Central, Peripheral, Autonomic Nervous Systems

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1 /11 Ababan, Abanilla, Abellar, Agustin, Amora

Central, Peripheral and Autonomic Nervous Systems

11 August 2014

Dra. Vivien Fe Fadrilan-Camacho 1

Nervous System I. Nervous System a. Physiology b. Structural Classification II. Central Nervous System a. Functional Anatomy of the Brain b. Protection of the CNS c. Spinal Cord III. Peripheral Nervous System a. Sensory/Afferent Subdivision b. Motor/Efferent Subdivision c. Structure of a Nerve d. Cranial Nerves e. Spinal Nerves IV. Nervous Tissue a. Neuroglia b. Neurons

I. NERVOUS SYSTEM

- master controlling and communicating system of the body - maintains body homeostasis with electrical signals; - provides sensation, higher mental functions, and emotional response; and - activate muscles and glands - communicates by electrical and chemical signals FUNCTIONS: 1. Uses sensory receptors to monitor changes occurring inside and outside the body. - Sensory input ! gathered info by the receptors 2. Processes and interprets sensory input and decides what should be done at each moment. (Integration) - Input may be stored as a memory or be ignored - Input may produce an immediate response 3. Activates effector organs which cause a response called motor output.

Figure 1 Functions of the Nervous System

STRUCTURAL CLASSIFICATION OF THE NERVOUS SYSTEM

Figure 2 Structural Classification of the Nervous System

II. THE CENTRAL NERVOUS SYSTEM - consists of the brain and the spinal cord - acts as the integrating and command centers of the nervous systems. - interprets sensory input and dictates motor responses based on past experience, reflexes, and current conditions

BRAIN - the largest and most complex mass of nervous tissue in the body. - two fistful of quivering pinkish gray tissue, wrinkled like a walnut, and somewhat the consistency of cold oatmeal - Males and females have equivalent brain sizes (in terms of brain mass per body mass) FOUR MAJOR REGIONS OF THE BRAIN A. CEREBRAL HEMISPHERE

- two halves of the brain that was divided on the middle lengthwise.

- Each hemisphere is divided into FOUR lobes SULCI – shallow grooves GYRI – elevated ridges of tissue FISSURES- deeper groves; less numerous LONGITUDINAL FISSURE- single deep fissure that separates the cerebral hemisphere

- Other fissures or sulci divide each cerebral hemisphere into a number of LOBES.

Central, Peripheral, and Autonomic Nervous Systems

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1 Ababan, Abanilla, Abellar, Agustin, Amora

LOBES OF THE BRAIN • FRONTAL LOBE

– in front of the central sulcus (major groove on the cerebrum) - reasoning, planning, parts of speech and movement (motor cortex), emotions, and problem solving

• TEMPORAL LOBE – below the lateral fissure - perception and recognition of auditory stimuli

(hearing) and memory (hippocampus)

• PARIETAL LOBE – behind the central sulcus - perception of stimuli related to touch, pressure, temperature, and pain

• OCCIPITAL LOBE - back of the brain, behind the parietal and temporal lobe - many aspects of vision

• INSULA - a fifth lobe on the cerebral hemisphere that is

buried deep within the lateral fissure[Human Anatomy and

Physiology 9th edition by Marieb and Hoehn]

Figure 3 Location of the insula lobe

Each cerebral hemisphere has three basic regions: a superficial CORTEX of gray matter, an internal WHITE MATTER, and the BASAL NUCLEI.

1. CEREBRAL CORTEX

Function: executive suite - thought - voluntary movement - language - reasoning - perception - It has three functional areas: motor areas, sensory areas, and association areas

Figure 4 Cerebral Cortex SOMATOTOPY – mapping of the body in the CNS. The entire body is represented spatially in the primary motor cortex of each hemisphere - most neurons in this gyri control muscles in body areas having the most precise motor control—face, tongue, and hands HOMONCULI – large motor regions of the caricature-like image - Motor innervations of the body are CONTRALATERAL. Left gyrus control right muscles of the body and vice versa.

