Nervous System Overview

Nervous System

• The nervous system is divided into two primary sections: the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS includes the brain and spinal cord. It controls and coordinates bodily functions.
• The PNS includes all the nerves outside of the CNS. It connects the CNS to the rest of the body.
• The nervous system monitors and processes sensory information from inside and outside the body. It combines new information with stored information to create responses.

Nervous System Divisions

• Central Nervous System (CNS):
  • Includes the brain and spinal cord.
• Peripheral Nervous System (PNS):
  • Includes nerves outside the CNS.
  • Includes: nerves, sensory receptors, and clusters of neuron cell bodies.

Nervous Tissue

• Despite its complexity, nervous tissue is made up of only two cell types.
• Neurons: Handle communication, sending signals.
• Neuroglia(or Glial Cells): Support and protect neurons.

Neuroglia (Glial Cells)

• Also called "nerve glue."
• Types in the CNS:
  • Astrocytes
  • Oligodendrocytes
  • Microglial cells
  • Ependymal cells
• Types in the PNS:
  • Schwann cells (also called neurolemmocytes)
  • Satellite cells
• Function: Support and protect the fragile neurons, acting as phagocytes (cleaning up waste and debris), producing myelin,helping with exchanges between capillaries and neurons, and controlling the chemical surroundings of neurons.

Neurons (Nerve Cells)

• These are the basic functional units of the nervous system, specialized to send messages (nerve impulses) throughout the body.
• They all have a cell body, from which extensions (axons and dendrites) come out.

Neuron Cell Body

• Biosynthetic center of the neuron, producing materials needed for the neuron.
• Receptive region that receives signals.
• Contains the nucleus and cytoplasm.
• Contains Neurofibrils and chromatophilic substance.
  • Neurofibrils: cytoskeletal elements supporting the neuron and transporting substances within it.
  • Chromatophilic substance: clusters of rough endoplasmic reticulum involved in protein synthesis.

Neuron Processes

• Dendrites: Receive signals from other neurons, have special receptors that detect chemicals (neurotransmitters) released by other neurons' axons.
• Axons: Responsible for sending out electrical signals (impulses) from the neuron.

Axon Terminals and Synapses

• Axon terminals connect to other neurons or cells (like muscles or glands) at junctions called synapses.
• These terminals store neurotransmitters in tiny sacs (vesicles). Neurotransmitters are released allowing the signal to continue to the next neuron or target..

Myelin and Myelinated Fibers

• Nerve fibers covered with myelin, a fatty material, are called myelinated fibers.

Schwann Cells and Myelin Sheath

• Myelin in the PNS is produced by Schwann cells. These cells wrap around the axon, forming the myelin sheath.

Discontinuous Myelin Sheath

• Gaps between Schwann cells are called myelin sheath gaps or nodes of Ranvier.

Types of Neurons

• Multipolar Neurons: Many processes (one axon, many dendrites). Found in the brain, spinal cord, and neurons carrying signals away from the CNS.
• Bipolar Neurons: Two processes (one dendrite, one axon). Rare, found in sensory organs (eyes, ears, and olfactory mucosa).
• Unipolar Neurons: One short process splits into peripheral (receptive) and central (axon) processes. Found in neurons sending signals to the CNS. Also called pseudounipolar neurons.

Classification by Function

• Sensory (Afferent) Neurons: Carry signals toward the CNS from sensory receptors. Cell bodies in ganglia outside the CNS. Typically unipolar
• Motor (Efferent) Neurons: Carry signals away from the CNS to muscles and glands. Usually multipolar, have cell bodies within the CNS.
• Interneurons: Found in the CNS, connecting sensory and motor neurons. Usually multipolar.

Structure of a Nerve

• Axon bundles can be called tracts in the CNS, or nerves in the PNS.

  • Sensory (afferent) nerves: Carry impulses to CNS
  • Motor (efferent) nerves: Carry impulses from CNS
  • Mixed nerves: Contain both sensory and motor fibers (most nerves, including spinal nerves)

• Connective Tissue Layers: Three layers surround nerves.

  • Endoneurium: Surrounds individual axons
  • Perineurium: Surrounds bundles of axons called fascicles
  • Epineurium: Encloses all fascicles and forms the outer nerve covering.

