2 cells of the nervous system

Neurons and Glial cells

Neurons

Basic signaling units of the nervous system -Dendrites, cell body, axon, axon terminals

Glial cells

- Support and protect neurons - Don't produce electrical signals 4 CNS types and 2 PNS types

4 Glial cells of the CNS

1. Oligodendrocytes 2. Astrocytes 3. Microglia 4. Ependymal Cells

Oligodendrocytes

Support and insulate axons by forming myelin -CNS

Astrocytes

-Help maintain homeostasis by taking up K+ and water -Provide neurons with substrates for ATP production -Form the blood brain barrier -Large portion of cells in the brain (50% of brain) -CNS

Microglia

Specialized immune cells in CNS; remove damaged cells and invaders since white blood cells can't pass the BBB -CNS

Ependymal cells

Creates the lining of the ventricles and make cerebrospinal fluid -CNS

2 Glial cells in the PNS

1. Schwann 2. Satellite

Schwann cells

-Insulate axons by forming myelin (speeds up signal) -Wraps around axon -PNS

Nodes of Ranvier

Gaps between Schwann cells (myelin sheaths)

Satellite cells

-Non-myelinating -Form supportive capsules around nerve cell bodies in ganglia (collection of nerve cell bodies outside CNS) -PNS

3 types of neurons

Sensory, motor and interneurons

Sensory, motor and interneurons

Sensory - conducts signals TO the spinal cord and brain and are AFFERENT neurons Motor - conduct signals FROM the spinal cord and brain and target an effector therefore are EFFERENT neurons Interneurons - connective and associative functions located in the brain and spinal cord

Cell body, dendrites, and axon

Cell body - contains genetic information and organelles and integrates postsynaptic potentials Dendrites - Receive messages from other neurons or from sensory receptors (input signal) Axon - Conduct electrical impulses from axon to the terminal buttons (output signal)

Resting membrane potential

The potential of an inactive neuron -70 mV

Mechanically gated ion channels

-Found in sensory neurons -Open in response to physical forces such as pressure or stretch

Voltage gated ion channels

-Respond to changes in the cell’s membrane potential -Voltage-gated Na+ and K+ channels are important in the initiation and conduction of electrical signals along the axon -Voltage required to open a channel depends on the channel type -Speed of opening also varies among channels -Na+ and K+ channels both activated by depolarization Na+ opens quickly K+ opens slowly -Most channels opened by depolarization close with repolarization

Ligand/Chemically gates ion channels

-Open when a specific ligand binds -Most neurons respond to a variety of ligands *Neurotransmitters *Neuromodulators *Intracellular signalling molecules, etc.

Action potential

- All or none response. A threshold must be reached in order for an action potential to proceed -Depolarization – Na rushes into cell making interior more positive -Repolarization – K channels open, K moves out of the cell and some positive charge is lost -Hyperpolarization – too much K moves out because their channels remain open for longer

Refractory period

Makes a nerve impulse travel in only one direction (unidirectional propagation) -Voltage-gated Na⁺ channels are inactivated. No matter how strong a stimulus is, another action potential cannot occur. - Some Na⁺ channels have reset, but K⁺ channels are still open. A stronger-than-normal stimulus is needed to trigger another action potential -The depolarization spreads in both directions electrically, but the region behind the action potential is in its refractory period

Saltatory conduction

the way electrical signals leap from gap to gap along a neuron wrapped in myelin, speeding up signal transmission and making it more energy efficient - Helps current stay inside the neuron

Electrical Synapses

-Synchronizes activity within a network of cells -Electrical signal passed from cytoplasm of one cell to another through pores of gap junction proteins -Rapid conduction of signals -Found in neurons of the CNS, cardiac & smooth muscle, and some non-excitable cells (e.g. Pancreatic β-cells)

Chemical Synapses

-Represents the majority of synapses -Neurocrine molecules carry signal from one cell to the next -Electrical signal of presynaptic cell converted to chemcial signal -Signal molecules cross synaptic cleft and bind to a receptor on the target cell

Synapses

The region where an axon terminal meets its target cell is called a synapse. It is filled with extracellular matrix whose fibers hold the presynaptic and postsynaptic cells in position -The neuron that delivers a signal to the synapse is known as the presynaptic cell,and the cell that receives the signal is called the postsynaptic cell -The narrow space between two cells iscalled the synaptic cleft

Neurotransmitters

a chemical substance released by a neuron to transmit a signal to another cell across a synapse - The same neurotransmitter can produce different responses depending on the receptor it activates - One neurotransmitter can act on multiple receptor types

What 3 things does a neurotransmitter effect depend on?

