Nervous System Organization

Organization of the Nervous System

• The nervous system has 2 initial divisions: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

Central Nervous System (CNS)

• The brain and the spinal cord make up the CNS.

Peripheral Nervous System (PNS)

• The PNS is the nervous system outside of the brain and spinal cord.
• The PNS consists of 31 spinal nerves and 12 cranial nerves.
• 31 spinal nerves carry information to and from the spinal cord.
• 12 cranial nerves carry information to and from the brain.

Brain Divisions

• The brain consists of the cerebrum, diencephalons (thalamus & hypothalamus), cerebellum, midbrain, pons, and medulla oblongata.

Spinal Cord Segments

• The spinal cord is divided into the following segments:
• 8 cervical segments
• 12 thoracic segments
• 5 lumbar segments
• 5 sacral segments
• 1 coccygeal segment

Spinal Nerves

• Spinal nerves arise from the spinal cord segments as 31 pairs.
• There are 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral nerves, and 1 coccygeal nerve.

Peripheral Nervous System Divisions

• The PNS is divided into the sensory division and the motor division.
• The sensory division is the afferent division.
• The sensory (afferent) division conducts impulses from receptors to the CNS.
• It informs the CNS of the body's interior and exterior state.
• Sensory nerve fibers can be somatic (from skin, skeletal muscles, or joints) or visceral (from organs in the ventral body cavity).
• The motor division is the efferent division.
• The motor (efferent) division conducts impulses from the CNS to effectors, like muscles or glands.
• Motor nerve fibers exist in the motor division.

Motor Efferent Division Subdivisions

• The motor efferent division can be further divided into the somatic nervous system and the autonomic nervous system.
• The somatic nervous system is generally voluntary.
• Somatic nerve fibers conduct impulses from the CNS to skeletal muscles.
• The autonomic nervous system is generally involuntary.
• It conducts impulses from the CNS to smooth muscle tissue, cardiac muscle tissue, and glands.

Somatic vs. Autonomic Divisions of PNS

• The somatic division consists of a single neuron between the central nervous system and skeletal muscle cells.
• The somatic division innervates skeletal muscle.
• The somatic division can only lead to muscle excitation.
• The autonomic division has a two-neuron chain connected by a synapse between the central nervous system and the effector organ.
• The autonomic division innervates smooth and cardiac muscle, glands, and GI neurons.
• The autonomic division can be either excitatory or inhibitory.

Autonomic Nervous System Divisions

• The autonomic nervous system is divided into the sympathetic nervous system ("Fight or Flight") and the parasympathetic nervous system ("Rest and Digest").
• The sympathetic and parasympathetic systems are antagonistic.

Sympathetic & Parasympathetic Commonalities

• The sympathetic and parasympathetic systems are both efferent (motor) systems, also known as "visceromotor".
• Both systems regulate the "internal" environment generally outside of conscious control, and are "autonomous".
• Both systems involve 2 neurons that synapse in a peripheral ganglion.
• Both systems innervate glands, smooth muscle, and cardiac muscle.

Sympathetic and Parasympathetic Differences

• Sympathetic cell bodies are located in thoracolumbar regions (T1-L2/L3) of the spinal cord.
• Parasympathetic cell bodies are located in craniosacral regions (Brain: CN III, VII, IX, X and Spinal Cord: S2-S4).
• Sympathetic preganglionic neurons are short, while postganglionic neurons are long.
• Parasympathetic preganglionic neurons are long, while postganglionic neurons are short.

Sympathetic and Parasympathetic Neurotransmitters

• All preganglionic nerves release acetylcholine (ACh) and are excitatory (+).
• Sympathetic postganglionic nerves release norepinephrine (NE) and are excitatory (+) or inhibitory (-).
• Parasympathetic postganglionic nerves release ACh and are excitatory (+) or inhibitory (-).
• Excitation or inhibition is a receptor-dependent and receptor-mediated response.
• Sympathetic nerves release NE (ACh at sweat glands), +/- alpha or beta receptors.
• Parasympathetic nerves release Ach, +/- muscarinic receptors.

Sympathetic and Parasympathetic Target Tissues

• Sympathetic targets include organs of the head, neck, trunk, and external genitalia, the adrenal medulla, sweat glands in the skin, arrector muscles of hair, and vascular smooth muscle.
• Parasympathetic targets include organs of the head, neck, trunk, and external genitalia.
• The Sympathetic system distributes to essentially all tissues due to vascular smooth muscle.
• The Parasympathetic system never reaches limbs or the body wall, except for external genitalia.

Functional Differences in the Sympathetic and Parasympathetic Systems

• Sympathetic system effects include dilation of pupils, inhibition of tears and salivation, activation of sweat glands, increased heart and respiration rate, release of adrenalin and glucagon, relaxation of the bladder, inhibited defecation, inhibited genitalia, and initiation of ejaculation.
• Parasympathetic system effects include constricted pupils, stimulated tears and salivation, decreased heart and respiration rate, constricted blood vessels, stimulated digestion, contracted bladder, stimulated defecation, and stimulated genitalia.

Autonomic Ganglia Definition

• A collection of nerve cells lies outside of the central nervous system.
• If these cells are inside the central nervous system, it is called the nucleus.

Types of Autonomic Ganglia

• There are 3 types of autonomic ganglia:
• Lateral ganglia (sympathetic chain ganglia): sympathetic only.
• Collateral ganglia: sympathetic only.
• Terminal ganglia: parasympathetic only.

Function of Autonomic Ganglia

• Autonomic ganglia serve as a distributing center where nerve impulses originating from limited areas in the CNS are distributed to most areas of the body.
• The ganglia are the site of action of drugs used to control autonomic functions.

Neurotransmitters in the Autonomic Nervous System

• Acetylcholine (ACh) is the preganglionic neurotransmitter for both sympathetic and parasympathetic systems.
• Acetylcholine (ACh) is the postganglionic neurotransmitter for the parasympathetic nervous system
• Acetylcholine (ACh) is the postganglionic neurotransmitter for the sympathetic supply to sweat glands
• Norepinephrine is the postganglionic neurotransmitter for the sympathetic nervous system.

Synapse Definition

• The synapse is the junction of pre and postsynaptic neurons.
• Neurotransmitters are produced in the presynaptic membrane, move across the synaptic space, bind with receptors on the postsynaptic membrane, and cause impulse potential.

Cholinergic Receptors

• Cholinergic receptors bind to acetylcholine.
• The two different forms of Cholinergic receptors are Nicotinic and Muscarinic
• Nicotinic receptors are located on postganglionic neurons, skeletal muscles, and adrenal glands,.
• Muscarinic receptors are located on parasympathetic effectors and receptors of some sweat glands.

Location of ANS Receptors

Adrenergic Receptors

• Adrenergic receptors bind norepinephrine/epinephrine found on the sympathetic effector organs.
• There are two types of Adrenergic receptors: alpha and beta receptors.
• Alpha and Beta receptors are further subdivided into categories: α₁ and β₁ usually have opposite affects than α₂ and β₂.