Fundamentals of Anatomy - The Nervous System

Chapter 7 - The Nervous System Role: Master controlling and communicating system Functions: 1. Monitor changes (stimuli) inside and outside of the body through sensory receptors (collect the sensory input) 2. Processes and interprets sensory input through integration 3. Effects a response by activating muscles or glands (motor output) Structural Classification: CNS: central nervous system - Made up of brain and spinal cord - Occupy the dorsal body cavity PNS: peripheral nervous system - Made up of nerves: cranial (impulses to/from brain) and spinal (impulses to/from spinal cord) - Link all parts of the body carrying impulses (sensory receptors to brain to muscles/glands) Functional Classification: only of the PNS Sensory (afferent) Division - nerve fibers that convey impulses to the CNS from sensory receptors - Somatic sensory fibers (nerves sending impulses from skin, skeletal muscle, joints) - Visceral sensory fibers (nerves sending impulses from visceral organs) Motor (efferent) Division - nerve fibers carrying impulses from the CNS to effector organs - Two subdivisions: - Somatic (Voluntary) Nervous System: conscious control of voluntary muscle (note: can be under autonomic control as well; i.e. reflexes) - Autonomic (Involuntary) Nervous System: regulates activity of smooth/cardiac muscle and glands - Two subdivisions: sympathetic and parasympathetic CNS Structure and Function Supporting Cells: neuroglia (“nerve glue”) that support, insulate, protect neurons; do not send impulses; retain ability to divide (from cancerous gliomas) Astrocytes- star shaped cells, make up 50% of nervous tissue, links to blood capillaries and neurons, barrier of protection (screen blood in a way), control chemical environment in the brain Microglia- line cavity of brain/spinal cord, ciliated to circulate cerebrospinal fluid Oligodendrocytes- produce fatty myelin sheaths to protect nerve fibers Supporting Cells in the PNS: Schwann cells - form myelin sheaths around PNS nerve fibers Satellite cells - protective, cushioning cells Neuron Anatomy: Cell body - contains organelles (no centrioles, no division) like the nucleus Nissl substance/body - the rough ER Neurofibrils - intermediate filaments to maintain shape Processes - fibers of varying length extending from the neuron Dendrites - processes that convey incoming messages to the cell body Axons - processes that conduct impulses away from the cell body; one per axon; originates at the axon hillock Axon terminals - end branches of axons; vesicles in terminii secrete neurotransmitters Synaptic cleft - separation between neurons, called a synapse Myelin - fatty material that covers nerve fibers; in PNS this is made by Schwann Cells; in CNS made by oligodendrocytes (affected in diseases like MS, ALS) Nodes of Ranvier - gaps in the myelin sheath along nerve fibers Neurilemma - external to the myelin sheath; only in PNS, can lead to nerve fiber regeneration Nuclei - clusters of cell bodies in the CNS; protected by bone Ganglia - small collections of cell bodies in the PNS Tracts - bundles of nerve processes through the CNS Nerves - bundles of nerve processes through the PNS White matter - dense collections of myelinated fibers/tracts Gray matter - dense collections of unmyelinated fibers/tracts or cell bodies Functional Classification of Neurons Sensory (afferent) neurons carry impulses from sensory receptors to the CNS - Cell bodies are found in a ganglion outside of the CNS - Dendrites of these neurons are found associated with sensory receptors (i.e. cutaneous, proprioceptors “own movement”, sense stretching) Motor (efferent) neurons carry info from the CNS to viscera/muscles/glands - Cell bodies always found in CNS Interneurons (association neurons) connect sensory and motor neurons - Cell bodies always located in the CNS Structural Classification of Neurons: based on number of processes from cell body Multipolar neuron - many processes (i.e. all motor, interneurons) Bipolar neuron - two processes (rate in adults) Unipolar neurons - single process that is short and divided into proximal (central) and distal (peripheral) portions (dendrites and axon) Physiology of Neurons Irritability - ability of a neuron to respond to a stimulus, convert to an impulse Conductivity - ability to transmit an impulse to other neurons/muscles/glands Polarized - unequal distribution of charge on the plasma membranes of a resting neuron - Fewer + ions on inner side (K+) of membrane than outer side (Na+) - Neuron stays inactive as long as inner side remains more negatively charged than outer side Stimuli “excite” neurons and generate an impulse (i.e. light, pressure, neurotransmitters) - Na+ channels become open and they move into the inner portion of the neuronal membrane (diffuse from HIGH to LOW concentration) - Leads to “depolarization” of the neuron (more + ions now on the inner side of the membrane) Creation of a graded potential may be strong enough to elicit a signal called an action potential (nerve impulse) - All or one response that goes the entire length of the axon (propagation) Repolarization - K+ ions move out across the neuron membrane restoring charge - sodium/potassium pumps helps restore Na+/K+ concentrations as well using ATP Saltatory conduction - occurs along myelinated neurons, as the action potential “jumps” from one Node of Ranvier to the next Sedatives, anesthetics alter ion permeability to stop action potential Pressure and cold block impulse conduction What about conductivity of the neurons? The action potential in most cases does not travel across the synapse, instead it stimulates the formation/release of neurotransmitters - The neurotransmitter is secreted into the synaptic cleft between neurons - Binds to receptors in the receiving neuron’s membrane, opening ion channels - This may generate depolarization and start a new action potential Effects of neurotransmitters are short-lived due to degradation by enzymes Overall, transmission of an action potential is ELECTROCHEMICAL Reflexes: rapid, predictable and involuntary responses to stimuli - Use neural pathways called reflex arcs - Involve the CNS and PNS Somatic - stimulate skeletal muscles Autonomic - regulate activity of smooth muscle, heart, glands Five elements to action - sensory receptor, effector organ, sensory neuron, motor neuron, integration center More neurons/synapses involved, the longer the reflex action takes to occur Degree of reflex action may indicate nervous system disorders CNS: starts as a embryonic neural tube, early brain forms at four weeks gestation Cerebral Hemispheres: the cerebrum Gyri (gyrus) - elevated ridges Sucli (sulcus) - shallow grooves Fissures - deeper grooves (i.e. longitudinal fissures) Lobes - created by fissures and sulci, named for cranial bones which cover them Gray matter - outermost part of cerebrum, unmyelinated neurons, ridged for more surface area White matter - deeper part of cerebrum, made up of myelinated fiber tracts Basal ganglia - help regulate voluntary motor activity Reticular Activating System (RAS) determines the level of alertness Meninges: three connective tissue membrane covering and protecting the CNS - Dura mater (outermost, double layered, periosteal layer forms periosteum, meningeal layer forms outer covering of brain, form sinuses) - Arachnoid (middle layer, web like) - Pia mater (“gentle mother”, delicate inner covering) Cerebrospinal fluid (CSF): like blood plasma in composition, but less protein, more vitamin C, altered ion composition Blood-brain barrier provided by very impermeable capillaries; only water, glucose, essential amino acids pass through easily; good protection, but what about drug treatments that need to enter the brain? Brain Dysfunctions: Alzheimer’s Disease - shortage of acetylcholine, plaques and tangles in the amydala/hippocampus Parkinson’s Disease - basal nuclei overstimulated due to low dopamine levels Huntington’s Disease - autosomal dominate genetic disease; “chorea” Traumatic brain injury - injury from impact, brain hitting opposite side of skull Cerebrovascular Accident - strokes; bloodflow to brain blocked due to plaques/clots/ruptures Spinal Cord: protected by meninges, which extend lower than vertebrae (for CSF testing) PNS: nerves, groups of neuronal cell bodies Autonomic Nervous System (ANS): motor subdivision of the PNS; regulate cardiac/smooth muscle and glands; “involuntary nervous system” - Somatic nervous system controls skeletal muscles, cell bodies of neurons are inside CNS, axons extend to muscles served - Autonomic nervous system uses 2 motor neurons Sympathetic vs Parasympathetic ANS: “fight or flight” vs “resting and digesting” Developmental Aspects of the CNS: - Measles infection in a pregnant woman can lead to damage in fetal CNS - Smoking decreases oxygen to the fetal CNS - Lack of oxygen during delivery may lead to cerebral palsy - Hydrocephalus, anencephaly (cerebrum does not develop) - Spina bifida (malformation of the neural tube) has many degrees of severity, impactedby levels of folate - Hypothalamus tends to be underdeveloped in premies (can’t control body temp well) - Max brain weight reached by late teens, can always develop new neural pathways - Sympathetic NS less efficient with age leading to orthostatic hypotension, etc. - Overall deterioration of CNS with age due to lack of oxygen