Physiology: Central Nervous System (CNS)

• Physiology of the nervous system outlines the network of nerve cells and fibers transmitting nerve impulses throughout the body
• The nervous system coordinates actions and sensory data by signal transmission, controlling essential functions within a delicate balance
• It consists of the central nervous system (CNS) and the peripheral nervous system (PNS).

Organization of Nervous System

• The nervous system divides into the Central Nervous System (CNS): brain and spinal cord, and a Peripheral Nervous System (PNS): somatic and autonomic systems
• The somatic system enables voluntary control of skeletal muscles
• The autonomic system regulates involuntary functions
• The brain is subdivided into the forebrain (telencephalon and diencephalon), midbrain (mesencephalon), and hindbrain (metencephalon and myelencephalon)

Central Nervous System (CNS)

• The CNS consists of the brain and spinal cord
• The brain is the body's central organ, consisting of the cerebrum, brainstem, and cerebellum, protected by skull bones and requiring 20% of the body's energy
• Approximately 100 billion neurons alongside 1,000 billion glial cells form the human brain.

Brain Composition

• The cerebrum controls higher functions like touch, vision, hearing, speech, reasoning, emotions, learning, and movement
• The cerebrum is divided into two halves: right and left hemispheres
• The cerebellum coordinates movement, posture, and balance
• The brainstem relays information; performing automatic functions like breathing, heart rate, and digestion

Cerebrum Hemispheres

• The cerebrum has right and left hemispheres
• The hemispheres are joined by the corpus callosum for transmitting messages
• Each hemisphere manages the opposite side of the body; damage to one side leads to weakness on the other
• Functions are not uniformly distributed on each side; the left is dominant in speech, math, hand use, and writing, versus spatial/artistic ability on the right
• The left hemisphere dominates in hand use and language in about 92% of people

Lobes of the Brain

• Each cerebral hemisphere contains four lobes: frontal, parietal, temporal, and occipital, each with specialized areas

Lobes and Their Function

• Frontal lobe handles personality, judgment, planning, speech (Broca's area), body movement, and intelligence
• Parietal lobe interprets language, touch, pain, temperature, and spatial/visual perception
• Occipital lobe interprets vision including color, light, and movement
• Temporal lobe is responsible for understanding language (Wernicke's area), memory, hearing, sequencing, and organization

Spinal Cord

• The spinal cord is a tubular nerve bundle that connects the body to the brain
• It extends from the brainstem's medulla oblongata to the lumbar vertebral region
• The spinal cord works like a telephone switchboard to aid brain-body communication in three major ways
  • It relays messages from the brain for actions
  • It transmits messages from sensory receptors to the brain
  • It coordinates reflexes independent of the brain

Spinal Cord Organization and Function

• The spinal cord contains 33 segments in five regions, each connecting nerves to body areas:
  • Cervical (head, neck, arms)
  • Thoracic (hands, chest, abdomen)
  • Lumbar (hips, knees, ankles)
  • Sacral (legs, bladder, anus)
  • Coccygeal (skin around coccyx)

Autonomic Nervous System (ANS)

• The ANS regulates involuntary functions such as heart rate, digestion, breathing, and sexual response, acting as the fight-or-flight control through sympathetic and parasympathetic systems
• The autonomic nervous system (ANS) consists of the sympathetic division, emerging from the thoracic and lumbar areas of the spinal cord, and the parasympathetic division

Sympathetic vs Parasympathetic Nervous System

• The sympathetic system promotes arousal; diverting blood from the GI/skin, enhancing skeletal muscle blood, dilating lung bronchioles
• The sympathetic system also increases heart rate and pupil dilation while inhibiting intestinal/urinary functions
• The parasympathetic system promotes rest/digestion; enhancing GI blood flow and constricting bronchioles, facilitating close vision
• The parasympathetic division is crucial to genital tissues and sexual arousal

Actions of Autonomic Systems

• Digestive system activity increases under parasympathetic and decreases under sympathetic influence
• Parasympathetic stimulation constricts lungs; sympathetic dilates
• Parasympathetic decreases heart rate; sympathetic increases it
• Eye muscles constrict pupils under parasympathetic, dilating pupil
• Parasympathetic system stimulates saliva; sympathetic inhibits it, causing dry mouth

Nerve Structure and Function

• Nerves transmit sensory/motor signals via electrical/chemical signals
• The fibrous sections are sheathed in myelin or neurilemma, comprising neuron collections and axons for electrochemical impulses
• Nerves, found throughout the peripheral nervous system, are structured in layers with an inner endoneurium, middle perineurium, and outer epineurium with interspersed blood vessels

Nerve Function and Types

• Nerves are cable-like structures relaying information throughout the body, consisting of fiber bundles wrapped in tissue/fat
• Sensory nerves relay messages from the sense organs to the brain
• Motor nerves carry responsive messages to muscles/glands
• Mixed nerves perform sensory and motor functions with impulses transmitted at ~120 m/sec

Neuromuscular Junction (NMJ)

• Function of nerves is to conduct and convey electrochemical impulses via individual neurons
• Nerve disorders include pain, muscle malfunction, sensation changes and mental disability Neuromuscular junction links motor neurons to muscle fibers, crucial for muscle contraction
• The motor neuron axon divides into terminals ending as synaptic end bulbs near the muscle's motor end plate
• A synaptic cleft separates neuron from and muscle fiber membrane (sarcolemma)

Neuromuscular Transmission Steps

• Neuromuscular transmission relies on nerve impulses to initiate muscle contraction via action potential transmission from motor neuron axon to muscle fiber
• The steps include:
  • Nerve action potential
  • Calcium entry into terminus
  • Release of acetylcholine (Ach)
  • Ach diffusion across the synaptic cleft
  • Receptor combination
  • Sodium/potassium channel opening
  • Postsynaptic depolarization
  • Muscle action potential