Organization of the Human Nervous System

Organization of the Human Nervous System

Overview

• The nervous system is a complex network of cells governing involuntary and voluntary behavior and maintaining homeostasis.
• Functions include:
  • Sensation and perception
  • Motor function
  • Cognition, thinking, and problem-solving
  • Executive function and planning
  • Language comprehension and creation
  • Memory
  • Emotion and emotional expression
  • Balance and coordination
  • Regulation of endocrine organs
  • Regulation of heart rate, breathing rate, vascular resistance, temperature, and exocrine glands
• The nervous system consists of over 100 billion cells that communicate and regulate signals.
• Action occurs when the body reacts to external stimuli using the nervous system.

Central and Peripheral Nervous System

• Three types of nerve cells:
  • Sensory neurons (afferent neurons): Transmit sensory information from sensory receptors to the spinal cord and brain.
  • Motor neurons (efferent neurons): Transmit motor information from the brain and spinal cord to muscles and glands.
  • Interneurons: Found between other neurons, predominantly in the brain and spinal cord, linked to reflexive behavior.
• Processing of stimuli and response generation:
  • May occur at the spinal cord level
  • May require input from the brainstem or cerebral cortex
• Reflexes only require processing at the level of the spinal cord.
  • Example: Patellar tendon reflex.
    • Sensory information goes to the spinal cord.
    • A motor signal is sent to the quadriceps muscle.
    • The leg jerks forward at the knee.
    • No brain input is required.
• Supraspinal circuits are used when input from the brain or brainstem is required.

Structure of the Human Nervous System

• Two primary components:
  • Central Nervous System (CNS)
  • Peripheral Nervous System (PNS)

Central Nervous System (CNS)

• Composed of the brain and spinal cord.
• Brain:
  • White matter: Axons encased in myelin sheaths, lies deeper than gray matter.
  • Gray matter: Unmyelinated cell bodies and dendrites.
  • Brainstem: Responsible for basic life functions (e.g., breathing).
• Spinal Cord:
  • Extends downward from the brainstem.
  • Divided into four regions: cervical, thoracic, lumbar, and sacral.
  • Provides sensory and motor innervation to structures below the neck.
  • Protected by the vertebral column.
  • White matter: Lies on the outside of the cord.
  • Gray matter: Lies deep within.
  • Sensory neurons bring information in from the periphery and enter on the dorsal side.
  • Cell bodies of sensory neurons are in the dorsal root ganglia.
  • Motor neurons exit the spinal cord ventrally.

Peripheral Nervous System (PNS)

• Made up of nerve tissue and fibers outside the brain and spinal cord.
• Includes all 31 pairs of spinal nerves and 10 of the 12 pairs of cranial nerves (excluding olfactory and optic nerves).
• Connects the CNS to the rest of the body.
• Subdivided into:
  • Somatic Nervous System
  • Autonomic Nervous System

Somatic Nervous System

• Sensory and motor neurons distributed throughout the skin, joints, and muscles.
• Sensory neurons transmit information through afferent fibers.
• Motor impulses travel along efferent fibers.

Autonomic Nervous System (ANS)

• Regulates heartbeat, respiration, digestion, and glandular secretions.
• Manages involuntary muscles associated with internal organs and glands.
• Regulates body temperature by activating sweating or piloerection.
• Functions are automatic or independent of conscious control.
• Peripheral component contains two neurons in series:
  • Preganglionic neuron: Soma in the CNS, axon travels to a ganglion in the PNS.
  • Postganglionic neuron: Cell body in the ganglion, stimulates the target tissue.
• A motor neuron in the somatic nervous system goes directly from the spinal cord to the muscle without synapsing.

Subdivisions of the Autonomic Nervous System

• Sympathetic Nervous System
• Parasympathetic Nervous System

• Conserves energy, associated with resting and sleeping states.
• Reduces heart rate and constricts the bronchi.
• Manages digestion by increasing peristalsis and exocrine secretions.
• Acetylcholine is the neurotransmitter released by preganglionic and postganglionic neurons.
• Vagus nerve (cranial nerve X) is responsible for much of the parasympathetic innervation of the thoracic and abdominal cavity.

• Activated by stress (fight or flight reactions).
• Increases heart rate.
• Redistributes blood to muscles of locomotion.
• Increases blood glucose concentration.
• Relaxes the bronchi.
• Decreases digestion and peristalsis.
• Dilates the eyes.
• Releases epinephrine into the bloodstream.
• Preganglionic neurons release acetylcholine, while postganglionic neurons release norepinephrine.

Reflexes

• Neural circuits (reflex arcs) control reflexive behavior.
• Example: Stepping on a nail.
  • Receptors in the foot detect pain.
  • Sensory neurons transmit the pain signal to the spinal cord.
  • Interneurons relay pain impulses to the brain.
  • Interneurons also send signals to muscles in both legs, causing withdrawal of the foot and support with the other foot.
  • The muscles respond before the brain receives the sensory information.

Types of Reflex Arcs

• Monosynaptic Reflex Arc
• Polysynaptic Reflex Arc

Monosynaptic Reflex Arc

• Single synapse between the sensory neuron and the motor neuron.
• Example: Knee-jerk reflex.
  • The patellar tendon is stretched.
  • Information travels up the sensory afferent neuron to the spinal cord.
  • Interfaces with the motor efferent neuron that causes contraction of the quadriceps muscles.
  • Results in the extension of the leg.
• Feedback loop in response to potential injury.

Polysynaptic Reflex Arc

• At least one interneuron between the sensory and motor neurons.
• Example: Withdrawal reflex (stepping on a nail).
  • The foot is stimulated to flex, pulling it away from the nail.
  • To maintain balance, the other foot must be planted firmly on the ground.
  • Interneurons connect the incoming sensory information to the motor neurons in the supporting limb.