Nervous tissue

Neurons

Transmit electrical signals

Support cells (neuroglial cells in CNS)

• Nonexcitable

• Surround and wrap neurons

Long definition of Neuron

Specialized cells conduct electrical impulses along the plasma membrane

Neurons are the basic structural unit of the nervous system

characteristics of Neurons

Longevity—can live and function for a lifetime

• Do not divide—fetal neurons lose their ability to

undergo mitosis; neural stem cells are an

exception

• High metabolic rate—require abundant oxygen

and glucose

Neurofibrils

• Are bundles of intermediate filaments

• Form a network between chromatophilic bodies

Ganglia

cluster of neuronal cell bodies in PNS

Axons

Neuron has only one

• Impulse generator and conductor

• Transmits impulses away from the cell body

• Chromatophilic bodies are absent

• No protein synthesis in axon

axonal transport

Neuron filaments from the axon Aid in transport of substances to and from the cell

body.

Terminal arboration (telodendria)

Multiple branches at end of axon

terminal boutons (axon terminals)

End in knobs called

Neurotransmitters

are released from vesicles at the terminal boutons

Synapses

Site at which neurons communicate

Most information passed through chemical messengers

Presynaptic neuron

Conducts signal toward a synapse

Postsynaptic neuron

Transmits electrical activity away from a synapse

Axodendritic

Between axon terminals of one neuron and dendrites of another

Most common type of synapse

Axosomatic

Between axons and neuronal cell bodies

Two types of synapses

Axosomatic, Axodendritic

Synaptic vesicles on presynaptic side

Membrane-bound sacs containing neurotransmitters

Synaptic cleft

Separates the plasma membrane of the two neurons

Structural Classification of Neurons

Multipolar, Bipolar, Unipolar

Multipolar

possess more than two processes

Numerous dendrites and one axon

Unipolar (pseudounipolar)

possess one short,

single process

Bipolar

possess two processes

• Rare neurons

• Found in some special sensory organs

Functional classification of neuron

According to the direction the nerve impulse travels relative to the CNS

Types of neurons

• Sensory neurons

• Motor neurons

• Interneurons

Sensory neurons, motor neurons, interneurons

The three functional types of neuron

Sensory neurons (bipolar)

Transmits impulses towards the CNS

The central process

One ends of sensory neuron, runs centrally into

the CNS

The peripheral process

second end of sensory neuron extends peripherally to the receptors

Motor (efferent) neurons

• Carry impulses away from the CNS to effector organs

• Most motor neurons are multipolar

• Cell bodies are within the CNS

• Form junctions with effector cells

Interneurons (association neurons)

most are multipolar

• Lie between motor and sensory neurons

• Confined to the CNS

Neuroglia in the CNS

Most neuroglial cells have branching processes and a central cell body

• Can divide throughout life

• Implications for cancer (gliomas)

Astrocytes, Microglia, Ependymal cells, Oligodendrocytes

Which are the Neuroglia in the CNS

Astrocytes

are the most abundant glial cell type

Sense when neurons release glutamate

Function of Astrocytes

Extract blood sugar from capillaries for energy

• Take up and release ions to control environment

around neurons

• Involved in synapse formation in developing

neural tissue

Microglia

Are the smallest and least abundant glial cell

• Phagocytes—the macrophages of the CNS

Derive from blood cells called monocytes

Ependymal cells

• Line the central cavity of the spinal cord and

brain

• Bear cilia—help circulate the cerebrospinal fluid

Oligodendrocytes

have few branches

• Wrap their cell processes around axons in CNS

• Produce myelin sheaths in the CNS

Neuroglia in the PNS

Satellite cells, Schwann cells

Satellite cells

surround neuron cell bodies within ganglia

Schwann cells (neurolemmocytes)

surround axons in the PNS

• Form myelin sheath around axons of the PNS

Myelin Sheaths

• Segmented structures composed of the lipoprotein myelin

• Form an insulating layer

• Prevent leakage of electrical current

• Increase the speed of impulse conduction

Nerves

cablelike organs in the PNS

• Consist of numerous axons wrapped in connective tissue

Endoneurium

layer of delicate connective tissue surrounding the axon

Perineurium

connective tissue wrapping surrounding a nerve fascicle

Nerve fascicles

groups of axons bound into bundles

Epineurium

whole nerve is surrounded by tough fibrous sheath

Gray matter

Is gray-colored and surrounds hollow central cavities of the CNS

White matter

• Lies external to the gray matter of the spinal cord

• Composed of myelinated axons

• Consists of axons passing between specific regions of the CNS

Tracts

are bundles of axons traveling to similar destinations

Reflex arcs

simple chains of neurons

• Explain reflex behaviors

• Determine structural plan of the nervous system

• Responsible for reflexes

Rapid, autonomic motor responses

• Can be visceral or somatic

a synapse

is a junction between two neurons that communicate the message from one neuron called presynaptic and postsynaptic neurons