Chapter 13: Spinal Cord

Spinal Cord Anatomy

The spinal cord is a crucial component of the central nervous system, responsible for transmitting neural signals between the brain and the rest of the body. Understanding its structure is essential for comprehending its function.

• Organization of White and Gray Matter: In the spinal cord, gray matter is located centrally in an H-shaped (or butterfly-shaped) region, while white matter surrounds it. In contrast, in the brain, gray matter forms the outer cortex, and white matter is found deeper within.

• White Columns and Gray Horns: The white columns (funiculi) are bundles of myelinated nerve fibers that carry information up and down the spinal cord. The gray horns are regions of gray matter and are divided into dorsal (posterior), ventral (anterior), and lateral horns, each with specific functions (sensory, motor, autonomic).

• Sensory and Motor Impulses: Sensory (afferent) impulses enter the spinal cord via the dorsal (posterior) root, while motor (efferent) impulses exit via the ventral (anterior) root.

• Main Rami of a Spinal Nerve: After exiting the vertebral column, a spinal nerve splits into three main branches: the dorsal ramus (serves the back), ventral ramus (serves the limbs and anterior trunk), and rami communicantes (connect to the sympathetic trunk).

Spinal Cord Pathways

Neural pathways in the spinal cord transmit motor and sensory information between the body and the brain.

• Somatic Motor Pathway (to brachioradialis or rhomboid major): Motor signals originate in the primary motor cortex, descend via the corticospinal tract, synapse with lower motor neurons in the anterior horn, and exit through the ventral root to reach skeletal muscles.

• Visceral Motor Pathway (to heart or sweat gland): Signals originate in the hypothalamus or brainstem, descend to preganglionic neurons in the lateral horn, exit via the ventral root, synapse in autonomic ganglia, and postganglionic fibers innervate target organs (e.g., heart, sweat glands).

• Sensory Pathway (from fingertip or back): Sensory receptors send signals via peripheral nerves to the dorsal root ganglion, then into the dorsal horn of the spinal cord, where they may ascend to the brain or synapse locally.

Spinal Reflexes

Reflexes are rapid, involuntary responses to stimuli, mediated by neural circuits called reflex arcs.

• Components of a Reflex Arc: 1) Receptor, 2) Sensory neuron, 3) Integration center (in CNS), 4) Motor neuron, 5) Effector.

• Stretch Reflex (e.g., knee-jerk): Involves muscle spindle (receptor), sensory neuron, synapse in spinal cord, motor neuron, and effector muscle. Function: maintains muscle tone and posture.

• Withdrawal (Flexor) Reflex: Initiated by painful stimulus; sensory neuron activates interneurons, which stimulate flexor muscles and inhibit extensors (reciprocal inhibition).

• Crossed Extensor Reflex: When a withdrawal reflex is triggered, interneurons also stimulate extensor muscles on the opposite side to maintain balance.

Chapter 15: Neural Integration

Sensory Pathways

Sensory pathways transmit information from receptors to the central nervous system, often involving multiple neurons and tracts.

• Three Neurons in a Sensory Pathway: 1) First-order neuron (from receptor to spinal cord/brainstem), 2) Second-order neuron (to thalamus), 3) Third-order neuron (to cerebral cortex).

• Tract Naming: The name indicates origin and destination (e.g., spinothalamic tract runs from spinal cord to thalamus).

• Posterior Column Pathway: Carries fine touch, vibration, and proprioception to the somatosensory cortex.

• Anterolateral (Spinothalamic) Pathway: Carries pain, temperature, and crude touch to the somatosensory cortex.

• Tracing the Anterior Spinothalamic Tract: First-order neuron (peripheral receptor to spinal cord), second-order neuron (crosses to opposite side, ascends to thalamus), third-order neuron (thalamus to cortex).

• Referred Pain: Perception of pain in an area distant from its source (e.g., left arm pain during a heart attack).

• Spinocerebellar Pathway: Carries proprioceptive information to the cerebellum for coordination of movement.

Motor Pathways

Motor pathways control voluntary and involuntary movements by transmitting signals from the brain to effectors.

• Somatic vs. Visceral Motor Systems: Somatic motor system controls skeletal muscles; visceral motor (autonomic) system controls smooth muscle, cardiac muscle, and glands.

• Two Neurons in Somatic Motor Pathway: 1) Upper motor neuron (from cortex to spinal cord), 2) Lower motor neuron (from spinal cord to muscle).

• Corticospinal Pathway: Upper motor neuron originates in the motor cortex, descends through the internal capsule and brainstem, decussates (crosses over), synapses with lower motor neuron in anterior horn, which then innervates skeletal muscle (e.g., brachialis).

Chapter 16: Autonomic Nervous System (ANS)

Overview of the ANS

The autonomic nervous system regulates involuntary functions, including heart rate, digestion, and glandular activity.

• Visceral Motor Neurons: Neurons that control involuntary effectors (smooth muscle, cardiac muscle, glands).

• Autonomic Ganglia, Preganglionic and Postganglionic Fibers: Autonomic ganglia are clusters of neuron cell bodies outside the CNS. Preganglionic fibers are axons from the CNS to the ganglion; postganglionic fibers are axons from the ganglion to the effector.

• Divisions of the ANS: 1) Sympathetic division ("fight or flight"), 2) Parasympathetic division ("rest and digest").

• "Fight or Flight" Response: Sympathetic stimulation increases heart rate, dilates pupils, inhibits digestion, and mobilizes energy reserves.

• "Rest and Repose" Response: Parasympathetic stimulation promotes digestion, reduces heart rate, and conserves energy.

Sympathetic Division

The sympathetic division prepares the body for stressful or emergency situations.

• Other Name: Thoracolumbar division, because its preganglionic neurons originate in the thoracic and lumbar spinal cord.

• Activation: The sympathetic nervous system becomes active during stress, exercise, or emergencies.

Additional info: Some details, such as the precise order of neurons and the names of specific tracts, have been expanded for academic clarity and completeness.