Muscle Physiology

Muscle Structure and Function

The muscular system is responsible for movement, posture, and heat production in the body. Muscles are classified based on their structure and function into skeletal, cardiac, and smooth muscle types.

• Skeletal muscle: Voluntary, striated muscle attached to bones; responsible for body movement.

• Cardiac muscle: Involuntary, striated muscle found only in the heart; responsible for pumping blood.

• Smooth muscle: Involuntary, non-striated muscle found in walls of hollow organs; controls movement of substances within the body.

Key Terms: sarcomere (functional unit of muscle), myofibril, actin, myosin

Muscle Contraction and Stimulation

Muscle contraction is initiated by a stimulus that leads to the sliding filament mechanism. The frequency and strength of stimulation affect the force generated by a muscle.

• Increased frequency of stimulation: Leads to summation and tetanus, increasing muscle tension.

• ATP Generation: Muscles generate ATP via creatine phosphate, glycolysis, and oxidative phosphorylation.

Equation for ATP production (aerobic):

• Fast glycolytic fibers: Contract quickly, fatigue rapidly, use anaerobic metabolism.

• Slow oxidative fibers: Contract slowly, resist fatigue, use aerobic metabolism.

Paralysis: Loss of muscle function due to nervous system damage; can be flaccid (loss of tone) or spastic (increased tone).

Nervous System Organization

Overview and Functional Divisions

The nervous system is organized into the central nervous system (CNS) and peripheral nervous system (PNS). It controls and coordinates body activities through electrical and chemical signals.

• CNS: Brain and spinal cord; processes and integrates information.

• PNS: Nerves and ganglia outside the CNS; transmits signals to and from the CNS.

• Functional divisions: Sensory (afferent), motor (efferent), and integrative functions.

Neurons and Neuroglia

Neurons are the functional units of the nervous system, transmitting electrical impulses. Neuroglia support and protect neurons.

• Neuron structure: Cell body, dendrites, axon.

• Neuroglia: Astrocytes, oligodendrocytes, microglia, ependymal cells (CNS); Schwann cells, satellite cells (PNS).

• Classification: Sensory, motor, and interneurons; multipolar, bipolar, unipolar types.

Resting Membrane Potential and Action Potentials

Neurons maintain a resting membrane potential due to ion gradients across the membrane. Action potentials are rapid changes in membrane potential that propagate along axons.

• Resting membrane potential: Typically -70 mV; maintained by Na+/K+ ATPase pump.

• Depolarization: Membrane potential becomes less negative.

• Hyperpolarization: Membrane potential becomes more negative.

• Action potential: All-or-none electrical signal; generated at the axon hillock and propagated along the axon.

Phases of action potential:

1. Depolarization (Na+ influx)

2. Repolarization (K+ efflux)

3. Hyperpolarization (K+ channels remain open)

Saltatory conduction: Action potentials jump between nodes of Ranvier in myelinated axons, increasing speed.

Synaptic Transmission

Neurons communicate at synapses via neurotransmitters. Synaptic potentials can be excitatory (EPSP) or inhibitory (IPSP).

• Presynaptic neuron: Releases neurotransmitter.

• Postsynaptic neuron: Receives signal; response depends on receptor type.

• Neurotransmitters: Acetylcholine, dopamine, serotonin, GABA, etc.

Integration: Neurons integrate multiple inputs to determine whether to fire an action potential.

Central Nervous System (CNS) Structure

Brain Development and Major Regions

The adult brain develops from embryonic structures: telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon.

• Telencephalon: Cerebral hemispheres

• Diencephalon: Thalamus, hypothalamus

• Mesencephalon: Midbrain

• Metencephalon: Pons, cerebellum

• Myelencephalon: Medulla oblongata

Cerebral Cortex and Functional Areas

The cerebral cortex is responsible for higher brain functions. It is divided into lobes and functional areas.

• Lobes: Frontal, parietal, temporal, occipital

• Gyri: Ridges; Sulci: Grooves; Fissures: Deep grooves

• Functional areas: Motor cortex, sensory cortex, association areas

Generalizations: Each hemisphere controls the opposite side of the body; specific areas have specialized functions.

Major Cortical Areas and Functions

Basal Nuclei and Thalamus

The basal nuclei are involved in motor control and coordination. The thalamus acts as a relay station for sensory and motor signals.

• Basal nuclei: Regulate voluntary motor activities, inhibit unnecessary movements.

• Thalamus: Receives, processes, and relays sensory information to the cortex.

Hypothalamus and Pituitary Gland

The hypothalamus regulates homeostasis and controls the pituitary gland, which secretes hormones affecting various body functions.

• Main nuclei: Supraoptic, paraventricular, etc.

• Pituitary gland: Secretes hormones such as growth hormone, ACTH, TSH, etc.

Brainstem and Cerebellum

The brainstem (midbrain, pons, medulla) controls vital functions. The cerebellum coordinates movement and balance.

• Midbrain: Visual and auditory reflexes

• Pons: Relays signals, regulates breathing

• Medulla oblongata: Controls heart rate, breathing, blood pressure

• Cerebellum: Coordinates voluntary movements, balance, and posture

Brain Waves and Sleep

Brain waves are electrical patterns measured by EEG. Sleep is essential for health and involves cycles of REM and non-REM stages.

• Seizures: Abnormal electrical activity in the brain

• Sleep cycle: Alternates between REM and non-REM sleep; important for memory and restoration

Cerebrospinal Fluid (CSF) and Circulation

CSF cushions the brain and spinal cord, circulates nutrients, and removes waste. It is produced by the choroid plexus and circulates through ventricles and the subarachnoid space.

• CSF flow: Lateral ventricles → third ventricle → fourth ventricle → subarachnoid space → arachnoid villi → venous blood

Spinal Cord and Pathways

Spinal Cord Structure

The spinal cord transmits signals between the brain and body. It contains gray matter (neuronal cell bodies) and white matter (myelinated axons).

• Cross-section: Central gray matter (H-shaped), surrounding white matter

• Motor neurons: Located in ventral horn

• Sensory neurons: Located in dorsal horn

Ascending and Descending Pathways

Ascending pathways carry sensory information to the brain; descending pathways carry motor commands from the brain.

Spinal Cord Injury and CNS Changes

Damage to the spinal cord can result in loss of sensation and motor function below the level of injury. The CNS undergoes changes throughout life, including neuroplasticity and age-related degeneration.

• Transection: Complete severing of the spinal cord; results in paralysis below the injury.

• Neuroplasticity: The CNS can adapt and reorganize in response to injury or learning.

Example:

Damage to the corticospinal tract results in loss of voluntary movement on the opposite side of the body.

Additional info: Some details, such as specific nuclei of the hypothalamus and the full list of neurotransmitters, were inferred based on standard Anatomy & Physiology curricula.