Which receptor is present Where the receptor is located How the receptor signals inside the cell

Neurotransmitter release process

1. Action potential depolarizes the axon terminal 2. The depolarization opens voltage gated Ca2+ channels and Ca2+ enters the cell 3. Ca entry triggers exocytosis of synaptic vesicle contents in the presynaptic neuron 4. Neurotransmitter diffuses across the synaptic cleft and binds with receptors on the postsynaptic cell, triggering a cellular response

Neurotransmitter termination process

1. Neurotransmitters can be returned to axon for reuse or transported into glial cells 2. Enzymes inactivate neurotransmitters 3. Neurotransmitters can diffuse out of synaptic cleft

Neurotransmitter receptors

-Receptor types can differ between tissues -Different receptors can activate different signaling pathway, so one neurotransmitter can have different (even opposite) effects in different tissues

Acetylcholine (ACh)

A neurotransmitter that is the chief neurotransmitter of the parasympathetic nervous system, the part of the autonomic nervous system that contracts smooth muscles, dilates blood vessels, increases bodily secretions, and slows heart rate. Acetylcholine can stimulate a response or block a response and thus can have excitatory or inhibitory effects

Types of receptors of acetylcholine and their effects on target tissue

-Nicotinic ACh receptors cause muscle contraction on target skeletal muscle -Muscarinic M2 receptors cause a decrease in heart rate (heart is target) -Muscarinic receptors cause contraction on smooth muscle targets - Muscarinic receptors can increase secretion on gland targets

2 divisions of the Nervous System

Central nervous system (CNS) and the peripheral nervous system (PNS)

CNS

- Brain and spinal cord - Processes, integrates, and stores information - Composed of neurons and supportive glial cells -Interneurons are those neurons completely contained within the CNS. -The tissues of the CNS are divided into gray matter and white matter. -Bundles of axons that connect different regions of the CNS are known as tracts - Neurons don't regenerate

Gray and white matter

-Gray matter consists of unmyelinated nerve cell bodies, dendrites, and axons. Has dorsal and ventral horns -White matter is mostly myelinated axons and contains very few neuronal cell bodies. Its pale color comes from the myelin that surround the axons.

2 horns of grey matter

Dorsal - deal with incoming information Ventral - carry efferent signals for motor neurons (going to muscles or glands)

What 4 things support the CNS

-Skull (cranium) protects the brain -Spinal cord runs through canal in the vertebral column -Nerves of PNS enter and leave spinal cord by passing through notches between the vertebrae - Meninges are three layers of connective tissue with fluid between that protect the CNS (dura mater, arachnoid, and pia mater)

3 CNS membranes (meninges) and 1 additional protective component

1. Dura mater 2. Arachnoid 3. Pia mater 1. Extracellular fluid

Dura mater (tough mother)

Thickest of the 3 membranes - It is associated with veins that drain blood from the brain through vessels or cavities called sinuses.

Arachnoid

Middle layer that is loosely tied to the inner membrane, leaving a subarachnoid space between the two layers.

Pia mater

The inner membrane that is a thin membrane, it adheres to the surface of the brain and spinal cord. Arteries that supply blood to the brain are associated with this layer

2 types of extracellular fluid in the CNS and their roles

It helps cushion the delicate neural tissue Cerebrospinal fluid - form the extracellular environment for neurons, found in the ventricles and in the space between the pia mater and the arachnoid space Interstitial fluid - form the extracellular environment for neurons, found inside the pia mater - cerebrospinal and interstitial fluid compartments communicate with each other across leaky junctions

2 main regions of the spinal cord

1) Cervical 2) Thoracic 3) Lumbar 4) Sacral

Nerve arrangement in the spinal cord

-Each spine segment has a bilateral pair of spinal nerves -Nerve divides into roots before spinal nerve joins spinal cord (dorsal and ventral roots)

