Nervous System: CNS and PNS

• The nervous system is the primary internal communication network of the body.
• It consists of the brain, spinal cord, and a network of nerves that transmit signals throughout the body.
• It's responsible for receiving, processing, and responding to sensory information from both the external environment and internal bodily functions.

Central Nervous System (CNS)

• The CNS consists of the brain and spinal cord.
• The brain is the control center of the body, responsible for higher-level functions such as thought, memory, emotion, and language.
• The spinal cord serves as a communication pathway between the brain and the peripheral nervous system (PNS).
• It also controls reflexes, which are rapid, involuntary responses to stimuli.

Peripheral Nervous System (PNS)

• The PNS consists of all the nerves outside the brain and spinal cord.
• It connects the CNS to the limbs and organs.
• It's divided into two main parts: the somatic nervous system and the autonomic nervous system.

Somatic Nervous System

• The somatic nervous system controls voluntary movements of skeletal muscles.
• It receives sensory information from the external environment.

Autonomic Nervous System (ANS)

• The autonomic nervous system regulates involuntary functions such as heart rate, digestion, and breathing.
• It's further divided into the sympathetic and parasympathetic nervous systems.

Sympathetic Nervous System

• The sympathetic nervous system prepares the body for "fight or flight" responses in stressful situations.
• It increases heart rate, dilates pupils, and inhibits digestion.

Parasympathetic Nervous System

• The parasympathetic nervous system promotes "rest and digest" functions, conserving energy.
• It slows heart rate, constricts pupils, and stimulates digestion.

Neurons

• Neurons are the basic functional units of the nervous system.
• They are specialized cells that transmit electrical and chemical signals.
• A typical neuron consists of a cell body (soma), dendrites, and an axon.
• Dendrites receive signals from other neurons, while the axon transmits signals to other neurons or target cells.

Glial Cells

• Glial cells provide support, insulation, and protection for neurons.
• Types of glial cells include astrocytes, oligodendrocytes, microglia, and Schwann cells. -Astrocytes maintain the chemical environment around neurons. -Oligodendrocytes and Schwann cells form myelin sheaths around axons. These increase the speed of signal transmission. -Microglia act as immune cells in the CNS.

Action Potential

• An action potential is a rapid electrical signal that travels along the axon of a neuron.
• It is caused by the movement of ions (such as sodium and potassium) across the neuron's cell membrane.
• It occurs when the neuron's membrane potential reaches a threshold, triggering a rapid depolarization followed by repolarization.

Synapse

• A synapse is the junction between two neurons, or between a neuron and a target cell.
• At the synapse, signals are transmitted chemically via neurotransmitters.
• The presynaptic neuron releases neurotransmitters, which bind to receptors on the postsynaptic neuron.

Neurotransmitters

• Neurotransmitters are chemical messengers that transmit signals across the synapse.
• Examples of neurotransmitters include acetylcholine, dopamine, serotonin, and norepinephrine.
• Neurotransmitters can have excitatory or inhibitory effects on the postsynaptic neuron.
• After they are released into the synapse, they are either degraded by enzymes, taken up by the presynaptic neuron (reuptake), or diffuse away.

Sensory Receptors

• Sensory receptors are specialized cells that detect stimuli from the environment or within the body.
• They convert various forms of energy (e.g., light, sound, pressure, chemicals) into electrical signals that can be interpreted by the nervous system.
• Sensory receptors are classified based on the type of stimulus they detect, such as mechanoreceptors (pressure), photoreceptors (light), chemoreceptors (chemicals), and thermoreceptors (temperature).

Reflex Arc

• A reflex arc is a neural pathway that controls a reflex action.
• Most reflex arcs involve a sensory receptor, sensory neuron, interneuron (in some cases), motor neuron, and effector (muscle or gland).
• Reflex arcs allow for rapid, automatic responses to stimuli without conscious involvement of the brain.

Brain Regions

• The brain is divided into several major regions, each responsible for specific functions.
  • The cerebrum is the largest part of the brain; it controls higher-level functions such as thought, memory, and voluntary movement.
  • The cerebellum coordinates movement and balance.
  • The brainstem controls basic life functions such as breathing, heart rate, and sleep-wake cycles.

Cerebral Cortex

• The cerebral cortex is the outer layer of the cerebrum and is highly folded, increasing its surface area. It is divided into four lobes:
  • The frontal lobe is involved in executive functions, decision-making, and motor control.
  • The parietal lobe processes sensory information such as touch, temperature, and pain.
  • The temporal lobe is involved in auditory processing, memory, and language.
  • The occipital lobe is responsible for visual processing.

Spinal Cord Structure

• The spinal cord is a long, cylindrical structure extending from the brainstem to the lower back.
• It is protected by the vertebral column and surrounded by the meninges (protective membranes).
• It contains a central canal filled with cerebrospinal fluid.
• The spinal cord consists of gray matter (containing neuron cell bodies and synapses) and white matter (containing myelinated axons).

Sensory and Motor Pathways

• Sensory pathways transmit sensory information from the PNS to the CNS.
• Motor pathways transmit motor commands from the CNS to the PNS.
• These pathways involve chains of neurons that relay signals to specific regions of the brain or spinal cord.
• The crossing over of sensory and motor pathways in the brainstem or spinal cord results in control of one side of the body by the opposite side of the brain.

Neuroplasticity

• Neuroplasticity is 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 growth of new neurons (neurogenesis).
• Neuroplasticity allows the nervous system to learn new skills, recover from brain damage, and adapt to changing environments.

Disorders of the Nervous System

• Many disorders can affect the nervous system, including:
  • Neurodegenerative diseases (e.g., Alzheimer's, Parkinson's)
  • Stroke
  • Epilepsy
  • Multiple sclerosis
  • Traumatic brain injury
  • Spinal cord injury

Neural Development

• Neural development is the process by which the nervous system forms during embryonic development.
• It involves several key steps:
  • Proliferation of neural stem cells
  • Migration of neurons to their correct locations
  • Differentiation of neurons into specific types
  • Formation of synapses
  • Pruning of unnecessary synapses
• Neural development is influenced by genetic factors, environmental factors, and interactions with other cells.