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Neurons: Basic Units of the Nervous System

Overview

• Nerve cells, or neurons, are the fundamental building blocks of the nervous system.

• Neurons are responsible for processing and transmitting information in the body, allowing for various human experiences such as feelings, thoughts, and actions.

• The human brain contains an estimated 10 to 120 billion neurons, which decrease with age.

• The average neuron connects with about 1000 other neurons, although some can connect with up to 30,000 neurons.

Types of Neurons

1. Sensory Neurons:

   • Transmit sensory information from receptors (sensory cells) to the brain or spinal cord.

   • Examples: Neurons that notify the brain of sensory experiences such as pain or temperature.

2. Interneurons:

   • Serve as the communication link between sensory and motor neurons.

   • Predominantly found in the brain and spinal cord, making up the majority of neurons.

3. Motor Neurons:

   • Convey commands from the brain or interneurons to muscles and glands across the body.

   • Responsible for voluntary movements and involuntary actions like heartbeat and digestion.

   • Also known as efferent neurons. Sensory neurons are often referred to as afferent neurons.

Anatomy of a Neuron

• Neurons consist of three main parts:

  • Dendrites: Branch-like extensions that receive signals from other neurons.

  • Cell Body (Soma): Houses the nucleus, which contains genetic material, and integrates incoming signals.

  • Axon: A long, thin extension that transmits signals to other neurons' dendrites, and may branch into collateral branches.

Myelin Sheath

• A protective covering around many axons made of lipid cells, essential for rapid information transmission.

• Myelinated axons appear white (known as white matter) in the brain, while unmyelinated parts appear gray (gray matter).

• The myelin sheath allows for faster transmission of electrical impulses via nodes of Ranvier, which facilitate jumping of signals from node to node.

Development and Diseases

• At birth, not all axons are myelinated, leading to slow nerve impulse transmission and poor motor control in infants.

• Diseases like multiple sclerosis damage the myelin sheath, hindering motor functions and sometimes causing paralysis or loss of coordination.

Synaptic Transmission

• The point of connection between two neurons is called a synapse, where neurotransmitters are released.

• When a neuron fires, an action potential travels down the axon, triggering neurotransmitter release from terminal buttons.

Electrical Activity in Neurons

1. Resting Potential:

• The state of a neuron when it is not firing, typically around -70 mV.

• Maintained by a selective permeability of the cell membrane and active ion pumps regulating ion concentrations inside and outside the neuron.

2. Graded Potentials:

• Changes in the membrane potential that occur in response to input from other neurons.

• Can be depolarizing (making it more positive and more likely to fire) due to an influx of positive sodium ions, making the charge inside the cell membrane less negative or hyperpolarizing (making it more negative and less likely to fire) due to the outflow of potassium ions or influx of negatively charged chloride ions, making the charge inside the cell membrane more negative.

3. Action Potentials

• Occur when a neuron's membrane potential reaches a certain threshold (around -50 mV).

• Characterized by a rapid influx of sodium ions followed by a rapid outflow of potassium ions, allowing the signal to travel down the axon.

• Action potentials operate on an all-or-nothing principle, meaning they trigger fully or not at all.

Synapse and Neurotransmitter Release

• At synapses, neurotransmitters are released when an action potential reaches terminal buttons.

• Neurotransmitters bind to receptors on the postsynaptic neuron, causing either depolarization (excitatory) or hyperpolarization (inhibitory) of that neuron.

• Some neurotransmitters are more specialized for particular functions, influencing various psychological processes such as mood and behaviour.

The Endocrine System

Overview

• The endocrine system is a network of glands that secrete hormones into the bloodstream to regulate various bodily functions.

• Hormones can have widespread effects throughout the body, similar to neurotransmitters but more slowly acting and longer-lasting.

Major Glands

1. Pituitary Gland:

   • Often referred to as the "master gland" because it regulates other endocrine glands.

2. Thyroid Gland:

   • Controls metabolism and energy levels.

3. Adrenal Glands:

   • Produce hormones such as adrenaline during stress responses.

4. Pancreas:

   • Regulates blood sugar levels through insulin secretion.

5. Gonads (Ovaries and Testes):

   • Influence sexual development and behaviour.

The Peripheral Nervous System (PNS)

Functions

• The PNS connects the CNS (brain and spinal cord) to the rest of the body through sensory and motor neurons, allowing communication between the CNS and peripheral areas.

Subdivisions

1. Somatic Nervous System:

   • Controls voluntary movements and conveys sensory information to the CNS.

2. Autonomic Nervous System:

   • Manages involuntary functions such as digestion and heart rate and is divided into:

     • Sympathetic Nervous System: Activates fight-or-flight responses.

     • Parasympathetic Nervous System: Promotes restful functions after stress.

The Central Nervous System (CNS)

Components

• Comprised of the brain and spinal cord, responsible for processing information and controlling behaviour.

Key Structures

1. Hindbrain:

   • Includes the medulla oblongata, cerebellum, and pons, crucial for vital life functions.

2. Midbrain:

   • Involved in processing visual and auditory information.

3. Forebrain:

   • Contains structures involved in complex processes such as emotion, memory, and cognition (e.g., thalamus, hypothalamus, cerebral cortex).

Cerebral Cortex

• Highly convoluted layer of neurons involved in higher brain functions.

• Divided into four lobes: occipital, parietal, temporal, and frontal, each associated with specific functions such as vision, touch, language, planning, and abstract thinking.

Neuroplasticity

Definition

• The brain's ability to reorganize itself by forming new neural connections in response to learning, experience, or injury.

Significance

• Neuroplasticity plays a vital role in recovery from brain injury and learning processes throughout life; it may lead to changes in how brain regions communicate.

• Neurogenesis: formation of new neurons in response to experience, particularly in the hippocampus, influencing learning and memory.

Conclusion

• Understanding the biological bases of behaviour, including the roles of the nervous and endocrine systems, provides insight into both normal psychological processes and disorders. The interplay between genetics and the environment shapes behaviour, demonstrating that both nature and nurture are critical in human psychology.