lecture exam 4: neural tissue and the nervous System

Neural Tissue and the Nervous System

Overview of Neural Tissue

Neural tissue is specialized for the conduction of electrical impulses and is essential for communication within the body. It consists of two main types of cells: neurons and neuroglia (glial cells).

• Neurons: Cells that send and receive electrical signals.

• Neuroglia (Glial Cells): Support and protect neurons.

The organs of the nervous system include the brain, spinal cord, sensory receptors (such as those in the eyes and ears), and nerves that connect the nervous system to other body systems.

Anatomical Divisions of the Nervous System

Central Nervous System (CNS)

• Consists of the brain and spinal cord.

• Contains neural tissue, connective tissues, and blood vessels.

• Functions:

  • Processes and coordinates sensory data from inside and outside the body.

  • Issues motor commands to control peripheral organs (e.g., skeletal muscles).

  • Responsible for higher functions such as intelligence, memory, learning, and emotion.

Peripheral Nervous System (PNS)

• Includes all neural tissue outside the CNS.

• Functions:

  • Delivers sensory information to the CNS.

  • Carries motor commands to peripheral tissues and systems.

• Peripheral nerves: Bundles of axons with connective tissues and blood vessels.

  • Cranial nerves: Connect to the brain.

  • Spinal nerves: Attach to the spinal cord.

Functional Divisions of the PNS

• Afferent division: Carries sensory information from PNS sensory receptors to CNS.

• Efferent division: Carries motor commands from CNS to PNS muscles and glands.

  • Somatic Nervous System (SNS): Controls voluntary and involuntary (reflex) skeletal muscle contractions.

  • Autonomic Nervous System (ANS): Controls subconscious actions, such as contractions of smooth and cardiac muscle, and glandular secretions.

    • Sympathetic division: Stimulating effect.

    • Parasympathetic division: Relaxing effect.

Neurons: Structure and Function

General Structure of a Multipolar Neuron

• Cell body (soma): Contains the nucleus, nucleolus, cytoplasm (perikaryon), mitochondria, rough endoplasmic reticulum (RER), and ribosomes.

• Cytoskeleton: Includes neurofilaments, neurotubules, and neurofibrils (support dendrites and axon).

• Nissl bodies: Dense areas of RER and ribosomes; give gray matter its color.

• Dendrites: Highly branched, with dendritic spines that receive information from other neurons (80–90% of neuron surface area).

• Axon: Long process that carries electrical signals (action potentials) to target cells.

  • Axoplasm: Cytoplasm of the axon.

  • Axolemma: Specialized cell membrane covering the axon.

  • Axon hillock: Thick section of cell body where the axon originates.

  • Initial segment: Attaches to the axon hillock.

  • Collaterals: Branches of a single axon.

  • Telodendria: Fine extensions at the distal end of the axon.

  • Synaptic terminals: Tips of the axon where communication with other cells occurs.

Synapse

• Area where a neuron communicates with another cell.

• Presynaptic cell: Neuron sending the message.

• Postsynaptic cell: Cell receiving the message.

• Synaptic cleft: Small gap between presynaptic and postsynaptic membranes.

• Synaptic knob: Expanded area of axon containing synaptic vesicles filled with neurotransmitters.

• Neurotransmitters: Chemical messengers released at the presynaptic membrane, affecting receptors on the postsynaptic membrane. They are broken down by enzymes and reassembled at the synaptic knob.

• Types of synapses:

  • Neuromuscular junction: Between neuron and muscle.

  • Neuroglandular junction: Between neuron and gland.

Structural Classification of Neurons

Type	Location	Features
Anaxonic	Brain, sense organs	Small, indistinguishable axons and dendrites
Bipolar	Special sensory organs (sight, smell, hearing)	One dendrite, one axon, cell body in the middle
Unipolar	Sensory neurons of PNS	Fused dendrites and axon, cell body to one side
Multipolar	CNS, skeletal muscle motor neurons	Multiple dendrites, one long axon

Functional Classification of Neurons

Type	Function	Structure
Sensory (Afferent)	Transmit sensory information to CNS	Unipolar, cell bodies in sensory ganglia
Motor (Efferent)	Carry instructions from CNS to effectors	Multipolar, axons extend to effectors
Interneurons	Connect sensory and motor neurons; involved in higher functions	Mostly multipolar, found in CNS

Sensory Receptors

• Interoceptors: Monitor internal systems (digestive, respiratory, cardiovascular, urinary, reproductive) and internal senses (taste, deep pressure, pain).

• Exteroceptors: Monitor external senses (touch, temperature, pressure) and distance senses (sight, smell, hearing).

• Proprioceptors: Monitor position and movement of skeletal muscles and joints.

Neuroglia (Glial Cells)

Neuroglia of the Central Nervous System (CNS)

Type	Function
Ependymal cells	Line central canal and ventricles; secrete, circulate, and monitor cerebrospinal fluid (CSF)
Astrocytes	Maintain blood-brain barrier, repair neural tissue, guide neuron development
Oligodendrocytes	Form myelin sheaths around CNS axons, increase speed of action potentials
Microglia	Migrate through neural tissue, clean up debris, waste, and pathogens

• White matter: Regions with many myelinated nerves.

