Unveiling the Nervous System: Anatomy, Neurons, and Pathways

HUMAN ANATOMY AND PHYSIOLOGY LECTURE CHAPTER 8 - NERVOUS SYSTEM NERVOUS SYSTEM • • Functions o Sensory input - sensory receptors respond to stimuli o Integration - brain and spinal cord process stimuli o Control of muscles and glands o Mental activity - brain o Homeostasis Main Divisions of Nervous System o Central Nervous System (CNS) - brain & spinal cord o Peripheral Nervous System (PNS) - all neurons outside CNS NEURON • • • • • • • Nerve cells Require oxygen and glucose Receive input, process input, produce a response Dendrite - receives stimulus from other neurons or sensory receptors Cell body processes stimulus; contains a nucleus Axon - transmits stimulus to a gland, muscle, organ, or other neuron Myelin Sheath o fatty, protective wrapping around axons o excellent insulator o Nodes of Ranvier - gaps in myelin sheath where action potentials develop o Saltatory conduction - jumping of action potentials o Myelinated axons conduct action potentials more quickly (3-15 meters/sec) than unmyelinated due to Nodes of Ranvier. o Multiple sclerosis - disease of myelin sheath that causes loss of muscle function TYPES OF NEURONS • • • Multipolar o many dendrites and a single axon o Ex. CNS and most motor neurons Bipolar o one dendrite and one axon o Ex. Eye and nasal cavity Pseudo-unipolar o one axon and no dendrites o Ex. Sensory neurons NEUROGLIA • • • • Supporting cells for neurons More numerous than neurons Can divide to produce more cells 5 types TYPES OF NEUROGLIA • • • • • Astrocytes - star-shaped; most abundant; form bloodbrain barrier Ependymal Cells - produce and circulate cerebrospinal fluid (CSF) Microglia - help remove bacteria and cell debris from CNS Oligodendrocytes - produce myelin sheath in CNS Schwann cells - produce myelin sheath in PNS • Neuronal Pathways o Converging: o two or more neurons synapse same neuron o allows info. to be transmitted in more than one neuronal pathway to converge into a single pathway o Diverging: o axon from one neuron divides and synapses with more than one neuron o allows info. to be transmitted in one neuronal pathway to diverge into 2 or more pathways ORGANIZATION OF NERVOUS TISSUE • • Gray matter - collection of dendrites and cell bodies White matter - collection of axons and their myelin sheath CNS PNS Oligodendrocytes Schwann cells Produce myelin sheath Nerve tracts Nerves Collection of axons Nuclei Ganglion Collection of cell bodies CENTRAL NERVOUS SYSTEM ELECTRICAL SIGNALS AND NEURAL PATHWAYS • • • • Resting Membrane Potential o Outside of cell is more + (Na+) o Inside of cell is more – (K+) o Leak ion channels - always open; K+ channels o Gated ion channels - closed until opened by specific signal; Na+ channels Action Potentials o “Electricity” that cause depolarization and repolarization o Change resting membrane potential by activating gated ion channels o Local Current - movement of Na+ which causes inside of cell to be more positive (depolarize) o If enough Na+ enters then threshold is reached and more Na+ channels open o Once threshold is reached all or none law applies o Action potentials continue until Na+ channels close, K+ channels open, and repolarization occurs o Sodium/potassium pump restores Synapse o where an axon attaches to a muscle, gland, organ, or other neuron o involved with release of neurotransmitters o Ex. Neuromuscular junction Reflexes o involuntary response to a stimulus o Reflex arc - path reflex travels ▪ Sensory receptors - pick up stimulus; in skin ▪ Sensory (afferent) neurons - send stimulus to interneurons in spinal cord ▪ Interneurons (Association) neuron - located in CNS and connect to motor neurons; process stimulus ▪ Efferent (motor) neurons - send response to effector ▪ Effector - muscle, gland, or organ • • • Consists of brain and spinal cord Brain in brain case Spinal cord in vertebral column • Spinal Cord o Extends from foramen magnum to 2nd lumbar vertebra o Protected by vertebral column o Spinal nerves allow movement o If damaged paralysis can occur o Gray Matter ▪ center of spinal cord ▪ looks like letter H or a butterfly ▪ Posterior horns - contain axons which synapse with interneurons ▪ Anterior horns - contain somatic neurons ▪ Lateral