Nervous System Basic Structure and Function Interesting facts

Nervous System Basic Structure and Function

Interesting facts about the Nervous System • There are millions of nerve cells in the human body. This number even exceeds the number of stars in the Milky Way. • The human brain alone consists of about a 100 billion neurons. If all these neurons were to be lined up, it would form a 600 mile long line. • In humans, the right side of the brain controls the left side of the body, while the left side of the brain controls the right side. • The diameter of the neurons can range between 4 to 100 microns. • In a child developing inside the womb, neurons grow at the rate of 250, 000 neurons per minute. • By the time of its birth, the baby's brains consists of around 10 million nerve cells. • The human spinal chord consists of around 13, 500, 000 neurons. The cluster of nerves located at the base of the spinal chord are most sensitive. • The weight of the brain in average adult males is 1375 grams, while in females it is 1275 grams. • And as we grow older the brain loses a gram each year. • At a given point of time, only four percent of the cells in the brain are active, the rest are kept in reserve. • Neurons, which are the largest cells in the human body, do not undergo the process of mitosis. • The nervous system is very quick. It can transmit impulses at a tremendous speed of 100 meters per second. The speed of message transmission to the brain can be as high as 180 miles per hour. • Sodium ions and potassium are necessary constituents to ensure proper functioning of the nervous system. Even Vitamin B is considered to be beneficial. • There are 43 different pairs nerves which connect the central nervous system to every part of our body. Twelve of these nerve pairs are connected to the brain, while the remaining 31 are connected to the spinal chord.

Divisions of the Nervous System 1) Central Nervous System (CNS) brain spinal cord 2) Peripheral Nervous System (PNS) peripheral nerves -cranial nerves (12 pairs) nerves that extend from the brain -spinal nerves (31 pairs) nerves that extend from the spinal cord Sensory Division (Afferent) picks up sensory information and delivers it to the CNS Motor Division (Efferent) carries information to muscles and glands Somatic – carries information to skeletal muscle Autonomic – carries information to smooth muscle, cardiac muscle, and glands Sympathetic and Parasympathetic divisions

Divisions of the Nervous System (Afferent) (Efferent) Voluntary Involuntary Sympathetic Parasympathetic 1. EXTERORECEPTORS gather info from EXTERNAL environment touch, temp, pressure, sight, smell, hear 2. INTERORECEPTORS gather info from INTERNAL environment body systems (cardio, resp, uri, reprod, dig) taste, deep pressure, pain 3. PROPRIORECEPTORS monitors POSITION of skeleton/muscles/joints

Organize the divisions of the nervous system into a flowchart. Provide information about the characteristics of each division. Place the following words into the boxes below: CNS, PNS, Efferent, Afferent, Somatic, Autonomic, Sympathetic, Parasympathetic

Organize the divisions of the nervous system into a flowchart. Provide information about the characteristics of each division. Place the following words into the boxes below: CNS, PNS, Efferent, Afferent, Somatic, Autonomic, Sympathetic, Parasympathetic Carry info to/from CNS V re olu sp nta on ry se Somatic (SNS) senso Motor/Efferent Autonomic (ANS) Effector organ: skeletal muscles Effector organs: cardiac smooth glands Sympathetic Fight-or-Flight response 12 pairs of cranial nerves 31 pairs of spinal nerves ry neu rons TO C NS Involuntary response Brain and Spinal Cord mo Process information tor FR neu OM ron CN s S PNS Parasympathetic Maintaining homeostatic conditions Sensory/Afferent Sends info TO CNS 3 types of sensory receptors: 1. EXTERORECEPTORS gather info from EXTERNAL environment touch, temp, pressure, sight, smell, hear 2. INTERORECEPTORS gather info from INTERNAL environment body systems (cardio, resp, uri, reprod, dig) taste, deep pressure, pain 3. PROPRIORECEPTORS monitors POSITION of skeleton/muscles/joints

