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Nervous System

Organization of Nervous System

The nervous system is organized functionally into two main parts:
- Peripheral Nervous System (PNS)
- Central Nervous System (CNS)

Functions of the Nervous System

Receives information from the surrounding environment (sensory) and generates a response (motor).
Integration areas: combine sensory perceptions and higher cognitive functions (memories, learning, emotion) to produce a response.
Communication among body parts is essential for efficiency.

Communication in the Nervous System

Nearly all multicellular organisms have communication systems.
Specialized cells, such as neurons, carry messages from one cell to another:
- Neurons use two types of signals to communicate:
- Electrical signals (long-distance)
- Chemical signals (short-distance)

Components of Nervous System

Structural Organization

Sensory Neuron: Receives sensory input (e.g., from skin)
Interneuron: Facilitates integration of sensory information
Motor Neuron: Generates a response by sending signals to muscles

Functional Classification

Classified into three main types:
- Sensory Input: Stimulus detected by sensory receptors.
- Integration: Processing sensory input and generating a response.
- Motor Output: Activation of muscles or glands.

Types of Neurons

Sensory Neurons: Transmit impulses from sense organs to the spinal cord and brain.
Interneurons: Connect sensory and motor neurons to facilitate communication.
Motor Neurons: Carry impulses from the brain and spinal cord to muscles and glands.

The Central Nervous System (CNS)

Control center for the body that:
- Relays messages
- Processes information
- Analyzes information
Composed of the brain and spinal cord.

Peripheral Nervous System (PNS)

Consists of nerves and associated cells not part of the brain or spinal cord.
Divided into:
- Sensory Division: Transmits information to the CNS.
- Motor Division: Carries signals away from the CNS.

Components of Neurons

Neurons bundle together to form nerves. The number of neurons in a nerve can vary significantly.
Neurons are composed of:
- Cell body
- Dendrites: Collect signals
- Axon: Passes signals

Glial Cells

Play a supporting role for nervous tissue. They provide a framework of tissue that supports the neurons and their activities.
Types of glial cells include:
- Astrocytes: Maintain the blood-brain barrier
- Microglial cells: Act as immune defense in the CNS
- Oligodendrocytes: Myelinate several axons in CNS
- Schwann cells: Myelinate axons in PNS

Action Potentials

Action potentials are generated based on ion movement across neuron membranes.
Threshold: Voltage at which an action potential is generated.
Typical resting potential of a neuron is $-70 mV$ .
Formation of action potentials includes:
1. Resting State:
- The neuron's membrane potential is primarily maintained by the Na+/K+ pump, which actively transports $3 Na^+$ ions out and $2 K^+$ ions into the cell, and by potassium leak channels, allowing $K^+$ to diffuse out.
- Most voltage-gated Na+ and K+ channels are closed.
1. Depolarization:
- A stimulus causes the membrane potential to rise to the threshold (e.g., $-55 mV$ ).
- Voltage-gated Na+ channels rapidly open, allowing a rapid influx of $Na^+$ ions into the cell, making the inside more positive.
1. Repolarization:
- At the peak of depolarization (around $+30 mV$ ), voltage-gated Na+ channels inactivate (close and cannot reopen immediately).
- Simultaneously, voltage-gated K+ channels open more slowly, leading to a rapid efflux of $K^+$ ions out of the cell, restoring the negative membrane potential.
1. Undershoot (Hyperpolarization):
- Voltage-gated K+ channels close slowly, allowing extra $K^+$ efflux.
- This causes the membrane potential to become transiently more negative than the resting potential (hyperpolarization).
1. Refractory Period / Restoration:
- During the undershoot and immediately after, the neuron enters a refractory period where it is difficult or impossible to generate another action potential.
- The Na+/K+ pump, along with ion diffusion, works to restore the correct ion concentrations and return the membrane to its resting potential.

Types of Electrical Signals

Graded Potentials: Slight changes in membrane potential.
Action Potentials: All-or-none responses; propagate along axons.

Synaptic Transmission

Neurons communicate with each other at synapses, which can be:
- Electrical Synapses: Allow for direct transmission via gap junctions.
- Chemical Synapses: Involve neurotransmitter release to communicate across the synaptic cleft.

Neurotransmission Process

Action potential arrives at the presynaptic terminal, opening voltage-gated $Ca^{2+}$ channels.
Influx of $Ca^{2+}$ ions triggers the fusion of synaptic vesicles (containing neurotransmitters) with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft.
Neurotransmitter diffuses across the synaptic cleft.
Binds to specific receptors on the postsynaptic cell, causing ion channels to open or close, thereby generating a postsynaptic potential (either excitatory or inhibitory).
Neurotransmitters are then removed from the synaptic cleft through enzymatic degradation, reuptake by the presynaptic neuron or glial cells, or diffusion, allowing the synapse to be ready for the next signal.

Major Neurotransmitter Types

Acetylcholine (ACh): Involved in muscle action, learning, and memory.
Dopamine: Influences movement, learning, attention, and emotion.
Serotonin: Affects mood, hunger, sleep, and arousal.
Norepinephrine: Involved in arousal and alertness.
GABA: Major inhibitory neurotransmitter.
Glutamate: Major excitatory neurotransmitter involved in memory.

Brain Regions and Functions

Brain Structure Overview

The vertebrate brain includes three major parts:
- Forebrain: Involved in complex functions; includes the thalamus and hypothalamus.
- Midbrain: Coordinates sensory input.
- Hindbrain: Controls involuntary functions and motor coordination.

Cerebral Cortex Lobes

Frontal Lobe: Associated with decision-making, motor functions, language, and executive functions.
Parietal Lobe: Processes somatosensory information (touch, temperature, pain).
Temporal Lobe: Involved in memory and auditory processing.
Occipital Lobe: Primarily responsible for visual processing.

Memory and Learning

Short-term Memory: Limited capacity (7-12 pieces of information); must be reinforced to persist.
Long-term Memory: Vast capacity; consolidated from short-term memory.
Long-Term Potentiation (LTP): Mechanism believed to be crucial for learning; involves increased synaptic strength.

Neurotransmitter Disorders

Schizophrenia: Linked to dysfunction in dopamine pathways.
Depression: Can involve serotonin and norepinephrine imbalances.
Alzheimer's Disease: Characterized by neurofibrillary tangles and amyloid plaques.
Parkinson's Disease: Associated with the death of dopamine-secreting neurons, leading to movement disorders.

Brain Imaging Techniques

Electroencephalography (EEG): Records electrical activity in the brain, useful for studying sleep cycles and brain functionalities.

Afferent Nerve Cells (Sensory Neurons)

Function: Transmit sensory information from peripheral receptors (e.g., skin, muscles, organs) to the central nervous system (CNS - brain and spinal cord).
Direction of Signal: Carry signals towards the CNS.
Role: Involved in sensing stimuli like touch, temperature, pain, pressure, and proprioception (body position).
Location: Cell bodies are typically located in the dorsal root ganglia (for spinal nerves) or sensory ganglia of cranial nerves.

Efferent Nerve Cells (Motor Neurons)

Function: Transmit motor commands from the CNS to effector organs such as muscles and glands.
Direction of Signal: Carry signals away from the CNS.
Role: Control muscle contractions (voluntary and involuntary) and gland secretions, allowing for movement and physiological responses.
Location: Cell bodies are typically located in the ventral horn of the spinal cord or in motor nuclei of the brainstem.