Download Document

Fundamentals of the Nervous System and
Nervous Tissue
Chapter 11
Marieb
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Nervous System
 The master controlling and communicating system of the
body
 Works with the endocrine system to coordinate the organ
systems of the body to maintain homeoastasis
 3 modes of functions
 Sensory input – monitoring stimuli occurring inside and
outside the body
 Integration – interpretation of sensory input
 Motor output – response to stimuli by activating effector
organs
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Nervous System
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.1
Organization of the Nervous System
 Central nervous system (CNS)
 brain
 spinal cord
 Peripheral nervous system (PNS)
 spinal and cranial nerves
 Ganglia – clusters of cell bodies in the PNS.
 sensory receptors
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Peripheral Nervous System (PNS): Two Functional
Divisions
 Sensory (afferent) division
 Somatic afferent axons – carry impulses from skin, skeletal
muscles, and joints to the brain
 Visceral afferent axons – transmit impulses from visceral organs to
the brain
 Motor (efferent) division
 Transmits impulses from the CNS to effector organs
 2 parts:
 Somatic nervous system- conscious control of skeletal muscles
 Autonomic nervous system (ANS)-regulates smooth muscle,
cardiac muscle, and glands
 2 Divisions – sympathetic, and parasympathetic.
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Nervous Tissue
The two principal cell types of the nervous system are:
Neurons – excitable cells that transmit electrical signals
Supporting cells – cells that surround and wrap neurons
(neuroglia or glial cells):
-Provide a supportive scaffolding for neurons
-Segregate and insulate neurons
-Protect neurons
-Provide conditions for proper functioning of neurons,
and promote the electrical activity.
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Types of support cells:
Astrocytes- most abundant, versatile, and highly branched glial cells
They cling to neurons and their synaptic endings, and cover capillaries;
functionally, they:
-Support and brace neurons
-Guide migration of young neurons
-Control the chemical environment
Microglia – small, ovoid cells with spiny processes; phagocytes that protect the
health of neurons
Ependymal cells – They line the central cavities of the brain and spinal column
(epithelial-like)
Oligodendrocytes – branched cells that wrap and myelinate CNS axons
Schwann cells– surround and myelinate axons of the PNS
Satellite cells- surround neuron cell bodies within ganglia
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Neurons (Nerve Cells)
Long-lived, amitotic cells, with a high metabolic rate that
function in communication through electrical signaling
 Neuron Cell Body (Soma)
•
Contains the nucleus and a nucleolus
•
Is the major biosynthetic center
•
Is the focal point for the outgrowth of neuronal processes
•
Has no centrioles (hence its amitotic nature)
•
Contains an axon hillock – cone-shaped area from which axons arise
 Neuron Processes
•
Armlike extensions from the soma
•
There are two types:
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
General Neuron structure
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Neuron Processes
 Dendrites (afferent)
•
Short, tapering, and diffusely branched processes
•
They are the receptive, or input, processes of the neuron
Most signals are from other neurons are received at synapses
located on dendrites.
•
Electrical signals carried by dendrites are graded potentials
(not action potentials)
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Axons
•
Processes of relatively uniform diameter arising from the hillock
•
They are the output processes of the neuron
•electrical
•secrete
•
signals carried by axons are usually action potentials
neurotransmitters from the axonal terminals
Long axons are called nerve fibers
Usually there is usually only one axon per neuron; branches, if
present, are called axon collaterals
•
• Axonal
•
terminal – branched terminus of an axon
Bundled axons are called tracts in the CNS and nerves in the PNS
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Myelinated axons
 Axons may be surrounded by a myelin sheath,
(neurilemma)
• Whitish, fatty (protein-lipid), segmented sheath around most
long axons
• It functions to:
Protect the axon
Electrically insulate fibers from one another
Increase the speed of nerve impulse transmission
Nodes of Ranvier- Gaps in the myelin sheath between adjacent
Schwann cells; promotes faster conduction of an electrical signal
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Gray & White Matter in the Brain and Spinal Cord
 White matter – dense collections of myelinated axons.
 Gray matter – mostly somas and unmyelinated axons.
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Neuron Classification
Structural:
 Multipolar — three or more processes
 Bipolar — two processes (axon and dendrite)
 Unipolar — single, short process
Functional:
 Sensory (afferent) — transmit impulses toward the CNS
 Motor (efferent) — carry impulses away from the CNS
 Interneurons (association neurons) — shuttle and integrate
signals through CNS pathways
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Neurophysiology, Synapses, Neurotransmitters
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Neurophysiology
 Neurons are excitable cells, as are muscle cells
Undergo a rapid change in membrane potential
 Changes in membrane potential are signals used by neurons (and
muscle cells) to send, receive, or integrate information
 Changes in membrane potential that occur in neurons and other
excitable cells are mainly produced by changes in membrane
permeability to ions through channels that occur in response to:
 - neurotransmitter release
 - change in membrane potential
 - pharmacological agents/toxins
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Membrane Potentials as Signals
Types of membrane potential changes that occur:
 Depolarization – the inside of the membrane becomes
less negative
 Repolarization – the membrane returns to its resting
membrane potential
 Hyperpolarization – the inside of the membrane
becomes more negative than the resting potential
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Changes in Membrane Potential
Figure 11.9
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Changes in Membrane Potential may be Graded
Potentials or Action Potentials (APs)
 Graded potentials:
 Short-lived, local changes in membrane potential-can
only travel over short distances
 Decrease in intensity with distance
 Their magnitude varies directly with the strength of the
stimulus
 Sufficiently strong graded potentials can initiate action
potentials
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Action Potentials (APs)
 Action Potentials::
 A rapid reversal of membrane potential above a threshold
level with a total amplitude of ~100 mV
 Action potentials are only generated by muscle cells and
neurons
 They do not decrease in strength over distance
 They are the principal means of neural communication
 An action potential in the axon of a neuron is often called
a nerve impulse
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Conduction Velocities of Axons
 Conduction velocities vary widely among neurons
 Rate of impulse propagation is determined by:
 Axon diameter – the larger the diameter, the faster the
impulse
 Presence of a myelin sheath – myelination dramatically
increases impulse speed
 Current passes through a myelinated axon only at the nodes of
Ranvier (salutatory conduction)
 Voltage-gated Na+ channels are concentrated at these nodessignal
“jumps”
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Multiple Sclerosis (MS)
 An autoimmune disease that mainly affects young
adults (onset 30-50 years old).
