neurones and muscles - medcol.mwcms.medcol.mw/cms_uploaded_resources/1919_2.pdf · 2011-01-06 ·...
TRANSCRIPT
Primary tissues 3
Neurones and muscles
Learning objectives
• 1, name the 4 primary tissues of the body
• 2, name the parts of a neurone
• 3, describe the function of a neurone
• 4, match the parts of a neurone to their function
• 5, recognise the structure of nerves
• 6, recognise the interaction between neurones and muscle
• 7, recognise the structure of muscle
• 8, identify the 3 types of muscle
• 9, identify the specific features that charachterise / are typical of the 3 types of muscle
Four Levels of Organization
Nervous TissueNeurons (nerve cells):
Specialized for action potential conduction.
Dendrites:
- Receive input.
Cell body:
- Nucleus.
- Metabolic center.
- Integrate inputs.
Axon:
- Conducts nerve impulses; output.
Nervous Tissue
• Detects stimuli, integrates information,
and relays commands for response
• Consists of excitable neurons and
supporting neuroglial cells
Neurons
• Excitable cells
• When stimulated, an electrical impulse
travels along the plasma membrane
• Arrival of the impulse at the neuron endings
triggers events that stimulate or inhibit
adjacent neurons or other cells
Muscle Tissues
Specialized for contraction.
3 types of muscle tissue:- Skeletal.
- Cardiac.
- Smooth.
Skeletal and cardiac muscle have similar mechanisms of contraction.
Musculoskeletal system
• The muscles themselves are made up of
• 1, muscle tissue [ muscle cells = muscle fibres ]
• 2,connective tissue
• 3, nerves
• 4, blood vessels
Striated muscle (skeletal)
Skeletal Muscles
Striated.
Voluntary.
Attached to bones by tendons.
Fibers arranged in parallel.
Produce graded contractions.
Cardiac Muscles
Striated.
Found only in the heart.
Co-ordinated contractions.
Intercalated discs couple cells together.
Smooth Muscles
Not striated.
Automatic.
Internal organs.
Muscle Tissues
The Neuron• The human body contains billions of neurons
– Basic functional unit of the nervous system• Specialized cells conduct electrical impulses along the plasma
membrane
– Nerve impulse: a series of action potentials
• Characteristics– Longevity – can live and function for a lifetime
– Do not divide – fetal neurons lose their ability to undergo mitosis; neural stem cells are an exception
– High metabolic rate – require abundant oxygen and glucose
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Parts of the Neuron• Cell Body. Aka Soma or Perikaryon
– Contains nucleus and nucleolus– Major biosynthetic center– No centrioles (hence its amitotic nature)
– Nissl bodies = chromatophilic substance = rough E.R: primary site of protein synthesis.
– Tapers to form axon hillock– Cytoskeleton of neurofilaments and
neurotubules• Dendrites: short, often highly branched.
– Receptive regions of the neuron• Axons. Long cytoplasmic process capable of
propagating a nerve impulse– Can branch to form collaterals.
– Axon hillock: Initial segment: beginning of axon
– Axoplasm - cytoplasm of the axon– Axolemma - membrane of the axon– Presynaptic terminals (terminal boutons)– Synaptic vesicles
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Parts of the Neuron
Axoplasmic Transport
• Anterograde:
– Axoplasm moved from cell body toward terminals.
– Supply materials for growth, repair, renewal.
– Can move cytoskeletal proteins, organelles away from cell body toward axon terminals.
• Retrograde
– Away from axonal terminal toward the cell body
– Damaged organelles, recycled plasma membrane, and substances taken in by endocytosis can be transported up axon to cell body.
– Rabies and herpes virus can enter axons in damaged skin and be transported to CNS.
Types of Neurons
Neurons Classified by Function
• So please note from above, when we talk about an interneurone, we are not talking about a different structural type of neurone i.e. We are not talking about it’s shape, we are talking about it’s functional type – it’s function, what it actually does. Which is to connect neurones.
• The complexity and amazing capacity of the nervous system largely does not come from differences in the shapes of cells but from the connections between cells and the circuits that are formed.
The Synapse• Junction between two neurons
• Site where nerve impulse in one cell cause excitation in the next cell
• Types of cells in synapse– Presynaptic neuron - conducts impulse toward the synapse
– Postsynaptic neuron - conducts impulse away from the synapse
• Two major types of synapses– Electrical synapses - rare in the CNS
– Chemical synapses - most common type in CNS
• Synapses between a neuron and its effector:– Neuromuscular junction - between a neuron and a muscle
– Neuroglandular synapse - between a neuron and a gland
Electrical Synapses• Gap junctions that
allow local current to flow between adjacent cells. Connexons: protein tubes in cell membrane that allow flow of ions from one cell to the next.
• Found in cardiac muscle and many types of smooth muscle. Action potential of one cell causes action potential in next cell, almost as if the tissue were one cell.