Figure 5 Functional Organization of the Cerebrum

PRIMARY SOMATIC SENSORY AREA - located in the parietal lobe; this is where impulses traveling from the body’s sensory receptors (except for the special senses) are localized and interpreted PRIMARY MOTOR AREA - located in the frontal lobe; allows us to consciously move our skeletal muscles. Axons of these motor neurons form the major voluntary motor tract- the CORTICOSPINAL OR PYRAMIDAL TRACT, which descends to the cord


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2. CEREBRAL WHITE MATTER - composed of fiber tracts carrying impulses to, from, or within the cortex.

CORPUS CALLOSUM - connects the cerebral hemispheres. It allows the cerebral hemispheres to communicate with each other.

ASSOCIATION FIBER TRACTS - connect areas within a hemisphere.

PROJECTION FIBER TRACTS - connect the cerebrum with lower CNS centers.

3. BASAL NUCLEI - islands of gray matter; also referred to as basal ganglia - helps in regulation of voluntary motor activities by modifying instructions sent to the skeletal muscles by the primary motor cortex. B. DIENCEPHALON - also known as interbrain, sits atop the brain stem and is enclosed by the cerebral hemispheres. MAJOR STRUCTURES OF THE DIENCEPHALON 1. THALAMUS - encloses the shallow third ventricle -receives sensory information and relays this information to the cerebral cortex (It is the neurons of the sensory cortex that actually localize and interpret the sensation) - cerebral cortex sends information to the thalamus which then transmits this info to other areas of the brain and spinal cord Fan: Sensory processing and movement 2. HYPOTHALAMUS - makes up the floor of the diencephalon. - center for many drives and emotions, and such it is an important part of the so-called LIMBIC SYSTEM or “emotional- visceral brain” Fan: autonomic control center, center for emotional response, body temperature regulation, regulation of food intake, regulation of water balance and thirst, regulation of sleep-wake cycle, control of endocrine system functioning

PITUITARY GLAND -hangs from the anterior floor of the hypothalamus by a slender stalk. MAMMILARY BODIES –“little breasts”; reflex centers involved in olfaction

3. EPITHALAMUS - forms the roof of the third ventricle. - Important parts of the epithalamus are the PINEAL BODY (influenced by light-dark cycle/ sleep-wake cycle) and the CHOROID PLEXUS of the third ventricle.

C. BRAIN STEM - has many small gray matter areas; provides a pathway for ascending and descending tracts 1. MIDBRAIN -conduction pathway between higher and lower brain centers (i.e. cerebral peduncles contain the fibers of the pyramidal tracts) - its superior and inferior colliculi are visual and auditory reflex centers - substantia nigra and red nuclei are subcortical motor centers - contains nuclei for cranial nerves III and IV

CEREBRAL AQUEDUCT- tiny canal that travels through the midbrain and connects that third ventricle of the diencephalon to the fourth ventricle below CEREBRAL PEDUNCLES- convey ascending and descending impulses CORPORA QUADREGEMINA- four rounded protrusions that are dorsally located

2. PONS -rounded structure that protrudes just below the midbrain -means “bridge” and it is mostly composed of fiber tracts -conduction pathway between higher and lower brain centers; pontine nuclei relay information from the cerebrum to the cerebellum - its respiratory nuclei cooperate with the medullary respiratory centers to control respiratory rate and depth; - houses nuclei of cranial nerves V-VII 3. MEDULLA OBLONGATA - conduction pathway between higher brain centers and spinal cord - site of decussation of the pyramidal tracts - houses nuclei of cranial nerves VIII - XII - contains nuclei cuneatus and gracilis (synapse points of ascending sensory pathways transmitting sensory impulses from skin and proprioceptors), and visceral nuclei controlling heart rate, blood vessel diameter, respiratory rate, vomiting, coughing, etc.

Figures 6 and 7 The Medulla Oblongata RETICULAR FORMATION - group of nuclei scattered throughout the brain stem - maintains cerebral cortical alertness (reticular activating system) and filters out repetitive stimuli - its motor nuclei help regulate skeletal and visceral muscle activity


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RETICULAR ACTIVATING SYSTEM (RAS) -plays a role in consciousness and the awake/sleep cycles. Damage to this area can result in permanent unconsciousness.

