Spinal Cord and Movement: Chapters 13-15 Review

Chapter 13 - Spinal Cord and Movement

Overview

In this chapter, you will learn about several crucial topics related to movement and control in the human body, including:

  • Muscle anatomy

  • Types of motor units

  • Excitation-contraction coupling

  • Spinal control

  • Reflexes

  • Central pattern generators

Muscle Basics

Major Muscles Involved in Movement

  • Triceps: Primary extensor of the elbow joint.

  • Biceps Brachii: Primary flexor of the elbow joint.

  • Brachialis: Assists in elbow flexion.

  • Anconeus: Assists in elbow extension.

Antagonistic Muscle Pairs

  • Biceps and Triceps: 1. Antagonistic Muscles: Opposing muscle groups with opposite actions; contraction of one results in the relaxation of the other.

    • Biceps cause flexion of the elbow.

    • Triceps cause extension of the elbow.

Muscle Fibers and Innervation

  • Each muscle fiber is innervated by a single alpha motor neuron (axon).

  • The connection between the nerve and muscle is called the neuromuscular junction, also known as the end plate.

  • Muscles can be further divided:

    • Flexor muscles: assist in bending a joint.

    • Extensor muscles: assist in straightening a joint.

    • Synergistic muscles: work together to assist the same movement (e.g., Flexor 1 and Flexor 2 in coordination).

Motor Unit

Structure and Function

  • Motor Unit: Defined as an alpha motor neuron and all the muscle fibers it innervates.

  • Motor Neuron Pool: The collection of all alpha motor neurons that innervate a single muscle.

Muscle Twitch

  • A single action potential in an alpha motor neuron initiates a muscle twitch, causing the muscle fiber to contract.

  • Sustained Contraction: The cumulative effect of action potentials leads to a sustained contraction through temporal summation.

Types of Motor Units

Three primary types of motor units are recognized:

  1. Fast fatigable

  2. Fast fatigue-resistant

  3. Slow resisting

  • Fast fatigable units generate high force but fatigue quickly.

  • Fast fatigue-resistant units balance force and endurance.

  • Slow units are designed for endurance with low force output.

Inputs to Alpha Motor Neurons

Alpha motor neurons receive inputs from:

  1. Sensory Input from muscle spindles

  2. Upper Motor Neurons in the brain

  3. Spinal Interneurons

Excitation-Contraction Coupling

  1. Acetylcholine Release: Release from synaptic terminals binds to nicotinic acetylcholine receptors (nAChRs), leading to muscle fiber depolarization.

  2. Action Potential Generation: Voltage-gated sodium channels in the muscle fiber's sarcolemma open, generating an action potential.

  3. Transmission to T-tubules: The action potential travels down the T-tubules, causing a release of calcium from the sarcoplasmic reticulum (SR) into the cytosol.

  4. Calcium Role: Calcium interacts with troponin and tropomyosin, allowing myosin heads to attach to actin, resulting in muscle contraction.

Molecular Basis of Muscle Contraction

  • Sliding Filament Theory: Myofibrils shorten as actin slides over the myosin filaments, initiated by increased intracellular calcium levels.

  • Cross-Bridge Cycle: Involves the binding of myosin heads to actin, power stroke, and detachment, which are regulated by the presence of calcium and ATP.

Spinal Control of Motor Units

  • Muscle Spindles: Proprioceptors embedded in muscle fibers that detect changes in muscle length and rate of change.

  • Stretch Reflex: Involves the muscle spindle's response to sudden weight addition, commanding rapid reflexive adjustments.

  • Knee-Jerk Reflex: A classic example showing the basic circuitry involving sensory input and corresponding motor output in response to stretch.

Gamma Motor Neurons

  • Innervate intrafusal fibers of muscle spindles, adjusting their sensitivity, which is crucial for maintaining proprioceptive feedback.

  • Alpha motor neuron activation leads to shortening of the extrafusal muscle fibers, while gamma motor neuron activation adjusts sensitivity without changing muscle length.

Golgi Tendon Organ

  • Functions as a strain gauge that monitors muscle tension and force exertion.

  • Associated with group Ib axons that relay tension information to the spinal cord and activate inhibitory interneurons, which inhibit alpha motor neurons of the contracting muscle to prevent damage.

Reflexes and Central Pattern Generators

Flexor Withdrawal Reflex

  • A reflex arc resulting from an adverse stimulus leading to withdrawal of a limb.

Crossed-Extensor Reflex

  • Activates opposite limb's extensor muscles to support body weight during withdrawal of a limb from a painful stimulus.

Central Pattern Generators

  • Networks of neurons that produce rhythmic outputs to control locomotion and other repetitive movements without sensory feedback.

Chapter 14/15 - Outputs of the Nervous System

Major Outputs

  1. Motor Output: Direct control over skeletal muscles.

  2. Hormonal Output: Via hypothalamic-pituitary axis, influencing hormone release.

  3. Autonomic Regulation: Involuntary processes including heart rate and digestion.

Autonomic Nervous System Divisions
Sympathetic Division

  • Prepares body for stress (fight-or-flight) by increasing heart rate, dilating airways, and increasing blood glucose.

Parasympathetic Division

  • Facilitates rest and digest by slowing heart rate and stimulating digestive functions.

Function of the Autonomic Nervous System

  • Sympathetic: Enhances alertness and energy use.

  • Parasympathetic: Decreases energy expenditure during restful periods.

Chapter 25 - Molecular Mechanisms of Learning and Memory

Key Concepts in Memory Formation

  • Distributed Memory Storage: Memory is stored across various brain regions.

  • Hippocampal Synaptic Plasticity: Essential for memory formation and retrieval.

Long-term Potentiation (LTP) and Depression (LTD)

  1. LTP: High-frequency stimulation leads to strong and lasting synaptic changes.

  2. LTD: Low-frequency stimulation promotes a weakening of the synapse.

Mechanisms of LTP

  • Requires Calcium influx through NMDA receptors, activating protein kinases that strengthen synaptic efficacy.

  • Changes in dendritic spine shape occur – often resulting in a larger number of synaptic receptors.

Consolidation and CREB

  • CREB (Cyclic AMP response element binding protein): an important molecule for initiating gene transcription related to long-term memory.

Metaplasticity

  • Refers to the possibility that activity history can alter how synapses behave in response to future stimuli.