Nervous System and Sensory System Overview

Neuron Anatomy

The structure of a neuron is critical for its function in the nervous system. One key component is the axon hillock, which is responsible for initiating the action potential in neurons. This area is densely packed with voltage-gated sodium (Na⁺) channels that open in response to membrane depolarization, making it essential for electrical signal transmission.

Function of the Axon Hillock

The axon hillock integrates various incoming signals and plays a pivotal role in deciding whether or not to fire an action potential, a critical process for neuronal communication.

Membrane Potential

In resting neurons, the membrane potential is typically around -70 mV, creating a polarized state. When the membrane potential reaches the threshold of -55 mV, an action potential is triggered.

Ion Movement

Diverse ions are involved in generating action potentials, notably sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), and chloride (Cl⁻). The Na⁺/K⁺ pump is crucial for maintaining this resting potential, utilizing ATP to transport Na⁺ out and K⁺ into the cell against their concentration gradients.

Types of Channels

Neuronal membrane channels reside in three main categories:

Voltage-gated: These channels open in response to changes in voltage across the neuronal membrane.
Ligand-gated: These open when specific neurotransmitter molecules bind to them.
Leaky channels: Generally, K⁺ channels that remain perpetually open, contributing to resting potential.

Graded Potentials

Graded potentials are changes in membrane potential that are not all-or-nothing responses like action potentials. Two critical types include:

EPSP (Excitatory Post-Synaptic Potential)
IPSP (Inhibitory Post-Synaptic Potential)

These potentials can summate in two significant ways:

Spatial summation: Multiple neurons firing simultaneously.
Temporal summation: A single neuron firing repeatedly in quick succession.

Both processes determine whether the threshold for an action potential is met.

Action Potential

Action potentials are characterized by their all-or-nothing nature. The process involves several steps:

Na⁺ ions enter the neuron, causing depolarization.
K⁺ ions exit the cell, leading to repolarization.
A brief period of hyperpolarization follows.

It is essential to be able to label and understand the phases of an action potential on a graph.

Synapses

Neural communication occurs at synapses, which can be categorized into:

Electrical synapses: Permit direct ion flow between neurons, facilitating rapid communication.
Chemical synapses: Involves neurotransmitters that diffuse across the synaptic cleft, formed by the axon terminal of one neuron and the dendrite of another.

Nervous System Organization

The nervous system is divided into the Central Nervous System (CNS), comprising the brain and spinal cord, and the Peripheral Nervous System (PNS), which connects the CNS to the rest of the body.

Brain Regions

The brain is categorized into three major regions:

Forebrain: Responsible for cognition, including structures such as the limbic system (hypothalamus, amygdala, thalamus, hippocampus).
Midbrain: Processes sensory information.
Hindbrain: Regulates vital autonomic functions such as breathing and heart rate.

Brain Lobes

The brain's functional areas are organized into lobes:

Frontal Lobe: Involved in decision-making and planning.
Parietal Lobe: Processes sensory touch information.
Temporal Lobe: Responsible for auditory information processing.
Occipital Lobe: Primarily handles visual stimuli.

Spinal Cord

The spinal cord is involved in reflex actions and includes:

Reflexes: Fast, automatic responses; can be monosynaptic (one synapse) or polysynaptic (more than one synapse).
Gray and White Matter: Gray matter is responsible for processing, while white matter is involved in signal transmission.

Peripheral Nervous System (PNS)

The PNS is divided into afferent (sensory) and efferent (motor) pathways:

Sympathetic Nervous System: Governs the fight-or-flight response, utilizing norepinephrine.
Parasympathetic Nervous System: Associated with rest and digest functions, primarily using acetylcholine as a neurotransmitter.

Glial Cells

Glial cells play supportive roles in both the CNS and PNS:

CNS: Include microglia (immune defense), astrocytes (support and blood-brain barrier), oligodendrocytes (form myelin), and ependymal cells (produce cerebrospinal fluid).
PNS: Primarily represented by Schwann cells that provide myelin insulation to peripheral neurons.

Sensory Systems

Types of Sensory Receptors

Various types of sensory receptors allow organisms to detect environmental stimuli, including:

Nociceptors: Sensitive to pain.
Baroreceptors: Detect pressure changes.
Proprioceptors: Inform the brain about body position.
Thermoreceptors: Sense temperature variation.
Chemoreceptors: Respond to chemical stimuli.