D. CEREBELLUM -has two hemispheres and a convoluted surface. -it has an outer cortex made up of gray matter and an inner region of white matter. - provides the precise timing of the skeletal muscle activity and controls our balance and equilibrium. - processes info from cerebral motor cortex and from proprioceptors and visual and equilibrium pathways, and provides “instructions” to cerebral motor centers that result in proper balance and posture and smooth, coordinated skeletal muscle movements PROTECTION OF THE CENTRAL NERVOUS SYSTEM 1. MENINGES - membranes that cover the brain and spinal cord. -There are three connective tissue membranes covering and protecting the CNS structures:

A. DURA MATER “tough or hard mother” - double-layered membrane where it surrounds the brain. PERIOSTEAL LAYER- attached to the inner surface of the skull. MENINGEAL LAYER- forms the outer most covering of the brain and continues as the dura mater of the spinal cord.

*The dural layers are fused together except in three areas where they separate to enclose dural venous sinuses that collect venous blood.

B. ARACHNOID MATER -middle meningeal layer - looks like a cobweb. -its threadlike extensions span the subarachnoid space to attach it to the innermost membrane. SUBARACHNOID SPACE- filled with cerebrospinal fluid ARACHNOID VILLI- specialized projections of the arachnoid membrane that protrudes through the dura mater. -through this structure the cerebrospinal fluid is absorbed into the venous blood in the dural sinuses. C. PIA MATER - innermost membrane

Figure 8 Meninges of the Brain 2. CEREBROSPINAL FLUID

- returns to the blood in the dural venous sinuses through the arachnoid villi. - found in and around the brain and spinal cord, forms a liquid cushion that gives buoyancy to the CNS structure - By floating the jellylike brain, the CSF effectively reduces brain weight by 97% and prevents the brain from crushing under its own weight - protects the brain and spinal cord from trauma - Ordinarily, CSF forms and drains at a constant rate so that its normal pressure and volume (150 mL- half a cup) are maintained.

Figure 9 Cerebrospinal Fluid Location

3. THE BLOOD- BRAIN BARRIER - Neurons are kept and separated from blood borne

substances by the so- called blood-brain barrier, composed of the least permeable capillaries in the whole body.

- Of water-soluble substances, only water, glucose, and essential amino acids pass easily through the walls of these capillaries.

- The blood- brain barrier is virtually useless against fats, respiratory gases, and other fat-soluble molecules that diffuse easily through all plasma membranes.


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SPINAL CORD - enclosed in the vertebral column. - extends from the foramen magnum of the skull to the level of the first or second lumbar vertebra, just inferior to the ribs. - 42cm (17inches) long and 1.8cm (3/4inch) thick - glistening-white spinal cord provides a two-way conduction pathway to and from the brain. - major reflex center. - protected by bone, meninges, and cerebrospinal fluid. - the dural and arachnoid membranes extend to the level of S2, well beyond the end of the spinal cord.

Figure 10 The Spinal Cord - 31 spinal cord segments in a human spinal cord

• 8 cervical segments forming 8 pairs of cervical nerves • 12 thoracic segments forming 12 pairs of thoracic

nerves • 5 lumbar segments forming 5 pairs of lumbar nerves • 5 sacral segments forming 5 pairs of sacral nerves • 3 coccygeal segments joined up becoming a single

segment forming 1 pair of coccygeal nerves - the main pathway for information connecting the brain and peripheral nervous system - 31 pairs of spinal nerves attached to the cord by paired roots Table 1 Spinal Nerves Functions

SC Level FUNCTION C1 – C6 Neck flexors C1 – T1 Neck extensors C3 – C5 Supply diaphragm (mostly C4)

C5 – C6 Shoulder movement, raise arm (deltoid);

flexion of elbow (biceps); C6 externally rotates the arm (supinates)

C6 – C7 Extends elbow and writs (triceps and wrist

extensors); pronates wrist C7 – T1 Flexes wrist C7 – T1 Supply small muscles of the hand T1 – T6 Intercostal and trunk above the waist

T7 – L1 Abdominal muscles L1 – L4 Thigh flexion L2 – L4 Thigh adduction L4 – S1 Thigh abduction L5 – S2 Extension of leg at the hip (gluteus maximus)

Figure 11 The Spinal Column with Vertebrae Dermatomes – somatic or musculotaneous areas served by fibers from specific spinal nerves Referred pain – caused when the sensory fibers from an internal organ enter the spinal cord in the same root as fibers from a dermatome. So pain in the heart is often interpreted as pain in the left arm or shoulder

KEY TERMS:

• Ganglion – a collection of cell bodies located outside the CNS; contain the cell bodies of sensory neurons entering the cord at that region.

