Nervous System Organization and Function

Nervous System Organization

Learning Outcomes

• Review of homeostasis and its importance in maintaining a stable internal environment.

• Identification of the structural and functional divisions of the nervous system, including the central nervous system (CNS), peripheral nervous system (PNS), somatic nervous system (SNS), autonomic nervous system (ANS), and enteric nervous system (ENS).

Homeostasis - Recap

• Definition: Equilibrium in the body’s internal environment, crucial for optimal cellular function.

• Disrupting Stimuli:

  • External: Heat, cold, lack of O_2, pathogens.

  • Internal: Psychological stress, exercise, abnormal blood pressure.

• Systems Responsible: Endocrine System (slower, longer-lasting responses) & Nervous System (faster, shorter-lasting responses).

Recap on Signaling

• Endocrine System:

  • Signals are sent via hormones, which travel through the bloodstream to target cells.

• Nervous System:

  • Signals are sent via nerve impulses (action potentials) along neurons.

• Speed of Signals:

  • The nervous system sends very rapid signals, allowing for quick responses to stimuli.

Components of a Feedback Loop

• Receptor: Monitors a controlled condition and detects changes.

• Control Center: Determines the next action based on the information received from the receptor.

• Effector: Receives directions from the control center and produces a response that changes the controlled condition, helping to restore homeostasis.

Functions of the Nervous System – Overview

1. Sensory Function (PNS):

   • Receptors sense changes in internal/external environment (e.g., temperature, pressure, light, chemicals).

   • Sensory (afferent) neurons conduct sensory impulses from receptors to the CNS.

2. Integrative Function (CNS):

   • Analyze and store (memory) sensory information.

   • Make decisions regarding appropriate responses.

   • Interneurons serve this function by processing sensory information and coordinating motor responses.

3. Motor Function (PNS):

   • Respond to stimuli by initiating action.

   • Motor (efferent) neurons conduct motor impulses to effectors.

   • Effectors = muscles & glands, which carry out the response.

Nervous System (NS) Divisions

• Central NS (CNS)

• Peripheral NS (PNS)

Nervous System - Structural Divisions

• Central Nervous System (CNS):

  • Brain: Control center for thought, memory, emotion, and regulation of bodily functions.

  • Spinal cord: Connects the brain to the peripheral nervous system; involved in reflexes.

• Peripheral Nervous System (PNS):

  • Cranial nerves (12 pairs - attached to base of brain): Innervate the head, neck, and torso, carrying sensory and motor information.

  • Spinal nerves (31 pairs - attached to spinal cord): Innervate the rest of the body, carrying sensory and motor information.

Structural Divisions - Functions

• CNS Functions:

  • Analysis and interpretation of neural input (sensory).

  • Coordination and execution of neural output (motor).

  • Reflexes (involuntary): Rapid, automatic responses to stimuli.

  • Conscious responses (voluntary): Intentional, controlled movements.

  • Logical thought.

  • Memory.

  • Emotions.

• PNS Functions:

  • Carry sensory (afferent) nerve impulses to the CNS.

  • Carry motor (efferent) nerve impulses away from the CNS.

Nervous System - Divisions (Functional)

• Central NS

  • Brain

  • Spinal cord

• Peripheral NS

  • Cranial nerves (12 pairs)

  • Spinal nerves (31 pairs)

• Somatic NS

• Autonomic NS

  • Sympathetic NS

  • Parasympathetic NS

• Enteric NS

Functional Divisions of PNS

• Somatic NS

• Autonomic NS

• Enteric NS (NOT EXAMINABLE)

Somatic Nervous System - SNS

• Structure:

  • Nerve fibers form part of both cranial and spinal nerves.

• Function:

  • Carries sensory input about body sensations and special senses to the CNS.

  • Carries motor output away from the CNS.

  • Allows conscious control of skeletal muscles (voluntary).

Autonomic Nervous System - ANS

• Function:

  • Subconscious control of organs and glands.

  • Carries sensory input from visceral (internal organs) receptors to the CNS.

  • Carries motor output to smooth muscle, heart muscle and/or glands (involuntary).

• Structure:

  • Nerve fibers part of both cranial and spinal nerves.

ANS - Sympathetic & Parasympathetic Divisions

• Sympathetic:

  • Prepares the body for emergencies or energy expenditure.

