Nervous System Overview

Nervous System Overview

• The nervous system has three main functions:
  • Sensory input – gathering information about changes inside and outside the body (stimuli)
  • Integration – processing and interpreting sensory input and deciding if action is needed
  • Motor output – a response to integrated stimuli activating muscles or glands

Structural Classification

• Central nervous system (CNS):
  • Brain
  • Spinal cord
• Peripheral nervous system (PNS):
  • Nerves outside the brain and spinal cord

Functional Classification of the PNS

• Sensory (afferent) division – carries information to the CNS
• Motor (efferent) division – carries impulses away from the CNS
  • Somatic nervous system – voluntary control of skeletal muscles
  • Autonomic nervous system – involuntary control of smooth muscle, cardiac muscle, and glands

Nervous Tissue: Support Cells (Neuroglia or Glia)

• Astrocytes: Star-shaped, abundant cells forming a barrier between capillaries and neurons, controlling the brain's chemical environment (CNS).
• Microglia (CNS): Spider-like phagocytes that dispose of debris.
• Ependymal cells (CNS): Line cavities of the brain and spinal cord, circulate cerebrospinal fluid.
• Oligodendrocytes (CNS): Produce myelin sheaths around nerve fibers in the CNS.
• Satellite cells (PNS): Protect neuron cell bodies in the PNS
• Schwann cells (PNS): Form myelin sheaths in the PNS

Neuroglia vs. Neurons

• Neuroglia divide; neurons do not.
• Most brain tumors are gliomas, involving neuroglia not neurons.
• Cell division is crucial in cancer.

Nervous Tissue: Neurons

• Neurons = nerve cells; specialized cells transmitting messages.
• Major regions of neurons:
  • Cell body – nucleus and metabolic center
  • Processes – fibers extending from the cell body (dendrites and axons)
  • Dendrites – conduct impulses toward the cell body
  • Axons – conduct impulses away from the cell body (only one per neuron)

Axons and Nerve Impulses

• Axons end in axonal terminals containing vesicles with neurotransmitters.
• Axonal terminals are separated by a synaptic cleft from the next neuron.
• Synapse – junction between nerves

Nerve Fiber Coverings

• Schwann cells – produce myelin sheaths around axons in the PNS in jelly-roll fashion.
• Nodes of Ranvier – gaps in the myelin sheath along the axon

Clinical Application: Multiple Sclerosis

• In multiple sclerosis, the myelin sheath is destroyed, hardening into scleroses.
• It is an autoimmune disease.

Neuron Cell Body Location

• Most neuron cell bodies are in the CNS (gray matter – cell bodies and unmyelinated fibers).
• Nuclei – clusters of cell bodies within the white matter of the CNS.
• Ganglia – collections of cell bodies outside the CNS.

Functional Classification of Neurons

• Sensory (afferent) neurons – carry impulses from sensory receptors (cutaneous and proprioceptors).
• Motor (efferent) neurons – carry impulses from the CNS to effectors.
• Interneurons (association neurons) – connect sensory and motor neurons within the CNS.

Structural Classification of Neurons

• Multipolar neurons – many extensions from the cell body
• Bipolar neurons – one axon and one dendrite
• Unipolar neurons – one short process leaving the cell body

How Neurons Function (Physiology)

• Irritability – ability to respond to stimuli
• Conductivity – ability to transmit an impulse.
• Plasma membrane at rest is polarized — fewer positive ions inside the cell than outside.

Continuation of the Nerve Impulse between Neurons

• Impulses cross the synapse to another nerve via neurotransmitters released from the axon terminal.
• The dendrite of the next neuron has receptors stimulated by the neurotransmitters.

The Reflex Arc

• Reflex – rapid, predictable, and involuntary response to a stimulus
• Reflex arc – direct route from a sensory neuron, to an interneuron, to an effector.

Types of Reflexes and Regulation

• Autonomic reflexes – regulation of smooth muscle, heart, blood pressure, glands, and the digestive system.
• Somatic reflexes – activation of skeletal muscles.

Regions of the Brain

• Cerebral hemispheres
• Diencephalon
• Brain stem
• Cerebellum

Cerebral Hemispheres (Cerebrum)

• Paired, superior parts of the brain, comprising more than half of the brain mass.
• Surface has ridges (gyri) and grooves (sulci).