• Nerve – group of fibers (axons) outside the CNS; contain the fibers of the sensory and motor neurons. A nerve does not contain cell bodies. They are located in the ganglion (sensory) or in the gray matter (motor).

• Tract – a group of fibers inside the CNS; carry information up or down the spinal cord, to or from the brain. Tracts within the brain carry information from one place to another within the brain. Tracts are always part of white matter.

• Gray matter – area of unmyelinated neurons where cell bodies and synapses occur. In the spinal cord, the synapses between the sensory and motor and interneurons occurs in the gray matter. Cell bodies of


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interneurons and motor neurons are found in the gray matter.

• White matter – area of myelinated fiber tracts. Myelination in the CNS differs from that in nerves.

SPINAL TRACTS

- located in the white matter. - travel up and down the spinal cord. - many of these travel to and from the brain to provide sensory input, or bring motor stimuli to control effectors. - Ascending tracts, which travel towards the brain, are sensory. - Descending tracts, which travel towards the spinal cord, are motor.

Proprioception – the perception of body position and muscle position necessary for coordinating movements. “Body sense” and “Muscle sense”.

Figure 12 Spinal Tracts Ascending: Fasciculus gracilis and Fasciculus cuneatus

- often called the posterior white columns. - carry discriminative touch, conscious proprioception, deep touch, pressure, and vibration

Posterior and Anterior spinocerebellar tracts - connects spinal cord and cerebellum - carry subconscious proprioceptive stimuli.

Lateral and Anterior spinothalamic tracts - lead to the thalamus. - pathway for crude touch, pain, temperature, pressure, tickle and itch sensations

Descending (Direct) Lateral and Anterior corticospinal tracts

- carry voluntary motor stimuli from the cerebral cortex to motor neurons in the spinal cord. - also called pyramidal tracts because some of them cross in the pyramids of the medulla. -Lateral: for skilled movements esp. of the hands -Anterior: for movement of trunk muscles

Descending (Indirect) - termed as a group as “extra-pyramidal tracts” - generally associated with balance and muscle tone.

Rubrospinal – movement coordination Anterior and Lateral Reticulospinal – posture adjustment Vestibulospinal – posture and balance Tectospinal tracts – in response to visual reflexes Olivospinal tract

III. PERIPHERAL NERVOUS SYSTEM - includes all neural structures outside the brain and spinal cord. - sensory receptors, peripheral nerves and associated ganglia and efferent motor endings. - its main function is to connect the CNS to the limbs and organs.

SUBDIVISIONS:

1. AFFERENT (SENSORY) DIVISION - Afferent = carrying toward - carry impulses from sensory receptors to CNS. - keeps the CNS informed of what is happening inside and outside of the body. - Somatic afferent fibers = conveys impulses from skin, skeletal muscles and joints. (soma = body) - Visceral afferent fibers = conveys impulses from visceral organs

2. EFFERENT (MOTOR) DIVISION - Efferent = carrying away - transmits impulses from the CNS to muscles and glands (efferent organs). - activates muscles to contract and glands to secrete. - brings about a motor response. TWO PARTS:

A. Somatic Nervous System -composed of somatic motor nerve fibers that conduct impulses from CNS to skeletal muscles. - voluntary nervous system B. Autonomic Nervous System -composed of visceral motor nerve fibers that regulate the activity of smooth muscles, cardiac muscles and glands.

- involuntary nervous system

Two functional subdivisions: 1. Parasympathetic - stimulates; survivability functions 2. Sympathetic -inhibits; fight or flight response

AUTONOMIC NERVOUS SYSTEM

• Autonomic means "a law unto itself“ • The involuntary nervous system • Motor division of the peripheral nervous system that

controls visceral activities, with the goal of maintaining internal homeostasis.


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• Innervates smooth and cardiac muscle and glands • In response to changing conditions, the ANS shunts

blood to "needy" areas. • With two functional subdivisions, the sympathetic and

the parasympathetic, which typically work in opposition to each other-what one subdivision stimulates, the other inhibits.