Steps of Sensation

The process of sensation consists of four fundamental stages:

Stimulus: The initial environmental input.
Transduction: Conversion of the stimulus into an electrical signal.
Transmission: The electrical signal is relayed to the brain.
Interpretation: The brain processes and makes sense of the sensory information.

Ear

Two main functions of the ear involve hearing and balance:

Hearing Process

Sound waves travel down the ear canal, striking the tympanic membrane, causing vibrations that move the ossicles, and ultimately lead to stimulation of hair cells in the cochlea, initiating action potentials that the brain interprets.

Balance

The inner ear detects motion through fluid and crystals that respond to acceleration and position changes.

Taste

Taste perception is facilitated by various receptor types, each responding to different flavors:

Sour: Detected by H⁺ channels.
Salty: Recognized via Na⁺ channels.
Sweet, Bitter, and Umami: These tastes are mediated by G protein-coupled receptors (GPCRs).

Smell

The olfactory system relies on chemoreceptors that detect airborne molecules, sending signals to the olfactory bulb for processing.

Vision

Visual perception begins when light strikes the retina, activating photoreceptors (rods and cones). The fovea, where cones are most densely concentrated, is crucial for sharp vision. Interestingly, phototransduction results in the hyperpolarization of photoreceptors in response to light, leading to visual signal generation.

Adaptation and Amplification

Adaptation refers to the reduction in sensitivity to a persistent stimulus over time, while amplification signifies the ability of a sensory receptor to convert a small stimulus into a robust electrical signal.

Neuromuscular Junction & Muscle

Activation of Muscles by Neurons

The neuromuscular junction is where motor neurons release acetylcholine (ACh) that binds to muscle fibers, leading to muscle contraction.

Sarcomere Anatomy

Understanding muscle contraction requires knowledge of sarcomere anatomy, which includes:

Z line
M line
H zone
A band
I band

Muscle Contraction Mechanism

The process of muscle contraction starts when a neuron fires, resulting in calcium ions (Ca²⁺) being released. These calcium ions bind to troponin, prompting a conformational change that moves tropomyosin away from actin binding sites, allowing myosin to bind to actin and initiate the cross-bridge cycle, which uses ATP as energy. Upon relaxation, calcium is pumped back into the sarcoplasmic reticulum (SR).

Rigor Mortis

This phenomenon occurs post-mortem, where lack of ATP production prevents myosin from detaching from actin, resulting in stiffening of the muscles.

Ecology

Basics of Ecology

Ecology studies the interactions between organisms and their environment, with considerations of evolution that show how species, like the Mexican Cavefish, adapt to their environments over time.

Population Ecology

Population dynamics involve several key concepts:

Dispersal

Dispersal can be classified into two categories:

Passive: Involving natural forces such as wind or water.
Active: Involving the movement of animals.

Population Size Factors

The size of a population is influenced by immigration (increases) and emigration (decreases).

Dispersion Patterns

Various patterns exist, including:

Clumped: The most common pattern, often due to resource availability.
Random: Occurs in an unpredictable distribution.
Uniform: Often results from territoriality or competition.

Climate Change

Climate change has significant implications for species distributions and abundance.

Demography

Growth Calculation

The change in population size (ΔN) can be computed with the equation:

ΔN = (births + immigration) - (deaths + emigration)
Survivorship Curves
Survivorship curves depict the likelihood of survival at various stages of life:
Type I: Characterized by high survival rates in early and middle life (e.g., humans).
Type II: Survival rates are relatively constant throughout life (e.g., birds).
Type III: High mortality rates in early life stages (e.g., insects).
Life History Strategies
Definition of how organisms allocate resources for growth, reproduction, and survival. The Principle of Allocation states that energy is limited, leading to inevitable trade-offs among these needs.
Fitness in Ecology
Fitness is defined as an organism's ability to survive and reproduce in its environment. Influencing factors can include:
Density-independent factors: Such as weather events and natural disasters.
Density-dependent factors: Include predation, resource competition, and disease.
Metapopulations
Metapopulations consist of isolated populations that interact and can enhance species continuity by decreasing extinction risks through migration among populations.
Population Pyramids
Population pyramids graphically represent age structures within a population and aid in predicting growth or stability of populations over time.

Comprehensive Overview of the Nervous, Sensory, Muscular Systems and Ecology

Comprehensive Overview of the Nervous, Sensory, Muscular Systems and Ecology