Motor Systems and Reflexes: Neuroanatomy & Neurophysiology Study Notes

Motor Systems: Organization and Control

Overview of Motor Systems

The motor system is responsible for initiating and regulating voluntary and involuntary movements in the body. It involves complex neural circuits that coordinate muscle activity, posture, and reflexes. Understanding the hierarchical organization of motor control is essential for grasping how the nervous system produces movement.

• Motor Unit: A motor neuron and all the muscle fibers it innervates.

• Motor Cortex: Region of the brain responsible for planning, controlling, and executing voluntary movements.

Levels of Motor Control

Three Levels of Motor Control

Motor control is organized into three hierarchical levels, each with distinct roles:

• Segmental Level (Lowest): Spinal cord circuits and reflexes that control automatic movements. Contains central pattern generators (CPGs) for rhythmic activities like walking.

• Projection Level (Middle): Motor cortex and brainstem nuclei send descending commands to spinal cord. Integrates direct (pyramidal) and indirect (extrapyramidal) pathways.

• Precommand Level (Highest): Cerebellum and basal nuclei plan, coordinate, and regulate movements before they are executed. They provide timing, patterns, and block unwanted movements.

Comparison to Sensory Systems

• Motor endings serve effectors (muscles/glands) rather than picking up sensory input.

• Motor pathways are descending (efferent), while sensory pathways are ascending (afferent).

• Focus is on motor behavior instead of perception.

Segmental Level

The segmental level consists of spinal cord circuits that control reflexes and automatic movements. Ventral horn neurons are activated to stimulate specific muscles. Central pattern generators (CPGs) are networks of oscillating neurons that produce rhythmic motor patterns, such as walking.

Projection Level

The projection level includes upper motor neurons in the motor cortex and brainstem nuclei. These neurons send commands via direct and indirect pathways to the spinal cord, controlling voluntary movements and reflexes. They also provide feedback to higher centers about ongoing movements.

Precommand Level

The precommand level involves the cerebellum and basal nuclei, which plan and coordinate movements before they are executed. They:

• Start and stop movements

• Coordinate movements with posture

• Block unwanted movements

• Monitor muscle tone

Cerebellum: Communicates with projection areas via the thalamus; calculates the best way to perform movements.

Basal Nuclei: Receive input from all cortical areas and send output to premotor and prefrontal cortex; involved in complex movement management.

Descending Motor Pathways

Overview

Descending motor pathways transmit efferent impulses from the brain to the spinal cord. They are divided into:

• Direct Pathways (Pyramidal Tracts): Originate in the motor cortex and control skilled, voluntary movements.

• Indirect Pathways (Extrapyramidal Tracts): Originate in the brainstem and regulate balance, posture, and coarse movements.

Motor Pathway Neurons

• Upper Motor Neuron: Begins in the motor cortex.

• Lower Motor Neuron: Located in the spinal cord or cranial nerve nuclei; innervates skeletal muscle.

Direct (Pyramidal) Pathways

• Axons descend without synapsing from the primary motor cortex through the spinal cord.

• Regulate fast and fine movements (e.g., typing, playing an instrument).

• Synapse with lower motor neurons in the anterior horn at the level where the neuron exits the spinal cord.

• 90% of these neurons are in the lateral corticospinal tract; 10% in the anterior corticospinal tract.

Indirect (Extrapyramidal) Pathways

• Include rubrospinal, vestibulospinal, reticulospinal, and tectospinal tracts.

• Regulate axial muscles for balance and posture, coarse limb movements, and head/eye movements.

• Pathways are complex and multi-synaptic.

Table: Major Descending (Motor) Pathways and Spinal Cord Tracts

Spinal Cord Tract	Location	Origin	Termination	Function
Lateral corticospinal	Lateral	Pyramidal neurons of motor cortex	Ventral horn (spinal cord)	Voluntary motor impulses to skeletal muscles (limbs)
Ventral corticospinal	Ventral	Pyramidal neurons of motor cortex	Ventral horn (spinal cord)	Voluntary motor impulses to axial muscles
Tectospinal	Ventral	Superior colliculus (midbrain)	Ventral horn (spinal cord)	Turns neck so eyes can follow moving objects
Vestibulospinal	Ventral	Vestibular nuclei (medulla)	Ventral horn (spinal cord)	Maintains muscle tone and balance
Rubrospinal	Lateral	Red nucleus (midbrain)	Ventral horn (spinal cord)	Helps regulate muscle tone
Reticulospinal	Ventral and lateral	Reticular formation (brainstem)	Ventral horn (spinal cord)	Muscle tone and unskilled movements

Spinal Cord Injury and Paralysis

• Paralysis: Loss of motor function.

• Paraplegia: Transection between T1 and L1; affects lower limbs.