• Efferent pathway = preganglionic and postganglionic neurons

• Preganglionic neurons, arise in the CNS and run to a ganglion in the body. Here they synapse with postganglionic neurons, which run to the effector organ (cardiac muscle, smooth muscle, or a gland).

Figure 13 Parasympathetic and Sympathetic effects on the body

" The sympathetic division is most active in times of

stress, and the parasympathetic division controls maintenance activities and helps conserve the body’s energy.

SYMPATHETIC NERVOUS SYSTEM

• The "fight-or-flight" system • evident when we are excited or find ourselves in

emergency or threatening situations

Involved in physical activity, excitement, emergency by: • Increasing heart rate and blood pressure • Dilating respiratory passageways • Stimulating perspiration • Stimulates the release of glucose from the liver • Inhibits digestive activities

" All preganglionic fibers of the sympathetic division arise

from cell bodies of preganglionic neurons in spinal cord segments Tl through L2 (thoracolumbar division)

" The axons of the preganglionic neurons exit and project to either:

- sympathetic chain ganglia (chain along the spinal cord)

- Collateral ganglia – nearer target organs (celiac, superior mesenteric and inferior mesenteric ganglia)

Unique roles of the sympathetic division:

1. Thermoregulatory response to heat • Mediates reflexes that regulate body temp. • applying heat to skin -> reflexive dilation of blood

vessels -> inc. systemic body temperature -> sympathetic nerves -> dilatation of skin’s blood vessels -> flushed skin -> activate the sweat glands -> perspiration

2. Release of renin from the kidneys • Sympathetic impulses stimulate the kidneys to

release renin, an enzyme that promotes an increase in blood pressure

3. Metabolic effects • Increases the metabolic rate of body cells • Raises blood glucose levels • Mobilizes fats for use as fuels

PARASYMPATHETIC NERVOUS SYSTEM

• Resting and digesting system • Conserves body energy and maintains body activities

at basal levels • Digestion, defecation, urination • Slows HR and RR • Constriction of the pupil

" Axons synapse with ganglionic neurons located in

terminal ganglia that lie very close to or within the target organs

" Preganglionic fibers run in the oculomotor, facial, glossopharyngeal, and vagus cranial nerves.

" Their cell bodies lie in associated motor cranial-nerve nuclei in the brain stem and spinal nerves S2-S4 (craniosacral division)

Figure 14 Cranial and Sacral Outflow


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CRANIAL OUTFLOW Oculomotor nerves (III)

• Innervate smooth muscles in the eyes for pupil constriction and the lenses for focusing

• The cell bodies of the ganglionic neurons are in the ciliary ganglia within the eye orbits

Facial nerves (VII)

• The preganglionic fibers synapse with pterygopalatine ganglia -> activate the nasal glands and the lacrimal glands of the eyes

• The preganglionic neurons that stimulate the submandibular and sublingual salivary glands originate in the pons and synapse with submandibular ganglia

Glossopharyngeal nerves (IX)

• Originate in the inferior salivatory nuclei of the medulla and synapse in the otic ganglia - activate the parotid salivary glands

• Cranial nerves III, VII, and IX supply the entire parasympathetic innervation of the head

Vagus Nerves (X)

• Account for about 90% of all preganglionic parasympathetic fibers

• Supplies motor parasympathetic fibers to all the organs except the suprarenal (adrenal) glands, from the neck down to the second segment of the transverse colon

• Responsible for heart rate, gastrointestinal peristalsis, sweating,

• Muscle movements in the mouth, including speech (via the recurrent laryngeal nerve) and keeping the larynx open for breathing

SACRAL OUTFLOW

• Innervates the distal half of the large intestine, urinary bladder; ureters, and reproductive organs

Table 2 Summary of Parasympathetic and Sympathetic Nervous System

Figure 15 Automatic Nervous System Ganglionic Fibers

NEUROTRANSMITTERS Acetylcholine (ACh) – released by all ANS preganglionic

axons - released by all parasympathetic

postganglionic axons - released by sympathetic postganglionic

fibers innervating sweat glands and some blood vessels in skeletal muscles

- CHOLINERGIC (ACh releasing fibers) Norepinephrine (NE) – released by all sympathetic

postganglionic axons (with some exceptions above)

- ADRENERGIC (NE releasing fibers) STRUCTURE OF A NERVE

- a nerve is a group of fibers (axons) outside the CNS • Axon – generates impulses and topically conducts

them away from the cell body. • Myelin sheath – insulating covering of the axon. • Endoneurium – connective tissue made up of

endoneurial cells that enclose a nerve fiber. • Fascicles – a small bundle of nerve fibers • Perineurium – encloses a bundle of fascicles. • Epineurium – outermost covering of a nerve.

