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Function

The Common Vein  Copyright 2009

Introduction

The brain is the central controlling system

principles of the brain the brain is a solid organ and is specifically designed for control and integration of organ function

Receive (afferent – sensory)

A specialized sensory receptive neuron acts as the stimulus detector. Sensory neurons are highly specialized and capable of detecting only specific stimuli. They are so specialized in fact that they have lost the ability to divide through mitosis. Sensory neurons are located dorsally in the spinal cord. Each sensory neuron has a limited sensory field area over which its dendrites are capable of detecting stimuli. Activity level of the receptor cells depends on the intensity of the stimulus. Sub-threshold stimuli are too weak for sensory detection. There is a refractory period in which no stimulus will create a response and a relative refractory period in which only sufficiently large stimulus would create a response following the most recent response to an above threshold stimulus. Sensory neurons adapt and become less responsive to constant stimuli over time. All sensory stimuli carry information about time and location.

 

 

The stimulus is created into action potentials as signals for additional processing. An action potential is the disturbance of the electric field across the neuron membrane. The action potentials begin at the axon hillock and are propagated through the axon and the signal is transmitted via neurotransmitters across the synapse. The action potential propagation is driven by the Na+/K+ gradient. When the voltage across the membrane is disturbed, gated sodium channels open causing depolarization to occur. Potassium gated channels take longer to respond to the voltage change and cause repolarization following hyperpolarization. Axon myelination speeds action potential propagation. Axons are myelinated by oligodendrocytes and Schwann cells. Ca2+ ion inflow releases stored neurotransmitters into synaptic cleft to bind postsynaptic receptors. Synaptic transmission is unidirectional. Neurotransmitters in the synaptic cleft diffuse via Brownian motion. Neurotransmitters bind only briefly before they are reabsorbed via active transport or degraded by enzymes in the synaptic cleft. Some of the most important neurotransmitters are: glutamate, aspartate, GABA, serotonin, acetylcholine, dopamine, and norepinephrine.

Process

 

There are receiving centers where neurons receive information from sensory neurons and intermediate receiving centers. Interneurons transfer signals from neuron to neuron and compose 90% of the neurons in the body. Once the nature and importance of the stimulus has been determined by the receiving centers, an appropriate response is initiated. Ninety-nine percent of sensory input is discarded by the brain. Perception of the meaning and significance of the stimulus is determined by evaluative comparison to previous experiences of similar or the same stimulus. Processing of feedback from internally regulated systems functions identically to those we have conscious control over in that a signal is transmitted via neurons, axons, and neurotransmitters, the signal is processed, and an appropriate response to the communication is elicited.

Export (efferent – motor)

Motor neurons carry stimulus from the brain to the muscles to elicit the conscious or unconscious response to sensory stimuli. Motor signals are sent to the motor cortex, through the thalamus and travel down to the spinal cord. Motor response requires continuous sensory feedback for continued successful execution. In addition to sensory feedback, proprioception is also involved in guiding motor response. Motor neurons are located ventrally in the spinal cord. Motor neurons primarily rely on the hormone acetylcholine which signals the muscle to contract. The postsynaptic acetylcholine receptor is an ionophore receptor, which when activated is permeable to calcium and sodium ions. Motor neurons and the muscle fibers they innervate are referred to as motor units.

 

Voluntary motor response performed by striated muscle. Red striated muscle has a large blood supply, many mitochondria, and lots of myoglobin. White muscle has less blood supply, mitochondria, and myoglobin but move more quickly than red muscle. Both kinds of striated muscle are innervated by motor neurons whose axons form motor nerves in the peripheral nervous system. Involuntary movements are performed by smooth muscle. An exception to this is cardiac muscle, which is characterized as unique striated muscle but still operates involuntarily.

Extenders and flexors are muscles that facilitate movement by opposing each others effect on the bone or organ. Reflex movements are wholly unconscious that occur before there is conscious awareness of their occurrence. Central program generators regulate reflex response and consist of sensory and motor neurons that automatically produce a specific response and are self monitoring. CPGs are directed by the mesencephalic locomotor region in the brain stem.

The motor cortex is located on the cerebral hemisphere near the central sulcus. Neurons in the motor cortex exhibit vertical columnar organization and columns of neurons near each other control related muscle groups. The prefrontal and sensory cortices guide the motor cortex in urgent situations.

The basal ganglia is composed of the striatum, globus pallidus, subthalamic nucleus, and substantia nigra. The striatum receives sensory and motor information from the cerebral cortex. The striatum is also a pre-processing receptor prior to signals being processed by the sensory cortex. The substantia nigra is responsible for dopamine transmission which is the primary area effected in Parkinson?s disease. Dopamine is required for effective control of motor activity. The lack of dopamine affects the functioning of the globus pallidus, as well as other structures. The subthalamic nucleus regulates movement preventing its occurrence in excess without conscious control.

The cerebellum receives information from the cerebral cortex, brain stem, and spinal cord. Purkinje cells in the cerebellum are involved in proprioception and signals from these cells are received at the deep cerebellar nuclei which subsequently modifies the motor cortex. The cerebellum directs fine and complex motor tasks. It is also involved in cognitive functions unrelated to motor behavior.

Two parallel motor systems.

 

Synaptic Cleft Requiring Chemical Transmitters eg Acetylcholine to Transmit Impulse Across Nerve Junctions
72046.800 mitochondria transmitter vesicles presynaptic terminal post synaptic terminal soma of neuron synaptic cleft acetyl choline norepinephrine dopamine serotonin forces chemical energy function principles Davidoff art Davidoff drawing Davidoff MD

 

Sodium Potassium Pump MEchanism in the Transmission of a Nerve Impulse
72045b04.800 nerve conduction force electricity electric force positive force negative force sodium pump Na pump Patassium pump K+ pump diffusion conduction of impulses Davidoff drawing Davidoff art Davidoff MD
Cisternogram
49689.800 CSF flow brain cisternogram radioisotope injected into subarachnoid space via lumbar puncture and over 6.5 hours progresses to base of brain. This patient had a CSF leak in the cribiform plate. Hernose was plugged with cotton gauze and the gauze was scanned after 6 hours – The left and right were both positive for leak greater on the right nuclear medicine Courtesy Alan Ashare MD
Normal – 4 months Prior and Folllowing Cardiopulmonary Arrest

Loss of Gray White Differentiation
70134c01 hx 52 F post cardiac arrest brain cerebrum gray matter white matter gray white differentiation gray white distinction sulci gyri loss of gray white differentiation hypodense dx anoxic injury with diffuse global cerebral edema Davidoff MD Loss of consciousness occurs within 10-15 seconds of cardio-pulmonary arrest. Irreversible brain damage can occur within 5 minutes. gray matter of the brain, particularly the frontal lobes have highest metabolic needsThe occipital, parietal, and temporal lobes and basal ganglia and cerebellum are lower. brainstem lowest needs

Appied Biology

Brain does not have metabolic reserve

Whatever the brain uses is what comes in

Loss of supply – 4 minutes is max can go before there is irreversible damage

Diving reflex cold – shutting off circ to all other parts

Carl Sagan

Old brain – primitive reflexes -strartle reflex

Inborn fear of snakes and reptiles  – from left over dinosaur ages