The Nervous system

• PNS = all neurons that lie outside or partially outside the CNS.
• CNS= brain & spinal cord
• Ganglia (ganglion, singular)= collection of nerve cell bodies outside the CNS.
• The brain sits in the bony cranium.
• The spinal cord runs down the back (dorsal side) inside the vertebral column.
• The bones of the cranium and vertebral column together with membranes (meninges) and fluid (cerebrospinal fluid) protect the nervous tissue.
• Nervous system - neurons directly contact (innervate) most organs.
• Endocrine system - Hormones are transported by the blood to body organs.
• Neurotransmitters are chemicals secreted by neurons that diffuse across a small gap to the target cell.
• Neurohormones are chemicals released by neurons into the blood for action at distant targets.
• Hormones are secreted by endocrine glands or cells into the blood. Only target cells with receptors for the hormone respond to the signal.
• Afferent/Sensory neurons: carry information (about light, temperature, pressure & other stimuli) from sensory receptors throughout the body into the CNS.
• Interneurons (interconnecting neurons): Lie entirely within the CNS where they integrate information. Often have complex branching processes to connect them with many other neurons.
• Efferent neurons: carry information away from the CNS to body tissues and organs (effectors).
• Somatic Motor Neurons: Motor neurons that innervate skeletal muscles and control voluntary movement.
• Autonomic Neurons are Innervate smooth muscle of many body organs (digestive tract, airways etc) and cardiac muscle of the heart and control function of these organs.
• Synapse- region of contact between two neurons or a neuron and muscle.
• Presynaptic neuron- delivers a signal to the synapse.
• Postsynaptic cell- receives the signal.
• The human CNS also contains electrical synapses where the presynaptic and postsynaptic cells are connected by gap junction channels. This cell to cell communication is bidirectional as well as faster than chemical synapses.
• The cell body is small, generally making up one-tenth or less of the total cell volume.
• Dendritic spines vary from thin spikes to mushroom shaped knobs and increase the surface area of dendrites.
• The axon cytoplasm is filled with many types of fibres and filaments but lacks ribosomes and endoplasmic reticulum. Proteins for the axon must be synthesized on the rough endoplasmic reticulum in the cell body.
• Neurons: Functional unit of the nervous system -100 billlion in human brain.
• Gilia: Support cells- more numerous than neurons, 10-50 times more.
• An Oligodendrocyte wraps myelin around axons.
• An Astrocyte extension touches the synapse.
• Astrocyte extensions contact the blood vessel.
• Glial cells are found in the central nervous system and the peripheral nervous system.
• Peripheral nervous system contains Schwann cells and Satellite cells.
• Central Nervous System contains the Ependymal cells, Astrocytes, Microglia (modified immune cells), and Oligodendrocytes.
• Satellite cells support cell bodies.
• Schwann cells form myelin sheaths and secrete neurotrophic factors.
• Ependymal cells create barriers between compartments and is a source of neural stem cells.
• Microglia act as scavengers.
• Astrocytes is a source of neural stem cells, take up K+, water, transmitters. secrete neurotrophic factors, help form Blood-brain barrier and provide substrates for ATP production.
• The law of conservation of electrical charge: net amount of electrical charge produced in any process is zero (overall human body is electrically neutral).
• Separating positive charges from negative charges requires energy.
• All living cells in the body maintain a potential (voltage) difference across the cell membrane. This is the cell's membrane potential.
• Neurons and muscles (excitable cells) can alter their membrane potential in response to stimulation.