Lobes of the Cerebrum

• Fissures divide the cerebrum into lobes.
• Frontal lobe, parietal lobe, occipital lobe, temporal lobe.

Specialized Areas of the Cerebrum

• Somatic sensory area – receives impulses from sensory receptors
• Primary motor area – sends impulses to skeletal muscles
• Broca's area – involved in speaking
• Gustatory area – (taste)
• Visual area
• Auditory area
• Olfactory area
• Interpretation areas of the cerebrum – speech/language region, language comprehension region, general interpretation area

Layers of the Cerebrum

• Gray matter – outer layer, composed mostly of neuron cell bodies
• White matter – fiber tracts inside the gray matter
  • Corpus callosum – connects hemispheres

Basal Nuclei

• Islands of gray matter within the cerebral white matter.
• Regulate voluntary motor activities by modifying information sent to the motor cortex.
• Problems with basal nuclei can cause jerky or spastic movements (Huntington's and Parkinson's diseases).

Diencephalon

• Situated above the brain stem, enclosed by the cerebral hemispheres.
• Composed of the thalamus, hypothalamus, and epithalamus.

Thalamus

• Surrounds the third ventricle; relay station for sensory impulses.
• Transfers impulses to the correct part of the cortex for localization and interpretation.

Hypothalamus

• Under the thalamus.
• Important autonomic nervous center.
• Helps to regulate body temperature and water balance.
• Regulates metabolism and part of the limbic system (emotions).
• Pituitary gland attached to it.

Epithalamus

• Forms the roof of the third ventricle and houses the pineal body (endocrine gland).
• Includes the choroid plexus for cerebrospinal fluid formation.

Brain Stem

• Attaches to the spinal cord.
  • Midbrain
  • Pons
  • Medulla oblongata

Midbrain ("Mesencephalon")

• Mostly composed of nerve tracts.
• Reflects vision and hearing, cerebral aqueduct connects 3rd and 4th ventricles.

Pons

• Bulging center part of the brain stem.
• Composd of fiber tracts.
• Nuclei control breathing.

Medulla Oblongata

• Lowest part of the brain stem.
• Merges into the spinal cord; contains important control centers for heart rate, blood pressure, breathing, swallowing , and vomiting.

Cerebellum

• Has two hemispheres with convoluted surfaces
• Provides involuntary coordination of body movements

Protection of the Central Nervous System

• Scalp and skin
• Skull and vertebral column
• Meninges:
  • Dura mater: Double-layered external covering (periosteum attached to the skull, meningeal layer covers the brain)
  • Arachnoid layer: Middle layer (web-like)
  • Pia mater: Internal layer, clings to the brain surface

Cerebrospinal Fluid

• Similar to blood plasma composition.
• Formed by the choroid plexus.
• Forms a watery cushion to protect the brain.
• Circulated in arachnoid space, ventricles, and central canal of the spinal cord.

Spinal Cord

• Extends from the medulla oblongata to the region of T12.
• Below T12: Cauda equina – collection of spinal nerves.
• Enlargements occur in the cervical and lumbar regions

Spinal Cord Anatomy

• Exterior white matter – conduction tracts.
• Internal gray matter – mostly cell bodies.
  • Dorsal (posterior) horns
  • Anterior (ventral) horns
• Central canal filled with cerebrospinal fluid.
• Meninges cover the spinal cord.
• Nerves leave at the level of each vertebra.
  • Dorsal root
  • Ventral root

Peripheral Nervous System

• Nerves and ganglia outside the CNS.
  • Nerve = bundle of neuron fibers
  • Neuron fibers bundled by connective tissue

Classification of Nerves

• Mixed nerves – both sensory and motor fibers.
• Afferent (sensory) nerves – carry impulses toward the CNS.
• Efferent (motor) nerves – carry impulses away from the CNS.

Spinal Nerves

• A pair at each vertebral level.

Explore the fundamental aspects of the nervous system, including its main functions: sensory input, integration, and motor output. Delve into the structural and functional classifications of the nervous system, distinguishing between the central and peripheral systems. Learn about the critical roles of neuroglia and the divisions of the peripheral nervous system.