Three Major Functions of Nervous System 1. Sensory Input Function • PNS • Sensory receptors (located at the end of peripheral nerves) gather information (detect changes occurring in their surroundings…. are stimulated) • Sensory neurons carry impulse/information to the CNS 2. Integrative Function • CNS (brain and/or spinal cord) interprets incoming sensory info • Interneurons use sensory information to create • sensations, memory, thoughts, decisions 3. Motor Function • PNS • decisions are acted upon (involves the response of a body part) • Motor neurons carry impulses to effectors (muscles contract) (glands secrete)

Neuron Structure Cell body (soma) 1. central portion of the neuron 2. contains organelles (nucleus, nucleolus) Nissl bodies = RER NO centrioles (cannot go through mitosis) 3. Axonal hillock sends impulse to axon, away from soma start of action potential. Axon 1. one per neuron 2. long, thin process 3. carry impulses AWAY from soma (cell body) 4. ends in branching axonal terminals, which end in synaptic knobs (contains neurotransmitters) DIRECTION OF NERVE IMPULSE TRANSMISSION Dendrites 1. many per neuron 2. short and branched 3. receptive portion of neuron 4. carries impulses TO cell body (soma)

Neuron Structure- Axons in PNS Click on diagram below for youtube video Large axons surrounded by myelin sheath produced by many layers of Schwann Cells neurilemma- cytoplasm & nucleus of Schwann Cell; outer layer of myelin sheath called “myelinated” nerve fiber myelin = lipoprotein interruptions in the myelin sheath between Schwann Cells = Nodes of Ranvier Small axons NO myelin sheath called “unmyelinated” nerve fibers still associated with Schwann Cell, just not wrapped by sheath (**All axons in the PNS are associated with Schwann Cells. )

Neuron Structure- Axons in CNS Myelin is produced by an oligodendrocyte rather than a Schwann Cell in the CNS. White Matter • appears white in color • contains myelinated axons Gray Matter • appears gray in color • contains unmyelinated structures cell bodies, dendrites, unmyelinated axons. LACK NEURILEMMA…

Multiple Sclerosis Click here for youtube video • The neurons in the brain and spinal cord lose their myelin coating in various sites becomes inflamed, eventually destroyed, leaving hard scars (scleroses) that block the underlying neurons from transmitting action potentials Muscles no longer receiving input stop contracting, then atrophy ”Short-circuiting” in one part of the brain may affect fine coordination, vision, speech, etc. Cause is not known for sure…maybe a virus? • Diagnosed by MRI scans (track development of lesions) • First symptoms: blurred vision and numb legs or arms (may be intermittent, so hard to diagnose) • Usual onset: 20 -40 (earliest age is 3, latest is 67) • Towards the end- paralysis leading to death • 300, 000 people in US are affected • Women 2 x more likely to develop MS than men • Caucasians higher risk Treatments • no cure • bone marrow transplant • interferon (anti-viral drug) • hormones

• Myelin begins to form on axons during the 14 th week of gestation • At time of birth, many axons are not completely myelinated • All axons have begun to develop myelin sheaths by the time a child starts to walk • Myelination continues into adolescence ------------------------------------------------------- • Excess myelination • Tay-Sachs Inherited, autosomal recessive disorder Myelin accumulates, burying neurons in fat (lysosomal enzyme disorder) Symptoms show by 6 months, death by about 4 years of age Gradually lose sight, hearing, muscle function Genetic screening possible Fewer than 10 children born with this per year In a healthy neuron, top, lysosomes act as the waste processing center of the cell. In Tay. Sachs disease, genetic deficiencies hobble lysosome enzymes that break down fatty cell products, also known as gangliosides, which build up and destroy the cell.