• Gray matter: Unmyelinated areas of the CNS.

Neuroglia of the Peripheral Nervous System (PNS)

Type	Function
Satellite cells (amphicytes)	Surround ganglia, regulate environment around neuron
Schwann cells (neurilemmacytes)	Form myelin sheath around peripheral axons; one Schwann cell sheaths one segment of axon

Ganglia

• Masses of neuron cell bodies surrounded by neuroglia, found in the PNS.

Neural Responses to Injury

PNS Regeneration

• Wallerian degeneration: Axon distal to injury degenerates.

• Schwann cells: Form a path for new growth and wrap new axon in myelin.

CNS Regeneration

• Limited by chemicals released by astrocytes that block growth and produce scar tissue.

Membrane Potential and Nerve Impulse

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the plasma membrane of a neuron at rest, typically about -70 mV. This is maintained by the sodium-potassium pump and differences in membrane permeability to ions.

• Sodium-potassium pump: Pumps 3 Na+ out and 2 K+ in, creating a net negative charge inside the cell.

• Membrane permeability: More permeable to K+ than Na+.

• Fixed anions: Negatively charged molecules inside the cell that cannot leave.

Equation for the resting membrane potential (Nernst equation):

$E = \frac{RT}{zF} \ln \left( \frac{[\text{ion outside}]}{[\text{ion inside}]} \right)$

Additional info: The actual resting potential is determined by the combined effects of all permeant ions, mainly K+, Na+, and Cl-.

Action Potential

• Depolarization: Membrane potential becomes less negative (towards zero).

• Hyperpolarization: Membrane potential becomes more negative.

• Threshold of excitement: The critical level (about -55 mV) that must be reached to trigger an action potential.

• All-or-none law: An action potential either occurs fully or not at all.

• Refractory period: Time after an action potential when a neuron cannot fire (absolute) or requires a stronger stimulus (relative).

Sequence of events in an action potential:

1. Depolarization to threshold opens voltage-gated Na+ channels.

2. Na+ influx causes rapid depolarization (up to +60 mV).

3. Na+ channels close, K+ channels open; K+ efflux repolarizes the membrane.

4. Resting potential is restored by the sodium-potassium pump.

Equation for the sodium-potassium pump:

$3 \text{Na}^+_{\text{in}} + 2 \text{K}^+_{\text{out}} + \text{ATP} \rightarrow 3 \text{Na}^+_{\text{out}} + 2 \text{K}^+_{\text{in}} + \text{ADP} + \text{P}_i$

Propagation of the Action Potential

• Action potentials are regenerated along each segment of the axon (unmyelinated: continuous conduction).

• In myelinated axons, action potentials "jump" from node to node (nodes of Ranvier) in a process called saltatory conduction.

• Myelination increases the speed of conduction and conserves energy.

• Diseases such as multiple sclerosis involve destruction of the myelin sheath, leading to impaired conduction.

Graded Potentials

• Local neurons with short axons produce graded potentials, which vary in magnitude and do not follow the all-or-none law.

• Graded potentials can depolarize or hyperpolarize the membrane in proportion to the stimulus.

The Synapse: Neuronal Communication

Structure and Function of Synapses

• Neurons communicate at specialized junctions called synapses.

• Components include the presynaptic cell, synaptic cleft, and postsynaptic cell.

• Neurotransmitters are stored in synaptic vesicles at the axon terminal.

• Action potentials trigger the release of neurotransmitters into the synaptic cleft via exocytosis, often stimulated by Ca2+ influx.

• Neurotransmitters bind to receptors on the postsynaptic cell, causing changes in its activity.

• Neurotransmitters are then removed by reuptake, enzymatic breakdown, or diffusion.

Sequence of Synaptic Transmission

1. Neurotransmitter is synthesized and stored in vesicles at the axon terminal.

2. Action potential reaches the axon terminal, causing vesicles to release neurotransmitter into the synaptic cleft.

3. Neurotransmitter diffuses across the cleft and binds to receptors on the postsynaptic cell.

4. Activated receptors cause changes in the postsynaptic neuron.

5. Neurotransmitter molecules are released from receptors and diffuse back into the synaptic cleft.

6. Neurotransmitter is reabsorbed by the presynaptic neuron (reuptake), broken down by enzymes, or diffuses away.

7. Postsynaptic cell may send negative feedback to slow further neurotransmitter release.

Types of Synapses

• Neuromuscular junction: Synapse between a neuron and a muscle cell.

• Neuroglandular junction: Synapse between a neuron and a gland cell.

Example: Local Anesthetics

• Local anesthetic drugs (e.g., Novocain) block sodium channels, preventing action potentials and thus blocking pain sensation.

Additional info: The process of synaptic transmission is fundamental to all nervous system functions, including reflexes, sensation, and higher cognitive processes.