The Nervous System: CNS

• The nervous system has two components: the central nervous system (CNS) and the peripheral nervous system (PNS).

Central Nervous System (CNS)

• Consists of the brain and spinal cord.

The Brain

• Six main parts of the brain include the cerebrum, cerebellum, brain stem, diencephalon, limbic system, and reticular activating system.
• The cerebrum is responsible for intelligence, learning, judgment, speech, memory, sensory processing (hearing, vision, taste, and smell), and voluntary muscle movements.
• The cerebellum controls balance, coordination, and posture.
• The brain stem regulates heart rate, breathing, blood pressure, digestion, swallowing, and vomiting.
• The diencephalon works with the endocrine system to release hormones, relays sensory and motor signals, and regulates circadian rhythms (sleep-wake cycles).
• The limbic system controls emotions, memory, and learning.
• The reticular activating system directs information to appropriate brain centers and is essential for maintaining consciousness.

The Vertebral Column & Spinal Cord

• Connects the brain and PNS, allowing information transfer.

Peripheral Nervous System (PNS)

• Carries information between the CNS and body organs.
• Afferent nerves (sensory) carry signals to the CNS from receptors.
• Efferent nerves (motor) carry signals from the CNS to body organs.

Autonomic vs. Somatic Nervous System

• The autonomic nervous system regulates automatic functions like heart rate and digestion.
• The somatic nervous system controls skeletal muscles.
• The sympathetic division of the autonomic nervous system triggers the "fight or flight" response by releasing adrenaline, increasing heart rate, dilating pupils, and redirecting blood to muscles.
• The parasympathetic division of the autonomic nervous system lowers heart rate and restores normal body function ("rest & digest").
• Reflexes are automatic responses to stimuli controlled by the somatic nervous system, such as the knee-jerk or withdrawal reflex.

The Reflex Arc

• The components of the reflex arc include the receptor, sensory (afferent) neuron, interneuron, motor (efferent) neuron, and effector organ.
• The receptor detects a stimulus like a pinprick or loud noise.
• The sensory neuron carries the impulse to the spinal cord or brain.
• The interneuron processes the signal and sends a response.
• The motor neuron sends the response from the CNS to a muscle or organ.
• The effector organ executes the action, such as moving a hand away from heat.

Proprioceptors – Control of Movement

• Golgi tendon organs detect tension changes where the tendon meets the muscle, positioned in series with muscle fibers, and respond to increased muscle tension.
• Muscle spindles help maintain muscle tension, are located inside muscle fibers parallel to them, and respond to changes in muscle length.

Somatic Reflexes

• Somatic reflexes include the stretch reflex, Golgi tendon reflex, withdrawal reflex, and crossed-extensor reflex.

The Stretch Reflex

• Simplest spinal reflex relies on a direct synapse between afferent and efferent neurons.
• An example is the knee-jerk reflex.

Golgi Tendon Reflex

• Prevents excessive tension in muscles to avoid injury, acting as a protective reflex.

Withdrawal & Crossed-Extensor Reflexes

• The withdrawal reflex allows for rapid pulling away from painful stimuli before the brain processes it.
• The crossed-extensor reflex ensures one limb automatically compensates for the movement of the opposite limb (e.g., stepping on a sharp object causes the opposite leg to stabilize).

Concussions

• Caused when the brain collides with the skull, damaging nerve fibers.
• Symptoms include: headache, fatigue, memory loss, slurred speech, dizziness, nausea, sensitivity to light/noise, confusion, loss of consciousness (in severe cases), and difficulty concentrating.

Spinal Cord Injuries

• Quadriplegia is paralysis of all four limbs (arms and legs).
• Paraplegia is paralysis of the legs only.