Motor Systems and Behavior

Muscle Types and Structure

Muscle tissue is specialized for contraction and is essential for movement in animals. There are three main types of muscle: skeletal, cardiac, and smooth, each with distinct locations and structural features.

• Skeletal Muscle: Attached to bones, responsible for voluntary movements. Contains sarcomeres, the repeating contractile units, giving it a striated appearance.

• Cardiac Muscle: Found only in the heart. Also striated due to sarcomeres, but cells are branched and connected by intercalated discs. Involuntary control.

• Smooth Muscle: Located in walls of hollow organs (e.g., intestines, blood vessels). Lacks sarcomeres and striations. Involuntary control.

Muscle Structure: Myofibrils and Sarcomeres

Muscle fibers contain bundles of myofibrils, which are composed of repeating units called sarcomeres. The sarcomere is the fundamental contractile unit of striated muscle.

• Sarcomere: Defined by Z lines; contains thin (actin) and thick (myosin) filaments.

• Actin: Protein forming the thin filament.

• Myosin: Motor protein forming the thick filament; interacts with actin for contraction.

• Z line: Boundary of each sarcomere; anchors actin filaments.

Sliding Filament Model of Muscle Contraction

The sliding filament model explains how muscles contract by the sliding of actin (thin) filaments over myosin (thick) filaments, shortening the sarcomere without changing the length of the filaments themselves.

• During contraction, Z lines move closer together as actin slides past myosin.

• Cross bridge cycling: Myosin heads bind to actin, pivot, detach, and repeat, powered by ATP hydrolysis.

Key Steps:

1. Myosin head binds to actin, forming a cross-bridge.

2. Power stroke: Myosin head pivots, pulling actin filament toward the center of the sarcomere.

3. ATP binds to myosin, causing it to detach from actin.

4. ATP hydrolysis re-cocks the myosin head.

Regulation: Troponin and tropomyosin regulate access of myosin to actin. Calcium ions bind to troponin, shifting tropomyosin and exposing binding sites.

Neuromuscular Junction and Neurotransmission

The neuromuscular junction is the synapse between a motor neuron and a muscle fiber. Acetylcholine (ACh) is the neurotransmitter released to trigger muscle contraction.

• Acetylcholine: Released from motor neuron, binds to receptors on muscle cell membrane, initiating an action potential.

• Acetylcholinesterase: Enzyme that breaks down acetylcholine, terminating the signal.

• Sarcoplasmic reticulum: Organelle that stores and releases calcium ions in response to action potentials.

• T tubule: Invagination of the muscle cell membrane that helps transmit the action potential into the cell.

Perturbations:

• Increased ACh release or receptor sensitivity enhances contraction.

• Inhibition of acetylcholinesterase prolongs contraction (e.g., nerve gas poisoning).

• Disruption of actin, myosin, troponin, tropomyosin, or calcium impairs contraction.

Autonomic Nervous System: Sympathetic vs. Parasympathetic

The autonomic nervous system (ANS) regulates involuntary body functions and is divided into the sympathetic and parasympathetic branches.

• Sympathetic Nervous System: Prepares the body for 'fight or flight' responses. Increases heart rate, blood pressure, and releases epinephrine and norepinephrine.

• Parasympathetic Nervous System: Promotes 'rest and digest' activities. Decreases heart rate and promotes digestion.

Stress Response: Increased stress activates the sympathetic system, raising epinephrine/norepinephrine and blood pressure. Relaxation activates the parasympathetic system.

Behavior: Innate vs. Learned

Animal behavior can be classified as innate (genetically programmed) or learned (acquired through experience).

• Innate Behavior: Present at birth, performed without prior experience (e.g., reflexes).

• Learned Behavior: Modified by experience (e.g., imprinting, conditioning).

Synaptic Plasticity and Learning

Learning involves changes in the strength of synaptic connections between neurons, a process known as synaptic plasticity.

• Strengthen Synapse: Increased neurotransmitter release or receptor sensitivity enhances synaptic transmission.

• Weaken Synapse: Decreased neurotransmitter release or receptor sensitivity reduces synaptic transmission.

• Hebb’s Rule: "Cells that fire together, wire together"—simultaneous activation of cells leads to pronounced increases in synaptic strength.

Experimental Evidence Linking Memory and Neural Activity

Memory formation is associated with changes in neural activity. Experimental approaches include:

• Optogenetics: Technique using light-sensitive proteins (e.g., channelrhodopsin) to control neuron activity with light.

• Evidence 1: Activating specific neurons with optogenetics can induce recall of a memory (e.g., mice freeze when neurons associated with fear memory are stimulated).

• Evidence 2: Blocking neurotransmitter release in certain brain regions impairs memory formation, as measured by behavioral tests (e.g., maze learning).

Measuring Memory: Behavioral assays (e.g., maze performance, freezing behavior) are used to assess memory in experimental animals.

Key Terms Summary

• PNS (Peripheral Nervous System): Nerves outside the brain and spinal cord.

• CNS (Central Nervous System): Brain and spinal cord.

• Troponin/Tropomyosin: Regulatory proteins controlling actin-myosin interaction.

• Channelrhodopsin: Light-gated ion channel used in optogenetics.

Additional info:

• Muscle contraction is an energy-dependent process, requiring ATP for both cross-bridge cycling and calcium reuptake into the sarcoplasmic reticulum.

• Synaptic plasticity underlies not only learning but also memory storage and retrieval.