Nervous System: General Principles PA 544 Clinical Anatomy Tony Serino. Ph.D.

Nervous System Controls and/or modifies all other systems Rapid response time Usually short duration

Functional Areas

Divisions of the Nervous System

Nervous Tissue Non-excitable Tissue (Supportive cells) –Neuroglia –present in CNS –Schwann and Satellite cells –present in PNS Neurons (excitable tissue) –Initiate and conduct electrical signals (action potentials)

Neuroglia (glial cells) Form BBB Regulate microenvironment Pass on nutrients; get rid of waste Phagocytic, protective

Neuroglia Line cavities Create CSF Secrete myelin in CNS

PNS Supportive Cells Schwann cells –secrete myelin in PNS Satellite cells –surround neuron cell bodies in PNS

Neuron Anatomy Axonal terminal Nerve ending Synaptic boutons Synaptic knobs

Functional Zones of a Neuron Receptor Zone Initial segment of Axon (trigger zone) Axon Nerve endings

Myelination In PNS, a Schwann cell wraps and individual segment of a single axon In the CNS, an oligodendrocyte performs the same function but can attach to more than one axon

Node of Ranvier: gaps in myelin sheath

Types of Neurons Anatomical classification –Based on number of process projecting from cell body Functional Classification –Based on location of neuron and direction of information flow

General Terms Ganglia vs. Nuclei –Areas of densely packed nerve cell bodies –Ganglia are usually found in PNS –Nuclei are found in CNS Nerve vs. nerve fiber –A nerve is a dissectible structure containing hundreds of axons –A nerve fiber is a single axon CT sheaths covering peripheral nerves:

Nerve CT sheaths

Synapses Areas where neurons communicate with other cells Can be chemical (with neurotransmitters) or electrical (gap junctions)

Anatomy of Synapse (chemical) Neurotransmission ends when NT diffuses away, re-absorbed by presynaptic neuron, or NT metabolized (degraded) by enzymes in cleft

Neurotransmission Electrical signal (action potential (AP)) descends axon to synaptic knob (nerve end) Depolarization opens Ca ++ channels to open in presynaptic membrane Triggers a number of synaptic vesicles to fuse with outer membrane Dumps neurotransmitter (NT) into synaptic cleft NT diffuses across cleft and binds to receptor on postsynaptic membrane This leads to channels opening on postsynaptic membrane changing the membrane’s potential

Types of Anatomical Synapses

Regeneration of Nerve Fibers Damage to nerve tissue is serious because mature neurons are post-mitotic cells If the soma of a damaged nerve remains intact, damage may be repaired Regeneration involves coordinated activity among: – remove debris –form regeneration tube and secrete growth factors –regenerate damaged part

Response to Injury Anterograde degeneration with some retrograde; phagocytic cells (from Schwann cells, microglia or monocytes) remove fragments of axon and myelin sheath Cell body swells, nucleus moves peripherally Loss of Nissl substance (chromatolysis) In the PNS, some Schwann cells remain and form a tubular structure distal to injury; if gap or scarring is not great axon regeneration may occur with growth down tube In the CNS, glial scar tissue seems to prevent regeneration If contact with tube is not established then no regeneration and a traumatic neuroma forms

Regeneration in PNS

Drug Intervention Possibilities A.Increase leakage and breakdown of NT from vesicles B.Agonize NT release C.Block NT release D.Inhibit NT synthesis E.Block NT uptake F.Block degradative enzymes in cleft G.Bind to post-synaptic receptor H.Stimulate or inhibit second messengers in post-synaptic cell