horns - contain autonomic neurons ▪ Central canal - fluid filled space in center of cord o White Matter ▪ outside of spinal cord ▪ contains myelinated fibers ▪ Contains 3 columns dorsal, ventral, lateral columns ▪ • • • • • • • Ascending tracts - axons that conduct action potentials toward brain ▪ Descending tracts - axons that conduct action potentials away from brain Spinal Nerves o Arise along spinal cord from union of dorsal roots and ventral roots o Contain axons sensory and somatic neurons o Located between vertebra o Categorized by region of vertebral column from which it emerges (C for cervical) o 31 pairs o Organized in 3 plexuses Cervical Plexus o Spinal nerves C1-4 o Innervates muscles attached to hyoid bone and neck o Contains phrenic nerve which innervates diaphragm Brachial Plexus o Originates from spinal nerves C5-T1 o Supply nerves to upper limb, shoulder, hand Lumbosacral Plexus o Originates from spinal nerves L1 to S4 o Supply nerves lower limbs Cerebrospinal Fluid o Fluid that bathes the brain and spinal cord o Provides a protective cushion around the CNS o Produced in choroid plexus of brain Brainstem o Medulla oblongata ▪ Location - continuous with spinal cord ▪ Function - regulates heart rate, blood vessel diameter, breathing, swallowing, vomiting, hiccupping, coughing, sneezing, balance ▪ Pyramids - involved in conscious control of skeletal muscle o Pons ▪ Location - above medulla, bridge between cerebrum and cerebellum ▪ Function - breathing, chewing, salivation, swallowing o Midbrain ▪ Location - above pons ▪ Function - coordinated eye movement, pupil diameter, turning head toward noise o Reticular Formation ▪ Location - scattered throughout brainstem ▪ Function - regulates cyclical motor function, respiration, walking, chewing, arousing and maintaining consciousness, regulates sleep-wake cycle Diencephalon o Located between the brainstem and cerebrum o Thalamus ▪ largest portion of diencephalon ▪ influences moods and detects pain o Epithalamus: ▪ above thalamus ▪ emotional and visceral response to odors o Hypothalamus ▪ below thalamus ▪ controls pituitary gland and is connected to it by infundibulum ▪ controls homeostasis, body temp, thirst, hunger, fear, rage, sexual emotions • • • Cerebrum o Largest portion of brain o Divisions: ▪ Right Hemisphere; Left Hemisphere ▪ separated by longitudinal fissure o Lobes: frontal, parietal, occipital, temporal o Cerebral Cortex ▪ surface of cerebrum, composed of gray matter ▪ controls thinking, communicating, remembering, understanding, and initiates involuntary movements o Gyri - folds on cerebral cortex that increase surface o area o Sulci - shallow indentations o Fissure - deep indentations o Left hemisphere ▪ controls right side of body ▪ responsible for math, analytic, and speech o Right hemisphere ▪ controls left side of body ▪ responsible for music, art, abstract ideas o Corpus callosum - connection bet. 2 hemispheres Lobes of Brain o Frontal lobe - front; controls voluntary motor functions, aggression, moods, smell o Parietal lobe - top; evaluates sensory input such as touch, pain, pressure, temp., taste o Occipital lobe - back; vision o Temporal lobe - sides; hearing, smell, memory Cerebellum o below cerebrum o means little brain o cortex is composed of gyri, sulci, gray matter o controls balance, muscle tone, coordination of fine motor movement SENSORY FUNCTIONS • • • • • CNS constantly receives sensory input We are unaware of most sensory input Sensory input is vital of our survival and normal functions Ascending Tracts o pathways in brain and spinal cord o transmit info. via action potentials from periphery to brain o each tract has limited type of sensory input (temp, touch, pain, etc.) o tracts are named indicating origin and termination o made of 2-3 neurons in sequence Sensory Areas of Cerebral Cortex o Primary sensory areas: ▪ where ascending tracts project ▪ where sensations are perceived o Primary somatic sensory cortex: ▪ general sensory area ▪ in parietal lobe ▪ sensory input such as pain, pressure, temp. SOMATIC MOTOR FUNCTIONS • Motor Areas of Cerebral Cortex o Primary motor cortex - frontal lobe; control voluntary motor movement o Premotor area - frontal lobe; where motor functions are organized before initiation