STRUCTURAL Classification of Neurons a) Multipolar • many dendrites • most neurons of CNS • one axon b) Bipolar • one dendrite • one axon • eyes, ears, nose c) Unipolar • one process from soma • Axon: forms central process (enters brain or spinal cord) & peripheral process (associated with dendrites near peripheral body part) • cell bodies of some aggregate to form specialized masses of nerve tissue called ganglia located outside the brain and spinal cord

FUNCTIONAL Classification of Neurons DIRECTION OF NERVE IMPULSE TRANSMISSION 1) Sensory Neurons • found in the PNS • afferent neurons (TO CNS) • carry sensory impulse from sensory receptors to CNS • Location of sensory receptors: 2) Interneurons (Association) skin and sense organs • found in CNS • • link most are unipolar neurons together • (ex: some are bipolar sensory neuron—interneuron—motor neuron • multipolar 3) Motor Neurons • found in PNS • efferent neurons (AWAY from CNS) • multipolar • carry impulses away from CNS to effectors (muscles and glands)

Types of Neuroglial Cells -Embryo: guide neurons to their positions; stimulate them to specialize -Nourish neurons; remove ions and neurotransmitters that accumulate between neurons. -Provide bulk of brain and spinal cord tissue -Uncontrolled growth is the cause of most brain tumors Astrocytes • CNS, star-shaped • nourishes neurons; • forms scar tissue (fills spaces/closes gaps) • mop up excess ions • metabolize glucose • induce synapse formation • connect neurons to blood Schwann Cells • PNS (only one in PNS) • myelinating cell Ependyma Oligodendrocytes • CNS; “eyeball” • epithelial-like layer; • myelinating cell cuboidal and columnar cells; (no neurilemma) ciliated • lines spaces in CNS: Microglia -central canal of spinal cord • CNS; “spider” -ventricles of brain • allows diffusion btwn CSF • structural suppor • phagocytic cell and nervous tissue

Regeneration of A Nerve Axon 1. Cell body injury = death of neuron (cannot divide to create new neuron) 2. Damage to an axon may allow for regeneration -proximal portion may survive, distal portion (after injury) degenerates (macrophages remove) -nerve growth factors from neuroglial cells help proximal axon regenerate -Schwann Cells proliferate along length of new axon growth Important the two cut ends reattach as quickly as possible. If the gap exceeds 3 mm, the regenerating axons may form a tangled mass called a neuroma. It is composed of sensory axons and painfully sensitive to pressure. Neuroma may complicate limb amputation

Click on diagram below to go to online tutorial to learn about the PHYSIOLOGY OF NERVE CELLS. Work through all 4 tutorials before moving on to the next slide. http: //www. getbodysmart. com/ap/nervoussystem/neurophysiology/menu. html

Nerve Impulses What are they? An action potential represents the START of a NERVE IMPULSE in one small portion of the neuron’s membrane. Now we will talk about how the action potential is transmitted throughout the entire neuron… NERVE IMPULSE • The propagation of action potentials along a nerve fiber (entire length of the neuron). • It is an electrical impulse. • Similar to a row of dominos falling once first falls, the entire row will fall • A nerve impulse begins on a dendrite (or cell body of a neuron), runs toward the cell body, through the http: //www. youtube. com/watch? v=DJe 3_3 Xs. BOg

Nerve Impulses Characteristics 1. All-or-none response if a nerve cell responds at all, it responds completely a. subthreshold stimulus (ex. 5 m. V)= no AP (action potential), no NI b. threshold stimulus (15 m. V) = yes AP, yes NI c. >threshold stimulus (ex. 20 m. V)= yes AP, yes NI, but no greater intensity than 15 m. V 2. Refractory Period a. The period following a NI when threshold stimulus cannot produce another NI. b. The RMP must be restored before it can be depolarized again. (ex: dominos must be set up again in order to be knocked down again) 3. Impulse Conduction a. unmyelinated nerve fibers: NI must travel the length of the nerve fiber; SLOW b. myelinated nerve fibers: NI jumps from node of Ranvier to node of Ranvier. VERY FAST “Saltatory Conduction”

The Synapse Nerve impulses pass from neuron to neuron at synapses. Synapse The junction btwn 2 neurons where a nerve impulse is transmitted. 1. Occurs between the axon of one neuron and the dendrite or cell body of a second neuron. 2. The two neurons do NOT touch! There is a gap between them called a synaptic cleft.

