The Nervous System: Structure and Function

• The nervous system is the primary internal communication network of the body.
• It consists of the brain, spinal cord, and peripheral nerves.
• The nervous system's primary function is to detect, process, and respond to stimuli.
• It coordinates various bodily functions and enables interactions with the external environment.

Cells of the Nervous System

• Neurons are the fundamental units of the nervous system.
• They're specialized cells that transmit electrical and chemical signals.
• Glial cells support, insulate, and protect neurons.
• Neurons include a cell body (soma), dendrites, and an axon.

Neuron Structure

• The cell body contains the nucleus and other organelles.
• Dendrites are branched extensions that receive signals from other neurons.
• The axon is a long, slender projection that transmits signals to other neurons or target cells.
• Axons can be myelinated (insulated) or unmyelinated.
• Myelination increases the speed of signal transmission.
• The myelin sheath is formed by glial cells (Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system).
• Gaps in the myelin sheath are called nodes of Ranvier, where the action potential regenerates during saltatory conduction.
• The axon terminal is the end of the axon, where it forms a synapse with another neuron or target cell.

Types of Neurons

• Sensory neurons transmit signals from sensory receptors to the central nervous system (brain and spinal cord).
• Motor neurons transmit signals from the central nervous system to muscles or glands.
• Interneurons connect sensory and motor neurons within the central nervous system.

Synapses

• Synapses are the junctions between neurons where signals are transmitted.
• Electrical synapses involve direct physical connections between neurons, allowing for rapid signal transmission.
• Chemical synapses involve the release of neurotransmitters from the presynaptic neuron.
• The neurotransmitters bind to receptors on the postsynaptic neuron, generating a response.

Neurotransmitters

• Neurotransmitters are chemical messengers that transmit signals across synapses.
• Examples include acetylcholine, dopamine, serotonin, norepinephrine, and GABA.
• Neurotransmitters can be excitatory or inhibitory.
• They can have a wide range of effects on the postsynaptic neuron.
• After release, neurotransmitters are either broken down by enzymes, taken back up into the presynaptic neuron (reuptake), or diffuse away.

Central Nervous System (CNS)

• Consists of the brain and spinal cord.
• The brain is the control center of the nervous system, responsible for processing information, making decisions, and coordinating bodily functions.
• The spinal cord transmits signals between the brain and the peripheral nervous system.
• It also controls reflexes.
• The CNS is protected by the skull and vertebral column.
• It's also protected by the meninges (dura mater, arachnoid mater, and pia mater).
• Cerebrospinal fluid cushions and nourishes the CNS.

Brain Regions

• The cerebrum is the largest part of the brain, responsible for higher-level functions such as thought, language, and voluntary movement.
• The cerebral cortex is the outer layer of the cerebrum, involved in sensory perception, motor control, and cognitive functions.
• The cerebellum coordinates movement and balance.
• The brainstem connects the brain to the spinal cord, controlling basic life functions such as breathing and heart rate.
• The diencephalon includes the thalamus (relays sensory information) and the hypothalamus (regulates body temperature, hunger, thirst, and hormone release).

Peripheral Nervous System (PNS)

• Includes all the nerves and ganglia outside of the brain and spinal cord.
• It connects the CNS to the rest of the body.
• The PNS is divided into the somatic nervous system and the autonomic nervous system.

Somatic Nervous System

• Controls voluntary movements of skeletal muscles.
• It consists of sensory neurons that carry information from sensory receptors to the CNS and motor neurons that carry information from the CNS to skeletal muscles.

Autonomic Nervous System

• Regulates involuntary functions such as heart rate, digestion, and breathing.
• It is divided into the sympathetic and parasympathetic nervous systems.
• The sympathetic nervous system prepares the body for "fight or flight" responses.
• The parasympathetic nervous system promotes "rest and digest" functions.

Sensory Receptors

• Specialized cells or structures that detect stimuli (e.g., light, sound, touch, chemicals).
• They convert stimuli into electrical signals that can be transmitted by neurons.
• Different types of sensory receptors respond to different types of stimuli (e.g., photoreceptors in the eye respond to light, mechanoreceptors in the skin respond to touch).

Motor Control

• Motor neurons transmit signals from the CNS to muscles, causing them to contract.
• The neuromuscular junction is the synapse between a motor neuron and a muscle fiber.
• Acetylcholine is the neurotransmitter released at the neuromuscular junction.
• The motor cortex in the brain controls voluntary movements.
• The cerebellum coordinates movement and balance.

Reflexes

• Involuntary, rapid responses to stimuli.
• They often involve a simple neural pathway called a reflex arc.
• Reflex arcs typically consist of a sensory neuron, an interneuron (in some cases), and a motor neuron.
• Examples include the knee-jerk reflex and the withdrawal reflex.

Neural Development

• The nervous system develops from the neural tube during embryonic development.
• Neurogenesis (the formation of new neurons) occurs primarily during development, but can also occur in certain brain regions in adults.
• Neurons migrate to their final locations and form connections (synapses) with other neurons.
• Synaptic pruning eliminates unnecessary or weak connections, refining neural circuits.

Neural Plasticity

• The ability of the nervous system to change and adapt in response to experience or injury.
• It involves changes in the strength of synaptic connections, the formation of new synapses, and the reorganization of neural circuits.
• Neural plasticity is important for learning, memory, and recovery from brain damage.

Common Disorders

• Neurodegenerative diseases (e.g., Alzheimer's disease, Parkinson's disease) involve the progressive loss of neurons.
• Stroke occurs when blood flow to the brain is interrupted, causing brain damage.
• Multiple sclerosis is an autoimmune disease that affects the myelin sheath, disrupting nerve signal transmission.
• Epilepsy is a neurological disorder characterized by recurrent seizures.
• Mental health disorders (e.g., depression, anxiety, schizophrenia) are associated with imbalances in neurotransmitter systems.

The nervous system is the body's communication network, comprising the brain, spinal cord, and nerves. Its primary role involves detecting, processing, and responding to stimuli. Neurons, the fundamental units, transmit signals, while glial cells offer support and insulation.