Nervous System 1 Nervous system is conservative n

Nervous System 1

Nervous system is conservative n Because of its role, the nervous system is resistant to evolutionary change. n Even if bones change shape, the nerves innervating the muscles must still work. n The system is therefore an ideal comparative tool to help us understand the evolution of vertebrates.

Nervous system is conservative n It does not fossilize, but it does leave its traces, particularly in the skull. n Behavior, psychology, and physiology are all tools we can use to help us decipher the evolutionary and functional history of vertebrates.

Neurons and neuroglia n Neurons – Nerve cell body – Nissl granules contribute to protein synthesis. – Multipolar – many filamentous processes (brain and spinal cord) – Bipolar (nose, eye, ear, and lateral line) – Pseudounipolar (spinal nerves)

Neurons and neuroglia n Dendrites n Axon n Schwann Cells

Neurons and neuroglia n Myelinn n Axon cylinder n Nodes of Ranvier n Neurilemma n Neuroglia

Nerve Impulse and Synapse n There is a potential of 60 m. V across the cell membrane. n Maintained by imbalance of K and Na. n Na outside of cell, K inside. n Excitation is all or none

Nerve Impulse and Synapse n Synaptic Knob n Presynaptic vesicle n Neurotransmitter – Acetylcholine – Noradrenalin – Serotonin – Dopamine – Glutamic Acid – ATP – Nitric Oxide

Tracts, Nerves, and Ganglia n Tracts n White Matter n Grey Matter n Neurilemma (around fiber) n Perineurium (around fascicles) n Epineurium n Ganglia / Plexus

Components n n n n CNS PNS Afferent – sensory Efferent – motor Association neurons Somatic Visceral Autonomic system

Function and Structure: Reflex arcs and Association neurons. Notice intersegmental nature of some fibers.

Function and Structure n n n Dorsal gray columns Ventral gray columns Gray commisure Dorsal funiculus Ventral funiculus Lateral funiculus n Central pattern generators – Modified by brain, but operate w/o brain as well. n Meninges

Evolution of the Spinal Cord n Gray matter – what happens to the organization of white and gray matter in lower and higher vertebrates? n Amniotes – cervical and lumbar enlargements. n Birds – glycogen body in expanded dorsal median sulcus of lumbar region.

Evolution of Spinal Nerves n Amphioxus – Paired dorsal spinal nerves, sensory and motor components, and no ganglia. They are intersegmental. n Lampreys – Intersegmental like amphioxus, but some cell bodies lie outside the cord. – Segmental ventral spinal nerves that contain only somatic motor fibers.

Evolution of Spinal Nerves n Fish and Amphibians – Dorsal and ventral nerves of each segment join outside the vertebral column, thus, one spinal nerve per segment. – Separate dorsal and ventral roots. – Dorsal ramus – structures of epaxial origin. – Vental ramus – structures of hypaxial origin. – Visceral ramus – structures derived from hypomere. – Nerve cell bodies are in dorsal root ganglion.

Evolution of Spinal Nerves n Amniotes – Dorsal and ventral roots of spinal nerves join insdte the vertebral column. – Each dorsal root joins at the same level as the corresponding ventral root, rather than posterior to it. – Usually all visceral motor fibers exit from the cord in the ventral root. So the shift is complete – leaving the dorsal root with only sensory neurons. – Brachial and lumbosacral plexuses are more complex.