Spinal Cord and Autonomic Nervous System

Spinal Cord Anatomy

• The CNS (Central Nervous System) includes the spinal cord and brain
• Spinal cord contains nuclei within it, and ganglia outside it
• Sensory nuclei responsible for receiving sensory information
• Sensory nuclei include somatic and visceral sensory nuclei
• Motor nuclei responsible for delivering motor commands
• Motor nuclei include autonomic and somatic efferent nuclei

Autonomic Nervous System

• The ANS is responsible for unconscious bodily functions
• The ANS is composed of: Somatic pathways, Parasympathetic pathway, Sympathetic pathways, Adrenal Sympathetic pathway

Autonomic Nervous System: Pathways

• The autonomic nervous system involves two branches; parasympathetic and sympathetic
• The ANS uses a two neuron pathway: Pre and Post Ganglionic Neuron
• Parasympathetic preganglionic neurons use acetylcholine (ACh) as a neurotransmitter
• Sympathetic preganglionic neurons use acetylcholine (ACh) as a neurotransmitter
• Sympathetic postganglionic neurons use norepinephrine (NE) as a neurotransmitter
• Both parasympathetic and sympathetic pathways use nicotine receptors
• The sympathetic pathways use alpha, beta 1 and beta 2 adrenoreceptors
• The parasympathetic pathways use muscarinic receptors

Response to Stimulus

• Reflex response is a rapid, involuntary reaction to a stimulus
• Homeostatic response is a slower response to maintain internal balance
• Homeostatic response involves sensory input, integration, and output
• Sensory input for both reflexes and homeostasis is received by sensory neurons
• Integration occurs in the CNS, potentially including the hypothalamus, pons, and medulla
• The output can be somatic, autonomic, or endocrine

Thermoregulation

• Thermoregulation is the process of maintaining body temperature
• Thermoregulation uses both the somatic nervous system and the autonomic nervous system
• Thermoregulation involves the hypothalamus and skeletal muscle
• Thermoregulation uses ACh and NE as neurotransmitters
• Thermoregulation involves smooth muscle which helps regulate blood flow to the skin

Summary

• The hypothalamus is crucial for maintaining body homeostasis
• Spinal reflexes are quick, involuntary actions that do not require brain integration
• Homeostatic reflexes are slower and often involve the autonomic nervous system
• The two divisions of the ANS, sympathetic and parasympathetic, have distinct effects on target cells
• Both branches of the ANS control the same target cells but produce opposite effects using different receptors

Muscle Types

• Three major types of muscle exist: skeletal, cardiac, and smooth
• Skeletal muscle is attached to bones
• Cardiac muscle is found in the heart
• Smooth muscle is found in internal organs and vessels
• Skeletal muscle is striated and multinucleated, while cardiac and smooth muscle have single nuclei

Skeletal Muscle

• Skeletal muscle is responsible for movement
• Skeletal muscle produces heat due to ATP use
• Skeletal muscle is composed of muscle fibers
• Skeletal muscle is innervated by somatic motor neurons

Organization of Skeletal Muscle

• Skeletal muscle is composed of fascicles, bundles of muscle fibers
• Muscle fibers are composed of myofibrils
• Muscle fibers contain nuclei, mitochondria, sarcoplasmic reticulum, and T tubules

Ultrastructure of Muscle

• Myofibrils are composed of repeating sarcomeres
• Sarcomeres are the functional units of muscle contraction
• Sarcomeres contain thick filaments (myosin) and thin filaments (actin)
• Myosin heads bind to actin
• Titin acts as a spring to hold the structure of the sarcomere
• Myosin tails form the M line and actin anchors to the Z disk
• Troponin is a calcium binding protein on the actin filament

Muscle Contraction

• Muscle contraction is a complex process involving calcium signaling, ATP hydrolysis and cross-bridge cycling
• The contraction cycle occurs at the level of the myosin head.

The Neuromuscular Junction

• The neuromuscular junction is the synapse where a motor neuron innervates a muscle fiber
• At the neuromuscular junction, the axon terminal releases acetylcholine (ACh)
• ACh binds to nicotinic receptors on the motor end plate of the muscle fiber
• Stimulation of the motor end plate initiates an action potential in the muscle fiber
• The action potential spreads across the sarcolemma and into the T tubules

Excitation-Contraction Coupling

• Excitation-contraction coupling is the process that converts an electrical signal (action potential) into a calcium signal
• The electrical signal triggers calcium release from the sarcoplasmic reticulum
• Binding of Ca2+ to troponin relieves the inhibition of actin
• Myosin heads can then bind to actin and initiate muscle contraction
• Muscle relaxation occurs when calcium is pumped back into the sarcoplasmic reticulum

The Calcium Signal

• Relaxation occurs when calcium is removed allowing the tropomyosin to block the actin binding site
• ATP is required for both the calcium pump and myosin detatchment

Spinal Cord Anatomy

• Ganglia are located outside the Central Nervous System (CNS)
• Nuclei are located within the CNS
• The dorsal root ganglion contains sensory neurons and is located outside the spinal cord
• The dorsal horn contains sensory nuclei
• The lateral horn contains autonomic efferent nuclei
• The ventral horn contains somatic motor nuclei
• The ventral root contains motor neurons and is located outside the spinal cord