Central and outer portions of the spinal cord

Central portion = grey unmyelinated neurons Outer portion = white myelinated neurons

2 roots of the spinal cord

•Dorsal root – carries incoming sensory info •Ventral root – carries info from CNS to muscles/ glands

Reflex arc (spinal reflexes)

Do not require sending info to the brain Sensory receptors sense stimulus - impulse is received by sensory neurons - stimulus is relayed to interneurons in the spinal cord - interneurons relays impulse to motor neurons - motor neuron carries impulse from the CNS (spinal cord) to the effector - gland or muscle produces a response

3 Sections of the brain

Forebrain, midbrain, and hindbrain

Metabolic requirements of the brain

-Nerves need constant supply of oxygen and glucose required to make ATP -15% of blood pumped by heart goes to brain -Glucose is main source of energy

Hypoglycemia

-Low blood glucose leads to confusion, unconsciousness and death

Structures in the hindbrain

Medulla Pons Cerebellum

Medulla Oblongata's role and what it contains

- Controls heart rate & respiration -Thoroughfare for sensory information Contains: -Somatosensory tracts Bring sensory info to brain -Corticospinal tracts Convey info from cerebrum to spinal cord -Neurons cross over -Area postrema lacks BBB to induce vomiting

Pons

Regulates sleep and arousal

Cerebellum

Coordinates fine muscle movement and is involved with balance

Midbrain role

-Important structure for visual and auditory processes - Reticular formation Sleep and arousal (attention) Connects lower and higher brain centres

Forebrain structures

Thalamus Hypothalamus Pituitary Basal Ganglia

Thalamus

-Sensory switchboard -Routes sensory information to appropriate place EX) visual goes to visual centres; auditory to auditory centres

Hypothalamus and 8 roles it has

-Center for homeostasis -Outputs affect behavioural drives, autonomic nervous system, and endocrine functions 1.Activates sympathetic nervous system Controls stress hormone release from adrenal medulla Helps maintain blood glucose 2. Maintains body temperature Stimulates shivering and sweating 3.Controls body osmolarity Motivates thirst and drinking behaviour -Stimulates vasopressin secretion 4. Controls reproductive functions Directs secretion of oxytocin, FSH, and LH 5) Controls food intake Stimulates satiety and feeding center 6) Interacts with limbic system Influences behaviour and emotions 7) Influences cardiovascular control center Effects on heart rate/breathing rate/blood pressure 8) Secretes trophic hormones Controls release of hormones from anterior pituitary

Pituitary gland and its roles

-Master gland that releases several hormones -Sexual development and behaviour -Metabolism -Stress

Hypothalamic-hypophyseal portal system

- a specialized network of blood vessels that connects the hypothalamus to the anterior pituitary gland - Hormones released from hypothalamus into capillaries, delivered to pituitary

Cerebrum and its role

Its the largest part of the brain made up of the frontal, parietal, temporal, and occipital lobes - Involved in conscious perception and voluntary motor control

Frontal, parietal, temporal, and occipital lobes role

Frontal - speech and motor cortex Parietal - somatosensory association and reading Temporal - smell, hearing, and auditory association Occipital lobe - Vision and visual association

Cerebral grey matter's 3 regions

Cerebral cortex, basal ganglia, and the limbic system

Cerebral cortex

- outermost layer of gray matter - 3mm thick - Convoluted (folded and wrinkled) structure enables more tissue to fit (increases surface area)

Basal Ganglia

Group of structures in the center of the brain that form important connections -Critical for voluntary motor control -Deliberate initiation of movement

Limbic system

A group of structures in the brain that regulate emotions, behaviour, motivation and memory - 3 structures

Limbic system structures and their roles in the system

-Hippocampus Forming and retrieving memories -Amygdala Fear and aggression -Nucleus Accumbens Reward and motivation

Corpus callosum

a bundle of nerve fibers that allow your brain’s left and right hemispheres to communicate. - plays a role in how you think, remember and coordinate your movements

Simple pathway in the CNS

Sensory input - integration - output - response

Complex pathway in the CNS

Sensory system (reflex) - CNS behavioural state system OR Cognitive system (voluntary) - Motor system output - physiological or behaviour response

