Neural Tissue: Structure, Function, and Physiology Study Guide

Neural Tissue

General Functions of Neural Tissue

Neural tissue is specialized for the conduction of electrical impulses that convey information from one area of the body to another. It is a key component of the nervous system, which includes the brain, spinal cord, and peripheral nerves.

• Sensory Input: Receives stimuli from internal and external environments.

• Integration: Processes and interprets sensory input and determines appropriate responses.

• Motor Output: Sends signals to effector organs (muscles and glands) to elicit a response.

Divisions of the Nervous System

• Central Nervous System (CNS): Consists of the brain and spinal cord.

• Peripheral Nervous System (PNS): All neural tissue outside the CNS.

Subcategories of the PNS (Motor Division)

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

• Autonomic Nervous System (ANS): Regulates involuntary functions by innervating smooth muscle, cardiac muscle, and glands.

Subcategories of the Autonomic Nervous System (ANS)

• Sympathetic Division: Prepares the body for 'fight or flight' responses.

• Parasympathetic Division: Promotes 'rest and digest' activities.

• Enteric Nervous System: Regulates the gastrointestinal tract independently of the CNS.

Target Organs of the ANS

• Cardiac muscle

• Smooth muscle (e.g., in blood vessels, digestive tract)

• Glands (e.g., salivary, sweat glands)

Types of Neurons: Structure and Function

Neurons are classified by their structure and function. Their structure determines how they process and transmit information.

• Multipolar Neurons: Have one axon and multiple dendrites; most common in CNS (e.g., motor neurons).

• Bipolar Neurons: One axon and one dendrite; found in sensory organs (e.g., retina of the eye).

• Unipolar (Pseudounipolar) Neurons: Single process that splits into two branches; found in sensory neurons of the PNS.

Functional Types:

• Sensory (Afferent) Neurons: Transmit impulses from receptors to the CNS.

• Motor (Efferent) Neurons: Carry impulses from the CNS to effectors (muscles/glands).

• Interneurons (Association Neurons): Connect sensory and motor neurons within the CNS.

Neuron Regeneration in the CNS

• Limited Regeneration: CNS neurons are generally not replaced after injury due to the absence of neurolemma, inhibitory factors in CNS myelin, and scar formation by astrocytes.

Glial Cells (Neuroglia)

Glial cells support, protect, and nourish neurons. There are six main types:

Type	Location	Function
Astrocytes	CNS	Maintain blood-brain barrier, provide structural support, regulate ion/nutrient concentrations.
Oligodendrocytes	CNS	Form myelin sheaths around CNS axons.
Microglia	CNS	Phagocytize debris and pathogens.
Ependymal cells	CNS	Line ventricles, produce and circulate cerebrospinal fluid (CSF).
Schwann cells	PNS	Form myelin sheaths around PNS axons, aid in regeneration.
Satellite cells	PNS	Support neuron cell bodies in ganglia.

Gray Matter vs. White Matter

• Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons; site of synaptic integration.

• White Matter: Composed mainly of myelinated axons; responsible for transmission of signals.

Ion Channels in Neurons

• Leak Channels: Always open; allow passive movement of ions down their concentration gradients.

• Gated Channels: Open or close in response to specific stimuli (e.g., voltage, chemicals, mechanical force).

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the plasma membrane of a resting neuron, typically around -70 mV.

• Created by differences in ion concentrations (mainly Na+ and K+) and selective permeability of the membrane.

• Maintained by the sodium-potassium pump ($3 \text{Na}^+$ out, $2 \text{K}^+$ in per ATP hydrolyzed):

$\text{Na}^+/\text{K}^+\ \text{ATPase}: 3\ \text{Na}^+_{\text{out}} : 2\ \text{K}^+_{\text{in}}$

Graded Potentials

• Small, localized changes in membrane potential that vary in magnitude and decay with distance.

• Occur in dendrites and cell bodies; can be depolarizing or hyperpolarizing.

Action Potentials: Depolarization, Repolarization, Hyperpolarization

• Depolarization: Membrane potential becomes less negative due to Na+ influx.

• Repolarization: Return to resting potential, mainly due to K+ efflux.

• Hyperpolarization: Membrane potential becomes more negative than resting, often due to continued K+ outflow.

Refractory Periods

• Absolute Refractory Period: No new action potential can be initiated, regardless of stimulus strength.

• Relative Refractory Period: A stronger-than-normal stimulus can initiate another action potential.

Factors Affecting Nerve Impulse Speed

• Axon diameter (larger = faster conduction)

• Myelination (myelinated fibers conduct faster via saltatory conduction)

• Temperature (higher temperature increases speed)

Synapses

• Definition: A synapse is a junction between two neurons or between a neuron and an effector cell, where information is transmitted.

• Types:

  • Electrical Synapse: Direct passage of ions via gap junctions; rapid, bidirectional.

  • Chemical Synapse: Neurotransmitter release from presynaptic neuron to postsynaptic cell; unidirectional, most common.

Parts of a Chemical Synapse:

• Presynaptic terminal (axon terminal with synaptic vesicles)

• Synaptic cleft (space between neurons)

• Postsynaptic membrane (with neurotransmitter receptors)

Example: The neuromuscular junction is a chemical synapse between a motor neuron and a skeletal muscle fiber.

*Additional info: Some explanations and examples have been expanded for academic completeness and clarity.*