Nervous System Functions: Sensory, Integrative, Motor

• The nervous system has three basic functions: sensory, integrative, and motor.

Sensory Functions

• Sensory receptors detect internal and external stimuli.
• Sensory or afferent neurons gather information by detecting changes.
• Monitor external factors like light and sound, and internal conditions like temperature and oxygen.

Integrative Functions

• The CNS integrates sensory input and makes response decisions.
• Interneurons or association neurons are the functional unit in the brain and spinal cord.

Motor Functions

• Response to integration decisions uses peripheral neurons to carry impulses to effectors.
• Motor or Efferent Neurons are the functional unit.
• Effectors, like muscles and glands, are outside the nervous system and are controlled or modified by nerve impulses.

Nerve Tissue

• The nervous system consists of neuroglia (supporting cells) and neurons (excitable cells).
• Neuroglia provide support, insulation (myelin), nutrients, and phagocytosis for neurons.

Types of Neuroglial Cells

• Oligodendrocytes wrap around nerve fibers, providing myelin sheaths.
• Ependymal cells form a membrane covering brain parts, lining brain spaces and spinal cord, and producing cerebrospinal fluid.
• Microglia (macrophages) support neurons, phagocytize debris, and form scars if there is damage.
• Astrocytes provide structural support, regulate nutrient and ion concentrations, and form scar tissue.
• Schwann cells form myelin sheaths around axons.
• Satellite cells have uncertain function.

Neurons

• Neurons are the nervous system's basic building blocks, made of a cell body, dendrites, and axons.
• Dendrites carry impulses towards the cell body, receiving stimuli from synapses or receptors.
• The cell body contains the nucleus and processes stimuli.
• Axons send information away from the cell body, interacting with muscles, glands, or other neurons.
• Larger axons are enclosed by myelin sheaths made by Schwann cells.
• Nodes of Ranvier are narrow gaps in the myelin sheath.
• Nerves are cable-like bundles of axons.

Neuron Classification

• Neurons are classified by function and structure.

Function

• Sensory (afferent) neurons detect changes and transmit information to the brain or spinal cord.
• Interneurons (association neurons) lie between sensory and motor pathways in the CNS, for processing information.
• Motor (efferent) neurons send signals to muscles and gland cells (effectors).

Structure

• Multipolar neurons have many processes arising from the cell body and are found in the brain or spinal cord.
• Bipolar neurons have two processes, one arising from each end of the cell body and are found in specialized parts, namely the ear, eyes, and nose.
• Unipolar neurons have a single process extending from the cell body and are found outside of the brain and spinal cord.

Synapse

• A synapse is a junction between communicating neurons, usually between an axon and a dendrite or cell body.
• Neurons at a synapse aren't in direct contact, but are separated by a synaptic cleft.

Synaptic Transmission

• Synaptic transmission is a one-way process using neurotransmitters.
• Axons have synaptic knobs with synaptic vesicles containing neurotransmitters.
• When an impulse reaches a knob, neurotransmitters are released and diffuse across a synaptic cleft.
• Neurotransmitters react with receptors on the postsynaptic neuron membrane, either exciting (turning on) or inhibiting (turning off) a process.

Nerves

• Nerves are cord-like bundles of nerve fibers within connective tissue layers.
• Nerves are bundles of axons, often referred to as nerve fibers.
• Epineurium is the outermost layer surrounding a peripheral nerve.
• Perineurium is a protective sheath surrounding nerve fascicles.
• Endoneurium is a connective tissue layer surrounding axons.
• Fascicles are small bundles of nerve fibers enclosed by the perineurium.

Nerve Pathways

• Nerve pathways are routes that nerve impulses follow through the nervous system.

Reflex Arcs

• The simplest pathway involves a few neurons and is called a reflex arc, the basis for involuntary actions (reflexes).
• A reflex arc begins with a receptor at the end of a sensory (afferent) neuron, leading to interneurons in the CNS (reflex center), the interneurons connect with motor (or efferent) neurons.

Components of a Reflex Arc

• Sensory receptor generates in a sensory neuron.
• A sensory neuron propagates the potential and synapses with neurons in the spinal cord or brain stem.
• There is an Integrating center located in the CNS gray matter, which transfers information to the motor neuron.
• Motor neuron carries the action potential from integrating center to the effector.
• An effector (skeletal muscle for somatic reflex, cardiac muscle, smooth muscle, gland for autonomic reflexes) responds to stimulation.

Reflexes

• Reflexes are automatic responses that help maintain homeostasis by controlling involuntary processes.
• Reflexes carry out actions like swallowing, sneezing, coughing, and vomiting.