Motor cortex

-Controls movement on opposite side of body -Controls movement of over 600 voluntary muscles -Located at rear of frontal lobe

Somatosensory Cortex

-Receives specific sense information from opposite side of body -At least one specific area for each sense -Located in parietal lobe

3 types of motor output (efferent division of the nervous system)

1) Skeletal muscle movement (somatic motor division) 2) Neuroendocrine signals (hypothalamus & adrenal medulla) 3) Visceral responses (autonomic division – smooth & cardiac muscle; endocrine/exocrine glands)

Skeletal Muscle Movement

-Controls voluntary and reflex movements -Uses the somatic motor system -Motor neurons directly activate skeletal muscle Examples: Walking, Reaching, Maintaining posture, Withdrawal from a painful stimulus

Neuroendocrine Signals

-Nervous system controls hormone release into the bloodstream -Primarily involves  hypothalamic neurons and the pituitary gland - Hormones travel through the blood to affect distant target tissues Examples: Hypothalamus → pituitary → hormone release Regulation of stress responses Control of growth, reproduction, and metabolism

Visceral Responses

-Regulates internal organs and body functions -Uses the autonomic nervous system -Controls smooth muscle, cardiac muscle, glands Examples: Changing heart rate, Adjusting blood, vessel diameter, Digestive activity, Sweating

PNS

-All neural structures outside the CNS (sensory receptors, motor receptors) - Relays sensory info to the CNS - 2 divisions

2 divisions of the PNS

Somatic and autonomic

Autonomic nervous system (ANS) of the PNS

- Automatically (involuntarily) regulates the body's internal organs, heart rate, digestion, breathing and pupil size - Has 3 more divisions (sympathetic, parasympathetic, enteric)

3 divisions of the autonomic nervous system

-Sympathetic = fight or flight, will dominate in stressful situations EX) Heart and blood pressure increases, respiration accelerates Mediated by hypothalamus -Parasympathetic = rest and digest. Dominates when body is relaxed/resting EX) Heartbeat slows, blood pressure reduces, respiration levels return to normal -Enteric

Antagonistic Control

Two biological agents such as hormones or autonomic nerves producing opposite effects on the same target to maintain homeostasis EX) insulin vs. glucagon or sympathetic vs. parasympathetic activity. which one autonomic branch is excitatory and the other branch is inhibitory -The two autonomic branches are usually antagonistic in their control of a given target tissue, but they sometimes work cooperatively on different tissues to achieve a common goal. -In some autonomic pathways, the neurotransmitter receptor determines the response of the target tissue

Components of autonomic pathways

- General route nerve signal takes from CNS in autonomic nervous system. Can be excitatory or inhibitory -2 types (parasympathetic and sympathetic) 2 neurons in series from the CNS to the target CNS -> preganglionic neuron (releases ACh) -> autonomic ganglion -> postganglionic neuron -> target tissue -The two neurons synapse in an autonomic ganglion - Neuroeffector = the synapse between a postganglionic autonomic neuron and its target cell Targets of autonomic neurons are: Smooth muscle, cardiac muscle, exocrine, lymphoid tissues, the liver, adipose tissue

Sympathetic ganglia

-Originate in thoracic & lumbar regions of spine -Found primarily in two chains on either side of spine - closer to the spinal cord -Usually have short pre-ganglionic neurons and long post-ganglionic neurons - Pre-ganglionic neurons release ACh onto nicotinic receptors, and post-ganglionic neurons secrete norepinephrine onto adrenergic receptors

Parasympathetic ganglia

-Most originate in brain stem -Some originate in sacral region (control pelvic -organs) -Ganglia usually located on or near target organ (long pre-ganglionic and short post-ganglionic neurons) and farther from the spinal cord -Communicate mostly with head, neck and internal organs - Pre-ganglionic and post-ganglionic neurons release ACh onto nicotinic receptors

Sympathetic pathway

Fight or flight - Originates in the thoracic and lumbar spinal cord. - Ganglia are close to the spinal cord. - Usually has a short preganglionic neuron and a long postganglionic neuron. - Prepares the body for stress. - Pre-ganglionic neurons release ACh onto nicotinic receptors, and post-ganglionic neurons secrete norepinephrine onto adrenergic receptors 4 receptors: α, β1, β2, β3

