Nervous and Muscle Tissue: Structure, Function, and Physiology

Nervous Tissue and the Nervous System

Divisions of the Nervous System

The nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). Each division has specialized functions and components.

• Central Nervous System (CNS): Consists of the brain and spinal cord; processes and integrates information.

• Peripheral Nervous System (PNS): Includes nerves outside the CNS; connects the CNS to the body.

• Afferent Division: Carries sensory information to the CNS.

• Efferent Division: Sends motor commands from the CNS.

• Somatic Division: Voluntary control of skeletal muscles.

• Autonomic Division: Involuntary control of organs and glands.

  • Sympathetic Division: "Fight or flight" response.

  • Parasympathetic Division: "Rest and digest" response.

Neuroglia (Glial Cells)

Neuroglia are supporting cells in the nervous system, each with specific roles.

• Microglia: Immune defense in the CNS.

• Ependymal cells: Produce cerebrospinal fluid (CNS).

• Schwann cells: Form myelin sheaths in the PNS.

• Satellite cells: Support neurons in the PNS.

Structure of the Neuron

Neurons are the functional units of the nervous system, specialized for communication.

• Soma: Cell body containing the nucleus.

• Axon hillock: Connects soma to axon; triggers action potential.

• Synaptic cleft: Space between neurons for neurotransmitter release.

• Axoplasm: Cytoplasm inside axon.

• Axolemma: Membrane surrounding axon.

• Axon terminal: End of axon; releases neurotransmitters.

Myelin Sheath

The myelin sheath is a fatty layer that insulates axons and speeds up action potential transmission.

• Oligodendrocytes: Form myelin in CNS.

• Schwann cells: Form myelin in PNS.

• Node of Ranvier: Gaps in myelin for action potential to jump.

• Internode: Myelinated segment between nodes.

Resting Membrane Potential

Neurons maintain a resting membrane potential, which is essential for electrical signaling.

• Resting potential: $-70\ \text{mV}$

• Inside of neuron is negative relative to outside.

• Polarized state maintained by Na+/K+ pump.

Changes in Membrane Potential

Movement of ions across the membrane alters the membrane potential.

• Depolarization: Cation influx; inside becomes less negative.

• Hyperpolarization: Anion influx; inside becomes more negative.

Action Potential Phases and Values

• Resting: $-70\ \text{mV}$

• Threshold: $-55\ \text{mV}$

• AP Peak: $+30\ \text{mV}$

Graded Potential vs. Action Potential

Electrical signals in neurons can be graded or all-or-none.

• Graded potential: Small, local changes; can sum.

• Action potential: Large, all-or-none; propagates along axon.

Propagation of Action Potentials

• Saltatory propagation: AP jumps from node to node (myelinated).

• Continuous propagation: AP travels the entire axon (unmyelinated).

Synapses and Neurotransmitters

• Chemical synapse: Neurotransmitter used to transmit signal.

• Presynaptic neuron: Sends signal.

• Postsynaptic neuron: Receives signal.

• Neurotransmitter: Chemical messenger.

• Absolute refractory period: Neuron cannot fire another AP.

Acetylcholine (ACh)

Acetylcholine is the first identified and best understood neurotransmitter in the body.

• Released at neuromuscular junctions and many synapses.

Events in Action Potential Generation

1. Neurotransmitter binds to receptor on postsynaptic membrane.

2. Ion channels open → graded potential.

3. Depolarization reaches threshold → AP initiated.

4. AP propagates along axon.

Muscle Tissue

Types of Muscle Tissue

Muscle tissue is specialized for contraction and movement. There are three main types:

Type	Location	Features
Skeletal	Attached to bones	Multinucleated, striated, voluntary
Cardiac	Heart	1-2 nuclei, striated, intercalated discs, involuntary
Smooth	Walls of organs	1 nucleus, non-striated, involuntary

Characteristics of Muscle Tissue

• Excitable

• Contractile

• Extensible

• Elastic

Muscle Structure

• Epimysium: Surrounds whole muscle.

• Perimysium: Surrounds fascicles.

• Fascicle: Bundle of muscle fibers.

• Endomysium: Surrounds individual fibers.

• Myofibril: Contractile organelle.

• Contractile unit: Sarcomere.

Muscle Fiber Anatomy

• Sarcoplasmic reticulum: Stores calcium.

• Sarcoplasm: Cytoplasm of muscle fiber.

• T-tubule: Helps propagate AP into fiber interior.

Myofilaments and Regulatory Proteins

Filament Type	Main Protein	Regulatory Proteins
Thick	Myosin	None
Thin	Actin	Troponin, Tropomyosin

Neuromuscular Junction

• Neuromuscular junction: Location where muscle meets motor neuron.

• Neurotransmitter released: Acetylcholine (ACh).

• Synaptic cleft: Space between axon terminal and muscle fiber.

Muscle Contraction: Steps and Mechanism

1. Neural stimulation of motor neuron → ACh release.

2. AP propagates along sarcolemma.

3. Calcium released from SR binds troponin.

4. Cross-bridge cycle occurs → contraction.

Depolarization: Occurs when inside of membrane becomes more positive.

Detailed Steps of Skeletal Muscle Contraction

• AP reaches axon terminal → ACh released → binds receptor → Na+ influx → depolarization → AP travels along sarcolemma → Ca2+ released from SR → Ca2+ binds troponin → exposes binding sites on actin → myosin binds actin → contraction cycle.

Rigor Mortis

Rigor mortis: Stiffening of muscles after death due to lack of ATP.

Types of Muscle Contractions

• Isotonic: Muscle changes length.

  • Concentric: Shortens.

  • Eccentric: Lengthens.

• Isometric: Muscle length stays the same.

Motor Unit

• One motor neuron and all muscle fibers it controls.

Factors Affecting Muscle Contraction Strength

• Number of fibers stimulated

• Size of fibers

• Frequency of stimulation

• Degree of stretch

Muscle tone: Slight contraction keeping muscle firm.

ATP Regeneration in Muscle

1. Creatine phosphate: $\text{CP} + \text{ADP} \rightarrow \text{ATP}$ (cytoplasm)

2. Glycolysis: $\text{Glucose} \rightarrow 2\ \text{ATP} + \text{pyruvate}$ (cytoplasm)

3. Aerobic respiration: $\text{Glucose} + O_2 \rightarrow 36\ \text{ATP}$ (mitochondria)

Muscle Fiber Types

• Slow oxidative: Endurance, aerobic activities.

• Fast oxidative: Moderate activities.

• Fast glycolytic: Short bursts, strength/sprinting.

Smooth Muscle vs. Skeletal Muscle Contraction

• No sarcomeres in smooth muscle.

• Smooth muscle contraction is slower and involuntary.

Hypertrophy and Atrophy

• Hypertrophy: Increase in muscle size.

• Atrophy: Decrease in muscle size.

Smooth Muscle Contraction Steps

1. Ca2+ enters cytoplasm.

2. Ca2+ binds calmodulin.

3. Activates myosin light chain kinase.

4. Myosin binds actin → contraction.

Intercalated Discs

• Connections between cardiac cells that allow AP to spread.

Clinical Conditions Affecting ACh Receptors

• Myasthenia gravis

• Curare poisoning