Nervous System Overview

Definitions and General Concepts

The nervous system is a complex network responsible for communication, control, and integration of body functions. It is divided into the central nervous system (CNS) and peripheral nervous system (PNS).

• Somatic vs. Autonomic Nervous System: The somatic system controls voluntary movements; the autonomic system regulates involuntary functions (e.g., heart rate, digestion).

• Excitability: The ability of neurons to respond to stimuli and convert them into nerve impulses.

• Amitotic: Most mature neurons do not undergo mitosis (cell division).

• Neuroplasticity: The ability of the nervous system to change its activity in response to intrinsic or extrinsic stimuli.

Example: Reflex actions such as the knee-jerk response are mediated by the somatic nervous system.

Neurons and Neuroglia

Structure and Function

Neurons are the functional units of the nervous system, specialized for the transmission of electrical impulses. Neuroglia (glial cells) support and protect neurons.

• Parts of a Neuron: Dendrites (receive signals), cell body (soma), axon (transmits signals).

• Axon Terminal: The endpoint of an axon where neurotransmitters are released.

• Ganglion: A cluster of neuron cell bodies in the PNS.

• Nucleus (in nervous system): A cluster of neuron cell bodies in the CNS.

• Types of Neurons: Sensory (afferent), motor (efferent), and interneurons (association).

• Neuroglia Types: Astrocytes, oligodendrocytes, microglia, ependymal cells (CNS); Schwann cells, satellite cells (PNS).

Additional info: Schwann cells form myelin in the PNS; oligodendrocytes form myelin in the CNS.

Neuronal Communication

Action Potentials and Synapses

Neurons communicate via electrical impulses (action potentials) and chemical signals at synapses.

• Resting Membrane Potential (RMP): The voltage difference across the neuron's membrane at rest, typically about -70 mV.

• Action Potential: A rapid change in membrane potential that travels along the axon.

• Depolarization: Na+ influx causes the inside of the neuron to become more positive.

• Repolarization: K+ efflux restores the negative membrane potential.

• Hyperpolarization: Membrane potential becomes more negative than the resting potential.

• Saltatory Conduction: Action potentials jump from node to node in myelinated axons, increasing speed.

• Continuous Conduction: Occurs in unmyelinated axons; slower than saltatory conduction.

• Synapse: The junction between two neurons; can be chemical (neurotransmitter-mediated) or electrical.

Equation:

Example: The neuromuscular junction is a chemical synapse between a motor neuron and a muscle fiber.

Neurotransmitters and Synaptic Transmission

Types and Functions

Neurotransmitters are chemicals that transmit signals across a synapse.

• Cholinergic Synapses: Use acetylcholine as the neurotransmitter.

• Adrenergic Synapses: Use norepinephrine or epinephrine.

• Excitatory Postsynaptic Potential (EPSP): Depolarizes the postsynaptic membrane, increasing the likelihood of an action potential.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes the postsynaptic membrane, decreasing the likelihood of an action potential.

Additional info: Neurotransmitter removal occurs via diffusion, enzymatic degradation, or reuptake.

Central Nervous System (CNS)

Brain and Spinal Cord

The CNS consists of the brain and spinal cord, responsible for processing and integrating information.

• Major Brain Regions: Cerebrum, diencephalon, brainstem, cerebellum.

• Ventricles: Four fluid-filled cavities in the brain containing cerebrospinal fluid (CSF).

• CSF: Cushions the brain, removes waste, and circulates nutrients.

• Spinal Cord: Begins at the foramen magnum and ends at L1-L2; transmits signals between the brain and body.

• Spinal Nerves: 31 pairs; each has dorsal (sensory) and ventral (motor) roots.

Example: The medulla oblongata controls vital centers for heart rate and respiration.

Peripheral Nervous System (PNS)

Cranial and Spinal Nerves

The PNS connects the CNS to limbs and organs. It includes cranial nerves (12 pairs) and spinal nerves (31 pairs).

• Cranial Nerves: Emerge from the brain; control sensory and motor functions of the head and neck.

• Spinal Nerves: Emerge from the spinal cord; involved in sensory and motor innervation of the body.

• Plexuses: Networks of intersecting nerves (e.g., brachial plexus).

Reflexes and Sensory Pathways

Reflex Arcs and Sensation

Reflexes are rapid, involuntary responses to stimuli. Sensory pathways transmit information from receptors to the CNS.

• Reflex Arc: Consists of receptor, sensory neuron, integration center, motor neuron, and effector.

• Types of Reflexes: Stretch, tendon, withdrawal, crossed extensor.

• Sensation: The conscious or unconscious awareness of external or internal stimuli.

• Receptors: Mechanoreceptors (touch), thermoreceptors (temperature), nociceptors (pain), chemoreceptors (chemicals), photoreceptors (light).

Table: Types of Sensory Receptors

Higher Order Functions

Sleep, Memory, and Learning

The brain is responsible for higher order functions such as sleep, memory, and learning.

• Sleep: Includes deep (NREM) and REM phases; REM is associated with dreaming and muscle paralysis.

• Memory: Short-term (working) and long-term memory; consolidation involves the hippocampus.

• Learning: The process of acquiring new knowledge or skills through experience or education.

Additional info: Lack of sleep can affect hormone levels and cognitive function.

Clinical Considerations

Disorders and Diseases

Various disorders can affect the nervous system, impacting function and quality of life.

• Multiple Sclerosis (MS): Demyelinating disease affecting CNS neurons.

• Bell's Palsy: Sudden weakness of facial muscles due to facial nerve dysfunction.

• Trigeminal Neuralgia: Severe facial pain due to trigeminal nerve irritation.

• ALS (Amyotrophic Lateral Sclerosis): Progressive degeneration of motor neurons.

• Alzheimer's Disease: Neurodegenerative disorder causing memory loss and cognitive decline.

• Epilepsy: Recurrent seizures due to abnormal electrical activity in the brain.

Example: Herpes zoster (shingles) infects dermatomes, causing pain and rash along affected nerves.

Summary Table: Key Nervous System Concepts