Parasympathetic pathway

Rest and digest - Originates in the brain stem and sacral spinal cord. - Ganglia are on or near the target organ. - Usually has a long preganglionic neuron and a short postganglionic neuron. - Supports resting functions such as digestion - Pre-ganglionic and post-ganglionic neurons release ACh onto nicotinic receptors - Contain only muscarinic cholinergic receptors (five difference receptor subtypes to modulate response)

Sympathetic and parasympathetic receptor types

Sympathetic: α receptors – respond strongly to norepinephrine, but weakly to epinephrine β1 receptors – respond equally strongly to norepinephrine and epinephrine (heart) β2 – more sensitive to epinephrine (muscle, blood vessels) β3 – more sensitive to norepinephrine (mostly adipose tissue) Parasympathetic: Only muscarinic cholinergic receptors (five difference receptor subtypes to modulate response)

Autonomic synapses

Postganglionic autonomic axons contain varicosities which contain vesicles full of neurotransmitters - Neurotransmitters then diffuse over a larger tissue area, therefore it is less direct than somatic but it allows one autonomic neuron to influence a larger area

How is an autonomic signal pathway stopped?

-Enzymatic degradation: enzymes break down the chemical messenger in the extracellular space. -Cellular uptake: neighboring cells take up the chemical messenger. - Endocytosis of Receptor-ligand complex: the receptor and ligand are taken into the cell by endocytosis; the receptor can subsequently be returned to the membrane by exocytosis.

Vagus nerve

-Contains ~75% of all parasympathetic fibers -Carries sensory info (afferent) from organs to brain and parasympathetic output from brain to organs -Innervation goes primarily to the head, neck and internal organs

Catecholamines

Epinephrine and norepinephrine -Norepinephrine – secreted mostly by sympathetic nervous system -Epinephrine – secreted mostly by adrenal medulla Released directly into the blood (fight/flight) General distribution

Somatic motor division of the PNS

Controls skeletal muscles, allowing voluntary movement - Always excitatory - Uses ONE neuron between the CNS and the muscle -Cell bodies located either in ventral horn of spinal cord or in the brain -Single long axon projecting to skeletal muscle target -Branch close to their targets (terminating on muscle surface) -Single motor neuron controls many muscle fibers

Neuromuscular Junction and what it consists of

Synapse of motor neuron on muscle fiber consists of: 1) presynaptic axon terminal with synaptic vesicles (containing ACh) and mitochondria 2) synaptic cleft (space between neuron and muscle cell) where ACh diffuses across 3) post-synaptic membrane (motor end plate) of skeletal muscle fiber

Motor end plate

-On muscle cell membrane opposite axon terminal -Series of folds that look like shallow gutters -ACh receptors found along upper edge of each gutter -Collagen matrix in synaptic cleft holds axon terminal and motor end plate together

Somatic motor division pathway

CNS -> One long motor neuron axon -> neuromuscular junction -> skeletal muscle fiber

Somatic motor neurons

-Somatic motor neurons branch close to their targets. -Each branch divides into a cluster of enlarged axon terminals that lie on the surface of the skeletal muscle fiber -Branching structure allows a single motor neuron to control many muscle fibers at once - Cell bodies located in ventral horn of spinal cord or brain -Long axon projects to skeletal muscle

Muscle contraction step-by-step

1. Action potential arrives down axon and opens voltage-gated Ca2+ channels in axon terminal membrane 2. Ca2+ diffuses into cell down concentration gradient 3. Release of ACh from synaptic vesicles triggered 4. ACh diffuses across synaptic cleft, combines with nicotinic receptor channels on muscle membrane 5. Binding opens gate that allows Na+ entry into the muscle cell, causing depolarization 6. Resulting action potential causes contraction of muscle cell 7. ACh is removed from the area (3 ways) to end the activation

3 types of muscle tissue and one similarity between all 3

1) skeletal 2) cardiac 3) smooth All are striated

Skeletal muscle

-Attaches to the bones of the skeleton -Controls body movement and positions the skeleton -Contracts only in response to a signal from a somatic motor neuron (can’t initiate own contraction) -Not directly influenced by hormones -Voluntary -40% of body weight

Cardiac muscle

-Found only in the heart -Moves blood through the circulatory system - Shorter than skeletal muscle fibers, and sometimes branched - Cells have single nucleus -Fibers are electrically linked to one another - Intercalated discs connects muscle - Under sympathetic, parasympathetic, and hormonal control - Has pace-maker potential

Intercalated discs

specialized junction that connects individual cardiac muscle cells within the heart muscle.

