Nervous Tissue Overview

• The nervous system detects and responds to changes inside and outside the body.
• The nervous system, along with the endocrine system, controls body functions and maintains homeostasis.
• Nervous system stimulation provides an immediate response, while endocrine activity is slower and prolonged.

Divisions of the Nervous System

• Central Nervous System (CNS): brain and spinal cord.
• Peripheral Nervous System (PNS): all other nerves.

Nervous Tissue Cells

• Neurons
• Schwann Cells
• Neuroglial/Glial Cells

Neuronal Anatomy

• Axons are communication routes between the cell body and axon terminals.
• Axons conduct electrical impulses away from the neuron's cell body and can be longer than 1 meter.
• Large axons in the peripheral nervous system are surrounded by a myelin sheath (series of Schwann cells).
• The myelin sheath is high in phospholipid (fatty) content, offering electrical insulation.
• Nodes of Ranvier are exposed areas between Schwann cells that assist rapid transmission of action potentials in myelinated neurons.
• Dendrites are extensions of the cell body that provide a large surface area to receive information to pass to the cell body.
• Dendrites have a similar structure to axons but are shorter, and the plasma has dense chemical ion channels for impulse transmission.
• Dendrites form synapses with other dendrites or terminate in specialized sensory receptors (e.g., skin).

Neuronal Properties

• Irritability: The ability to initiate nerve impulses in response to stimuli from inside or outside the body.
• Conductivity: The ability to transmit an impulse.

Glial Cell Types

• Neuroglial cells act as a support network.
• Three types of non-excitable glial (glue-like) cells support the neurons of the central nervous system and continue to replicate throughout life.
• Microglia are found mainly in the area of blood vessels and become phagocytic in areas of inflammation and cell destruction.
• Astrocytes form the main supporting tissues of the central nervous system, are star-shaped with fine branching processes, and lie in a mucopolysaccharide ground substance, found in large numbers adjacent to blood vessels.
• Oligodendrocytes are smaller than astrocytes and are found in clusters around the neuron cell wall, and adjacent to, and along the length of many myelinated nerve fibers in the CNS, forming and maintaining myelin with same function as Schwann cells in the peripheral nerves.

Blood-Brain Barrier

• It is a selective barrier protecting the brain from potentially toxic substances and chemical variations in the blood.
• Oxygen, carbon dioxide, glucose, and other lipid-soluble substances (e.g., alcohol) quickly cross the barrier into the brain.
• Some large molecules, many drugs, inorganic ions, and amino acids pass more slowly or do not pass at all from the blood to the brain.

Synaptic End Bulbs

• They are important for nerve impulse conduction from one neuron to another or neuron to an effector.
• They contain enclosed sacs called synaptic vesicles, which store neurotransmitters.

Synapse

• Consists of presynaptic neuron, presynaptic knob, neurotransmitters (acetylcholine/noradrenaline), calcium, mitochondria, neurotransmitter receptors, postsynaptic knob, and postsynaptic neuron.

Synaptic Transmission

• Nerves aren't joined or connected.
• The synapse is where the nerve impulse jumps from one nerve to another.
• Approximately 1000 trillion synapses exist in the brain.
• Synaptic knobs contain spherical, membrane-bound synaptic vesicles which store a chemical neurotransmitter that is released into the synaptic cleft.

Role of Neurotransmitters

• Synthesized by nerve cell bodies.
• Actively transported along the axons.
• Stored in the synaptic vesicles.
• Released by exocytosis in response to the action potential.
• Diffuse across the synaptic cleft.
• Act on specific receptor sites on the post-synaptic membrane.
• Action is short lived.
• After action, inactivated by enzymes or taken back into the synaptic knob.

Clinical Relevance to Dentistry

• Local anesthetics block nerve transmission to pain centers in the central nervous system by binding to and inhibiting the function of an ion channel (sodium channel) in the cell membrane of nerve cells, obstructing nerve impulses near the injection site without changes in awareness and sense perception in other areas.
• Excessive nerve impulses can result in neuralgia, most commonly trigeminal neuralgia.
• Herpes Simplex Virus lies dormant in the trigeminal ganglion, when re-activated it travels down the trigeminal nerve emerging on the lip and causing a cold sore.

CNS: Brain and Spinal Cord

• The brain weighs around 1.4 kg and lies in cranial cavity
• Comprises of Cerebrum, Thalamus, Hypothalamus, Midbrain, Pons, Medulla oblongata, Cerebellum

Cerebrum

• Largest part of the brain
• Three main functional areas associated with mental activities (memory, intelligence, problem solving, reflection, judgement, thinking, reasoning, initiative, inhibition, moral sense of responsibility and learning), sensory perception (perception of pain, temperature and touch), and initiation and control of voluntary muscle contraction.
• The cerebral cortex is highly wrinkled to increase the surface area of the brain and the amount of neurons within it.
• A deep furrow divides the cerebrum into two halves, known as the left and right hemispheres, connected by the corpus callosum (a bundle of axons).

Thalamus

• Small structure
• Sensory receptors in skin & viscera send info about touch, pain & temperature with input from the special sense organs travels here too where there is perception but only in basic form
• Thought to be involved in arousal and the processing of some emotions and complex reflexes

Hypothalamus

• Situated within the cerebrum and attached to the pituitary gland
• Controls the output of hormones
• Controls effectors of the autonomic nervous system including emotional reactions, hunger and thirst, body temperature and biological clocks

Midbrain

• Consists of nuclei and nerve fibres which connect the cerebrum with the lower parts of the brain and spinal cord.
• Nuclei act as relay stations for the ascending and descending nerve fibres and have important roles in auditory and visual reflexes.

Pons

• Contains nerve fibres, which form a bridge between the two hemispheres of the cerebrum
• Processes info from several of the cranial nerves

Medulla Oblongata

• Is continuous with the spinal cord
• Controls the cardiovascular centre, respiratory centre, and reflex centres: coughing, vomiting, sneezing, swallowing

Spinal Cord

• Nerves conveying impulses from the brain to various organs and tissues descend through it.
• Starts in the medulla oblongata and descends through the spinal column.
• Elongated, almost cylindrical part of the CNS.
• Suspended in the vertebral canal, surrounded by meninges & cerebro-spinal fluid
• Approx the thickness of a little finger and approx 45cms long

Functions of Cerebrospinal Fluid

• Cerebrospinal fluid (CSF) is a clear colourless fluid that:
  • Occupies the subarachnoid space in the brain
  • Acts as a "cushion" or buffer for the cortex
  • Is renewed 4 times daily
  • Contains proteins, glucose and lymphocytes
• The CSF protects the brain by ‘buffering’ the brain, lessening the impact when there is a blow to the head.
• Because the brain is immersed in fluid, the net weight of the brain is reduced from about 1400g to about 50g, thus reducing the pressure at the base of the brain.
• The one-way flow from the CSF to the blood takes potentially harmful metabolites, drugs and other substances away from the brain
• The CSF serves to transport hormones to certain areas of the brain, hormones released into the CSF can be carried to remote sites of the brain where they may act.

Autonomic Nervous System

• The sympathetic nervous system activates the “fight or flight” response during a threat or perceived danger and is needed for short term survival.
• The parasympathetic nervous system restores the body to a state of calm, the “rest and digest” phase and is needed for long term survival.