Bio review

Topic 1: Muscular System

Overview of Muscles

Muscles play a crucial role in supporting movement and joints within the body. There are three main types of muscles:

  1. Cardiac Muscles
       - Location: Found in the walls of the heart.
       - Movement Type: Involuntary (not under conscious control).

  2. Smooth Muscles
       - Location: Present in the walls of hollow organs, such as the stomach.
       - Movement Type: Involuntary.

  3. Skeletal Muscles
       - Location: Attached to bones, as well as in facial muscles or skin.
       - Characteristics: Cigar-shaped, striated, and multinucleate (possessing multiple nuclei).
       - Movement Type: Voluntary (under conscious control).

Connective Tissue in Skeletal Muscles

Skeletal muscle cells are surrounded and bundled by connective tissue.

Function of Connective Tissue

Connective tissue serves various roles:

  • It connects cells, tissues, and organs.

  • It links bones to muscles, thereby facilitating movement.

Types of Connective Tissue Bundling Skeletal Muscles

Skeletal muscles are organized by three types of connective tissue, varying in location within the muscle, and most are anchored to tendons and bones.

Hierarchical Structure of Skeletal Muscle

The organization of skeletal muscle can be divided as follows:

  1. Epimysium:
       - The outer layer that wraps around the whole skeletal muscle.

  2. Perimysium:
       - Surrounds groups of muscle fibers, known as fascicles.

  3. Fascicle:
       - A bundle of muscle fibers.

  4. Endomysium:
       - Encloses a single muscle fiber.

  5. Muscle Fiber:
       - Also known as a muscle cell.

  6. Myofibril:
       - An organelle within the muscle fiber, composed of sarcomeres.

  7. Sarcomere:
       - The contractile unit of a muscle fiber, comprised of actin and myosin (myofilaments/protein filaments).

Myofilaments of Skeletal Muscle

  1. Actin:
       - Description: A thin filament anchored to the Z disc.

  2. Myosin:
       - Description: A thick filament that contains ATPase enzymes which facilitate the splitting of ATP.
       - Components: Possesses myosin heads that are referred to as “cross bridges.”

Sarcomere Structure

  • I Band:
       - The light band consisting of the Z-disc and actin filaments.

  • A Band:
       - The dark band containing the entire length of myosin filaments, as well as the H zone and M line.

  • Sarcoplasmic Reticulum (SR):
       - A specialized form of smooth endoplasmic reticulum that surrounds myofibrils and is responsible for storing and releasing calcium.

Sliding Filament Theory

Key Steps of Muscle Contraction

  1. The sarcoplasmic reticulum releases calcium ions.

  2. Calcium binds to troponin, aiding in the exposure of myosin binding sites on the actin filaments.

  3. ATP attaches to the myosin heads.

  4. ATP undergoes hydrolysis, breaking down into ADP and phosphate via ATPase activity.

  5. The myosin heads switch to a high-energy configuration and stand upright.

  6. The myosin heads then bind to specific sites on actin.

Topic 2: The Nervous System

Overview of the Nervous System

The nervous system serves as the body’s rapid communication system, enabling the brain to send and receive signals, which control everything from simple reflexes to complex movements. It works alongside the endocrine system to maintain homeostasis.

Functions of the Nervous System

  1. Sensory:
       - Responsible for receiving sensory input.

  2. Integrative:
       - Integrates the sensory information received.

  3. Motor:
       - Involves motor output that produces a response.

Structure of the Nervous System

The nervous system is comprised of two main components:

  1. Central Nervous System (CNS):
       - Includes the brain and spinal cord.

  2. Peripheral Nervous System (PNS):
       - Consists of nerves connecting the CNS to the body's other parts.

Types of Neurons

  1. Afferent Neurons:
       - Carry signals toward the CNS (sensory).

  2. Efferent Neurons:
       - Carry signals away from the CNS (motor).

Autonomic Nervous System (ANS)

The ANS regulates involuntary body functions and consists of three divisions:

  1. Sympathetic Division:
       - Responsible for “fight or flight” responses.

  2. Parasympathetic Division:
       - Manages “rest and digest” functionalities.

  3. Enteric Division:
       - Focuses on the management of the digestive system.

Somatic Nervous System (SNS)

The SNS controls voluntary movements of skeletal muscles.