Pacemaker potential

The slow rise in membrane potential -Some cells have oscillating membrane potential -Regular depolarizations always reach threshold & fire an action potential -Creates a regular rhythm of contraction

Smooth muscle

-Internal organs and tubes (stomach, bladder, blood vessels) -Influences movement of material into, out of, and within the body

Tendons

-Made of collagen -Dense connective tissue that attach muscle to bone Origin: end of muscle attached closest to trunk (or to the more stationary bone) -nsertion: the more distal or mobile element

Flexors and extensors

Flexors: brings the centre of connected bones closer together when it contracts Extensors: muscle that moves bones away from each other when it contracts

Antagonistic muscle groups

flexor-extensor pairs (one shortens when the other lengthens) EX) bicep- triceps (flexing = extensor triceps and bicep flexes)

Skeletal muscle fibers

-Fibers are long, cylindrical, multi-nucleated cells, found in bundles -Largest cells in the body, each sheathed in connective tissue - Made up of fascicles - Entire muscle has connective tissue sheath

Fascicles and what is between them

-Bundled groups of adjacent muscle fibers -Collagen, elastic fibers, nerves & blood vessels found between fascicles

Sarcolemma, Sarcoplasm, Sarcoplasmic reticulum

-Sarcolemma = cell membrane of muscle fiber -Sarcoplasm = cytoplasm of muscle cell -Sarcoplasmic reticulum = modified endoplasmic reticulum that wraps around each individual myofibril, and concentrates and stores Ca2+ so its important in muscle contraction

T-tubules

-Continuation of muscle fiber membrane (Sarcolemma) -Lumen of tubules is continuous with ECF -Allow action potentials to move rapidly from cell surface to interior of fiber

Myofibrils and what they consist of

-Muscle fiber contractile structures - >1000 myofibrils/muscle fiber 1) contractile proteins (actin & myosin) 2) regulatory proteins (tropomyosin & troponin) 3) accessory proteins (titin & nebulin)

Myosin

-Thick Filament -Has ability to create movement (motor protein) -Each molecule has protein chains that intertwine to form a long tail & a pair of heads -Heads have an elastic hinge region (allows them to swivel around point of attachment) -Each head has a heavy chain & smaller light chain (wrap around the neck to stabilize it) -Heavy chain has binding site for actin -Consist of ~250 myosin molecules -Heads are clustered at each end of the filament -Central region of filament is a bundle of myosin tails

Actin

-Thin filament -Consists of a double strand of actin twisted together -Also contains troponin and tropomyosin -Crossbridges are formed when myosin heads bind to actin (have binding site for myosin heads)

Sarcomere and what it contains

Arrangement of thick & thin filaments makes a pattern of light and dark bands Sacromere = one repeat of the pattern - 2 Z-disks, two I-bands, A-band, H-zone, and an M-line

Z-disks

zigzag protein structures that serve as attachment site for thin filaments

I-bands

regions only occupied by thin filaments (lightest colour bands)

A-band

darkest of sarcomere’s bands

H zone

central region of A band that contains only thick filaments (lighter than outer edges)

Titin

-Huge elastic molecule stretching from z-disk to m-line -Stabilizes the position of the contractile filaments -Elasticity returns stretched muscles to their resting length

Nubulin

-Inelastic protein (doesn’t stretch like titan) -Lies alongside thin filaments and attaches to Z-disks -Helps align actin filaments within the sarcomere

Isotonic Contraction and isometric Contraction

-Isotonic Contraction: creates force and moves a load EX) carrying groceries into the house -isometric Contraction: creates force without movement EX) Trying to lift heavy groceries but they aren’t lifting EX) Flexing

Muscle tension

force created by contracting muscle Load = weight or

Load

weight or force that opposes contraction of a muscle

Contraction

Creation of tension in a muscle -Active process requiring ATP -Shortening of sarcomeres (actin & myosin sliding past each other)

