROHISTOLOGY

Central Nervous System

(CNS)

Brain, Spinal cord

Peripheral Nervous System

(PNS)

A. Somatic division (voluntary)

B. Autonomic division

Neurons:

Conducting cells

Neuroglia:

Accessory supporting
cells

Meninges:

Coverings for brain
and spinal cord

Basic functional units
• Cells that receive and
transmit information
• Classified according to
functional and structural
characteristics

neurons

dendrites:

Stimulated by

environmental

changes or activities

of other cells

Cell body:

Contains nucleus

and other

organelles

Synaptic

terminals

Connection with

other neuron or

organ

Axon:

Conducts

nervous

impulse to

distal end of

neuron

Contains typical cellular

organelles: Nucleus,

mitochondria, golgi, etc.

• No mitosis-related

organelles: Neurons

don’t divide.

• Neurofibril cytoskeleton

• Supports & maintains

neuron shape

cell body

Neuronal cell bodies

often found in

grouped

clusters

Nucleus (pl. nuclei):

Cluster of neuronal

cell bodies in CNS

Example:

Cranial nerve

nuclei in brain

stem

• Ganglion (pl.

ganglia):

Cluster of neuronal

cell bodies in PNS

Example:

Paravertebral

chain ganglia in

thorax

Receive impulses

& transmits to cell

body

Dendrites:

One per neuron.

Transmits impulse

from body to

termina

Axon ( nerve

fiber)

Transmits impulse to

tissue or another

neuron

Terminal

Nerves are bundled together like

muscle fibers.

(with similar CT coverings too!)

Bundles of axons in CNS

Example:

Tract 

example:  Dorsal column

medial lemniscus tract in

CNS

Nerve:

and Example: 

Bundles of axons in PNS

Example:

Sciatic nerve

in posterior

thigh

Unipolar Neuron

• Single axon extending from cell body.

• Predominantly found in invertebrates

Pseudounipolar Neuron

• Short axon, two processes

• One process from sensory structures

• One process to spinal cord

• Associated with sensation

Bipolar Neuron

Two processes: 1 Dendrite, 1 Axon, body in the middle

• Involved in special sensation

• Ex. Retina of eye, inner ear, olfactory nerve (smell)

Multipolar =

most common type • More than two processes & multiple dendrites • Found in CNS and motor neurons that control muscle

Sensory (Afferent: A=towards)

Pseudounipolar neuron

• Impulse transmission direction:

• Peripheral sensory receptor  CNS

• Cell body located in sensory ganglia

2. Motor (Efferent: E=away from)

Multipolar neuron

• Impulse transmission direction:

• CNS  effector

• Skeletal/smooth/cardiac

muscle, glands

• Cell bodies in CNS

Interneuron (associative)

• “Link” between sensory & motor

neurons in CNS

• Modifies responses & relays info to

multiple other neurons

• Primarily Inhibitory (“off switch”)

• Important part in some reflexes

Ion concentration differences: • More potassium (K+) within neuron • More sodium (Na+) outside neuron • Concentration difference creates cell polarity • More positive outside • More negative inside

Neuron at rest: •

caused by influx of Na+ ions into cell through ion channels • Now more negative outside of cell, more positive within. • “Wave of negativity” travels along cell membrane in one direction • Repolarization: caused by influx of K+ ions to outside of cell

Depolarization

Repolarization:

caused by influx of K+ ions to outside of cell

The Myelin Sheath •

Function: • Insulates axon for faster impulse conduction • Produced by 2 different cells • Oligodendrocyte in CNS • Schwann Cell in PNS. • White color to due Lipids

Node of Ranvier

• Area of axon not covered by myelin. • Allows FOR RAPID CONDUCTON

Continuous conduction

• Slower conduction speed • Unmyelinated neurons • Action potential travels uninterrupted along axon • Exchange of ions moves like a wave along axon

 Saltatory (“jumping”) conduction

• Faster conduction • Occurs in myelinated neurons • Ions exchange at nodes of Ranvier only • Mylein insulates axon

Multiple sub-types • Motor, pressure, sharp pain, etc. • Large myelin sheath • Fast conduction

A fibers •

• Intermediate amount of myelin • Moderate speed • Autonomic nervous system

B fibers

Least myelin or no myelin • Slow speed • Dull /aching pain, itching, warmth

C fibers

Chemical signaling molecules • Allows communication between neurons • Two types: • Excitatory (“on switch”) • Inhibitory (“off switch”)

Neurotransmitters •

Synaptic vesicles

Structures that store and carry neurotransmitters Released at a synapse or voltage-gated sodium ion channels

Synapse:

Region of contact between neuron and another cel

1. Presynaptic neuron (“before the synapse) •

Transmits impulse to synapse

2. Synaptic cleft

• Space between: • Presynaptic neuron’s axon • Postsynaptic neuron’s dendrites OR different type of cell

3. Postsynaptic neuron

• Receives impulse and conduct it to target tissues • Postsynaptic cell can be non -neuronal (muscle, gland, etc.)

Neurotransmitters:

Cross cleft and bind to postsynaptic dendrite

Neurotransmitters

•  On/off switches 

• Excitatory: On switch 

• Depolarizing impulse 

• Acetylcholine (AcH) & Norepinephrine (NE) • Both involved with muscle contraction and autonomics

• Serotonin • Smooth muscle, mood, cyclic body processes

•  Dopamine • Reward and pleasure centers • Stimulate hormone secretion

Acetylcholine (AcH) & Norepinephrine (NE) •

Both involved with muscle contraction and autonomics

Serotonin

• Smooth muscle, mood, cyclic body processes

. Dopamine •

Reward and pleasure centers • Stimulate hormone secretion

Inhibitory:

Off Switch

Hyperpolarize postsynaptic neuron: Prevents impulse transmission across synapse

1.Gamma Aminobutyric acid (GABA) • Regulate muscle tone

2.Enkephalins • Bind to opiate receptors • regulates pain perception 3.Endorphins • Hormones similar to enkephalins • Bind to opiate receptors • Analgesic effect (pain relief)

4.Dopamine : • Inhibit Production of certain hormones • Regulate movement and emotional responses • Deficiency: Parkinson’s disease

.Gamma Aminobutyric acid (GABA) •

Regulate muscle tone

2.Enkephalins

Bind to opiate receptors • regulates pain perception

.Endorphins

• Hormones similar to enkephalins • Bind to opiate receptors • Analgesic effect (pain relief)

4. Dopamine

Inhibit Production of certain hormones • Regulate movement and emotional responses • Deficiency: Parkinson’s disease

Neuroglia

Support cells

Oligodendrocyte

Produces myelin sheath (covering) around axons in CNS

Schwann Cells

Produces myelin sheath around axons in PNS

Microglia

Macrophages of CNS:

• Engulf cellular debris,

microorganisms, dead or

damaged tissue.

Astrocyte

• Nourishment of neurons in CNS • Formation of the Blood Brain Barrier.

Ependymal cells

• Cells line the ventricles of brain • Ventricles: Interconnected cavities • Responsible for regulation of cerebrospinal fluid (CSF)