Hippocampal Dynamics

The ability of HPCs to produce synchronous bursting is determined after hyper-polarization of the cell. During the burst, the intracellular calcium level increases, triggering a potassium conductance mechanism. This results in potassium loss and hyperpolarization. Hyperpolarization lasts until calcium is cleared by the cell making it relatively refractory to further activity. This process has a net effect of producing synchronous bursting among HPCs with a frequency based on the rate-limiting steps of: a) relatively slow calcium conductance, b) post-hyperpolarization, and c) calcium clearance. These steps determine the rate of repetitive firing and cells become synchronized by clustering together temporally in the slow phase of calcium clearance prior to the next firing (Prince 1983). Clearly, the hippocampus has potentially powerful excitatory output. This excitatory output is held in check by the inhibitory GABA-mediated granule cells. The inhibitory granule neurons have the continuous job of attenuating and controlling the output of a region capable of producing spontaneous discharges, as well as self-propagating and self-perpetuating circuit rhythms. If spontaneous discharges are allowed to inappropriately propagate beyond the domain of the hippocampus, they can significantly interfere with patients with so called "mild" head injuries (Varney 1995). Secondary pathological effects include edema, hemorrhage, and hypoxia.
Given the above, it can be proposed that tertiary effects may also occur when axons have suffered shearing due to closed head injury and attempt to regenerate and reestablish their appropriate connections (Povlishock & Coburn 1989). Recent data suggest that lesion-induced synaptogenesis in the hippocampus creates circuitry that is physiologically maladaptive and may result in aberrant recurrent excitatory connections by mossy fibers onto granule cells in the dentate gyrus-(Ribak et al. 1982). This synaptic rearrangement can lead to burst firing in the granule cells which previously did not have such capacities (Prince 1983, Ribak et al. 1982). Not only could neuronal injury due to closed head trauma result in direct loss of inhibitory control of HPCs, but it could also contribute to the post-injury establishment of inappropriate dysfunctional excitatory circuits. This tertiary effect requires a regenerative process which may explain why some of the symptomatology is delayed following the original insult.
Summary - The excitatory output of the hippocampus is amplified following the loss of inhibitory influence of granule cells. The susceptibility of the hippocampus, particularly the inhibitory granule cells, to injury makes it a useful explanatory model. In this model, gross structural changes are not necessary to produce gross unchecked excitatory discharges from HPCs, the behavioral effect of which is a multidimensional symptom complex. The model also helps explain the relationship between injury to anatomical structures and the resulting pathophysiology which in turn produces substantive neurobehavioral dysfunction without clear evidence on standard EEGs or radiographic imaging.
Where EEGs and imaging techniques fail, patients with ESD can be identified via a number of different behavioral indices. Possibly the most effective and most direct is to determine the number and frequency of symptoms with which they present. This model also has implications for treatment strategies. It suggests that medication (e.g., normal electrophysiological activity and effect sensory, cognitive, affective behavior due to the diverse connections of the hippocampus with cortical and subcortical structures (Isaacson 1975).

Next: Hippocampal Pathology

Why subtlebraininjury.com?

subtlebraininjury.com is a website with a mission to educate with respect to the magnitude of brain injury which does not involve coma - injuries that have been labeled by names which grossly minimize their potential impact upon the life of the injured person. This site is brought to you by the advocates of the Brain Injury Law Group, a community of plaintiff's trial lawyers across the United States united by a common interest in serving the rights of persons with traumatic brain injuries and a common commitment to fully understanding the anatomic, medical and psychological aspects of TBI.

Additional Information

For a full treatment of the topic of brain injury, and recovering adequate compensation for those who have survived such injury, please visit our other pages. tbilaw.com A general treatment of all types of brain injury, including severe brain injury and concussion, with a special focus on the legal aspects of recovering full and adequate compensation for such injuries. tbilaw.com has been at the cornerstone of the web advocacy of the Brain Injury Law Group since it went online in 1996. waiting.com A page designed to assist those with issues regarding coma, especially in the acute phase when the doctors are saying "I just don't know." vestibulardisorder.com Addressing vertigo and dizziness resulting from trauma as well as information and resources for vestibular disorders.

Sensory Brain

Disclaimer:

The materials on this World Wide Web site are provided purely for informational purposes and are not legal advice. These materials are intended, but not promised or guaranteed, to be correct, complete, and current. This web site is not intended to be a source of advertising, solicitation or legal advice. Therefore, the reader should not consider this information an invitation for an attorney-client relationship. Readers should not act or rely upon any information contained in this World Wide Web site and should always seek the advice of competent counsel.
The owner of this web site is a law firm, the Johnson Law Office which organized the Brain Injury Law Group. The Johnson Law Office is licensed to practice in the States of Wisconsin , Illinois and Michigan. The Brain Injury Law Group does not wish to represent anyone desiring representation based upon their viewing any portion of this World Wide Web site that fails to comply with all legal and ethical rules in such individuals state. While not intended to do so, but in a good faith effort to comply with all rules and regulation which may be applicable to it, the Brain Injury Law Group hereby informs readers that this site may be construed as advertising and promotional materials. The Brain Injury Law Group makes no representation that it can obtain the same results as reported in this web site in other legal matters.
The transmission of an e-mail request for information does not create an attorney-client relationship, and you should not send us via e-mail any information or facts relating to your legal problem. If you are a client, remember that e-mail may not be secure. WE BELIEVE THAT THE BY FAR PREFERRED METHOD FOR YOU TO CONTACT US IS BY PHONE AT: 1-800-992-9447.