Nervous Tissue and Neuroglia

Histology of Nervous Tissue

The nervous tissue is composed of two main cell types: neurons and neuroglia. Neurons are the functional units responsible for transmitting electrical signals, while neuroglia provide support and protection.

• Neurons: Specialized cells for communication via electrical impulses.

• Neuroglia (Glial Cells): Support, nourish, and protect neurons; found in both CNS and PNS.

• Types of Neurons: Multipolar, bipolar, and unipolar neurons, classified by the number and arrangement of their processes.

Example: Multipolar neurons are common in the CNS, while unipolar neurons are typical in sensory pathways.

Structure of a Neuron

• Cell Body (Soma): Contains the nucleus and organelles.

• Dendrites: Receive incoming signals.

• Axon: Transmits impulses away from the cell body.

• Myelin Sheath: Insulating layer formed by Schwann cells (PNS) or oligodendrocytes (CNS).

Additional info: Myelination increases the speed of impulse conduction via saltatory conduction.

Membrane Potentials and Action Potentials

Resting Membrane Potential

Neurons maintain a voltage difference across their membrane, known as the resting membrane potential, due to differences in ion composition and membrane permeability.

• Typical Value: -70 mV (inside negative relative to outside)

• Key Ions: Sodium (Na+), Potassium (K+), Chloride (Cl-)

Equation:

(Nernst equation for potassium)

Action Potential

An action potential is a rapid change in membrane potential that propagates along the axon.

• Depolarization: Na+ channels open, Na+ enters the cell.

• Repolarization: K+ channels open, K+ exits the cell.

• Hyperpolarization: Membrane potential becomes more negative than resting.

• All-or-None Phenomenon: Action potentials occur fully or not at all.

Propagation: Action potentials travel along the axon, enabling communication between neurons.

Graded Potentials

• Definition: Small changes in membrane potential that vary in size and decay with distance.

• Role: Initiate action potentials if threshold is reached.

Synapses and Neurotransmitters

Chemical Synapse

A chemical synapse is a junction where neurotransmitters are released from one neuron to stimulate another.

• Presynaptic Neuron: Releases neurotransmitter.

• Postsynaptic Neuron: Receives signal via receptors.

Neurotransmitter Classifications and Receptors

• Neurotransmitters: Chemical messengers such as acetylcholine, dopamine, serotonin, and GABA.

• Receptors: Proteins on the postsynaptic membrane that bind neurotransmitters and initiate a response.

Additional info: Neurotransmitters can be excitatory or inhibitory, affecting the likelihood of action potential generation.

Central Nervous System (CNS)

Embryonic Development and Ventricles

The CNS develops from the neural tube, forming structures such as the brain and spinal cord. Ventricles are fluid-filled cavities within the brain.

• Ventricles: Lateral, third, and fourth ventricles; contain cerebrospinal fluid (CSF).

Cerebral Hemispheres and Cortex

• Cerebral Hemispheres: Right and left halves of the brain.

• Cortex: Outer layer responsible for higher functions (motor, sensory, association areas).

• Gray Matter: Neuron cell bodies.

• White Matter: Myelinated axons.

Diencephalon and Brainstem

• Diencephalon: Includes thalamus, hypothalamus, epithalamus.

• Brainstem: Includes midbrain, pons, medulla oblongata; controls vital functions.

Cerebellum

• Function: Coordination of movement and balance.

Meninges and Protection of the Brain

• Meninges: Three layers—dura mater, arachnoid mater, pia mater.

• Function: Protect the brain and spinal cord.

Cerebrospinal Fluid (CSF) and Blood-Brain Barrier

• CSF: Cushions the brain, provides nutrients, removes waste.

• Blood-Brain Barrier: Selective barrier preventing harmful substances from entering the brain.

Spinal Cord and Spinal Nerves

Structure and Function

• Spinal Cord: Transmits signals between brain and body; contains gray and white matter.

• Spinal Nerves: 31 pairs; each has a dorsal (sensory) and ventral (motor) root.

Reflex Arcs

• Components: Receptor, sensory neuron, integration center, motor neuron, effector.

• Example: Knee-jerk (patellar) reflex.

Peripheral Nervous System (PNS)

Cranial Nerves

• Number: 12 pairs.

• Function: Sensory, motor, or mixed.

• Origin: Brainstem.

Spinal Nerves

• Number: 31 pairs.

• Function: Connect CNS to limbs and trunk.

Autonomic Nervous System (ANS)

Divisions and Pathways

The ANS regulates involuntary functions and is divided into sympathetic and parasympathetic divisions.

• Sympathetic: Prepares body for 'fight or flight'.

• Parasympathetic: Promotes 'rest and digest'.

• Pathways: Involve preganglionic and postganglionic neurons.

Neurotransmitters and Receptors in ANS

• Sympathetic: Uses norepinephrine and acetylcholine.

• Parasympathetic: Uses acetylcholine.

Comparison Table: Sympathetic vs. Parasympathetic

Special Senses

Eye and Vision

• Accessory Structures: Eyelids, lacrimal apparatus, eye muscles.

• Layers of the Eye: Fibrous (sclera, cornea), vascular (choroid, ciliary body, iris), neural (retina).

• Retina: Contains photoreceptors—rods (dim light) and cones (color vision).

• Focusing: Accommodation of the lens for near and distant vision.

Additional info: The fovea centralis is the area of sharpest vision.

Ear and Hearing/Balance

• External Ear: Auricle and external auditory canal.

• Middle Ear: Tympanic membrane, ossicles (malleus, incus, stapes).

• Inner Ear: Cochlea (hearing), vestibular apparatus (balance).

Example: The cochlea converts sound waves into electrical signals for the brain.

Microscopic Anatomy

• Retina: Layers include photoreceptors, bipolar cells, ganglion cells.

• Cochlea: Contains hair cells that detect sound vibrations.

Summary Table: Major Structures and Functions