Neuron Structure and Function

  1. Soma (Cell Body):
       - Contains the nucleus and is responsible for making RNA for protein synthesis.

  2. Dendrites:
       - Structures that receive signals.

  3. Axon:
       - Transmits impulses away from the cell body.

  4. Myelin Sheaths:
       - Insulate the axon and block ion flow.

  5. Nodes of Ranvier:
       - Gaps between myelin sheaths where ion flow can occur.

  6. Axon Terminals:
       - End points of the axon that send signals to other neurons.

  7. Synapse:
       - The interface where axon terminals transmit signals to adjacent neurons and release neurotransmitters.

Types of Neurons

  1. Sensory Neurons:
       - Transmit impulses from sensory receptors to the CNS.

  2. Interneurons/Relay Neurons:
       - Connect sensory and motor neurons, facilitating communication between the brain and spinal cord.

  3. Motor Neurons:
       - Carry impulses from the CNS to muscles and glands.

Support Cells in the Nervous System (Neuroglia)

Neuroglia serve as supporting cells for neurons and are important for the health and maintenance of neuronal function.

  1. CNS Neuroglia:
       - Astrocytes:
         - Star-shaped and most common; forms the blood-brain barrier, supports brain function, recycles neurotransmitters, clears dead neurons, regulates blood flow, coordinates axon activity, and stores glucose for brain energy.
       - Oligodendrocytes:
         - Cells that form the myelin sheath and speed up information transmission along axons.
       - Microglial Cells:
         - Act as the brain's immune system, detecting injuries and diseases, removing dead cells, and pathogens.
       - Ependymal Cells:
         - Line the central canal of the spinal cord and the cavities within the brain (ependyma) and create cerebrospinal fluid, which delivers nutrients and eliminates waste.

  2. PNS Neuroglia:
       - Schwann Cells:
         - Similar to oligodendrocytes; provide a protective pathway for axon regeneration.
       - Satellite Cells:
         - Maintain the chemical environment around neurons, regulate nutrients, absorb toxins, and respond to injury and inflammation, although their exact role in repair is unclear.

Action Potentials and Nerve Impulses

Definition of Nerve Impulse

  • A message that travels along a neuron using electrical signals for long distances and chemical signals for short distances; it is the primary form of communication within the nervous system.

Cell Membrane and Membrane Potential

  • Cell Membrane:
       - A semi-permeable membrane maintaining ion gradients between the inside and outside of the cell, utilizing transport proteins to carry ions across (requiring ATP).

  • Membrane Potential:
       - Voltage difference across the cell membrane, usually noted in millivolts (mV).

  • Resting Potential:
       - The voltage of a resting neuron, typically at -70 mV, where the cytoplasm is more negative due to higher concentrations of negative ions and proteins, aided by sodium (Na+), potassium (K+), and ATPase mechanisms.

Action Potential Dynamics

  • An Action Potential is a rapid sequence of voltage changes across the membrane.

  • Threshold Potential:
       - The level that must be reached for an action potential to occur, typically -55 mV, leading to the opening of gated sodium channels and triggering a positive feedback loop for Na+ channel opening.

Sequence of Action Potential

  1. When a neuron is at rest, voltage-gated ion channels for Na+ and K+ are closed.

  2. A stimulus opens some Na+ channels causing depolarization of the membrane. If sufficient depolarization occurs and the threshold is reached, an action potential is triggered.

  3. Most Na+ channels open, resulting in a positive internal membrane potential.
       - The peak mV of action potential is approximately +33 mV.
       - The action potential response is an all-or-none event and is independent of the stimulus strength.
       - The frequency of action potentials can vary, depending on the intensity of the stimulus.

  4. As K+ channels open, potassium exits, returning the membrane potential to the resting state (-70 mV).

Role of Neurotransmitters

  • Excitatory Neurotransmitters:
       - Chemicals that facilitate signal transmission to the next neuron.
       - In the presynaptic region, nerve impulses open Ca+ channels, triggering exocytosis of synaptic vesicles that release neurotransmitters into the synaptic cleft, where they bind to receptors on the postsynaptic membrane and promote depolarization (leading to impulse generation).

  • Inhibitory Neurotransmitters:
       - Chemicals that oppose the effects of excitatory neurotransmitters, resulting in hyperpolarization and inhibiting nerve impulses.
       

Synapses and Impulse Transmission

  • Synapses can either excite or inhibit nerve impulse transmission, playing a regulatory role in nerve activity.

Voluntary vs Reflex Responses

  • Voluntary Responses: Controlled by the brain.

  • Reflex Responses: Controlled by the spinal cord.

Conclusion

These notes cover the muscular and nervous systems comprehensively, detailing their structures, functions, and interrelationships within the body. Further study and discussion are encouraged, especially regarding action potentials and synaptic transmissions. Good luck with your studies, Batch 29!