Relaxation

release of tension created by contraction

Steps in contraction

1) Events at the neuromuscular junction ACh released from motor neuron converted to electrical signal in the muscle fiber 2) Excitation-contraction (E-C) coupling Muscle action potentials initiate Ca2+ signals that activate a contraction relaxation cycle 3) Contraction-relaxation cycle Sliding filament theory

Sliding filament theory

the mechanism of muscle contraction in skeletal muscles. According to this theory, muscles contract when thin actin filaments slide over thick myosin filaments, resulting in the shortening of muscle fibres. The filaments themselves do not change in length; instead, the degree of overlap between actin and myosin increases.

Relaxed state of sliding filament theory

(no signal) -Ends of thick and thin (actin) filaments overlap slightly -Sarcomere has large I-band -A-band is the length of the thick (myosin) filament

Contracted state of sliding filament theory

Thick and thin filaments slide past each other Z-disks of sarcomere move closer together as sarcomere shortens I-band and H-zone almost disappear Length of A-band stays constant

Crossbridging

-Myosin heads bind to actin molecules -Ca2+ signal initiates the “power stroke” Myosin cross-bridges swivel and push the actin filaments towards the centre of the sarcomere -At the end of the power stroke, myosin releases actin and swivels back into position so that it can bind with a new molecule -Analogy – several hands pulling a rope -The heads don’t all release at the same time -Several “power strokes” occur during a single contraction

Power stroke

Force-producing step in muscle contraction where a myosin head pivots and pulls actin filament after releasing inorganic phosphate - begins when tropomyosin moves off binding site

Where does the energy for the power stroke come from?

Comes from ATP -Myosin ATPase converts ATP to ADP and Pi -Energy released is trapped as potential energy in the angle of the myosin head/long axis of myosin filament -Potential energy becomes kinetic energy during the power stroke

What happens to tropomyosin when at rest and when Ca2+ signal occurs (troponin and tropomyosin roles in muscle contraction)

-At rest, tropomyosin wraps around actin filaments and actin’s binding sites for myosin are partially covered -When Ca2+ signal occurs: Ca2+ binds to troponin and pulls tropomyosin away from the myosin binding sites -Myosin head binds, crossbridges carry out power stroke to move actin filament

Contractile cycle

1) Power stroke (begins when tropomyosin moves off the binding site), myosin binds 2) Myosin releases ADP; myosin tightly bound to actin until another ATP binds, head pivots 3) ATP binds and myosin detaches from actin 4) ATP hydrolyzed provides energy to rotate the myosin head into the cocked position (weak cross-bridge is formed)

3 ways of classifying smooth muscle

1) Location: each location has slightly different physiology & function 2) Contraction pattern (2 patterns) 3) By their communication with neighboring cells (2 ways)

6 major locations of smooth muscle

Vascular Gastrointestinal Urinary Respiratory Reproductive Ocular

2 patterns of smooth muscle contraction

A) Phasic smooth muscle: undergo periodic contraction and relaxation cycles E.g. Lower esophagus, GI tract B) Tonic smooth muscle: always maintain some level of contraction (tone) E.g. Urinary bladder sphincter

2 ways smooth muscle communicates with neighbouring cells

-Single unit smooth muscle: Cells electrically connected by gap junctions Contract as a coordinated unit -Multi-unit smooth muscle: Cells not electrically linked Each cell functions independently Each cell associated with an axon terminal varicosity Allows fine control of contraction (e.g. Iris of the eye)

Differences between smooth and skeletal muscle

Lots of differences on slides 26-32 Lecture 11

Cardiac muscle contraction

1 Current spreads through gap junctions to contractile cell 2 Action potentials travel along plasma membrane and T tubules 3 Ca2+ channels open in plasma membrane and SR 4 Ca2+ induces Ca2+ release from SR 5 Ca2+ binds to troponin, exposing myosin-binding sites 6 Crossbridge cycle begin (muscle fiber contracts) 7 Ca2+ is actively transported back into the SR and ECF 8 Tropomyosin blocks myosin-binding sites (muscle fiber relaxes)

Differences between skeletal and cardiac muscle

Slides 37-40 Lecture 11