Nervous System: Anatomy and Root Words

• The nervous system controls other body systems by monitoring, integrating, and responding to environmental information
• It has two main divisions: the Central Nervous System (CNS), and the Peripheral Nervous System (PNS)
• Sensory signals go from receptors via the PNS to the CNS, and motor signals travel from the CNS through the PNS to effectors.
• Somatic signals relate to the outer body, while visceral signals relate to the inner body

Roots to Remember

• af- = towards
• arbor = tree
• astro = star
• axo = axle, axis
• chroma = color
• dendro = tree
• ef- = away
• ependymal = wrap
• -feren = to carry
• gangli = swelling, knot
• glia = glue
• lemma = sheath
• mono = one
• myel, myelo = marrow, the spinal cord
• neur = nerve
• oligo = few
• phil, philo = loving
• poly = many
• propri = one's own
• sclero = hard

Nervous Tissue

• Its primary cell types are neurons, which transmit electrical signals, and neuroglia, which support neurons

Neurons

• Nerve cells that conduct electrical signals
• These signals travel along the neurilemma as nerve impulses or action potentials
• Neurons are long lived, can function for over 100 years
• Neurons cannot divide, thus the characteristic can be problematic
• Neurons have high metabolic rate, needing continuous supplies of oxygen and glucose
• Neurons feature a cell body or soma from which processes project

The Cell Body

• Varies in size from 5 to 140 µm in diameter
• Features a single nucleus surrounded by cytoplasm
• Nissl bodies are chromatophilic substance consisting of rough endoplasmic reticulum and free ribosomes and stain darkly
• Neurofibrils consist of intermediate filaments that run through cytoplasm's network
• Most neuron cell bodies are in the CNS
• Ganglia are clusters of cell bodies along nerves in the PNS

Neuron Processes

• Armlike extensions of cell bodies
• There are two types called dendrites and axons
• Typically, motor neurons contain several dendrites, processes branching out like tree limbs
• Dendrites are receptive, provide increased surface area for receiving signals from other neurons
• Dendrites conduct electrical signals towards the cell body

Axons

• Each neuron typically features a single axon
• The axon arises from the axon hillock
• This is the region of the cell body shaped like a cone
• Axons generate and conduct impulses, transmitting away from the cell body
• Substance and Golgi do not occur in the axon and hillock
• Ribosomes and some organelles are absent
• Axons rely on the cell body for protein components
• Neurofilaments, actin microfilaments, and microtubules provide structure and aid the transport of substances
• Neurofilaments, actin microfilaments, and microtubules is called axonal transport
• A nerve fiber is a long axon
• Axon diameter varies among different neurons
• Large axon diameters conduct impulses faster
• Axon collaterals are occasional branches along axon length
• The terminal arborization is the end of axon, branching profusely
• Axon terminals/terminal boutons are knobs
• Nerve impulses generate at the axon hillock, transmits along the axon
• The terminal boutons release neurotransmitters into extracellular space
• These neurotransmitters affect nearby neurons and target organs

Synapses

• This the site of neuron communication
• Synapses typically use chemical transmitters
• Synapses allow signals to go in one direction
• A presynaptic neuron conducts signals toward a synapse
• A postsynaptic neuron transmits signals away from a synapse
• In the CNS, most function as both presynaptic and postsynaptic
• Axodendritic synapses occur between terminal boutons and dendrites
• Axosomatic synapses occur between axons and neuron cell bodies
• In axodendritic synapses, the terminal bouton features synaptic vesicles
• Neurotransmitters are contained in these vesicles
• Mitochondria occur in the terminal bouton because secretion of neurotransmitters requires energy
• A synaptic cleft separates neurons at the synapse
• When an impulse goes along the presynaptic axon, vesicles fuse with membrane
• Vesicle rupture releases neurotransmitter molecules, which diffuse across the synaptic cleft to postsynaptic membrane
• Neurotransmitter binding affects the ability to generate a nerve impulse

Classification of Neurons Structurally

• Multipolar: more than two processes, many dendrites and single axon
• Bipolar: two processes extending from opposite sides of cell body
• Unipolar: short, single process splitting into two long branches