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Ch 11: Nervous System &
Nervous Tissue
Section 1 – Functions & Divisions of the
Nervous System (pp. 386-387)
The Nervous System
This is your brain…
The Nervous System
And this is your brain – IN ANATOMY!!
Or better yet…
The Nervous System
Functions of the Nervous System
1) Sensory input
- information about internal/external changes gathered by
sensory receptors
2) Integration
- interpretation of sensory input
3) Motor output
- activation of effector organs (muscles, glands, etc.)
- production of response
The Nervous System
Divisions of the Nervous System
1) Central nervous system (CNS)
- brain & spinal cord
- integration & command center
The Nervous System
Divisions of the Nervous System
2) Peripheral nervous system (PNS)
- spinal & cranial nerves
- carry messages to & from CNS
Functional divisions of PNS:
a) Sensory (afferent) division
- carry information to the CNS from effector organs
b) Motor (efferent) division
- carry information away from CNS to effector organs
The Nervous System
Divisions of the Nervous System
2) Peripheral nervous system (PNS)
- spinal & cranial nerves
- carry messages to & from CNS
Motor divisions of PNS:
a) Somatic (voluntary) nervous system
- provides conscious control of skeletal muscles
b) Autonomic (involuntary) nervous system
- regulates smooth muscle, cardiac muscle, & glands
- divided into sympathetic & parasympathetic systems
Ch 11: Nervous System &
Nervous Tissue
Section 2 – Histology of Nervous Tissue
(pp. 388-395)
Neurons
Neurons (aka “Nerve Cells”)
- Fundamental units of the nervous system
- Cells that are capable of carrying electrical signals
Special characteristics:
1) long-lived (capable of living 100 years or more)
2) amitotic…cannot divide (with a few exceptions)
3) high metabolic rate (require continuous O2 & sugar supply)
4) plasma membrane designed for electrical signaling
Neurons
Typical neurons have 4 distinct regions:
1) Dendrites
- Receive & respond to signals from other neurons
- Use special receptors to respond to neurotransmitters
- Deliver electrical signal to cell body
2) Cell body (aka “perikaryon” or “soma”)
- Neuron’s integration center
- Combines all incoming electrical signals
- If incoming signals are positive enough, cell body
allows signal to continue to axon
Neurons
Typical neurons have 4 distinct regions:
3) Axon
- Long, thin fiber…makes neurons longest cells in body
- Carries electrical signal away from cell body
- Allows signals to be carried large distances
- Multiple axons are bundled together to form “nerves”
4) Synaptic terminals
- Endings of the axons
- Contain neurotransmitters (NTMs)
- Release NTMs to other neurons, glands, or muscles
Other Cells of Nervous System
Neuroglia
- Literally means “nerve glue”
- Cells that support the function of the nervous system
- Are not capable of carrying electrical impulses
Examples found only in Central Nervous System:
1) Astrocytes
- Abundant, star-shaped cells that brace neurons
- Control chemical environment of brain
- Form barrier between capillaries & neurons
Other Cells of Nervous System
Astrocyte
Other Cells of Nervous System
2) Microglia
- Spider-like phagocytes that dispose of
debris
3) Ependymal cells
- Line cavities of the brain & spinal cord
- Circulate cerebrospinal fluid
Other Cells of Nervous System
4) Oligodendrocytes
- Produce myelin sheath
around nerve fibers in
central nervous system
*Myelin sheath
- Acts like insulation
- Prevents short circuits
Other Cells of Nervous System
Other Cells of Nervous System
Examples of Neuroglia in the Peripheral Nervous System:
1) Satellite cells
- protect neuron cell bodies in PNS
2) Schwann cells
- form myelin sheath around axons in PNS
- vital to regeneration of damaged peripheral nerves
Other Cells of Nervous System
More on Myelin Sheath of PNS:
- formed in a “Jelly roll”-like fashion
- insulates the axons
- enhances & increases speed of the
electrical signal
Nodes of Ranvier
- Gaps in the myelin sheath formed by
spaces between Schwann cells
Structural Classification of Neurons
Multipolar Neurons
- Characterized by many extensions from cell body
- All of the motor neurons
Structural Classification of Neurons
Bipolar Neurons
- Consist of only one dendrite & one axon
- Found only in nose (smell) & eyes (vision)
Structural Classification of Neurons
Unipolar Neurons
- Consist of a single, short extension leaving the cell body
- All sensory neurons
Ch 11: Nervous System &
Nervous Tissue
Section 3 – Neuron Function & Action
Potentials (pp. 399-404)
Information Processing
Information Processing Requires 4 Basic Operations:
1) Determine the type of stimulus
- Distinguished by various wiring patterns in the brain
2) Determine the intensity of the stimulus
- Either by the number of times a single neuron “fires” or
the total number of neurons “firing” at once
3) Integrate information from many different sources
4) Initiate & direct a response
Neuron Function
Basic Neuron Function:
- Neurons are highly irritable (responsive to stimuli)
- Send signals over long distances by generating “action
potentials”
Action Potential
- “nerve impulse”
- a brief change in the electrical charges found on either side
of the nerve cell membrane
- travels from the cell body to the end of the axon
- always the same strength regardless of stimulus
Neuron Function
More on Action Potential:
- created by the movement of positively charged sodium &
potassium ions across the cell membrane of the axon
- as charged particles move, they create electrical impulses
- considered “all-or-none phenomenon”…either happen
completely or not at all
Threshold stimulus
- minimum stimulus required to create an action potential
Neuron Function
Conduction velocity:
- the speed action potentials travel
- vary widely…some faster than others
- fastest occur at 100 meters/sec or more!