Cranial Nerves

• 12 pairs of nerves that emerge from the brain, primarily serving the head and neck (except the vagus nerve which extends further).

Spinal Cord

• Protected by meninges.
  • Dura Mater
  • Arachnoid Mater
  • Pia Mater
• CSF (Cerebrospinal Fluid) fills the subarachnoid space cushioning the brain and spinal cord.
• 31 pairs of spinal nerves emerge from the spinal cord. They serve areas near the point where they emerge.
• There is a collection of nerves called the cauda equina below the spinal cord.

Spinal Cord Damage

• Damage to the spinal cord results in loss of motor and sensory functions in the affected body areas.
  • Paraplegia: Paralysis in both legs
  • Quadriplegia: Paralysis in all four limbs

Spinal Nerves (further detail)

• Divided into 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal pairs.
• Emergence of Spinal Nerves: C1 exits between skull and atlas. All others exit the spinal cord below the associated vertebrae.

Nerve Plexuses

• Networks of nerves formed by ventral rami (except thoracic).
• Primary serve the muscles and skin of the limbs.

Cervical Plexus

• Network of nerves formed by ventral rami of C1-C5.
• Important for supplying the shoulder and neck muscles, including the phrenic nerve which is essential for breathing.

Brachial Plexus

• Large and complicated network of nerves formed by ventral rami of C5-T1.
• Controls the muscles and sensations in the arms, shoulders and hands.

Sciatic Nerve

• Largest nerve in the body; serves the lower limb.
• Injury can result from falls or disc herniation. The injury can cause impairments in the lower limb.

Autonomic Nervous System (ANS)

• A subdivision of the PNS, regulating involuntary bodily functions.

• Controls smooth muscles, cardiac muscles and glands; two-neuron chain.

  • Preganglionic neurons: Cell bodies in the brainstem or spinal cord.
    • Their axons leave the CNS to synapse with the next neuron.
  • Postganglionic neurons: Cell bodies are in autonomic ganglia outside the CNS.
    • Their axons extend to the organ they serve.

• Has two main divisions:

  • Sympathetic division: Prepares the body for fight or flight

• Parasympathetic division: Promotes rest and digest

Aging and Sympathetic Nervous System

• As we age, the efficiency of the sympathetic nervous system, specifically its ability to cause vasoconstriction (narrowing of blood vessels) decreases.
• This can lead to light-headedness or fainting when standing up quickly (orthostatic hypotension).

Reflexes

• Rapid, predictable, involuntary motor responses to stimuli, occurring through neural pathways called reflex arcs.
• Inborn (intrinsic) reflex: Automatic, often not requiring conscious thought.
• Learned (acquired) reflex: Developed through experience.

Types of Reflexes

• Autonomic (visceral) reflexes: Involve smooth and cardiac muscles, and glands; controlling digestion and elimination.
• Somatic reflexes: Involve skeletal muscles.

Five Components of Reflex Arcs

• Receptor: Site where the stimulus acts.
• Sensory neuron: Carries afferent impulses to the CNS.
• Integration center: Processes information (monosynaptic: direct connection; Polysynaptic: more than one interneuron).
• Motor neuron: Transmits efferent impulses from the CNS.
• Effector: Muscle or gland that responds to the impulse by contracting or secreting.

Cranial Nerve Reflexes

• Corneal reflex: Mediated by the trigeminal nerve. Protects the eye, absence can indicate brainstem damage.
• Gag reflex: Tests somatic motor responses of cranial nerves IX & X. Assesses the cranial nerves' and brainstem health.

Autonomic Reflexes

• Pupillary reflex: Controls pupil size in response to light; involves the consensual reflex.
• Ciliospinal reflex: A reflex involving pupillary responses, and demonstrates the nervous system's link between brain stem and eye/face regions.

Reaction Time of Reflexes

• Several factors influence reaction time, including receptor sensitivity, speed of nerve impulse conduction, the number of neurons/synapses involved, and effector activation speed.
• Intrinsic reflexes are fast because they are hardwired; learned reflexes are slower and involve higher brain activities.

Explore the fundamental concepts of the nervous system, including its divisions, the roles of the central and peripheral nervous systems, and the types of nervous tissue. This quiz will test your knowledge on how the nervous system functions and its components.