neurones and muscles - medcol.mwcms.medcol.mw/cms_uploaded_resources/1919_2.pdf · 2011-01-06 ·...

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.