Nervous system cells Flashcards
Central nervous system
Brain and spinal cord
- structural and functional center of the nervous system
- integrates sensory information
- evaluates sensory infor and initiatives an outgoing response
Peripheral nervous system
Nerves that lie outside of CNS
- cranial nerves: originate from brain and brain stem (12 pairs)
- spinal nerves: originate from spinal cord (31 pairs)
Afferent nerves
All incoming sensory pathways
Efferent nerves
All outgoing motor pathways (initiate response to stimulus)
Somatice nervous system
Motor, body
- motor: carries information to the somatice effectors (skeletal muscles)
- sensory: carries feedback information from somatic effectors to somatic integration centers in CNS
Autonomic nervous system
Involuntary
- efferent: carries into to the autonomic or visceral efferrctors (smoooth cardiac muscles and glands)
1. Sympathetic
2. Parasympathetic - afferent/visceral: sensory division-carries feedback info to autonomic integrating centers in CNS
Enteric nervous system
Lumped into autonomic
Sympathetic division of ANS
Prepares body for immediate threats to its internal environment
- fight - flight - freeze
Parasympathetic division of ANS
Coordinates body’s normal resting activities
- rest - repair
Cells of nervous system
- Neurons: excitable cells that make all nervous system functions possible - form the wiring of the nervous systems information circuits
- Glial cells (neuroglia): do not usually conduct information - support the function of neurons in various ways “helper cells”
- astrocytes - microglia - ependymal cells - oligodendrocytes - Schwann cells
Astrocytes
- star-shaped, largest, most numerous
- transfer nutrients from the blood to the neurons (glucose)
- help recycle neurotransmitters at synapses
- form tight sheaths around brain capillaries with tight junctions between capillary endothelial cells, help form the blood-brain barrier
BBB
- helps maintain a very stable environment required for normal functioning of the brain
- forms a barrier that regulates passage of most ions between the blood and brain tissue
- som ions could disrupt the transmission of nerve impulses if BBB did not exist
Microglia
- small, usually stationary
- engulf and destroy microorganisms an celular debris
- macrophages of the nervous system
Ependymal cells
- form thin sheets that line fluid-filled cavities in the CNS
- some take part in producing the fluid that fulls these spaces
- some have motile cilia that help circulate the fluid
Oligodendroctyes
- smaller than astrocytes with fewer processes
- hold nerve fibers together
- produce the mullein sheath around nerve fibers in CNS ONLY
Schwann cells
- PNS ONLY
- support nerve fibers and form myelin sheaths
- gaps in the myelin sheath are called Nodes of Ranvier
- neurolemma: nucleus and cytoplasm squeezed to perimeter. Essential in normal nerve growth and regeneration
Neurons
Excitable cells that initiate and conduct impulses to make all nervous system functions possible
- cell body - dendrites - axon
Cell body
Perikaryon or soma
- provides neurotransmitters needed for transmission of nerve signals. Proteins packed in vesicles
- provide proteins for maintaining and regenerating nerve fibers
- contains mitochondria, which are transported to the end of the axon to provide energy for nerve signaling
Dendrites
- each neuron has 1+ and they branch from soma
- distal ends of dendrites of sensory neurons may be receptors because they receive the stimuli that initiate nerve signals
- dendrites conduct nerve signals to the cell body of the neuron
- do not have myelin sheath
Axon
- single process extending form the axon hillock, sometimes covered by a myelin sheath
- the axon hillock is the portion of the cell body from which the axon extends
- myelination affects speed of impulse conduciotn
- axons conduct nerve impulses away from the cell body of the neuron
Regions of neuron (4)
- Input zone: dendrites and cell body receive stimulus and initiate impulse
- Summation zone: axon hillock collects all the nerve impulses arriving from the cell body and dendrites and decides whether to send the impulse any farther along the neuron
- Conduction zone: axon conducts the nerve impulse from the axon hillock all the way to the end of the neuron
- Output zone: telodendria and synaptic knobs release neurotransmitters
Structural classification of neurons
Number of processes extending from cell body
- multipolar: one axon and several dendrites
- bipolar: only one axon and one dendrite; least numerous kind of neuron
- unipolar: one process comes off neuron cell body but divides almost immediately into two fibers; central process and peripheral process
Functional classification of neurons
Direction in which they conduct impulses
- afferent: sensory. Conduct impulse to spinal cord and brain
- efferent: motor. Conduct impulses away from spinal cord or brain toward muscles or glandular tissue
- interneurons: conduct impulses from afferent neurons to or toward motor neurons. Lie within CNS
Reflex arc
Signal conduction route to and from the CNS with the electrical signal beginning in receptors and ending in effectors
Two neuron arc
Simplest form, consists of an afferent neuron and an efferent neuron
Three neuron arc
Most common; consists of an afferent neuron, an interneuron, and an efferent neuron
Synapse
The location where nerve signals are transmitted from one neuron to another
- location: typically the junction of the synaptic knob of one neuron and the dendrites or cell body of another neuron
Nerves and tracts
- nerves: bundles of peripheral nerve fibers held together by several layers of connnective tissue that form a sheath
- tracts: within the CNS, bundles of nerve fibers are called tracts rather than nerves
Nerve facts
- Endoneurium: surround Schwann cells
- Perineurium: surrounds a fasicle
- Epineurium: surrounds a whole nerve (numerous fascicles and the blood vessels that supply that)
- most nerves are mixed and contain both afferent and efferent fibers
Tracts facts
Nerves within the CNS and do not have connective tissue coverings
White matter
Bundles of myelinated fibers
- PNS: consists of myelinated nerves
- CNS: consists of myelinated tracts
Gray matter
Made up of cell bodies and unmyelinated fibers
- PNS: bundles of cell bodies referred to as ganglia
- CNS: bundles of cell bodies referred to as nuclei
Repair of nerve fibers (replacement vs repair)
- Replacement: evidence has shown that mature neurons can be replaced, and that fresh new neurons are added to the existing network
- Repair: neurons have a limited capacity to repair themselves. Nerve fibers can be repaired if the damage is not extensive, cell body and Schwann cell have to be intact
Stages of repair 1-4
- Following an injury, the distal portion of the axon and the myelin sheath degenerate
- Macrophages remove the debris
- Remaining neurolemma (Schwann cells) and endoneurium form a tunnel from the point of injury to the effector
- New Schwann cells grow in the tunnel to maintain a path fro regrowth of the axon
Stages of repair 5-8
- Cell body reorganizes itself to provide the necessary proteins to extend the remaining healthy portion of the axon
- Axon sprouts appear
- When sprout reaches tunnel, its growth rate increases a lot
- The skeletal muscle scell atrophied until the nervous connection is reestablished
CNS repair
CNS neurons lack the neurolemma, so no tunnel to guide repair from the point of injury
- astrocytes fill the damaged areas and block regrowth
- most injuries to the brain and spinal cord cause permanent damage
