Study Notes: The Nervous System (Anatomy & Physiology)

The Nervous System

General Functions of the Nervous System

• Detection of internal and external environment stimuli: The nervous system monitors changes inside and outside the body.

• Analysis of detected changes: Sensory input is processed and interpreted.

• Organization of information for immediate and future use: The nervous system integrates sensory input and determines appropriate responses.

• Initiation of appropriate actions: Motor output is generated to respond to changes.

Divisions of the Nervous System

Anatomical Divisions

• Central Nervous System (CNS): Consists of the brain and spinal cord. It is the main integration center, processing information and initiating action.

• Peripheral Nervous System (PNS): Located outside the CNS, includes cranial and spinal nerves, ganglia, and sensory receptors. It transmits sensory input to the CNS and carries motor output from the CNS to effectors (muscles and glands).

Functional Divisions of the Peripheral Nervous System

• Sensory (Afferent) Division: Carries action potentials from sensory receptors to the CNS.

  • Somatic sensory division: Sensory input from skin, skeletal muscles, joints.

  • Visceral sensory division: Sensory input from viscera (organs), such as the heart and blood vessels.

• Motor (Efferent) Division: Carries action potentials from the CNS to effectors.

  • Somatic division: Voluntary (conscious) and involuntary (subconscious) control of skeletal muscles.

  • Autonomic division: Involuntary control of cardiac muscle, smooth muscle, and glands.

Neurons

Structure and Function

• Neurons: The structural and functional units of the nervous system, specialized to generate and transmit action potentials.

• Nerve cell body (soma): Contains the nucleus and most organelles.

• Processes: Dendrites and axons extend from the cell body.

Dendrites

• Short, highly branched processes that receive impulses (electrochemical signals) from other neurons or sensory receptors.

Axon

• Long process that carries impulses away from the cell body.

• May have side branches (collaterals) and terminal boutons at the distal tip for synaptic transmission.

• Some axons are insulated by a myelin sheath, increasing the speed of transmission.

Types of Neurons

1. Multipolar neurons: Several dendrites and a single axon; most common in CNS.

2. Bipolar neurons: One dendrite and one axon; found in sensory organs (e.g., retina).

3. Pseudounipolar neurons: Single process that splits into two branches; found in sensory ganglia of PNS.

Functional Types

• Sensory (afferent) neurons: Carry action potentials from sensory receptors to the CNS. Structure: Mostly pseudounipolar, some bipolar.

• Interneurons: Located entirely within the CNS; process and interpret information. Structure: Multipolar.

• Motor (efferent) neurons: Carry action potentials from the CNS to effectors (muscles, glands, adipose tissue). Structure: Multipolar.

Neuroglia (Glial Cells)

• Support and protect neurons. Four types in the CNS, two in the PNS.

Types of Neuroglia

• Schwann cells (PNS): Form myelin sheath around PNS axons; aid in axon regeneration.

• Satellite cells (PNS): Support neuron cell bodies in ganglia.

• Oligodendrocytes (CNS): Form myelin sheath in CNS; do not form neurilemma, limiting regeneration.

• Astrocytes (CNS): Support neurons, regulate the blood-brain barrier, and maintain extracellular environment.

• Microglial cells (CNS): Phagocytic cells that remove debris and pathogens.

• Ependymal cells (CNS): Line ventricles and central canal; produce cerebrospinal fluid (CSF).

Neuron Physiology

Membrane Potential

• Resting membrane potential (RMP): Voltage created by electrical charge differences across the plasma membrane. Typical RMP is around -70 mV.

• Maintained by sodium (Na+) and potassium (K+) gradients, and selective permeability of the membrane.

Action Potential Formation

• Neurons exhibit an all-or-none response when stimulated.

• Action potentials are identical in magnitude; they travel along the axon or do not occur at all.

• Depolarization: Na+ channels open, Na+ flows in, membrane potential becomes less negative.

• Repolarization: K+ channels open, K+ flows out, membrane potential returns to RMP.

• Restoration: Na+/K+ pumps reestablish ion gradients.

Equation for Resting Membrane Potential:

$V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out} + p_{Na}[Na^+]_{out} + p_{Cl}[Cl^-]_{in}}{[K^+]_{in} + p_{Na}[Na^+]_{in} + p_{Cl}[Cl^-]_{out}} \right)$

Additional info: This is the Goldman-Hodgkin-Katz equation, which accounts for the permeability of multiple ions.

Protection for the Central Nervous System

Meninges

• Three membranes: dura mater (outer), arachnoid mater (middle), pia mater (inner).

• Dura mater: Tough, outermost layer; forms protective tube in vertebral canal.

• Arachnoid mater: Thin, web-like; subarachnoid space contains CSF.

• Pia mater: Delicate, innermost; adheres to brain and spinal cord surface.

Cerebrospinal Fluid (CSF)

• Clear, watery fluid in subarachnoid space; cushions CNS and removes waste.

The Brain

• Large, complex organ with about 100 billion neurons.

• Four major components: cerebrum, cerebellum, diencephalon, brain stem.

Cerebrum

• Functions: Sensations, voluntary actions, reasoning, planning, problem solving.

• Structure: Two hemispheres joined by the corpus callosum; separated by longitudinal fissure.

• Functional areas: Sensory, motor, and association areas.

Sensory Areas

• Receive action potentials from sensory receptors and interpret sensations.

• Examples: Visual areas in occipital lobes, auditory areas in temporal lobes, somatosensory areas in parietal lobes.

Motor Areas

• Located in the frontal lobes; control voluntary skeletal muscle movements.

• Primary motor cortex (precentral gyrus) initiates movement.

Association Areas

• Integrate sensory input and motor output; involved in reasoning, memory, and emotion.

Table: Major Glial Cells and Their Functions

Cell Type	Location	Function
Schwann cell	PNS	Myelinates axons, aids regeneration
Satellite cell	PNS	Supports neuron cell bodies in ganglia
Oligodendrocyte	CNS	Myelinates axons, no neurilemma
Astrocyte	CNS	Supports neurons, regulates blood-brain barrier
Microglial cell	CNS	Phagocytosis of debris and pathogens
Ependymal cell	CNS	Lines ventricles, produces CSF

Example: Damage to the myelin sheath (as in multiple sclerosis) impairs the rapid conduction of action potentials, leading to neurological deficits.