|
TABLE OF CONTENTS
KEY CONCEPTS IN THIS LECTURE 1. The Autonomic Nervous System (ANS) and the Somatic Nervous System make up the efferent paths of the Peripheral Nervous System. The ANS is further subdivided into the sympathetic and parasynpathetic nervous systems, both of which are involved in integration of involuntary physiological processes, such as control of heart rate. Often, but not always, these two subsystems work in opposition to regulate a physiological event. The Somatic Nervous System controls either voluntary or involuntary motor events which always involve muscle contraction. 2. Nerves releasing Ach at the neuromuscular junction (=end plate) cause the contraction of skeletal muscle. The functional unit of a muscle organ is the muscle fiber (=muscle cell). The muscle fiber contracts in an "all-or-none" fashion when stimulated by an action potential. The action potential first causes intracellular Ca++ release from the sarcoplasmic reticulum and the Ca++ activates a cascade of events which results in the movement of actin over myosin (=sliding filament theory). The contractile process is achieved by the repeated formation of cross bridges between the myosin and actin myofibrils along with expenditure of ATP. 3. Muscles tend to maintain a status quo of not contracting until instructed to do otherwise. Some simple reflexes, such as the knee jerk reflex, illustrate this point. In this reflex, stretch of the muscle spindle in the muscle activates a neural circuit which causes contraction of extrafusal muscle. There are many reflexes, some of which have been incorporated into basic circuits associated with simple involuntary behaviors, such as walking in humans or swimming in the lamprey. Neural circuits which control such simple behaviors are often controlled by central pattern generators. 4. The brain controls most behavior. Sensory inputs from different cortical regions collect in the prefrontal cortex. Using this information, the secondary motor area (SMA) and the premotor area (PMA) organize the behavior and the primary motor cortex (M1) executes it. The descending paths include the lateral spinal columns which are important in hand and limb movements while the ventrolateral columns are involved with involuntary behaviors, such as walking, breathing, and eye movements. The basal ganglion also receive output from a number of cortical areas; these structures provide positive feedback to the SMA and PMA and may help in the initiation of a behavior. The cerebellum also receives cortical input and provides M1 with information on the status of a behavior as well as information on its feasibility. LECTURE OBJECTIVES 1. The organization and function
of the sympathetic and parasympathetic nervous systems will be discussed.
2. How a nerve innervates and activates the muscle cell 3. Establish the molecular basis for muscle contraction 4. Discuss how the CNS alters the status quo of a neural circuit to cause contraction of muscle 5. Establish how central pattern generators control basic rhythmic behaviors LECTURE OUTLINE I. INTRODUCTION **
A. Review: The nervous system is made up of the CNS and PNS
1. CNS:
a. Brain controls motor behavior
b. Spinal cord relays information to and from the brain
1) Ascending and decending paths in the spinal cord
2) Spinal cord is also involved in reflexes and pattern generators
2. PNS
a. Autonomic Nervous System (ANS, involuntary regulation)
b. Somatic Nervous System (involved in muscle contraction
and voluntary behavior)
II. AUTONOMIC: SYMPATHETIC & PARASYMPATHETIC
A. Organization and general functions of this system
B. Neuronal and functional organization of the ANS
1. Organized as a two neuron system
a. Differences exist for: 1) orgin of nerves, 2) postsynaptic
neurotransmitter (NE verses Ach), and 3) position
of ganglion
2 . What does the ANS regulate?