Chemical Synapse• Presynaptic bulb has secretory
vesicles that contain neurotrans-mitter chemical (NT)
• NT must pass across the synaptic cleft, space that separates pre- and postsynaptic membranes
• Postsynaptic membrane contains receptors specific for each type of NT
• Binding of NT to its receptorcauses ion channels to open or close
• Postsynaptic membrane is thus either stimulated or inhibited
Nerves
• Nerves - bundles of axons– If only sensory axons, called sensory nerves– If only motor axons, called motor nerves– If both sensory and motor axons, called mixed nerves
• Connective Tissue Coverings– Endoneurium – layer of delicate connective tissue
surrounding the axon• Nerve fascicles – groups of axons bound into bundles
– Perineurium – connective tissue wrapping surrounding a nerve fascicle
– Epineurium – whole nerve is surrounded by tough fibrous sheath
Structure of a Nerve
A. Note the similarity of a nerve to a muscle
1. Just as a muscle is a collection of muscle fibers, a nerve is a collection of nerve fibers (axons).
2. Each is broken up in smaller units known as fascicles
3. Each is covered by connective tissue:• Epimysium vs. Epineurium• Perimysium vs. Perineurium• Endomysium vs. Endoneurium
• The skeletal muscle fiber
– Fibers are long and cylindrical
• Are huge cells – diameter is 10–100µm
• Length – several centimeters to dozens of centimeters
– Each cell formed by fusion of embryonic cells
– Cells are multinucleate
– Nuclei are peripherally located
Part of a skeletal Muscle Fiber
Cardiac Muscle
• Present only in the heart
• Cells are striated and
branching
• Ends of cells are joined by
communication junctions
Cardiac Muscle 1000X
intercalated disc
striations
short branching cells; intercalated discs at cell junctions
nucleus
Smooth Muscle
• In walls of many internal
organs and some blood
vessels
• Cells are not striped and
taper at the ends
• Muscles can cause either locomotion of the organism itself or movement of internal organs, that includes contraction -> decreased diameter of the organ that causes movement of the substance within it while the organ maintains it’s original position
• Cardiac and smooth muscle contraction occurs without conscious thought and is necessary for survival.
• i.e. contraction of the heart to keep blood flowing and peristalsis which pushes food through the digestive system.
Organization of Connective Tissues
• Muscles have 3 layers of connective tissues:
1. Epimysium-Exterior collagen layer
• Connected to deep fascia
• Separates muscle from surrounding tissue
2. perimysium- Surrounds muscle fiber bundles (fascicles)
• Contains blood vessel and nerve supply to fascicles
3. endomysium
3. Endomysium
• Surrounds individual muscle cells (muscle fibers)
• Contains capillaries and nerve fibers contacting muscle cells
Muscle Attachments
• Endomysium, perimysium, and epimysium come together:
– at ends of muscles
– to form connective tissue attachment to bone matrix
– i.e., tendon (bundle) or aponeurosis (sheet)
• A tendon has connective tissue all going in one direction / aligned in one direction,
• An aponeurosis has the connective tissue aligned in all directions
• Skeletal muscle cells are called fibers
Organization ofSkeletal Muscle Fibers
Level 1: Skeletal Muscle
Level 2: Muscle Fascicle
Level 3: Muscle Fiber
Figure 10–6 (3 of 5)
Level 4: Myofibril
Myofibrils- 1-2um in diameter • Lengthwise subdivisions within muscle fiber
• Made up of bundles of protein filaments (myofilaments)
• Myofilaments - are responsible for muscle contraction
2 Types of Myofilaments• Thin filaments:
– made of the protein actin
• Thick filaments: – made of the protein myosin
Muscle Contraction
• Is caused by interactions of thick and thin filaments
• Structures of protein molecules determine interactions
Skeletal Muscle Contraction
Fiber Shortening
• As sarcomeres shorten, muscle pulls together, producing tension
Skeletal Muscle Innervation
The Neuromuscular Junction
• Is the location of neural stimulation
• Action potential (electrical signal):– travels along nerve axon
– ends at synaptic terminal
Synaptic Terminal• Releases neurotransmitter (acetylcholine or
ACh)
• Into the synaptic cleft (gap between synaptic terminal and motor end plate)
Innervation of Skeletal Muscle
What are the structural and functional differences between
skeletal muscle fibers and cardiac muscle cells?
Cardiac muscle is striated, found only in the heart
Characteristics of Cardiocytes
– are aerobic (high in myoglobin, mitochondria)
– have intercalated discs
Intercalated Discs
• Are specialized contact points between cardiocytes
• Join cell membranes of adjacent cardiocytes (gap junctions, desmosomes)
Functions of Intercalated Discs
• Maintain structure
• Enhance molecular and electrical connections
• Conduct action potentials
Coordination of Cardiocytes
• Because intercalated discs link heart cells mechanically, chemically, and electrically, the heart functions like a single, fused mass of cells
4 Functions of Cardiac Tissue
1. Automaticity:
– contraction without neural stimulation
– controlled by pacemaker cells
2. Variable contraction tension:
– controlled by nervous system
3. Extended contraction time
4. Prevention of wave summation and tetanic contractions by cell membranes
Role of Smooth Muscle in Body Systems
• Forms around other tissues
• In blood vessels:– regulates blood pressure and flow
• In reproductive and glandular systems:– produces movements
• In digestive and urinary systems:– forms sphincters
– produces contractions
• In integumentary system:– arrector pili muscles cause goose bumps
What are the structural and functional differences between
skeletal muscle fibers and smooth muscle cells?
Structure of Smooth Muscle
• Nonstriated tissue
Comparing Smooth and Striated Muscle
• Different internal organization of actin and myosin
• Different functional characteristics
8 Characteristics ofSmooth Muscle Cells
1. Long, slender, and spindle shaped
2. Have a single, central nucleus
3. Have no T tubules, myofibrils, or sarcomeres
4. Have no tendons or aponeuroses
8 Characteristics of Smooth Muscle Cells
5. Have scattered myosin fibers
6. Myosin fibers have more heads per thick filament
7. Have thin filaments attached to dense bodies
8. Dense bodies transmit contractions from cell to cell
Smooth Muscle Tone
• Maintains normal levels of activity
• Modified by neural, hormonal, or chemical factors
• While skeletal muscles are arranged in regular, parallel bundles, cardiac muscle connects at branching, irregular angles (called intercalated discs).
• Striated muscle contracts and relaxes in short, intense bursts, whereas smooth muscle sustains longer or even near-permanent contractions.