Nervous System III: Autonomic & Somatic Motor Control

Proprioceptors and Muscle Sensory Structures

• Definition and Types of Proprioceptors: Proprioceptors are specialized sensory receptors that provide information about the position and movement of body parts. There are three primary types:     * Muscle Spindle: Located within skeletal muscles.     * Golgi Tendon Organ: Located within skeletal muscles at the junction with tendons.     * Joint Receptors: Found in the capsules and ligaments surrounding joints.

• Muscle Spindle Anatomy and Function:     * Location: Muscle spindles are buried among the extrafusal muscle fibers, which are the normal contractile fibers of the muscle.     * Structure: They consist of intrafusal fibers.     * Central Region: This area of the spindle lacks myofibrils and is the site where tonically active sensory neurons originate.     * Innervation:         * Alpha Motor Neurons: Innervate the extrafusal muscle fibers to trigger contraction.         * Gamma Motor Neurons: Originate from the CNS and innervate the contractile ends of the intrafusal fibers.     * Sensory Role: The muscle spindle sends information regarding muscle stretch to the CNS. The sensory neurons are tonically active, meaning they send a steady stream of action potentials to the CNS even when the muscle is at a resting length.

• The Stretch Reflex:     1. Muscle Stretch: When a muscle length increases, the muscle spindle is stretched.     2. Increased Firing: The afferent sensory neurons in the spindle increase their firing rate of action potentials.     3. CNS Integration: The spinal cord integrates this signal.     4. Efferent Output: There is an increased efferent output through alpha motor neurons.     5. Reflex Response: The muscle contracts, returning to its initial length to prevent damage from over-stretching. This constitutes a negative feedback loop.

• Alpha-Gamma Coactivation:     * Purpose: To maintain spindle function and sensitivity when a muscle contracts and shortens.     * Mechanism: When the alpha motor neuron fires to contract extrafusal fibers, the gamma motor neuron fires simultaneously to contract the ends of the intrafusal fibers.     * Outcome: The center of the intrafusal fiber does not slacken; therefore, the stretch on the central region remains unchanged, and the firing rate of the afferent neuron remains constant during contraction.     * Consequence of Failure: Without gamma motor neurons, muscle contraction would cause the spindle to become slack, the center would experience less stretch, and the firing rate of the spindle sensory neuron would decrease or stop, rendering the spindle unable to detect further changes in length.

• The Golgi Tendon Organ (GTO):     * Anatomy: Consists of sensory nerve endings interwoven among collagen fibers within a capsule. It links the muscle to the tendon.     * Function: It is sensitive to muscle tension rather than stretch.     * The Golgi Tendon Reflex:         1. Muscle Contraction: Contraction of the muscle pulls on the tendon, stretching the Golgi tendon organ.         2. Firing: The neuron from the Golgi tendon organ fires action potentials.         3. Inhibition: The signal travels to the spinal cord and stimulates an inhibiting interneuron.         4. Relaxation: The alpha motor neuron is inhibited, the muscle relaxes, and the load is dropped.     * Protective Role: This reflex prevents damage by causing relaxation if an excessive load is placed on the muscle.

General Organization of the Efferent Division

• The Efferent Division consists of two main branches:     * Autonomic Division: Controls visceral functions, cardiac muscle, smooth muscle, glands, and adipose tissue. It is involved in maintaining homeostasis via antagonistic controls.     * Somatic Motor Division: Controls skeletal muscles through neuromuscular junctions for posture and movement.

• Autonomic Role in Homeostasis:     * Dynamic Balance: Homeostasis is maintained by the balance between the two autonomic branches.     * Parasympathetic Branch: Known for "Rest and Digest" activities; focuses on restoring body functions.     * Sympathetic Branch: Known for "Fight or Flight" responses; focused on energetic action.

• Central Control of Autonomic Functions:     * The Hypothalamus, Pons, and Medulla initiate autonomic, endocrine, and behavioral responses.     * Hypothalamus: Regulates water balance, temperature control, hunger (eating behavior), and urinary bladder control.     * Pons: Acts as a secondary respiratory center.     * Medulla: Controls blood pressure, respiration (primary center), cardiac function, vomiting, and swallowing.