 Symptoms include visual disturbances, weakness,
loss of muscular control, and urinary incontinence
 myelin sheaths in the CNS become nonfunctional
scleroses and nerve fibers degenerate
 Shunting and short-circuiting of neuron electrical
impulses occurs
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Synapses
 Junctions that mediate information transfer from one
neuron:
 To another neuron
 To an effector cell
 Presynaptic neuron – conducts impulses toward the
synapse
 Postsynaptic neuron – transmits impulses away from the
synapse
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Synapses
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.17
Types of Synapses (structural classification)
 Axodendritic – synapses between the axon of one
neuron and the dendrite of another
 Axosomatic – synapses between the axon of one neuron
and the soma of another
 Other types of synapses include:
 Axoaxonic (axon to axon)
 Dendrodendritic (dendrite to dendrite)
 Dendrosomatic (dendrites to soma)
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Types of Synapses (functional classification)
Electrical synapses:
 Are less common than chemical synapses
 Made of gap junctions - ions pass directly from one neuron to
another
Chemical Synapses:
 Specialized for the release and reception of neurotransmitters
 Typically composed of two parts:
 Axonal terminal of the presynaptic neuron, which contains
synaptic vesicles
 Receptor region on the dendrite(s) or soma of the postsynaptic
neuron
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Synaptic Cleft of a Chemical Synapse
 Fluid-filled space separating the presynaptic and
postsynaptic neurons
 Prevents neuron electrical impulses from directly
passing from one neuron to the next
 Transmission across the synaptic cleft:
 Is a chemical event (as opposed to an electrical one)
 Ensures unidirectional communication between neurons
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Information Transfer at a Synapse
Figure 11.17
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Termination of Neurotransmitter Effects
 Removal of neurotransmitters occurs when they:
 Are degraded by enzymes
 Are reabsorbed by astrocytes or the presynaptic terminals
 Diffuse away from the synaptic cleft
 This results in termination of the neurotransmitter effect
Synaptic delay
 Time needed for neurotransmitter to be released, diffuse across
the synapse, and bind to receptors is 0.3-5.0 ms
 This synaptic delay is the rate-limiting step of neuronal
transmission
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Neurotransmitters and their Receptors
 Chemicals used for neuronal communication with the
body and the brain
 50 different neurotransmitters have been identified
 Classified chemically and functionally
Chemical Classification of Neurotransmitters
_ Acetylcholine (ACh)
_ Biogenic Amines
_ Amino Acids
_ Peptides
_ Novel Messengers: ATP, Nitrous oxide.
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Neurotransmitters: Acetylcholine
 First neurotransmitter identified, and best understood
 Released at the neuromuscular junction
 Synthesized and enclosed in synaptic vesicles
 Degraded by the enzyme acetylcholinesterase (AChE)
 Released by:
 All neurons that stimulate skeletal muscle NMJs
 Some neurons in the autonomic nervous system
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Neurotransmitters: Biogenic Amines
 Include:
 Catecholamines – Dopamine, norepinephrine (NE), and
epinephrine.
 Indolamines – serotonin and histamine.
 Broadly distributed in the brain
 Play roles in emotional behaviors and our biological
clock
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Synthesis of Catecholamines
*
 Enzymes present in the
cell determine length of
biosynthetic pathway
 Norepinephrine and
dopamine are synthesized
in axonal terminals
 Epinephrine is released by
the adrenal medulla
Figure 11.22
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Neurotransmitters: Amino Acids
 Include:
- GABA (gamma amino butyric acid)
- glycine
- aspartate
- glutamate
 Found only in the CNS
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Neurotransmitters: Peptides
 Include:
- Substance P – mediator of pain signals.
- Beta endorphin, enkephalin
 Act as natural opiates, reducing our perception of
pain
 Bind to the same receptors as opiates and morphine
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Functional Classification of Neurotransmitters
 Two classifications:
 Excitatory neurotransmitters cause Depolarization.
(e.g., glutamate)
 Inhibitory neurotransmitters cause hyperpolarization.
(e.g., GABA and glycine) More negative than the resting
membrane potential. Less likely to undergo AP.
 Some neurotransmitters have both excitatory and
inhibitory effects
 Determined by the receptor type of the postsynaptic neuron
 Example: acetylcholine
 Excitatory at neuromuscular junctions with skeletal muscle
 Inhibitory in cardiac muscle
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Neurotransmitter Receptor Mechanisms
 Direct: neurotransmitters that open ion channels
 Promote rapid responses
 Examples: ACh and amino acids
 Indirect: neurotransmitters that act through second
messengers. (ANS).
 Slower onset of response; promote longer-lasting effects
 Examples: biogenic amines, peptides, and dissolved gases
 Many are G-protein-linked receptor mediated
PLAY
InterActive Physiology®:
Nervous System II: Synaptic Transmission
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings