Divisions and Functions of the Nervous System: Anatomy & Physiology Study Notes

Nervous System: Divisions by Anatomy

Central Nervous System (CNS)

The Central Nervous System (CNS) consists of the brain and spinal cord. It is responsible for integrating sensory information and responding accordingly.

• Brain: The control center for thoughts, emotions, and coordination of movement.

• Spinal Cord: Conducts signals to and from the brain; controls reflex activities.

• Nuclei: Clusters of neuron cell bodies within the CNS.

• Tracts: Bundles of axons in the CNS that transmit signals.

Example: The spinal cord mediates the knee-jerk reflex.

Peripheral Nervous System (PNS)

The Peripheral Nervous System (PNS) includes all neural structures outside the CNS. It connects the CNS to limbs and organs.

• Nerves: Bundles of axons that transmit signals between the CNS and the rest of the body.

• Ganglia: Clusters of neuron cell bodies in the PNS.

Example: The sciatic nerve is a major nerve in the PNS.

Gray Matter vs. White Matter

Gray matter contains neuron cell bodies, dendrites, and unmyelinated axons. White matter consists mainly of myelinated axons, which facilitate rapid signal transmission.

• Gray matter: Found in the cortex of the brain and central region of the spinal cord.

• White matter: Located in deeper brain regions and outer spinal cord.

Example: The corpus callosum is a white matter tract connecting the two cerebral hemispheres.

Nervous System: Divisions by Function

Sensory (Afferent) Division

The Sensory Division transmits information from sensory receptors to the CNS.

• Somatic Sensory: Carries signals from skin, muscles, and joints.

• Visceral Sensory: Carries signals from internal organs.

Example: Touch receptors in the skin send signals to the CNS via somatic sensory fibers.

Motor (Efferent) Division

The Motor Division transmits commands from the CNS to effectors (muscles and glands).

• Somatic Motor: Controls voluntary movements via skeletal muscles.

• Autonomic (Visceral) Motor: Regulates involuntary functions (e.g., heart rate, digestion).

  • Sympathetic Division: Prepares body for 'fight or flight' responses.

  • Parasympathetic Division: Promotes 'rest and digest' activities.

Example: The autonomic nervous system increases heart rate during exercise.

Main Functions of the Nervous System

The nervous system is the master controlling and communicating system of the body. Its main function is to maintain homeostasis (dynamic constancy of the internal environment).

• Integration: Processes and interprets sensory input.

• Sensory Input: Detects changes inside and outside the body.

• Motor Output: Activates effectors to produce a response.

Example: Withdrawal of hand from a hot surface is a motor response to sensory input.

Homeostasis and Feedback Loops

Negative Feedback Loop

Negative feedback is a mechanism that reverses a change to maintain homeostasis. It is the most common feedback system in the body.

• Stimulus: Produces a change in variable.

• Sensor: Detects the change.

• Control Center: Processes information and determines response.

• Effector: Produces a response to restore balance.

Example: Regulation of body temperature.

Positive Feedback Loop

Positive feedback amplifies the original stimulus, leading to an even greater response. It is less common but important in specific physiological processes.

• Example: Blood clotting and childbirth (oxytocin release).

Homeostatic Variables and Control Table

The following table summarizes key homeostatic variables, their set points, sensors, and effectors:

Variable	Set Point	Sensor	Effector
Blood Pressure	<120/80	Stretch receptors in vessel walls	Small vessels, heart
Body Temperature	37°C (98.6°F)	Temperature sensors	Behavior/invest
Blood Glucose Concentration	70-140 mg/dL	Stretch receptor in pancreas	Insulin/glucagon
Water Volume in Body	70% of body weight	Stretch receptor in hypothalamus	Kidney/thirst center in brain

Reflexes and Reflex Arcs

Somatic Reflex Arc

A reflex arc is the neural pathway that mediates a reflex action. Reflexes are rapid, stereotyped responses to stimuli.

• Receptor: Detects stimulus.

• Sensory Neuron: Transmits impulse to CNS.

• Integration Center: Processes information (may involve interneurons).

• Motor Neuron: Transmits impulse to effector.

• Effector: Muscle or gland that responds.

Types of Reflexes:

• Ipsilateral Reflex: Response occurs on the same side as the stimulus.

• Contralateral Reflex: Response occurs on the opposite side of the body.

Example: The withdrawal reflex is an ipsilateral somatic reflex.

Key Terms and Definitions

• Homeostasis: Maintenance of a stable internal environment.

• Neuron: Nerve cell specialized for signal transmission.

• Effector: Organ or cell that acts in response to a stimulus.

• Integration Center: Region within CNS that processes sensory input.

• Feedback Loop: Mechanism for regulating physiological variables.

Relevant Equations

• Blood Pressure: $\text{Blood Pressure} = \text{Cardiac Output} \times \text{Peripheral Resistance}$

• Homeostatic Control: $\text{Variable} \xrightarrow{\text{Stimulus}} \text{Sensor} \xrightarrow{\text{Input}} \text{Control Center} \xrightarrow{\text{Output}} \text{Effector} \xrightarrow{\text{Response}} \text{Balance}$

Additional info: Academic context and definitions have been expanded for clarity and completeness.