Figure 16 Structure of a Nerve


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CRANIAL NERVES • Oh, Oh, Oh, To Touch And Feel A Girl’s Very Soft Hands • Some Say Marry Money, But My Brother Says Big

Business Makes Money (Sensory/Motor/Both)

Table 3 Cranial Nerves and Their Function Cranial Nerve Major Function

I Olfactory (S) Smell II Optic (S) Vision III Oculomotor (M) Eyelid and eyeball movement IV Trochlear (M) Innervates superior oblique,

Turns eye downward and laterally

V Trigeminal (B) Chewing, Face and mouth, touch and pain

VI Abducens (M) Turns eye laterally VII Facial (B) Controls most facial

expressions, Secretion of tears and saliva, Taste

VIII Vestibulocochlear or Auditory (S)

Sense of balance and hearing

IX Glossopharyngeal (B) Throat movement, taste (back 1/3 of the tongue)

X Vagus (B) Sensation in the throat and windpipe; movement of muscles of the throat, windpipe, organs of the chest and abdomen

XI Spinal Accessory (M) Neck muscle movement XII Hypoglossal (M) Movement of tongue muscles SPINAL NERVES

- 8 cervical nerves; 12 thoracic nerves; 5 lumbar nerves; 5 sacral nerves; 1 pair of coccygeal nerves

Figure 17 The Brain in Your Gut

ENTERIC NERVOUS SYSTEM • A meshwork of nerve fibers that innervate the viscera

(gastrointestinal tract, pancreas, and gall bladder). • Linked to the CNS by afferent visceral fibers and by

sympathetic and parasympathetic branches (motor fibers) of the ANS

IV. NERVOUS TISSUE

2 principal types: 1) Neuroglia – supporting cells that insulate and protect the delicate neurons. 2) Neurons – transmit electrical signals. A) NEUROGLIA In CNS: a) Astrocytes - star-shaped - most abundant/most versatile - control chemical environment around neurons b) Microglia - “scavenger” - spider-like with long “thorny” processes - turns into a macrophage that phagocytizes neuronal debris. c) Ependymal cells - “wrapping garment” - glial cells that line the central cavities of the brain and the spinal cord

- they are squamous (protection) or columnar (absorption); ciliated

- beating of cilia helps circulate CSF d) Oligodendrocytes - glia that wrap their flat extensions (processes) tightly around the nerve fibers

- same like astrocytes but with fewer processes. In PNS a) Schwann cells - regeneration of damaged peripheral nerve fibers - also called neurolemmocytes b) Satellite cells - act as protective, cushioning cells

Figure 18 Glial Cells and their Function


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B) NEURONS • they are amitotic • have extreme longevity • high metabolic rate • requires continuous and abundant supplies of Oxygen

and Glucose. Parts of a Neuron: 1) Cell body - metabolic center of the neuron. - contains abundant Nissl bodies (Rough ER) and neurofibrils (intermediate filaments) 2) Dendrites - convey incoming messages 3) Axons

- generate nerve impulses and typically conduct them away from the cell body.

4) Axon hillock - cone-like region - connects cell body to axon. 5) Presynaptic terminals - swollen, distal end of an axon. - contains a neurotransmitter substance within synaptic vesicles

- Synaptic ending or synaptic bouton Classification of Neurons Structural Classification Basis: the quantity of processes extending from neuron cell body

1. Unipolar – Single process extending from cell body – Very short and divides immediately into

proximal (central) and distal (peripheral) – Only small branches at the end of

peripheral process are dendrites; Remainder function as axons

– Sensory, *spinal cord 2. Bipolar

– 2 processes: 1 axon (conducts action potential to CNS) and 1 dendrite (receives specific stimulus)

– Rare (eyes and ears) – Act in sensory processing as receptor

cells 3. Multipolar

- 1 axon and many dendrites - Most common - Motor and interneurons

Functional Classification Basis: the direction the nerve impulse is travelling relative to the CNS

