Nervous Tissue and Nervous System: Study Guide Notes

Nervous System Organization

CNS vs PNS

The nervous system is divided into two main parts:

• Central Nervous System (CNS): Consists of the brain and spinal cord. It is responsible for processing and integrating information, and directing responses.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS. It connects the CNS to limbs and organs.

Afferent vs Efferent Pathways

• Afferent (Sensory) Pathways: Carry sensory information from receptors to the CNS.

• Efferent (Motor) Pathways: Transmit commands from the CNS to effectors (muscles and glands).

Somatic vs Autonomic Nervous System

• Somatic Nervous System (SNS): Controls voluntary movements by innervating skeletal muscles.

• Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., heart rate, digestion) by controlling smooth muscle, cardiac muscle, and glands.

Flow of Information

• Information typically flows: Receptor → CNS Processing → Motor Response

Nervous Tissue & Neuroglia

Structure and Function of a Neuron

• Neurons are the functional units of the nervous system, specialized for communication.

• Key parts:

  • Dendrites: Receive incoming signals.

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

  • Axon: Conducts electrical impulses away from the cell body.

Neuroglial Cells

• Neuroglia (glial cells) support and protect neurons. Major types include:

• Astrocytes: Maintain the blood-brain barrier, provide structural support, regulate ion and nutrient concentrations, and repair tissue. They do NOT conduct electrical impulses.

• Other glial cells: oligodendrocytes (CNS myelination), Schwann cells (PNS myelination), microglia (immune defense), ependymal cells (produce cerebrospinal fluid).

Electrical Properties of Neurons

All-or-None Principle

• An action potential either occurs fully or not at all, depending on whether the threshold is reached.

Graded Potentials vs Action Potentials

• Graded Potentials: Local changes in membrane potential; vary in size and decay with distance.

• Action Potentials: Large, uniform electrical signals that travel along the axon without decreasing in strength.

Threshold

• The threshold is the critical level of depolarization required to trigger an action potential.

Sequence of Events in an Action Potential

1. Resting membrane potential

2. Depolarization to threshold

3. Rapid depolarization (Na+ influx)

4. Repolarization (K+ efflux)

5. Return to resting potential

Continuous vs Saltatory Conduction

• Continuous Conduction: Occurs in unmyelinated axons; action potential moves along every part of the membrane.

• Saltatory Conduction: Occurs in myelinated axons; action potential jumps between nodes of Ranvier, increasing speed.

• Myelination greatly increases conduction velocity.

Synapses & Summation

Synapse

• A synapse is the junction where a neuron communicates with another cell (neuron, muscle, or gland).

• Neurotransmitters are released to transmit the signal across the synaptic cleft.

EPSPs vs IPSPs

• Excitatory Postsynaptic Potentials (EPSPs): Depolarize the postsynaptic membrane, increasing the chance of an action potential.

• Inhibitory Postsynaptic Potentials (IPSPs): Hyperpolarize the membrane, decreasing the chance of firing.

Temporal vs Spatial Summation

• Temporal Summation: Multiple signals from one synapse in rapid succession.

• Spatial Summation: Signals from multiple synapses at the same time.

Neuronal Firing Decision

• Neurons integrate all incoming EPSPs and IPSPs; if the threshold is reached at the axon hillock, an action potential is generated.

Spinal Cord & Nerve Anatomy

Posterior (Dorsal) vs Anterior (Ventral) Roots

• Posterior (Dorsal) Roots: Carry sensory (afferent) information into the spinal cord.

• Anterior (Ventral) Roots: Carry motor (efferent) commands out of the spinal cord.

Posterior Root Ganglion

• Contains cell bodies of sensory neurons; essential for transmitting sensory information to the CNS.

Organization of Sensory vs Motor Neurons

• Sensory Neurons: Enter via dorsal roots; cell bodies in dorsal root ganglia.

• Motor Neurons: Exit via ventral roots; cell bodies in the spinal cord.

Somatic vs Visceral Motor Pathways

• Somatic Motor Pathways: Control skeletal muscles.

• Visceral Motor Pathways: Control smooth muscle, cardiac muscle, and glands (autonomic functions).

Reflexes & Neural Circuits

Simple Reflex Arc

1. Receptor detects stimulus

2. Sensory neuron transmits signal to CNS

3. Integration center processes information

4. Motor neuron carries command to effector

5. Effector produces response

Classification of Reflexes

• Somatic vs Visceral: Somatic reflexes affect skeletal muscles; visceral reflexes affect internal organs.

• Spinal vs Cranial: Spinal reflexes are processed in the spinal cord; cranial reflexes in the brain.

• Monosynaptic vs Polysynaptic: Monosynaptic reflexes have one synapse (e.g., knee-jerk); polysynaptic have multiple synapses.

• Innate vs Acquired: Innate reflexes are present at birth; acquired are learned.

Neuronal Circuit Patterns

• Convergence: Multiple neurons synapse on a single neuron.

• Divergence: One neuron synapses on multiple neurons.

• Reverberation: Feedback loops that maintain activity in a circuit.

Brain Regions & Functions

Brainstem Functions

• Controls vital functions such as heart rate, breathing, and consciousness.

Diencephalon: Thalamus vs Hypothalamus

• Thalamus: Relay station for sensory information.

• Hypothalamus: Regulates homeostasis, endocrine activity, and autonomic functions.

Cerebellum vs Cerebrum

• Cerebellum: Coordinates movement and balance.

• Cerebrum: Responsible for higher cognitive functions, voluntary movement, and sensory perception.

Midbrain Reflex Centers

• Control visual and auditory reflexes (e.g., tracking moving objects, startle response).

Limbic System

• Involved in emotion, motivation, and memory formation.

Sensory Receptors & Sensory Processing

Types of Sensory Receptors

• Interoceptors: Monitor internal environment (e.g., blood pressure).

• Exteroceptors: Detect external stimuli (e.g., touch, temperature).

• Proprioceptors: Sense body position and movement.

Pain Pathways

• Fast Pain: Sharp, immediate pain carried by Type A fibers (myelinated, rapid conduction).

• Slow Pain: Dull, aching pain carried by Type C fibers (unmyelinated, slower conduction).

Sensory Adaptation

• Sensory Adaptation: Decreased sensitivity to a constant stimulus over time.

• Tonic Receptors: Respond continuously to stimuli (e.g., pain receptors).

• Phasic Receptors: Respond only when stimulus changes (e.g., pressure receptors).

• Adaptation prevents the brain from being overwhelmed by unimportant stimuli.

Key Diagrams to Review

• Functional steps of the nervous system

• Distribution of sensory and motor commands in the spinal cord

• Simple reflex arc

• Action potential graph

• Structure of a neuron

Summary Table: Comparison of Key Nervous System Divisions

Division	Main Components	Function
CNS	Brain, Spinal Cord	Integration, processing, coordination
PNS	Cranial & Spinal Nerves	Communication between CNS and body
Somatic NS	Motor neurons to skeletal muscle	Voluntary movement
Autonomic NS	Sympathetic & Parasympathetic divisions	Involuntary control of organs

Key Equations and Concepts

• Resting Membrane Potential:

  • $V_m = V_{inside} - V_{outside}$

• Ohm's Law (applied to neurons):

  • $I = \frac{V}{R}$

• Action Potential Threshold:

  • Typically around $-55 \ \text{mV}$

Additional info: Some explanations and the summary table were expanded for academic completeness and clarity.