Nervous System Pathways

• Somatic Motor Pathway: CNS to skeletal muscle, uses acetylcholine (ACh) as neurotransmitter, receptor is nicotinic on skeletal muscle
• Parasympathetic Pathway: CNS to autonomic effectors, uses ACh at both pre-ganglionic and post-ganglionic synapses, pre-ganglionic receptor is nicotinic, post-ganglionic receptor is muscarinic
• Sympathetic Pathways: CNS to autonomic effectors, uses ACh pre-ganglionically, uses norepinephrine (NE) post-ganglionically, pre-ganglionic receptor is nicotinic, post-ganglionic receptor is adrenergic
• Adrenal Sympathetic Pathway: CNS to adrenal medulla primarily, uses ACh pre-ganglionically, uses NE and epinephrine (E) from adrenal medulla, pre-ganglionic receptor is nicotinic, post-ganglionic receptor is adrenergic
• Autonomic effectors include smooth muscle, cardiac muscle, some endocrine and exocrine glands, and some adipose tissue

Response to Stimuli

• Reflex response: Sensory input triggers integration and action
• Homeostatic response: Sensory input triggers hypothalamic receptors and integration processing in the pons, medulla, and hypothalamus, resulting in autonomic, somatic motor, and endocrine responses

Thermoregulation

• Afferent sensory neurons detect temperature changes and send signals to the hypothalamus.
• The hypothalamus integrates information and sends signals through descending tracts to somatic motor neurons and efferent autonomic neurons.
• Somatic motor neurons activate skeletal muscle for shivering (generating heat).
• Autonomic neurons activate smooth muscle in blood vessels, either constricting or dilating to regulate heat loss.

The Hypothalamus

• The hypothalamus is essential for maintaining homeostasis.

Reflexes

• Spinal reflexes are fast, involuntary actions that do not require direct integration by the brain.
• Homeostatic reflexes are slower and often involve the autonomic nervous system.

The Autonomic Nervous System

• The autonomic nervous system has two branches: parasympathetic and sympathetic.
• Within the CNS, the control of these branches is spatially segregated.
• Both branches affect the same target cells but produce antagonistic effects by interacting with different receptors.

Skeletal Muscle

• Skeletal muscle is attached to the skeleton and is responsible for movement.
• It is striated, multinucleated, and exhibits the fastest contraction speed.
• It is controlled by the somatic motor nervous system.

Fundamental Functions of Muscle

• Muscle converts neural information into mechanical energy (force and displacement).
• Muscle contraction produces heat as a byproduct.

Organization of Skeletal Muscle

• Skeletal muscle consists of muscle fibers bundled into fascicles, surrounded by connective tissue.

Ultrastructure of Muscle

• Muscle fibers contain myofibrils, which are composed of sarcomeres.
• Sarcomeres are the basic contractile units of muscle.
• Sarcomeres contain thick filaments (myosin) and thin filaments (actin).

Contraction Cycle

• The contraction cycle involves a series of steps that require ATP.
• Myosin heads bind to actin (cross-bridge formation) and perform a power stroke, pulling the thin filaments toward the center of the sarcomere, causing muscle shortening.
• Myosin detaches from actin when ATP binds to myosin, allowing the cycle to repeat.

Neuromuscular Junction

• The neuromuscular junction is a synapse between a motor neuron and a muscle fiber.
• The axon terminal of the motor neuron releases acetylcholine (ACh), which binds to nicotinic receptors on the motor end plate of the muscle fiber.
• This binding triggers an action potential in the muscle fiber, leading to muscle contraction.

Excitation-Contraction Coupling

• Excitation-contraction coupling refers to the conversion of an electrical signal (action potential) in the sarcolemma (muscle fiber membrane) into a calcium signal in the cytoplasm.
• This occurs when the action potential triggers the release of calcium from the sarcoplasmic reticulum (SR).
• The release of calcium initiates the contraction cycle by allowing myosin to bind to actin.

Calcium Signal

• The calcium signal is critical for muscle contraction.
• Calcium release from the SR triggers muscle contraction, while the removal of calcium from the cytoplasm allows the muscle to relax.

Muscle Relaxation

• Muscle relaxation occurs when calcium is removed from the cytoplasm, which is achieved through the action of calcium pumps in the SR membrane.
• The removal of calcium allows the muscle to relax and return to its resting length.

Three Types of Muscle

• Skeletal muscle: Attached to skeleton, striated, multinucleated, fastest contraction speed
• Cardiac muscle: Found in heart, striated, uninucleated, intermediate contraction speed
• Smooth muscle: Located in internal organs and vessels, smooth, uninucleated, and slowest contraction speed

ATP in Muscle Contraction

• ATP is essential for muscle contraction.
• It is required for the following processes:
  • Myosin detachment from actin, breaking the cross-bridge
  • The power stroke of myosin
  • Calcium pumps to move calcium back into the sarcoplasmic reticulum, allowing for muscle relaxation.