Nervous System Basics
1
2
Nervous System
•
•
Nervous system is divided into:
– Central nervous system (CNS):
™ Brain.
™ Spinal cord.
– Peripheral nervous system (PNS):
™ Cranial nerves.
™ Spinal nerves.
Made up of nervous tissues
3
1
4
5
Nervous Tissue
•
Nervous Tissue contains two types of cells.
– Neurons transmit nerve impulses between
parts of the nervous system.
– Neuroglia support and nourish neurons.
6
2
Neurons
•
•
•
•
•
Basic structural and functional units of the
nervous system.
– Cannot divide by mitosis.
Respond to physical and chemical stimuli.
Produce and conduct electrochemical impulses.
Release chemical regulators.
Nerve:
– Bundle of axons located outside CNS.
™ Most composed of both motor and sensory
fibers.
7
Functional Classification of Neurons
•
•
•
•
Based upon direction
impulses conducted.
Sensory or afferent:
– Conduct impulses
from sensory
receptors into CNS.
Motor or efferent:
– Conduct impulses
out of CNS to
effector organs.
Association or
interneurons:
– Located entirely
within the CNS.
– Serve an integrative
function.
8
Neuron Structure
•
Neurons contain three basic parts.
– Cell body contains nucleus and other
organelles.
– Dendrites receive signals from sensory
receptors or other neurons.
– Axon conducts nerve impulses.
9
3
10
Neurons
•
•
•
(continued)
Cell body (perikaryon):
– “Nutrition center.”
Dendrites:
– Provide receptive area.
– Transmit electrical impulses to cell body.
Axon:
–
–
Conducts impulses away from cell body.
Axoplasmic flow:
™ Proteins and other molecules are transported
by rhythmic contractions to nerve endings.
11
12
4
Neuron Structure
•
Neurons are classified according to function.
– Sensory neurons take impulses from a
sensory receptor to the CNS.
– Interneurons receive input from sensory
neurons, and other neurons, and then
communicate with motor neurons.
– Motor neurons take nerve impulse away
from the CNS to an effector that carries out
responses to environmental change.
13
Types of Neurons
14
Structural Classification of Neurons
Based on the # of processes
that extend from cell body.
– Pseudounipolar:
™ Short single process
that branches like a T.
¾ Sensory neurons.
– Bipolar neurons:
™ Have 2 processes.
¾ Retina of the eye.
– Multipolar:
™ Have several
dendrites and 1 axon.
¾ Motor neuron.
15
5
16
CNS Supporting Cells
•
Oligodendrocytes:
– Process occurs mostly postnatally.
– Each has extensions that form myelin sheaths
around several axons.
™ Insulation.
17
CNS Supporting Cells
•
Astrocytes:
–
Most abundant glial cell.
–
Vascular processes
terminate in end-feet that
surround the capillaries.
–
Stimulate tight junctions,
contributing to bloodbrain barrier.
–
Regulate external
environment of K+ and
pH.
–
Take up K+ from ECF,
NTs released from
axons, and lactic acid
(convert for ATP
production).
™ Other extensions
adjacent to synapses.
(continued)
18
6
CNS Supporting Cells
•
•
(continued)
Microglia:
– Phagocytes, migratory.
Ependymal cells:
– Secrete CSF.
– Line ventricles.
– Function as neural stem cells.
– Can divide and progeny differentiate.
19
20
21
7
Blood-Brain Barrier
•
•
Capillaries in brain do not have pores between
adjacent endothelial cells.
– Joined by tight junctions.
Molecules within brain capillaries moved
selectively through endothelial cells by:
– Diffusion.
– Active transport.
– Endocytosis.
– Exocytosis.
22
PNS Supporting Cells
•
•
•
Schwaan cells:
– Successive wrapping of the cell
membrane.
– Outer surface encased in glycoprotein
basement membrane.
– Provide insulation.
Nodes of Ranvier:
– Unmyelinated areas between adjacent
Schwaan cells that produce nerve
impulses.
Satellite cells:
– Support neuron cell bodies within ganglia.
23
Myelin Sheath
•
Some axons are covered by a protective
myelin sheath.
– Formed by Schwann cells (in peripheral
N.S.) containing myelin in plasma
membranes.
– Nodes of Ranvier are gaps on the axon
with no myelin sheath.
24
8
Myelin Sheath
25
26
Nerve Regeneration
•
Schwann cells:
– Act as phagocytes, as the distal
neuronal portion degenerates.
– Surrounded by basement membrane,
form regeneration tube:
™ Serve as guide for axon.
™ Send out chemicals that attract the
growing axon.
™ Axon tip connected to cell body
begins to grow towards destination.
27
9
Nerve Regeneration
•
(continued)
CNS has limited
ability to regenerate:
– Absence of
continuous
basement
membrane.
– Oligodendrocytes
molecules inhibit
neuronal growth.
28
Neurotrophins
•
•
Promote neuron growth.
Nerve growth factors include:
– Nerve growth factor (NGF), brainderived neurotrophic factor (BDNF),
glial-derived neurotrophic factor
(GDNF), neurotrophin-3, and
neurotrophin-4/5.
29
Neurotrophins
•
•
(continued)
Adult:
– Maintenance of sympathetic ganglia
(NGF).
– Mature sensory neurons need for
regeneration.
– Required to maintain spinal neurons
(GDNF).
– Sustain neurons that use dopamine
(GDNF).
Myelin-associated inhibitory proteins:
– Inhibit axon regeneration.
30
10