Introduction to Neurophysiology

Introduction to Neurophysiology

• The nervous system is the highest body control system, sharing this role with the endocrine system
• The functional unit is the neuron, responsible for integration and transmission
• The nervous system has two primary divisions:
  • Central Nervous System (CNS): Includes the brain and spinal cord
  • Peripheral Nervous System (PNS): Includes sensory receptors, sensory nerves, and ganglia outside the CNS
  • The CNS and PNS extensively communicate with each other.

Objectives

• Students will be able to identify the nervous system as the highest control system
• Students will be able to identify the functional divisions of the nervous system
• Students will be able to name and describe the parts of a neuron and their functions
• Students will be able to list and describe different nerve fiber types in mammals
• Students will be able to define reflex action and describe the components of a reflex arc
• Students will be able to outline the functional organization of the sensory and motor system
• Students will be able to identify the higher functions of the brain

Anatomical Division of the Nervous System

• The CNS consists of the brain and spinal cord
• The PNS consists of spinal nerves and cranial nerves
• There are 12 pairs of cranial nerves
• There are 31 pairs of spinal nerves

Functional Division of the Nervous System

• The nervous system has two major functional divisions:
  • Somatic nervous system: Communicates with sensory organs and voluntary muscles
  • Autonomic nervous system: Communicates with internal organs and glands and further divides into: Sympathetic and Parasympathetic division
  • Sympathetic division (arousing)
  • Parasympathetic division (calming)
• Sensory (afferent) nervous system (sensory input)
• Motor (efferent) nervous system (motor output)

Sensory System

• Sensory receptors detect sensory information in the periphery
• Sensory receptors include visual, auditory, chemoreceptors, and somatosensory (touch) receptors
• Sensory nerve pathways transmit sensory information to sensory areas of the CNS (e.g. cerebral cortex)
• The sensory cortex perceives the transmitted sensory information as different sensations (touch, pressure, electromagnetic waves, warmth, cold, pain, etc.)

Motor System

• The motor or efferent division carries information out of the nervous system to the periphery
• The motor response involves the contraction of skeletal muscles, smooth muscles, or cardiac muscles, or secretion by endocrine and exocrine glands

Basic Tasks

• Sensory receptors monitor internal and external environments
• The CNS analyzes sensory information and integrates it often with stored information
• The PNS signals effector organs such as muscles or glands when necessary

Integration of Sensory Information

• The CNS decodes incoming sensory impulses
• Sensory information from several sources may be integrated
• The CNS may integrate information with information stored in memory

Signaling Effectors to Respond

• The CNS sends nerve impulses to effector organs (muscles, glands, etc.).
• This allows the body to appropriately respond
• Sometimes the process of signal transmission and decoding by effector organs happens only via simple spinal reflexes

Neurons

• Nerve cells (neurons) are the structural units of the nervous system
• Neurons have a cell body, dendrites, axon, axon hillock, myelin sheath, nodes of Ranvier, and terminal branches/bulbs
• Neurons are specialized for impulse transmission
• There are four functional domains of a neuron: input region, trigger zone, propagation region, and chemical release zone.

Types of Neurons

• Neurons are classified based on structural differences into bipolar, pseudo-unipolar, and multipolar types
• Neurons also have several ways to functionally classify them.

Types of Nerve Fibers

• Nerve fibers are classified by conduction velocity (dependent on fiber size and presence/absence of myelin) into two classification systems: A, B, C fibers and I, II, III, IV fibers.
• Large myelinated fibers have fast conduction velocities
• Small unmyelinated fibers have slow conduction velocities.

Reflex Action

• Reflexes are involuntary responses to stimuli
• Reflexes are an essential component of homeostasis
• Reflex actions occur via a pathway called the reflex arc
• The reflex arc involves a receptor, afferent nerve, central nervous system, efferent nerve, and effector

General Functions of the Nervous System

• The nervous system has three basic functions: sensory, integrative, and motor functions
• Sensory function involves reception of stimuli and sending of information to the brain and spinal cord via the PNS
• Integrative function analyses sensory information to create a response
• Motor function sends instructions via the PNS to effector organs

Sensory & Motor Systems

• Sensory and motor systems work in conjunction to allow actions
• Sensory systems include many receptors on the skin, inside the body, and in the muscles
• Some prominent sensory systems involved are somatic, eye, ear

Functional Integration of CNS

• Brain functions high intellectual functions (Neocortex), behavior and emotions (Limbic system), and vegetative functions (Paleocortex)
• Primitive brain

Functional Organization of the Sensory System

• Sensory receptors receive sensory information and convert them to electrical signals
• Sensory pathways transmit the information to sensory areas of the CNS
• The sensory cortex then perceives these signals as different sensations

Functional Organization of the Motor System

• Voluntary movements are initiated by the cerebral cortex and carried through descending motor tracts
• Muscle tone is involuntary activity of skeletal muscles and is maintained through stretch reflexes
• Coordination of movement stems from the cerebellum and basal ganglia

Higher Functions of the Brain

• Higher functions include language, speech, perception, memory, learning, consciousness, sleep, arousal, high intellectual functions, and behavior.

Schwann Cells

• Schwann cells form the myelin sheath in the PNS
• Schwann cells aid in the regeneration of axons

Glial Cells (Neuroglia)

• Glial cells (neuroglia) are much more common than neurons
• Astrocytes form the blood-brain barrier and maintain the chemical environment for nerve impulse generation
• Oligodendrocytes form the myelin sheath in the CNS, increasing speed of nerve impulse conduction
• Microglial cells act as phagocytes in the CNS

What is a Nerve?

• A nerve is a bundle of axons outside the CNS
• There are varying types of nerves: mixed nerves, sensory nerves

Cranial and Spinal Nerves

• There are 12 pairs of cranial nerves, most connecting with the brain stem
• 31 pairs of spinal nerves branch out from the spinal cord to the different parts of the body

References

• Lippincott's Illustrated Reviews: Physiology, 2013
• Linda S. Costanzo, Physiology (6th Ed), 2018