Rate determined by…
1) Axon diameter
- larger diameter = faster conduction velocities
2) Degree of myelination
- more myelin = faster conduction velocities
Ch 11: Nervous System &
Nervous Tissue
Section 4 – The Synapse &
Neurotransmitters (pp. 406-421)
The Synapse
Synapses:
- junctions that regulate information between two neurons
- also found between neurons & effector cells/organs
Presynaptic neuron
- conduct impulses toward synapse
Postsynaptic neuron
- conduct impulses away from synapse
Types of Synapses
Two most common types of synapses:
1) Axodendritic synapse
- between the axon of one neuron & the dendrite of another
2) Axosomatic synapse
- between the axon of one neuron & the cell body of another
*These may be either “electrical” or “chemical” synapses…
Animation: Synapses
Electrical Synapses
Electrical Synapses:
- not as common as chemical synapses
- neurons are physically connected by gap junctions
- electrical signal travels directly through the gap junction
- very rapid; utilized in very fast events…reflexes
Chemical Synapses
Chemical Synapses:
- specialized for releasing & receiving neurotransmitters
Typically composed of two parts:
1) Axon terminal
- found on presynaptic neuron
- contains synaptic vesicles w/ neurotransmitters
2) Receptor region
- found on postsynaptic neuron
- has special receptors that receive neurotransmitters
Synaptic Cleft
Synaptic Cleft:
- fluid-filled space between pre- & post-synaptic neurons
- prevents nerve impulses from directly passing from one
neuron to the next
Synaptic Cleft
Transmission of Electrical Impulse Between Neurons:
- Electrical signal reaches end of axon
- Neurotransmitters are released from synaptic terminals
- Neurotransmitters diffuse through synaptic cleft
- Neurotransmitters bind to receptor sites on adjacent
dendrites
Animation: Neurotransmitters
Synaptic Cleft
Terminating effects of neurotransmitters:
- occurs within a few milliseconds of NTM release
- happens as a result of either the…
1) NTMs being broken down by enzymes
2) NTMs being reabsorbed into the axon terminal
3) NTMs diffusing away from synaptic cleft
Neurotransmitters
Types of neurotransmitters:
- most neurons make 2 or more NTMs
- the number of times per second that a neuron “fires”
determines which NTM is actually released
- over 50 different NTMs have been identified
Specific Neurotransmitters
Neurotransmitters & Functions:
1) Acetylcholine - Activate skeletal muscles
2) Dopamine - Important in general movement
3) Epinephrine - Activates organs of sympathetic nervous system
4) Serotonin - Influences mood (“mellow”) & sleep
5) Endorphins - Influences mood (“feel good”), reduces pain
6) Nitric oxide - Important in forming memories
Ch 11: Nervous System &
Nervous Tissue
Section 5 – Neural Integration…Circuits
(pp. 422-423)
Organization of Neurons
Types of circuits:
1) Diverging circuits
- one incoming nerve fiber stimulates multiple fibers
- often acts as an “amplifying” circuit
Ex = single neuron in brain can activate hundreds of motor
neurons in spinal cord and ultimately thousands of skeletal
muscle fibers
Organization of Neurons
Types of circuits:
2) Converging circuits
- multiple nerve fibers in different areas combine signals
into one fiber
- often have a “concentrating” effect; strong stimulation
or strong inhibition
Ex = seeing a baby smile, smelling baby powder, hearing baby
laugh all combine to trigger “warm/fuzzy” feelings in parents
Organization of Neurons
Types of circuits:
3) Reverberating (oscillating) circuits
- chain of neurons arranged in a loop
- results in signal that is sent through the circuit over &
over in a rhythmic pattern
Ex = sleep-wake cycles, breathing, arms swinging when walking
Organization of Neurons
Processing information in the nervous system:
1) Serial processing
- input travels along one path to a specific destination
- works in all-or-none manner producing specific response
Ex = spinal reflexes
Neural Pathways Direct Behavior
Reflex
- Simplest behavior
- Involuntary movement of body part in response to stimulus
- Occur without involving the conscious portions of the brain
- Signal sent to spinal cord & immediately back to source
- Usually help keep you from being hurt
Organization of Neurons
Processing information in the nervous system:
2) Parallel processing
- input travels along several pathways
- one stimulus promotes many responses
- crucial for higher-level mental functioning
Ex = a smell usually isn’t processed as just an odor; also usually
triggers memories of experiences associated w/ the smell
Ch 11: Nervous System &
Nervous Tissue
Section 6 – Homeostatic Imbalances
(pp. 422-423)
Homeostatic Imbalances
Multiple Sclerosis (MS):
- autoimmune disease; mainly affects young adults
- myelin sheaths in the CNS become destroyed
- sheaths turn into nonfunctional, hardened lesions
- leads to short-circuiting of nerve impulses
- eventually impulse conduction ceases
Symptoms
- visual/speech disturbances, weakness, loss of muscle
control, & loss of bladder control
Homeostatic Imbalances
Neuroblastoma:
- malignant tumor that occurs in the peripheral nervous
system of children
Rabies:
- viral infection of nervous system
- transmitted through bites from infected animals
- causes inflammation of brain, delirium, & death
Homeostatic Imbalances
Shingles:
- viral infection of the sensory neurons found in the skin
- scaly, painful blisters; similar to chickenpox but w/ pain
- seen mostly in adults over 50 years old