Download Nervous System Intro

NERVOUS SYSTEM
INTRODUCTION
BIO 137 Anatomy & Physiology I
Nervous System
• One of body’s 2 main control systems
• Detects a stimulus inside or outside of body & coordinates
a response
• Acts by electrical signals, called nerve impulses
• Nervous tissue is excitable
• Controls most of body’s activities to help maintain
homeostasis
Organs of the Nervous System
• Brain
• Spinal Cord
• Nerves
• Cranial
• Spinal
• Peripheral
Functions of the Nervous System
• 3 main functions
• Sensory function
• Integrative function
• Motor function
Sensory Function of the Nervous System
• Sensory receptors are located at the ends of peripheral
nerves
• Sensory receptors detect a change outside or inside the body
• Monitor internal, external and environmental factors
• Light and sound, temperature, oxygen concentration
• Information from sensory receptors is converted into a nerve
impulse, which is transmitted from peripheral nerves to the
CNS
Integrative Function of the Nervous
System
• The nervous system processes sensory information by
analyzing it and making decisions for an appropriate
response
• ‘Sensory information is integrated ‘
Motor Function of the Nervous System
• Conscious and subconscious decisions are made and
acted upon
• Response is carried out by effectors through cranial and
spinal nerves
• Muscles and/or glands
Divisions of Nervous System
• Central Nervous System,
CNS
• Brain
• Spinal cord
• Peripheral Nervous
System, PNS
• Peripheral nerves
• Cranial nerves
• Spinal nerves
• PNS connects CNS to
body parts
Divisions of Nervous System
Peripheral Nervous System Divisions
• Sensory Division
• Sensory receptors in the PNS pick up sensory
information from the body and deliver it to the CNS as a
nerve impulse
• Motor Division
• Neurons carry information from the CNS to effectors
PNS: Motor Division
•Somatic Division
• Oversees voluntary
activities
• Skeletal muscle
contraction
• Autonomic Division
• Controls visceral
activities
• Involuntary activities
• Including: Heart rate,
blood pressure,
breathing, body
temperature
PNS:Motor:Autonomic
• Sympathetic
• Active under stressful
situations
• ‘Fight or Flight’
• Parasympathetic
• Active under normal,
restful conditions
• Returns body to
homeostatic levels
following a stressful
experience
Divisions of the Nervous System
• Ganglia are small masses of neuronal cell bodies located
outside the brain and spinal cord, usually closely associated
with cranial and spinal nerves.
• There are ganglia which
are somatic, autonomic,
and enteric (that is, they
contain those types
of neurons.)
Histology of Nervous System
• 2 main types of cells
• Neurons
• A single nerve cell
• Neuroglia
• Support and nutrition of
brain cells
• ‘neuron helpers’
• Found in both CNS and
PNS
Neuronal Structure
• Dendrite(s) – receive information from within or outside the body
• Cell body – contains nucleus, organelles
• Axon hillock - Axon arises from here on the cell body
• Axon – transmits received information as a nerve impulse
• A neuron may have many dendrites but only 1 axon
• Axon may have branches towards its end called collaterals
• Ends of each axon collateral are called axon terminals which end
at a synaptic knob
• Vesicles of neurotransmitter located here
• Synapse exists between end of an axon and what it innervates
Neuronal Structure
Flow of Information in a Neuron
• Dendrite(s)
• Cell Body
• Axon hillock
• Axon
• Collaterals
• Axon Terminals
• Synaptic knob
Axon Terminals
• Axons can terminate at three locations
1. Muscle
2. Gland
3. Another neuron
•
The site of communication between two
neurons or between a neuron and another
effector cell is called a synapse.
The Synapse: Chemical Transmission
 Neurotransmitter is stored
in the synaptic end.
 The synaptic cleft is the
gap between the pre and
post-synaptic cells.
Classification of Neurons
• Neurons can be classified structurally and
functionally
• Structural classification
• Relationship of number of dendrites to the axon
• Functional classification
• Where information is carried
• Carry info to CNS, entirely in CNS or out of CNS
Structural Classification of Neurons
• Multipolar
• Many dendrites
• Most common in CNS; motor neurons in PNS
• Bipolar
• One dendrite and one axon extend from cell body
• Eyes, ears, nose
• Unipolar
• One process extends from the cell body
• Touch, stretch receptors
Structural Classification of Neurons
Functional Classification of Neurons
• Sensory (Afferent) Neurons
• Carry info from a body part to the CNS
• Detect change outside or inside the body
• Usually unipolar
• Interneurons
• Lie entirely in the CNS
• Link other neurons together (within CNS or CNS to motor
neurons) - Integrators
• Usually multipolar
• Motor (Efferent) Neurons
• Carry impulses from the CNS to effectors (muscles, glands)
• Usually mulitpolar
Functional Classification of Neurons
Neuroglia
• Neuroglia do not generate or conduct nerve
impulses.
• They support neurons by:
• Forming the Blood Brain Barrier (BBB)
• Forming the myelin sheath (nerve insulation) around neuronal
axons
• Making the CSF that circulates around the brain and spinal cord
• Participating in phagocytosis
Neuroglia
• There are 4 types of neuroglia in the CNS:
• Astrocytes - support neurons in the CNS
• Maintain the chemical environment (Ca2+ & K+)
• Oligodendrocytes - produce myelin in CNS
• Microglia - participate in phagocytosis
• Ependymal cells - form and circulate CSF
• There are 2 types of neuroglia in the PNS:
• Satellite cells - support neurons in PNS
• Schwann cells - produce myelin in PNS
Neuroglia
PNS: Neuroglial Cells
• Schwann Cells
• Produce myelin on peripheral myelinated axons
• Satellite Cells
• Support neurons
CNS: Neuroglial Cells
• Astrocytes
• Star shaped, found
between neurons and
vessels
• Form scar tissue in
response to brain injury
• maintain blood brain
barrier
• Regulate nourishment
of neurons
CNS: Neuroglial Cells
• Oligodendrocytes
• Form myelin sheath in CNS neurons
• Ependyma
• Line ventricles in brain and central canal in cord
• Cerebrospinal fluid circulation
• Microglia
• Support neurons and phagocytize bacteria and debris
Functions of Neurons
• Conduct nerve impulses
• Axonal transport
• Transport of biochemicals produced in the cell body to
the end of the axon
Myelination of Axons
• Myelin sheath found around some large neuronal axons in
PNS & CNS
• Myelin has a high lipid content that insulates axons and increases
nerve impulse speed
• CNS: Myelin formed by Oligodendrocytes
• PNS: Myelin formed by Schwann Cells
Schwann Cell Myelination in PNS
• Gaps between
regions of myelin
called Nodes of
Ranvier
• Nerve impulse jumps
from node to node in
a myelinated axon
• This is called Saltatory
Conduction
Multiple Sclerosis
• Disease resulting from destruction of myelin
sheath in some neurons of brain & spinal cord
• Leaves scar tissue called sclerosis
• Nerve fibers may also be damaged
• Prevents nerve impulse transmission to/from brain
• Muscles atrophy without stimulation
• Possible causes
• *Autoimmune
• Viral infection (no direct link to one specific virus)
• Symptoms reflect specific neurons in the CNS that
are affected
Regeneration of Neuronal Axons
• CNS neuronal axons can not regenerate
• PNS may allow regeneration of axons
• Nerve tissue regeneration is largely dependent on Schwann Cells
• Cell body damaged – no regeneration
• Axon damaged – may be regenerated
• Regeneration is slow and the axon may not always grow in correct
location
• May still lose some function, but not all
Neuronal Regeneration
• The outer nucleated cytoplasmic layer of the Schwann cell,
which encloses the myelin sheath, is the neurolemma
(sheath of Schwann).
• When an axon is injured, the neurolemma aids regeneration by
forming a regeneration tube that guides and stimulates
regrowth of the axon.
Gray and White Matter
• White matter of the brain and spinal cord is formed from
aggregations of myelinated axons from many neurons.
• The lipid part of myelin imparts the white appearance.
• Gray matter (gray because it lacks myelin) of the brain and
spinal cord is formed from neuronal cell bodies
and dendrites.
Neuronal Physiology
• Neurons are excitable cells that have an electrical
charge
• Cell membrane is polarized (electrically charged)
• A difference in charge exists between inside/outside cell
• Inside of cell negative relative to outside
• The difference in charge is called membrane potential,
measured in millivolts
• MP due to differences in distribution & permeability
of Na+, K+ and other anions between the inside and
outside of the neuron
Resting Membrane Potential, RMP
• At rest, RMP is -70 millivolts
• Inside cell
• High potassium, K+
• Anions
• Outside cell
• High sodium Na+
• These ions can only move through ion channels in the
plasma membrane
• Cell membrane impermeable to anions
- anions
- anions
- anions
Resting Membrane Potential, RMP
• RMP maintained by normal diffusion (80%) and
Sodium/ Potassium ATPase pump (20%)
• By normal diffusion, what way would Na+ travel?
• By normal diffusion, what way would K+ travel?
RMP: Sodium-Potassium ATPase Pump
• 3 Na+ out for every 2 K+ in
• ACTIVE TRANSPORT
• Results in charge separation between inside and outside
a neuron
Nerve Impulse
• When a neuron responds to a stimulus, it affects
the resting membrane potential
• Begins on dendrites or cell body BUT effect is on
gated ion channels in the axon cell membrane
(K+/Na+)
• Gated ion channels open in response to an
electrical impulse
• Recall which way Na+ and K+ travel by normal
diffusion
Membrane Responses to Ion Movement
• Hyperpolarized – inside more (–) than resting
• Depolarized – inside more (+) than resting
• Repolarized – return to -70mV
Stimulus - Depolarization
1. Stimulus is detected
• must cause RMP to reach
threshold potential, -55mV
(depolarized)
2. Voltage gated Na+
channels open, Na+ rushes
into cell
• Inside more positive than
RMP, up to +30mV
• Result is beginning of
Action Potential - rapid
reversal of membrane
potential
Repolarization
• When the MP reaches
+30mV, potassium (K+)
channels open (while
sodium channels close)
• Potassium ions (K+) flow
out of the cell until RMP is
restored back to -70mV
•
Repolarization is required
before the neuron can be
stimulated again.
Action Potential
• Sufficient Stimulus
• Threshold reached
• Depolarization when
Na+ gated channels
open, Na+ enters cell
• Na+ channels close,
K+ channels open
• K+ exits cell
• Repolarization
Action Potentials
• Only occurs on the axon
• AP is an all or none response
• May be summation of many stimuli to reach
threshold (graded potentials)
• Many AP per single axon
• AP goes in one direction
• AP can not be stimulated during absolute
refractory period
Nerve Impulse
• A nerve impulse is the
propagation of action
potentials along an
axon
• Unmyelinated axons
• Impulse conducted over
entire surface
• Myelinated axons
• Impulse occurs only at
nodes of ranvier
Saltatory Conduction
• Regions of axon
undergoing AP simulate
adjacent areas to reach
threshold
• Impulse jumps from
node to node
• Occurs through length
of axon
• Saltatory conduction
The Synapse
• Nerve impulses pass
from neuron to
neuron at synapses
• Gap between the
pre-synaptic neuron
axon terminal and
post-synaptic neuron
dendrite called
synaptic cleft
• Is a type of chemical
transmission
Synaptic Transmission
• Pre-synaptic axon terminal synaptic knob
contains vesicles that store chemical
messengers called neurotransmitters
• Nerve impulse reaches knob, Ca++ channels
open, Ca++ enters knob
• Neurotransmitter released into synaptic cleft
• NT combines with receptor on post-synaptic
membrane
• Inhibits or excites post-synaptic neuron
Fate of Neurotransmitter
• Must be removed or inactivated
1. Diffuses away from synaptic knob
2. Inactivated by enzymes
3. Taken back up into synaptic knob
Synaptic Transmission
•Neurotransmitters
(chemicals) are released
when impulse reaches
synaptic knob
•Nervous system produces
at least 100 different
neurotransmitters
•A single neuron can
release one or more types
of neurotranmitters
Neurotransmitters
• Both excitatory and inhibitory neurotransmitters are present
in the CNS and PNS.
• The same neurotransmitter may be excitatory in some
locations and inhibitory in others.
• For example, acetylcholine (ACh) is a common neurotransmitter
released by many PNS neurons (and some in the CNS). Ach is
excitatory at the NMJ but inhibitory at other synapses.
Commonly Known Neurotransmitters
• Acetylcholine
• Stimulates skeletal muscle contraction
• Epinephrine
• Adrenalin (↑ heart rate, ↑ blood pressure)
• Dopamine
• Involved in controlling fine motor movements