1. Sensory/Afferent Neurons - “To go toward” - Cell bodies are always found in a

ganglion outside CNS - Unipolar - Send information from sensory receptors

to CNS

2. Motor/Efferent Neurons - Multipolar - Cell bodies are always located in the

CNS - Send information to muscles or glands

from CNS 3. Interneurons or Association Neurons

- Multipolar - Link between the afferent and efferent

interneurons - Mostly found in CNS

Physiology Action Potential

- Principal way neurons send signals over long distances by generating and propagating AP’s

- Generated by cells with excitable membranes – neurons and muscle cells - Brief reversal of membrane potential with 100mV (from -70mV to +30mV)

Depolarization repolarization hyperpolarization (short period)

- Over in a few milliseconds - Myelinated: faster - Unmyelinated: slower - Polarized: plasma membrane of a resting neuron - Major positive ion inside the cell: Potassium - Major positive ion outside the cell: Sodium As long as the inside remains more negative than the outside, the neuron will stay inactive.

ACTION POTENTIAL PROCESS 1. Resting State (Polarized)

- Internal face of membrane is slightly negative due to a higher concentration of Na+ ions outside the neuron

2. Depolarization - Stimulus (initiator) changes the

permeability of a local “patch” of the membrane

- Na+ ions diffuse into the cell causing the internal face more positive

3. Repolarization - Reverses the membrane polarity - K+ ions exit the axon

4. Hyperpolarization - Chain Reaction - Influx of sodium depolarizes the axon

and the outflow of potassium repolarized the axon

- Decrease in membrane potential


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Sodium-potassium Pump - Stabilizes the resting membrane potential by maintaining

the concentration gradients for sodium and potassium ions Threshold and the All-or-None Phenomenon - Either action potential is generated or not - Depolarization must reach threshold values if an axon is to “fire” - Threshold (dictator) – membrane potential at which the outward current created by K+ movement is exactly equal to the inward current created by Na+ movement - Usually reached when membrane has been depolarized by 15-20mV from resting value Saltatory Conduction - Jump jump! - Allows AP to propagate faster and more efficiently - Occurs in myelinated nerve fibers (Na+ channels are found at the Nodes of Ranvier) Synapse - “Clasp or join”, a junction or a connection between two terminals - Mediates information transfer (neuron neuron OR neuron

effector cell)

Transmission of Signal at the Synapses - AP opens voltage gated calcium channels allowing calcium influx into the axon terminal synaptic vesicles fuse with presynaptic membrane neurotransmitter diffuses and attaches to receptors Ion channels open as a result of the binding of neurotransmitter, which causes graded potentials enzymes destroy neurotransmitter Ion channels close

Figure 19 The Synapse http://anthropology.net/2008/01/20/dopamine-transporter-gene-and-primate-social-behavior/neuron-synapse/ Reflexes - Involuntary reactions in response to a stimulus - Reflex Arc: neural pathway by which a reflex occurs - Smallest and simplest pathway capable of receiving a stimulus and yielding a response - Dorsal root (receptor) to Ventral root (effector)

Steps 1. Stimulus + receptor 2. Sensory neuron activation 3. CNS processing 4. Motor neuron activation

Figure 20 The Reflex Arc http://www.tooloop.com/nervous-system-reflex-arc/nervous-system-reflex-arc/ *All reflex arcs have a minimum of five elements: sensory receptor, effector organ, sensory and motor neurons and integration center.” Somatic Reflexes - Reflexes that stimulate skeletal muscles Autonomic Reflexes

- Regulate activity of smooth muscles, the heart and glands - Secretion of saliva, changes in the size of eye pupils - Digestion, elimination, blood pressure and sweating

REFERENCE Marieb, E. & Hoehn, K. (2011). Essentials of Human

Anatomy and Physiology (9th ed.). Pearson Education Inc.

Marieb, E. & Hoehn, K. (2012). Human Anatomy and Physiology (9th ed.). Pearson Education Inc. VanPutte, C., Regan, J. & Russo, A (2013). Seeley’s

Essentials of Anatomy & Physiology (8th ed.). McGraw-Hill ed.

Central, Peripheral, Autonomic Nervous Systems

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