Examples of Reflexes

• The patellar reflex (knee-jerk) is a simple reflex involving a sensory neuron communicating directly with a motor neuron when striking the patellar ligament.
• The withdrawal reflex occurs when touching something painful, activating skin receptors.

Meninges

• Meninges are layered membranes protecting the brain and spinal cord.

Layers of Meninges

• Dura mater: the outermost layer, and tough fibrous connective tissue.
• Arachnoid mater: thin, weblike membrane without blood vessels.
• Pia mater: thin, contains nerves and blood vessels, hugging the organ surfaces.
• Cerebrospinal fluid floats the brain, cushions it, maintains ionic concentration, and provides a waste pathway.

Central Nervous System (CNS)

• The central nervous system consists of the brain and spinal cord.
• The neurons cannot regenerate if damaged.

Brain

• The brain is a soft organ of about 100 billion multipolar neurons communicating with each other.

Surface Anatomy of the Brain

• The surface anatomy includes gyri (ridges), sulci (shallow grooves), cortex, and fissures (deeper grooves).

Anatomical Classification of the Brain

• The brain can be divided into the cerebrum, diencephalon, brainstem, and cerebellum.
• Cerebrum is the largest part, with centers for sensory, motor, and higher mental functions.
• Diencephalon processes sensory information.
• The Brainstem connects the nervous system parts and regulates visceral activities.
• Cerebellum coordinates voluntary muscular movements.

Cerebral Cortex

• The cerebral cortex is a layer of neural tissue that covers the gyri and dips into the sulci and fissures.

Cerebrum

• The Cerebrum is highly developed, making up 80% of brain weight.
• The Outer surface, consisting of the cerebral cortex plays a key role in most neural functions.
• The function is primarily for higher brain functions, sensory perception, voluntary control of movement, language, personality traits and sophisticated mental events such as memory and decision making.

Lobes of Cerebral Hemispheres

• Frontal lobe: reasoning, planning, movement, emotion and problem solving
• Parietal lobe: movement, orientation, recognition, arithmetic, perception of stimuli
• Temporal Lobe perception and recognition of auditory stimulus, memory and speech
• Occipital Lobe visual processing
• Receives relays input from both the visual cortex and auditory cortex and is important for understanding written or spoken language (Wernicke's area).
• Broca's area generates movements of muscles necessary for speech.
• Basal nuclei produce the inhibitory neurotransmitter dopamine, facilitating voluntary movement.

Diencephalon

• The diencephalonis the middle brain part between the cerebral hemispheres and above the midbrain.

Main Regions

• Thalamus: central relay station for sensory impulses to the cerebral cortex.
• Hypothalamus: coordinates activities of endocrine and nervous systems; homeostasis by regulating visceral activities.
• Heart rate and arterial blood pressure.
• Body temperature
• Water and electrolyte balance
• Control of hunger and body weight
• Control of movements and glandular secretions of the stomach and intestines

Limbic System

• Controls emotional experiences and expressions.
• Modifies acts by producing feelings like fear, anger, pleasure, and sorrow.
• Recognizes upsets in physical or psychological condition.

Brain Stem

• The brain stem connects the cerebrum to the spinal cord.

Regions

• Midbrain visual and auditory reflexes.
• Pons autonomic functions, facial expressions and sleep
• Medulla oblongata control vital visceral activities (cardiac, vasomotor, and respiratory centers).
• Cerebellum coordinates skeletal muscle movements, balance, and posture.

Spinal Cord

• Passes bundled neurons which pass information between the body and brain.
• Spinal cord consists of 31 segments: cervical, thoracic, lumbar, sacral, and coccygeal.

Peripheral Nervous System (PNS)

• Peripheral nerves connect the CNS to the rest of the body, and can regenerate if damaged,.
• Nerves are axons bundles, and ganglia are aggregates of neuron cell bodies.
• The function is to receive info from the environment, and to carry responses to effectors such as muscle cells .
• Sensory neurons carry signals to the CNS from sensory organs.
• Motor neurons carry signals from the CNS to different parts.
• Somatic motor neurons stimulate skeletal muscles to contract, and antagonist of the muscle is inhibited
• The Autonomic Nervous System Stimulate Smooth or cardiac muscle or glands.
• Is Composed of the sympathetic and parasympathetic divisions, plus the medulla oblongata.

Sympathetic Nervous System

• "Fight or flight" - prepares body for stressful or emergency situations
• Increase heart and breathing rates, slows down muscles of the stomach and intestines
• Dilates pupils.
• Constricts blood vessels - increases blood pressure

Parasympathetic Nervous System

• Prepares body for rest and digesting
• Activate muscles of stomach and intestine
• Slows heart and breathing rates, constricts pupils