• Quadriplegia: Transection in cervical region; affects all limbs.

• Hemiplegia: Usually due to brain injury; affects one side of the body.

• Flaccid Paralysis: Damage to ventral roots or anterior horn; no voluntary or involuntary muscle control.

• Spastic Paralysis: Damage to upper motor neurons; loss of voluntary control, but spinal reflexes remain.

Reflex Arcs

Types of Reflexes

• Inborn (Intrinsic) Reflex: Unlearned, automatic responses (e.g., withdrawal from pain).

• Learned (Acquired) Reflex: Result from practice or repetition (e.g., driving a car).

Somatic Reflexes: Activate skeletal muscle. Autonomic (Visceral) Reflexes: Activate smooth muscle, cardiac muscle, or glands.

Stretch and Tendon Reflexes

Stretch Reflex

Helps coordinate skeletal muscle activity by responding to changes in muscle length. The nervous system monitors:

• Muscle Spindles: Detect muscle length.

• Tendon Organs: Detect muscle tension.

Each muscle spindle contains 3-10 intrafusal fibers (modified muscle fibers) enclosed in a connective tissue capsule. The central region is noncontractile and serves as the receptive part. Two types of afferent endings:

• Annulospiral endings: Large axons, monitor rate and degree of stretch.

• Flower spray endings: Smaller axons, monitor degree of stretch only.

Intrafusal fibers have actin and myosin at their ends and can contract when stimulated by small motor neurons. The rest of the muscle (extrafusal fibers) is stimulated by large motor neurons.

How Muscle Stretch is Detected

• Muscle can be stretched by external force or contraction of antagonistic muscle.

• Activating motor neurons can put stretch on the spindle.

• Descending pathways activate both alpha and gamma motor neurons to keep the brain informed about muscle status.

Stretch Reflex Mechanism

• Stretched muscle spindles initiate a reflex causing contraction of the stretched muscle and inhibition of its antagonist.

• All stretch reflexes involving the agonist muscle are monosynaptic and ipsilateral.

• Example: The patellar (knee-jerk) reflex prevents knees from buckling when standing upright.

Tendon Reflex (Polysynaptic)

• Acts in the opposite direction: muscles relax and lengthen in response to tension.

• Contracting muscle relaxes as its antagonist is activated (reciprocal activation).

• Protects against tearing and ensures smooth muscle contraction.

Flexor and Crossed Extensor Reflexes

• Flexor (Withdrawal) Reflex: Initiated by painful stimulus; protective and important for survival.

• Crossed Extensor Reflex: Important for weight-bearing limbs; ipsilateral withdrawal reflex plus contralateral extensor reflex.

• These reflexes can be overridden voluntarily.

Superficial Reflexes to Test for Spinal Cord Damage

• Abdominal Reflexes: Stroking the skin near the umbilicus causes contraction of abdominal muscles; tests integrity of T8 to T12.

• Plantar Reflex: Stroking the sole induces plantar flexion; Babinski sign (dorsiflexion of great toe) indicates damage to the primary motor cortex or corticospinal tract.

The Autonomic Nervous System (ANS) and Autonomic Reflexes

Organization of the Nervous System

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

• Peripheral Nervous System (PNS): Sensory (afferent) and motor (efferent) divisions.

• Motor Division: Somatic (skeletal muscle) and autonomic (smooth/cardiac muscle, glands) systems.

Comparison of Somatic and Autonomic Motor Neurons

System	Neuron Pathway	Neurotransmitter	Effector
Somatic	Single, thick, myelinated axon from CNS to muscle	Acetylcholine (ACh)	Skeletal muscle
Autonomic	Two-neuron chain: preganglionic (thin, lightly myelinated) and postganglionic (thinner, unmyelinated)	ACh or Norepinephrine (NE)	Smooth muscle, cardiac muscle, glands

Pathways and Ganglia

• Somatic: Rapid conduction, no ganglia.

• ANS: Preganglionic neuron originates in CNS, synapses with postganglionic neuron in ganglion outside CNS; slower conduction.

Divisions of the ANS

• Sympathetic Division: Mobilizes body during extreme situations ("fight or flight"). Increases heart rate, dilates pupils, causes cold, sweaty skin.

• Parasympathetic Division: Maintains body functions during rest ("rest and digest"). Promotes digestion, defecation, and diuresis.

• The two divisions usually counterbalance each other.

Summary Table: Sympathetic vs. Parasympathetic

Division	Main Function	Key Effects
Sympathetic	"Fight or flight"	Increased heart rate, rapid breathing, dilated pupils, energy mobilization
Parasympathetic	"Rest and digest"	Promotes digestion, energy conservation, normal body maintenance

Example: After a meal, the parasympathetic system predominates, promoting digestion and relaxation. During a stressful event, the sympathetic system takes over, preparing the body for action.