Synaptic Transmission animation link here Neurotransmitters are released when impulse reaches synaptic knob 1. NI reaches axonal terminal of presynaptic neuron causing depolarization of synaptic knob 2. Ca ion channels open and calcium ions rush into axonal terminal causing synaptic vesicles (filled with neurotransmitter (NT)) to release NT via exocytosis into the synaptic cleft. 3. NT diffuses across synaptic cleft and depolarizes the post-synaptic neuron’s membrane. 4. An action potential is triggered and NI begins in the post-synaptic neuron. Pre-synaptic terminal = axon Post-synaptic terminal= cell body or dendrite

Synaptic Potentials 1. Post-synaptic neurons response to NT binding: a) May be depolarized= Excitatory Post-Synaptic Potential (EPSP) b) May be hyperpolarization= Inhibitory Post-Synaptic Potential (IPSP) 2. Summation= many subthreshold stimuli received one after another may allow threshold potential to be reached, trigger an AP and begin a NI on a neuron. a) +15 m. V = threshold = AP = NI b) +5 m. V, +5 m. V = +15 m. V = threshold = AP = NI EPSP • excitatory postsynaptic potential • graded • depolarizes membrane of postsynaptic neuron • action potential of postsynaptic neuron becomes more likely IPSP • inhibitory postsynaptic potential • graded • hyperpolarizes membrane of postsynaptic neuron • action potential of postsynaptic neuron becomes less likely • EPSPs and IPSPs are added together in a process called summation • More EPSPs lead to greater probability of action potential

Neurotransmitters 1. At least 30 different NT produced by CNS 2. Some neurons produce/release only one while others release many 3. Most typical NT is ACh (Acetylcholine) released by all motor neurons (stimulate skeletal muscles)

Fate of Neurotransmitters in Synaptic Cleft Both of the following processes prevent continual stimulation of the post-synaptic membrane. Destruction of NT Enzymes that are present in the synaptic cleft destroy NT EX: acetylcholinesterase destroys ACh Reuptake of NT NT is transported back into pre-synaptic knob

Disorder Alzheimer’s Clinical Depression Epilepsy Huntington’s Disease Hypersomnia Insomnia Mania Myasthenia gravis Parkinson’s disease Schizophrenia SIDS Tardive dyskinesia (uncontrollable movements of facial muscles) Cause deficient ACh deficient norepinephrine/serotonin excess GABA leads to excess norepinephrine/dopamine deficient GABA excess serotonin deficient serotonin excess norepinephrine deficient ACh receptors at NMJ’s deficient dopamine deficient GABA leads to excess dopamine deficient dopamine

DRUG NT AFFECTED MECHANISM OF ACTION EFFECT Tryptophan Serotonin Stimulates NT synthesis sleepiness Reserpin Norepinephrine Packages NT vescicles limb tremors Curare ACh Decreases NT in NMJ muscle paralysis Valium GABA Nicotine ACh Stimulates synthesis of AChase increased alertness Cocaine Norepinephrine Blocks reuptake Euphoria Tricyclic Antidepressants Norepinephrine Blocks reuptake Mood elevation Enhances receptor decreased anxiety binding

Impulse Processing Neuronal Pools • groups of interneurons that make synaptic connections with each other • interneurons work together to perform a common function (results in facilitation-makes stimulation easier to achieve) • each pool receives input from other neurons, generates output to other neurons Convergence • neuron receives input from several neurons (summation occur) • typical of motor pathways • many inputs from brain, but usually one motor response Divergence • one neuron sends impulses to several neurons (signal amplies) • impulse from a single neuron in CNS may be amplified to activate enough motor units needed for muscle contraction