3. The ANS is important in stress and other behavioral responses
a. Response to an acute stressor: Cannon's "fight or flight" response
b. Response to a chronic stressor: CRH, ACTH, and Corticosterone
II. SOMATIC NERVOUS SYSTEM CONTROLS MUSCLE CONTRACTION **
A. Somatic Nervous System
1. Somatic nerves always innervate skeletal muscle
B. Neuromuscular junction are only excitatory
1. A unique mechanism occurs at the neuromuscular junction
a. Ca++ initiates Ach release from presynaptic neuron
b. Ach --> opens Na+/K+ channel (transmitter-gated) --> End Plate
Potential (EPP) --> opens Na+ channel (voltage-gated) -->
Action Potential --> Muscle fiber contraction
C. Motor unit
1. Motor unit recruitment permits a graded response in the muscle
organ
D. Slow (Type I) and fast (Type IIa and IIb) twitch muscle fibers
1. Functional properties of Type I and Type II
a. Fast twitch can be oxidative (IIa) or glycolytic (IIb)
b. Significance of slow and fast twitch muscle fibers
IV. MOLECULAR BASIS FOR MUSCLE CONTRACTION **
A. Basic muscle microanatomy
1. Muscle organ --> Muscle fiber (the cell) --> Myofibril --> Sarcomere
--> Myofilaments (actin, myosin) are within the sarcomere
2. Cytoarchitecture of a Sacromere--the basic contractile unit in
the myofibril
a. There is functional organization of the myofibrils
(actin and myosin)
B. Molecular Basis for Muscle Contraction (Sliding filament mechanism)
1. Myosin (thick filaments; site of ATPase activity)
2. Actin (thin filaments)
3. Tropomyosin and Troponin
4. T tubules and Sacroplasmic Reticulum (SR) --
AP mediation
5. Ca++ release from the SR initiates contraction
a. Ca++ acts on Troponin to reveal cross-bridge
binding sites
b. Cross-bridge formation causes contraction of the muscle fiber
1) ATP is utilized with cross-bridge formation
2) Cross-bridge formation is repeated over and over (=cycling)
C. Summary of these processes (Animation)
V. REFLEXES AND CENTRAL PATTERN GENERATORS **
A. What is a reflex and how are they important in behavior?
1. Example 1: Withdrawal reflex
2. Example 2: Crossed extensor reflex
B. Muscle Spindles and the Stretch Reflex
1. Monosynaptic reflex
a. Neural circuitry tends to maintain the status quo
2. This circuit includes a:
a. Primary afferent (senses stretch and rate of
change in stretch)
b. Secondary afferent (stretch only)
c. Alpha motor neuron (efferent which innervates
extrafusal muscle)
d. Gamma motor neuron (efferent which innervates
intrafusal muscle)
3. The CNS intervenes to elicit contraction and
motor behavior via alpha-gamma coactivation **
C. Central Pattern Generators (CPG) and behavior
1. The circuitry for some involuntary behaviors,
such as walking, reside in the spinal cord
a. Early experimental evidence
b. Role of the higher brain centers in control of
these spinal circuits
c. Spinal cord circuitry responsible for lamprey swimming and human walking
are similar
d. Can the spinal cord learn?
VI. THE BRAIN CONTROLS MOST BEHAVIOR
A. Voluntary behavior: Idea --> Program --> Execution --> Feedback
B. Voluntary behavior involves many brain areas (Overview)
1. Prefrontal cortex--Idea
a. Retrieves relevant information from various cortices
2. Supplementary motor area (SMA)--Program
a. Example of SMA function
3. Premotor area (PMA)--Program
a. Integrates numberous sensory inputs in guiding behavior
4. Primary motor cortex (M1)--Execution
5. Basal ganglia and Cerebellum--Feedback
C. Motor output passes through one of several
descending spinal tracts
1. Some parallel processing exists
D. Cortical input to the basal ganglia forms a loop back
to the SMA, PMA, and M1
1. Basal ganglia output is excitatory
E. Cortical input to the cerebellum forms a loop back to M1
1. The cerebellum monitors but does not execute behavior
a. How does behavior change following a cerebellar lesion?
2. The cerebellum has a number of functions
a. Involved in balance and posture
b. Has a memory for past behaviors
c. Uses various sensory inputs to assess the status of a
executed behavior
d. Modifies behavior via M1 **
F. Summary on neural control of voluntary behavior
ADDITIONAL INFORMATION ON THE INTERNET Pharmacology and Physiology of the Autonomic Nervous System. A very detailed treatment of the subject from the Washington University School of Medicine in St. Louis. Links to related subjects. Motor Systems Slide Presentation Animations on muscle receptors and reflexes from the University of Western Ontario Muscle Structure and Function in detail from the University of California at San Diego. Network Patterns and Locomotion. A general article on pattern generators and vertebrate locomotion.
Go to: |
