Nervous System Part 1

central nervous system

brain and spinal cord

neurons

conduct impulses but generally cannot divide Most can not divide, some can repair

Sensory neurons

conduct impulses from sensory receptors, in the periphery, to the CNS

peripheral nervous system

cranial, spinal, and peripheral nerves conducting impulses to and from the CNS

sympathetic nervous system

emergency situations; "fight or flight"

anterograde transport

movement of organelles and molecules along microtubules from soma to axon terminals

retrograde transport

movement of organelles or molecules along microtubules from axon terminals to soma; • Facilitated by dynein proteins

parasympathetic nervous system

normal functions; "rest and digest"

what produces the myelin sheath in the CNS

oligodendrocytes

glial cells (neuroglia)

support the neurons and can not conduct impulses, but can divide

Autonomic motor neurons two sections

sympathetic and parasympathetic

CNS axons are NOT able to regenerate due to:

1. Death receptors form that promote apoptosis of oligodendrocytes 2. Inhibitory proteins in the myelin sheath prevent regeneration 3. Glial scars from astrocytes form that also prevent regeneration

neuron general function

1. Respond to chemical and physical stimuli 2. Conduct electrochemical impulses 3. Release chemical regulators 4. Enable perception of sensory stimuli, learning, memory, and control of muscles and glands

neurilemma (sheath of Schwann)

All axons in the PNS are surrounded by a sheath of Schwann cells called the neurilemma (sheath of Schwann), nucleated cytoplasmic layer of Schwann cell.

Axonal transport

An active bidirectional transport of large and small molecules within the axons

Regeneration of a Cut Neuron

When an axon in the PNS is cut or damaged, 1) Nerve fibers and myelination distal to the injury degenerates and phagocytosed by macrophages. 2) Weeks after the injury, Schwann cells proliferate and form a regeneration tube. 3) Growth factors are released that stimulate growth of axon sprouts within the tube (at a rate of 0.5-3.0 mm/day) 4) After a few months, myelination is completed and the new axon reestablishes synapses with the effector

dendrites

extensive processes branching from the soma that act to receive and conduct signals from other neurons toward the cell body • Contain mainly cytoskeleton

Oligodendrocytes

form myelin sheaths around the axons of CNS neurons

Bipolar neurons

have two processes, one on either end; found in special sensory neurons

ependymal cells

line the ventricles and secrete cerebrospinal fluid Ciliated columnar or cuboidal cells that line the fluid-filled ventricles of the brain and central canal of the spinal cord Function in filtration of blood to create an ultrafiltrate called cerebral spinal fluid (CSF) Apical surface contains cilia to create current for CSF movement Basal end doesn't attach to basement membrane but contain elongated, branching processes

Association/interneurons

located completely within the CNS and integrate functions of the nervous system

axon

long, large process, varying in length (a few millimeters to a meter), originating from a pyramid-shaped region of the soma, called the axon hillock Plasma membrane of the axon (axolemma) also contains large amounts of voltage-gated ion channels

microglia

macrophage of CNS - migrate around CNS tissue and phagocytize foreign and degenerated material Phagocytic cells (resident macrophage) of the CNS These mobile neuroglia function to remove debris and microbial pathogens

astrocyte

regulate the external environment of the neurons Most abundant glial cell Contain processes with end-feet that associate with blood capillaries and axon terminals Influence interactions between neurons and capillaries that supply blood to neural tissues

Autonomic motor neurons

responsible for autonomic reflexes and involuntary control of smooth muscle, cardiac muscle, and glands

Somatic motor neurons

responsible for somatic reflexes and voluntary control of skeletal muscles

multipolar neurons

several dendrites and one axon; most common type (most interneurons and all motor neurons)

Pseudounipolar

single short process that branches like a T to form 2 longer processes (peripheral and central axons); sensory neurons

function of myelination

to increased the conduction velocity of electrical impulses

Neuroglia

(Glial Cells) of the PNS Cells that are non-conducting but support neurons

Satellite cells

(ganglionic gliocytes) - support cell bodies of peripheral neurons located in ganglia

Schwann cells

(neurolemmocytes) - support peripheral axons and form myelin sheaths to insulate certain axons

four types of neuroglia in the CNS

oligodendrocytes, ependymal cells, microglia, astrocytes

cell body (neurons)

(perikaryon or soma) that contains: • Nucleus, Nissl bodies (highly concentrated rER and polyribosomes), Golgi apparatus, and mitochondria (located throughout neuron) • Abundant microtubules, actin filaments, and Type IV intermediate filaments (formed by protein subunits called neurofilaments or neurofibrils)

Astrocytes and Neural Activity

- although astrocytes do not produce action potentials, they are excited by changes in intracellular Ca(2+) concentration - when some neurons are active, they release ATP, which increases the Ca(2+) of adjacent astrocytes; creates a Ca(2+) wave - a rise in Ca(2+) can also cause the astrocyte release prostaglandin E2 from the end-feet on a blood capillary, increasing blood flow -increased neural activity is accompanied by increased blood flow to that region

astrocyte function

1. Regulate and maintain ionic environment around neurons 2. Take up extra neurotransmitter released from axon terminals, particularly glutamate. 3. End-feet around capillaries take up glucose from blood and stored as glycogen for use by neurons to make ATP; converted first to lactic acid 4. Form the blood-brain barrier via interactions of end-feet and capillaries 5. Regulate neurogenesis through regulation of synapse formation in the CNS

Classification of Nerves in the PNS

A collection of neuronal cell bodies is called a ganglion A bundle of axons is called a nerve Note: most nerves are composed of both sensory and motor neurons.

Classification of Nerves in the CNS

A collection of neuronal cell bodies is called a nucleus A bundle of axons is called a tract.

axon collaterals

Axons can form many branches called axon collaterals, which allow for communication with multiple neurons

Astrocytes and Blood-Brain Barrier

Capillaries in the brain do not have pores between adjacent cells but are joined by tight junctions. Substances can only move transcellularly by selective processes of diffusion through endothelial cells, active transport, and bulk transport Astrocytes influence the production of ion channels, to selectively transport certain ions, and enzymes, that can destroy toxic substances by secreting glial-derived neurotrophic factor. Creates problems with chemotherapy of brain diseases because many drugs can not penetrate the blood-brain barrier.

what does the cytoplasm of axon contain

Cytoplasm of axon (axoplasm) contains microtubules, neurofilaments and transport vesicles, but very few rER and ribosomes

initial segment

End of the axon hillock and beginning of axon is known as the initial segment, containing large amounts of voltage-gated ion channels, which generate an action potential (nerve impulse)

terminal bouton

Ends of the axon undergo terminal arborization, with each small branch ending with a dilation called a terminal bouton (axon terminal), which contains abundance of mitochondria

nodes of ranvier

Gaps between myelin sheaths (exposed axons) are called nodes of Ranvier.

gray matter

Gray matter is cell bodies, dendrites, or axons that lack myelin sheaths

myelin sheath

Many but not all axons contain a thick insulating layer, called the myelin sheath, formed by the Schwann cell wrapping itself around a portion of the axon.

function of satellite cells

May have a role in regulation of environment around neuronal cell bodies to augment neuronal activities Currently under research to determine full functions of these supporting cells

white matter

Myelin gives these tissues (axons) a white color = white matter.

myelinated axons in the pns are surrounded by what

Myelinated axons in the PNS are surrounded by myelin sheath and neurilemma. • Myelinated axons conduct electrical impulses along the axons more rapidly.

one oligodendrocyte projects how many extensions

One oligodendrocyte projects multiple extensions to several axons and each extension wraps around a section of an axon to form a myelin sheath

what are two types of glial cells in the PNS

Schwann cells and satellite cellsRegeneration of a Cut Neuron

Unmyelinated axons in PNS are surrounded by what

Unmyelinated axons in the PNS are surrounded by neurilemma (Schwann cells) but lack myelin sheath • Small axons (2 micrometers in diameter) are usually unmyelinated.

two types of axonal transport

anterograde transport and retrograde transport

nervous system has two parts

central and peripheral

Motor neurons

conduct impulses from the CNS to the effectors (muscles or glands)

do Myelinated axons in CNS lack neurilemma

yes