Lecture 15

Functions of NS

stimulate glands and muscles
contribute to homeostatic feedback loops
produce quick effects by electrochemical mechanisms

what is the central nervous system composed of

brain = central processing center
spinal cord = gateway between brain and limbs

what is the peripheral nervous system composed of

ganglia = relay centers, where neurons synapse and transmit info
nerves = conducting wires, conduct info

what are nerves composed of

axons of neurons

what are ganglion composed of

neuron cell bodies

major anatomical and functional subdivisions of NS

CNS = brain and spinal cord
PNS = sensory (afferent) => visceral sensory and somatic sensory | motor (efferent) => visceral motor => sympathetic and parasympathetic and somatic motor

somatic sensory PNS

carries signals from receptors in the skin, muscles, bones, and joints

what is the motor division PNS responsible for

voluntary muscle contractions
involuntary somatic reflexes
motor nerves that innervate skeletal muscles

visceral sensory PNS

detects changes in viscera (internal organs)

what does the visceral motor PNS control

autonomic NS
controls... cardiac muscle, smooth muscle, and glands

what are the 3 functional properties found in all neurons

excitability = responds to stimuli
conductivity = produce electrical signals that travel along axons
secretion = nerve fiber endings release chemical NTs

structures in PNS

ganglia = contain neuron cell bodies
nerves = bundles of axons

structures in CNS

neural cortex
nuclei = collections of neuron cell bodies in the interior of CNS
tracts = bundles of CNS axons that share common origin, destination, and function
columns = several tracts that form a distinct mass

lower center controls?

essential body functions

high center controls?

sophisticated information processing

what is the role of the 3 most basic functional categories of neurons?

1. sensory neurons conduct signals from receptors to CNS
2. interneurons are confined to the CNS
3. motor neurons conduct signals from the CNS to effectors such as muscles and glands

what are the parts of a neuron?

soma = produces neurotransmitters

axon = "conducting region" , unbranched

dendrite = receive signals from other neurons, chemically regulated ion gates

trigger zone= axon hillock + initial segment (initiating nerve signal)

nissl bodies = stained masses of rough ER and ribosome separated by actin filaments

lipofuscin granules = products of lysosomal activity "aging pigment"

multipolar neuron

one axon, multiple dendrites

most neurons of the spinal cord and brain are in what neuron class?

multipolar

bipolar neuron

one dendrite and one axon

where are bipolar neurons located?

special sense organs (smell and sight)

unipolar neuron

a neuron with one process extending from its cell body

what class of neuron are most neurons in

first class

anaxonic neuron

many dendrites but no axon

what materials use fast anterograde transport

organelles, enzymes, synaptic vesicles, small molecules

what material use slow anterograde transport

enzymes, cytoskeletal componenets, supplies new axoplasm

what materials use fast retrograde transport

toxins, pathogens, recycled materials

what are the general functions of the cells that aid neurons

bind neurons together
in fetus, guide migrating neurons to their destination
cover mature neurons - gives precision to conduction pathways
provide physical and metabolic support to neurons

what are the 2 types of cells in PNS that aid neurons

Schwann cells = assist in the regeneration of damaged fibers and myelinate certain axons

satellite cells = provide electrical insulation, and regulate the chemical environment

what are the four types of cells in the CNS that aid neurons

oligodendrocytes = form myelin sheaths
astrocytes = from blood-brain barrier, regulate extracellular material, and convert glucose to lactat
ependymal = secrete cerebrospinal fluid
microglial = defensive cells (remove damaged neurons and infections)

what is the composition of a myelin sheath?

20% protein and 80% lipid
formed by oligodendrocytes in CNS and Schwann cells in PNS

what is the importance of the myelin sheath

increases action potential conduction velocity

how do oligondendrocytes myelinate

they reach out to several nerve fibers (like a hand reaching out to a bunch of people grabbing them)

how do Schwann cells myelinate nerve fibers

repeatedly spiral around a single nerve fiber

when can regeneration of damaged PNS axons occur?

soma intact and at least some neurilemma remains

what happens in local potentials

produced by ligand-gated sodium channels on dendrites and soma

can be excitatory or inhibitory

graded = proportional to stimulus strength

reversible = returns to RMP if stimulation ceases before threshold is reached

local = has effects for only a short distance from point of origin

decremental = signal grows weaker with distance

what happens in action potentials

produced by voltage-gated ions channels on trigger zone and axon

always begins with depolarization

all-or-none law = if a stimulus depolarizes the neuron to threshold, neurons fire at its max voltage

irreversible = goes to completion once it begins

self-propagating = has effects a great distance from point of origin

non-decremental = signal maintains same strength regardless of distance

how is the nerve signal conducted down an axon (propagation of AP)

1. when the local current arrives at the axon hillock, it depolarizes the membrane

2. if reaches the threshold voltage; neuron fires producing an AP

what factors affect conduction velocity of nerve signals

diameter = larger axons, faster conduction of signals

presence or absence of myelin = myelin increases conduction

what is the difference between action potential propagation in nonmyelinated and myelinated axons

nonmyelinated axons = continuous conduction (uninterrupted)

myelinated axons = saltatory conduction (jumps from node to node)