  • ‘Fight or flight’ response: Increased heart rate, dilated pupils, release of glucose.

• Parasympathetic:

  • ‘Restores’ body at rest - ‘rest & digest’.

  • Energy conservation and restoration: Decreased heart rate, increased digestive activity.

Organization of the Nervous System

• Central Nervous System (CNS): brain and spinal cord

• Peripheral Nervous System (PNS): all nervous tissue outside the CNS

  • Somatic Nervous System (SNS):

  • Somatic and special sensory receptors and somatic sensory neurons

  • Somatic motor neurons (voluntary)

  • Skeletal muscle

  • Autonomic Nervous System (ANS):

  • Autonomic sensory receptors and autonomic sensory neurons

  • Autonomic motor neurons (involuntary): sympathetic and parasympathetic divisions

  • Smooth muscle, cardiac muscle, and glands

  • Enteric Nervous System (ENS)

  • Enteric sensory receptors and enteric sensory neurons in GI tract and enteric plexuses

  • Enteric motor neurons (involuntary) in enteric plexuses

  • Smooth muscle, glands, and endocrine cells of GI tract.

Nervous System Functions - General Model

• Receptors

• Sensory neurons

• CNS (interneurons)

• Motor neurons

• Effectors

Steps in Detail

1. Sensory Receptor: Responds to a stimulus by producing a generator or receptor potential

2. Sensory Neuron: Axon conducts impulses from receptor to integrating center

3. Integrating Center: (one or more regions within the CNS that relay impulses from sensory to motor neurons)

4. Motor Neuron: Axon conducts impulses from integrating center to effector

5. Effector: Muscle or gland that responds to motor nerve impulses

Receptors

• Location:

  • External - on body surface e.g. skin, eye

  • Internal - within body tissue/organs e.g. baroreceptors, chemoreceptors

• Function:

  • Detect stimulus (change in internal/external environment)

  • Generate nerve impulses in sensory neurons

Receptor Types (by Stimulus Detected)

• Mechanoreceptors

• Thermoreceptors

• Nociceptors

• Photoreceptors

• Chemoreceptors

• Osmoreceptors

Receptors – Classified According to Stimulus Detected

• Mechanoreceptors:

  • Stimuli detected: Deformation, bending, stretching of cells

  • Sensations of touch, pressure, vibration, proprioception, hearing

• Thermoreceptors:

  • Stimuli detected: Changes in temperature

• Nociceptors:

  • Stimuli detected: Painful stimuli; Physical / chemical damage to tissues

• Photoreceptors:

  • Stimuli detected: Light; Retina of eye

• Chemoreceptors:

  • Stimuli detected: Chemicals (taste, smell, body fluids)

• Osmoreceptors:

  • Stimuli detected: Osmolarity of body fluids

Sensory (afferent) Neurons

• Location:

  • Part of the PNS

• Function:

  • Carry nerve impulses from receptors to the CNS

CNS - Interneurons

• Location:

  • CNS (brain & spinal cord - grey matter)

• Function:

  • Link sensory, motor and CNS neurons

  • Essential for coordination between various divisions of the nervous system

Motor (efferent) Neurons

• Location:

  • Part of the PNS

• Function:

  • Carry nerve impulses from the CNS to effectors

Effectors

• Are NOT part of the nervous system

• Are the:

  • muscles (skeletal, cardiac & smooth) and

  • glands

• …that alter their activity in response to motor nerve impulses

Nervous System Function - Example

• Receptors: detect increase in skin temperature (stimulus)

• Sensory neurons: carry impulses about T_{sk} to the CNS

• CNS: interprets/analyzes sensory nerve impulses; ‘decides’ on an appropriate response (e.g. sweating)

• Motor neurons: carry impulses to sweat glands, sweat

• Effectors: sweat glands produce more sweat and the skin cools (stimulus is reversed)

Summary

• Structural divisions of the NS

  • Central nervous system (CNS)

  • Peripheral nervous system (PNS)

• Functional divisions of the NS

  • Somatic nervous system

  • Autonomic nervous system (ANS) – sympathetic (fight or flight) & parasympathetic (rest & digest)

  • Enteric nervous system (information only)

• General model of NS function

  • Receptor, sensory neuron, CNS (interneuron), motor neuron, effector

• Receptors – classified relating to location or stimulus detected