Spinal Locomotor Generator

My other websites: Children of the Amphioxus   Boys without Fathers

Cross references:  
Early Behavior     Pre-Chordate Locomotion     Chordate Locomotion    
Central Pattern Generators     Mesencephalic Locomotor Region       Perinatal Behavior       

One of my major sources has been
"An Introduction to Brain and Behavior" by Bryan Kolb & Ian G. Whishaw, First edition.  (See:
Historical Background & Free Book .)   On page 358 it says: "The frontal lobe of each hemisphere is responsible for planning and initiating sequences of behavior."  

This is the majority view and represents a "top down" understanding of human behavior. I will argue for a "bottom up" interpretation in which the frontal lobe plays an almost minor role in determining our behavior.

The cerebral cortex is a relatively new evolutionary addition to our nervous systems. Amphibians and their predecessors have no cortex at all, yet they have no trouble initiating, and perhaps even planning, sequences of behavior. Our brains are essentially amphibian brains on top of which has been added a cerebral cortex.

In my presentation of a "bottom up" analysis of behavior, I will start with the spinal cord. The spinal locomotor generator is responsible for coordinating the "axial" muscles of our trunk. Some online references for the spinal locomotor generator are:  

Spinal Locomotion (Wiki) 

    "Spinal locomotion results from intricate dynamic interactions between a central program in lower thoracolumbar spine and proprioceptive feedback from body in the absence of central control by brain as in complete spinal cord injury (SCI).[1][2] [3] Following SCI, the spinal circuitry below the lesion site does not become silent rather it continues to maintain active and functional neuronal properties although in a modified manner.[4] [5]  

Centrally Generated Patterns

    "The spinal cord executes rhythmical and sequential activation of muscles in locomotion.
Central Pattern Generator   (CPG) provides the basic locomotor rhythm and synergies by integrating commands from various sources that serve to initiate or modulate its output to meet the requirements of the environment.  
    My comment
So my new focus is the command(s) that the CPG integrates to initiate behavior.     
    CPG within the lumbosacral spinal cord segments represent an important component of the total circuitry that generates and controls posture and locomotion.[6] This spinal circuitry can function independently in the absence of descending input from the brain to generate stable posture and locomotion and even modulate activity to match changing conditions (e.g., stepping over obstacles).[7]  
    This capability improve with training (spinal plasticity)[8] and therefore it is believed that spinal cord has the capability to learn and memorize.[9][10]

1.2  Sensory Feedback   

The sensory feedback originates from muscles, joints, tendons and skin afferents as well as from special senses and dynamically adapts the locomotor pattern of spinal cord to the requirements of the environment. These afferent sensory receptors perceive deformation of tissue the amount of pressure (stretch or simply, placement), direction of movement, speed and velocity at which movement is occurring.

File:Nervous system organization en.svg

Simplified schema of basic nervous system function: signals are picked up by sensory receptors and sent to the spinal cord and brain, where processing occurs AT EACH LEVEL AND results in MODULATION OF signals sent FROM the spinal cord and out to motor neurons

The spinal cord processes and interprets proprioception in a manner similar to how our visual system processes information.[14]  
    When we view a painting, the brain interprets the total visual field, as opposed to processing each individual pixel of information independently, and then derives an image. At any instant the spinal cord receives an ensemble of information from all receptors throughout the body that signals a proprioceptive “image” that represents time and space, and it computes which neurons to excite next based on the most recently perceived “images.”  
   The importance of the CPG is not simply its ability to generate repetitive cycles, but also to receive, interpret, and predict the appropriate sequences of actions during any part of the step cycle, i.e., state dependence. The peripheral input then provides important information from which the probabilities of a given set of neurons being active at any given time can be finely tuned to a given situation during a specific phase of a step cycle.

The spinal locomotor generator
Only abstract available online. 
In several vertebrate species it has been clearly demonstrated that neural circuits within the spinal cord are capable of generating the signals underlying locomotory rhythms.

Localization and Organization of the Central Pattern Generator for Hindlimb Locomotion in the New Born Rat (Goog)          
Full length PDF available online for free.  Download to copy.
from the abstract:   
    "The network located at the Ll/L2 level was found to be responsible
not only for generating the rhythm but also for organizing its alternating pattern. We demonstrated that the rhythmic synaptic drive that the motoneurons receive during locomotor-like activity comes directly from the Ll/L2 network and that there is no relay at the segmental level."  

Ion channels of importance for the locomotor pattern generation in the lamprey brainstem-spinal cord.
    "The intrinsic function of the spinal network that generates locomotion can be studied in the isolated brainstem-spinal cord of the lamprey, a lower vertebrate. The motor pattern underlying locomotion can be elicited in the isolated spinal cord. The network consists of excitatory glutamatergic and inhibitory glycinergic interneurones with known connectivity. The current review addresses the different subtypes of ion channels that are present in the cell types that constitute the network. In particular the roles of the different subtypes of Ca2+ channels and potassium channels that regulate integrated neuronal functions, like frequency regulation, spike frequency adaptation and properties that are important for generating features of the motor pattern (e.g. burst termination), are reviewed. By knowing the role of an ion channel at the cellular level, we also, based on previous knowledge of network connectivity, can understand which effect a given ion channel may exert at the different levels from molecule and cell to network and behaviour."
    183 Related citations:
    5 Cited by's:
    Free Full-length article: 

Spinal cord pattern generators for locomotion (Goog)         
Full length PDF available online for free.  Download to copy. 
from the abstract:   
    "It is generally accepted that locomotion in mammals, including humans, is based on the activity of neuronal circuits within the spinal cord (the
Central Pattern Generator , CPG). Afferent information from the periphery (i.e. the limbs) influences the central pattern and, conversely,
the CPG selects appropriate afferent information according to the external requirement. Both the CPG and the reflexes that mediate afferent input to the spinal cord are under the control of the brainstem."  

Intersegmental Coordination of Rhythmic Motor Patterns

Full length HTML available online for free. 
from the abstract   
Numerous studies have shown that the isolated nervous system is capable of producing rhythmic motor output in the absence of sensory feedback. In addition, in many cases these rhythmic motor patterns are similar to those required for specific behaviors in the intact animal.  
    For example, in the lamprey, forward swimming is accomplished by means of side-to-side undulations that travel from anterior to posterior along the length of the body. In the lamprey, as in many other animals, wavelike motor patterns arise from a network of neurons that is distributed longitudinally along the neural axis. This type of neural network consists of a chain of coupled segmental oscillators, local neural networks that are capable of independently generating rhythmic output. The appropriate phase relationships between these segmental oscillators arise as an emergent property of the segmental oscillators and the coupling between them.

From swimming to walking: a single basic network for two different behaviors (PubMed)

Only abstract available online. 
In this paper we consider the hypothesis that the spinal locomotor network controlling trunk movements has remained essentially unchanged during the evolutionary transition from aquatic to terrestrial locomotion. The wider repertoire of axial motor patterns expressed by amphibians would then be explained by the influence from separate limb pattern generators, added during this evolution.
Taken together, our findings support the hypothesis of a phylogenetic conservatism of the spinal locomotor networks generating axial motor patterns from agnathans to amphibians.

Spinal locomotor networks in the cat and monkey  (Goog)          
Only abstract available online.
    "It is now well established that spinal networks have the capability of generating the fundamental rhythm for locomotion in a large number of vertebrates."