Functional Organization of the Nervous System PDF

NEUROSCIENCE I FUNCTIONAL ORGANIZATION OF THE NERVOUS SYSTEM OBJECTIVES Explain how the organization & structure of the nervous system relate to its function Describe the processes following nerve lesion i.e. Wallerian degeneration and regeneration Define the terms: axoplasmic transport, denervation...

NEUROSCIENCE I FUNCTIONAL ORGANIZATION OF THE NERVOUS SYSTEM OBJECTIVES Explain how the organization & structure of the nervous system relate to its function Describe the processes following nerve lesion i.e. Wallerian degeneration and regeneration Define the terms: axoplasmic transport, denervation supersensitivity Introduce the resting membrane potential NERVOUS SYSTEM CENTRAL N.S. BRAIN PERIPHERAL N.S. SPINAL CORD AFFERENT SYSTEM EFFERENT SYSTEM FUNCTIONS Integration Centre of the living organism Controls all activities of the body: ✓Glandular secretions ✓Growth ✓Behaviour ✓Muscle contractions How is the Nervous System able to accomplish these functions? signals Sense Organs Afferents CNS Appropriate commands Efferents Effector Organs e.g. skeletal muscles Appropriate Action Arrangement enables the animal to: 1. Function as a coordinated unit 2. Adapt to changes in the environment Functional Units Principal functional units are called NEURONES Human N.S. has 10 11-12 neurones 10X as many non-neuronal cells Non-Neuronal Cells 1. ASTROGLIA: form the blood–brain barrier; produce neurotrophins –survival & growth 2. OLIGODENDROGLIOCYTES : ~75% of glial cells in the white matter, Form myelin sheaths on central neurons 3. MICROGLIA: phagocytic; scattered in the grey matter 4. EPENDYMAL CELLS: line the fluid-filled ventricles of the brain; direct cell migration during brain development Types of Neurones ▪ Dendrites – receptor zone, arborize extensively; dendritic spines ▪ Cell body or Soma – maintains the functional & anatomic integrity of the axon; contains nucleus, Nissl granules, neurofibrils etc. ▪ Axon Hillock – most excitable axonal region ▪ Axon – long, fibrous, usually myelinated, formed by Schwann cells in the periphery (cp. role of oligodendrocytes) ▪ Telodendria or terminal buttons – involved in neurotransmission AXOPLASMIC TRANSPORT Axons do not contain ribosomes Soma synthesizes the proteins involved in neurotransmission Anterograde transport: movement of proteins from soma to terminal endings Retrograde transport: movement from the terminal endings to soma Wallerian Degeneration & Regeneration Cell body maintains the functional integrity of the neuron If axon is severed, the part distal to the lesion degenerates 5 grades of injury: Grade 1 – least harmful Grade 5 – most harmful Wallerian Degeneration & Regeneration Changes in the Cell Body: 1. Chromatolysis within 24 hrs of injury 2. Cell body swells and assumes a spherical shape 3. Nucleus pushed to one side 4. Retention leads to regeneration of the neuron Wallerian Degeneration & Regeneration Changes in the Axon: 1. Orthograde or Wallerian degeneration occurs distal from site of injury 2. Schwann cells divide mitotically & form cords of cells in the endoneurial tube 3. Macrophages invade the degenerating stump and remove debris Wallerian Degeneration & Regeneration 4. Schwann cell cytoplasm then fills the endoneurial tubes. Process ~ 3 mths 5. Schwann cells elongate and send processes outward to unite stumps, 6. Schwann cells spin new myelin around the new axon. Called: Regenerative sprouting Wallerian Degeneration & Regeneration Central axon elongates & grows out in several directions = Regenerative sprouting Schwann cells of the peripheral endoneurial tube guide the regeneration fibrils Peripheral nerve achieves ~ 80% of original diameter Transection of axons in the brain does not normally occur lead to regeneration Due to: 1. Lack of Schwann cells 2. Oligodendrocytes wrap myelin sheaths around several neurons in the CNS DENERVATION SUPER- or HYPER-SENSITIVITY Increased responsiveness of the end-organs or muscles following injury (denervation) of the peripheral nerve 1. Regenerative sprouting 2. Lack of re-uptake of neurotransmitters 3. Hyper-responsive to neurotransmitters 4. Up-regulation (increased numbers) of receptors on effector tissue When higher centres in the CNS are destroyed, activity of lower centres are often increased Known as “Release phenomenon” (Partly due to denervation supersensitivity) Many of the signs/symptoms of neurologic disease are due to denervation supersensitivity All excitable cells, e.g. nerves and muscles, have, at rest, a potential difference across their cell membranes Resting Membrane Potential How do we know that this RMP of the nerve fibre exists? Oscilloscope records 0 mV ~ A constant negative RMP is recorded ~ For large nerves, RMP = –70mV Range: - 45mV to –95mV