Autonomic Pathway Structure and Neurochemistry

• Two-Neuron Chain: Autonomic pathways consist of two neurons in series that synapse in an autonomic ganglion:     1. Preganglionic Neuron: Runs from the CNS to the ganglion.     2. Postganglionic Neuron: Runs from the ganglion to the target tissue.

• Neurotransmitters and Receptors:     * Sympathetic Pathways:         * Preganglionic neurotransmitter: Acetylcholine ($ACh$).         * Ganglionic receptor: Nicotinic cholinergic receptor ($nAChR$).         * Postganglionic neurotransmitter: Norepinephrine ($NE$).         * Target tissue receptor: Adrenergic receptors ($\alpha$ and $\beta$).     * Parasympathetic Pathways:         * Preganglionic neurotransmitter: Acetylcholine ($ACh$).         * Ganglionic receptor: Nicotinic cholinergic receptor ($nAChR$).         * Postganglionic neurotransmitter: Acetylcholine ($ACh$).         * Target tissue receptor: Muscarinic cholinergic receptor ($mAChR$).

• Neurotransmitter Synthesis and Inactivation (Table 11-1):     * Norepinephrine (Sympathetic): Synthesized from Tyrosine. Inactivated by Monoamine Oxidase (MAO) in the mitochondria of the varicosity. It is also removed from the synapse via active transport (reuptake).     * Acetylcholine (Parasympathetic): Synthesized from Acetyl CoA + Choline. Inactivated by Acetylcholinesterase (AChE) in the synaptic cleft. Choline is then recycled.

Autonomic Effector Organs and Neuron Structure

• Targets of Autonomic Control:     * Smooth and Cardiac muscle.     * Exocrine and Endocrine glands.     * Lymphoid and Adipose tissue.

• Varicosities: Unlike the standard axon terminals in somatic neurons, autonomic postganglionic axons have varicosities. These are swollen regions along the axon containing vesicles of neurotransmitter, allowing the signal to be broadcast over a wider area of the target tissue (neuroeffector junction).

• Steps of Norepinephrine Release at a Sympathetic Varicosity:     1. Action potential arrives at the varicosity.     2. Depolarization opens voltage-gated $Ca^{2+}$ channels.     3. $Ca^{2+}$ entry triggers exocytosis of synaptic vesicles containing $NE$.     4. $NE$ binds to adrenergic receptors on the target cell.     5. Receptor activation ceases when $NE$ diffuses away from the synapse.     6. $NE$ can be taken back into the varicosity for re-release.     7. $NE$ can be metabolized by monoamine oxidase ($MAO$).

Specific Actions of Autonomic Branches

• Sympathetic Branch Actions:     * Stimulation: Pupil dilation, increased heart rate and volume, blood vessel and bronchiole dilation, fat breakdown, ejaculation, and salivation.     * Inhibition: Digestion, pancreas secretion, and urination.

• Parasympathetic Branch Actions:     * Stimulation: Digestion, insulin release, urination, and erections.     * Inhibition/Constriction: Pupil and bronchiole constriction, and slowing of the heart.

• The Adrenal Medulla:     * Considered a modified sympathetic ganglion.     * Chromaffin Cells: These are modified postganglionic sympathetic neurons that lack axons.     * Function: Upon stimulation by preganglionic sympathetic neurons, they release Epinephrine ($E$), a neurohormone, directly into the blood to act on distant targets.

Somatic Motor Division Characteristics

• Pathway Structure: Consists of a single neuron originating in the CNS.

• Myelination: These neurons are myelinated for fast conduction.

• Neurotransmitter: Always uses Acetylcholine ($ACh$).

• Receptor: Always Nicotinic receptors on the skeletal muscle.

• Functional Effect: Excitatory only (causes muscle contraction).

Questions & Discussion

• Question 1: When the muscle shown in the muscle spindle diagram is relaxed, which neurons are firing?     * Answer: (b) muscle spindle sensory neuron. This is due to their tonic activity at resting length.

• Question 2: Which neuron fires to cause contraction of the extrafusal muscle fibers?     * Answer: (a) muscle alpha motor neuron.

Comparison Summary: Somatic vs. Autonomic (Table 11-5)