neuroanatomy and physiology
TRANSCRIPT
Neuroanatomy and physiologyVSR 605, VSR 606Dr. Rekha PathakSenior scientist
Division of Surgery
•Neurons conduct impulses•The passage of Action potential•RMP of muscle and nerves: The resting membrane potential in skeletal muscle cells is similar to that in neurons, i.e. −70 to −90 mV. Unlike nerve cells, where the resting membrane potential is predominantly a result of K+ permeability, skeletalmuscle cell resting membrane potential receives a significant contribution from Cl− conductance. (structure of cell memberane protein bilipid and stacked with protein molecules)
•Neurons are the most sensitive cells to change in pH, pCO2,pO2 and blood glucose levels•Neurons are classified into
• Sensory neuron: cell body outside the CNS and function is to conduct impulses from periphery to CNS (for eg. Irritative impulses)
• Depends on the direction of the action potential (afferent / efferent, so afferent cell bodies are outside the CNS)
Interneurons: completely within the CNS: pass impulses within the CNS to the motor neuron network (very complex) (eg. Motor functions centrally
governed and glandular secretions)
• Motor neurons: cell body in the CNS and axons in outside the CNS (muscle movement)
• Motor neurons are classified into UMN and LMN
• UMN: cell bodies in the cerebral cortex or brain stem and axons forming the descending fasciculi (long tracts)
• The LMN: Cell bodies in the brainstem/spinal cord and axons for the motor fibers which innervate the muscular structures.
• The clinical signs of UMN and LMN are different
• It can be related with the fact that if nerve cells are destroyed no regeneration but it is possible if the axon is destroyed.
•Axons and some dendrites and its branches are covered with a layer of white fatty substance known as myelin sheath. Size of the fibres 0.5-20 microns
•The smallest axon has very thin film of myelin sheath so they are called unmyelinated (because the sheath is not visible by staining)
• Slow nerve conduction in unmyelinated fiber• 0.5M/SECOND in very small unmyelinated fiber• 100m/second very large myelinated fiber• Velicity increases approximately with the fuber dia in myelinated nerve
fiber and approx with the square root of fiber dia. In unmyelinated fiber• In young ones after birth slow pick up several weeks after birth• CD, allergic reactions ------destruction of myelin• Axons outside the CNS have one more covering called endoneuriuim
above the myelin
•Nerve damage---if distal (may regenerate)•If cell body damages---does not regenerate•The degeneration may be ascending or descending degeneration•Nissils substance undergo chromatolysis•The myelin is converted into lipid droplets and aligned along the broken axon•The Schwann cells elongate and interconnect with each other•Axons grow and join•Glial scarring in CNS prevents regeneration
• Due to– Trauma– acute compression
• Signs & symptoms– Loss of motor function– Loss of sensory function
• Pathology– Demyelination/axonal degeneration– Disruption of the sensory/motor function of the injured nerve– Remyelination with axonal regeneration– Reinnervation of the sensory receptors & muscle end plates
• Degenerative changes• Axonal injury
– Degenerative changes at proximal & distal end• Anterograde degeneration (Wallerian Degeneration)
– Affecting the» Injured neuron» Neurons functionally connected to the injured neuron
– Transneural degeneration» also degenerate neurons that synapses with the injured neuron
– Starts in 24 hours
Peripheral nerve injury
• Retrograde Degeneration– Extends up to the first node of Ranvier proximal to the injury– Changes in the dendritic tree
» the parent cell body & the part of the axon still attached to the cell body
– Chromatolytic changes– Swelling of the cell– Displacement of the nucleus to periphery
» sometimes extruded out– Fragmentation & reduction of Golgi apparatus– Disappearance of neurofibrils
• Chromatolysis• disintegration of the Nissl substance
– begins within 24 – 48 hours– begins near the axon hillock & spreads to other parts
• occurs in certain infectious or degenerative diseases of the nervous system– poliomyelitis– progressive muscular atrophy
• degree of chromatolysis depends on– proximity of the site of injury to the nerve cell
• more in motor neurons• Wallerian degeneration
• Reg. Rate of nerves• 0.5-2.0 mm / day in canine. There is down growth of
neurobifibrils from the proximal portion of severed nerve (if connected with other nerves and AP is there), then chances of regeneration will be more.
• The nerve impulse: AP, RMP• Thicker fibers carry faster impulse• Synapse: one neuron joins the other/ effectors• May transmit signals/ may block signals/ may change it from
single impulse to repetitive impulses• Reduce the signals• Integrate the impulses with other neuron• Excitatory transmitter: hyperesthesis, hyperalgesia and
excessive motor responses visual and tactile stimuli
• Inhibitory transmitter: very important because CNS continuously receives a barrage of stimuli from the peripheral nerves . Inhibition plays a role in selecting the imp. Signals at a particular set of circumstances
• Strychnine poisoning : muscular rigidity• Sensory receptors• They turn specific form of energy into nerve impulses• Mechanoreceptors: touch, pressure, kinesthetic, sound
equilibrium, lung stretch, heart stretch• Thermo receptors: heat and cold• Chemoreceptor• Pain , taste smell and oxygen tension in blood • Electromagnetic radiations receptors; sight—Rods and
cones of eye
• Effectors• Reflexes: myotactic or stretch reflex- knee jerk • The antigravity muscles are responsible for muscle tone• Knee jerk: when patella is hammered the stifle goes
into extension • Neuroglia• Meninges• Internal cavities of the brain• CSF
• Hydrocephalous• BBB
Choroid plexus; made of ependydymal cells and responsible for the synthesis
of CSF
•Blood CSF barrier-----At the choroid plexus•CSF brain barrier------- at the CSF
•BBB--- Is actually due to the pedicular processes of the astrocytes which cover the capillaries of the CNS. These interposition of the glial cells provides the selectivity of the entrance of materials into the CNS tissue.
The Blood –CSF barrier : is based on organization of the choroid plexus its capillaries
•The CSF brain barrier: selectivity of the ependymal cells lining the ventricles and ependymal cells lining the ventricles and the cerebral aqueduct and the astrocytes adjacent to the ependyma.
•Certain regions are devoid of BBB like pineal body, optic recess of the third ventricle, the saccular eminence of the hypophyseal stalk and posterior lobe of hypophysis.
Blood supply•Cerebrum: Anterior , middle and posterior cerebral arteries•Cerebellar: Anterior and posterior cerebellar artery
• Important for the clinician who does cerebral angiograms for diagnostic purposes to recognize the normal vascular pattern
• Clinical sign with sudden flaccid hemi- paresis suggest an acute vascular accident
• Subdural hemorrhage (Video)
Venous drainage is from the Dural sinuses (vascular channels within the structure of the Dura mater)
• Brain metabolism utilizes about 14 per cent of the total oxygen consumption of the body
• Carbohydrate oxidation to glucose is the chief energy source
• Autoregulation of blood flow to the CNS• Autonomic system (sympathetic and parasympathetic)• Co2 is dominant than O2• When carbon dioxide partial pressure PCO2 increases in the
capillaries the BV dilate allow rapid blood flow to remove the excess co2 and other metabolites which can lower the Co2 back to normal
• In brain edema: Vasoconstriction is brought about by hyperventillation and oxygenation at the tissue site
• This process reduces the blood flow to tissue and edema subsides
• It should not be excessive otherwise problem occurs