SBMS 1431- Organization of the Nervous System and Autonomic Nervous System

Neural Tissue

• Conducts electrical impulses

• Carries information

Overview of the Nervous System

• It regulates personal responses to adapt to changes in both the internal and external environment.

• It forms the prime communication and coordination network inside the body systems to reflect, respond, and react to the external environment and maintain body’s homeostasis.

How does the nervous system work? (Overview)

Sensory input: The gathered information is called sensory input.

• The nervous system uses its millions of sensory receptors to monitor changes occurring both inside and outside the body.

Integration: The nervous system processes and interprets sensory input and decides what should be done at each moment

• an integration process.

Motor output: The nervous system sends a motor output by activating effector organs, including muscles and glands.

Central Nervous System (CNS)

Includes brain and spinal cord

Involved in decision-making processes, including a variety of functions:

• Interpretation of sensory information

• Planning and monitoring movement

• Maintenance of homeostasis

• Higher mental functions such as language and learning

Peripheral nervous system (PNS)

Consists of spinal nerves, cranial nerves, and sensory receptors.

• Motor functions- include stimulation of a muscle cell contraction or a gland secretion; performed by the peripheral nervous system (PNS)

• Sensory functions- detection of sensations within and outside the body; also performd by PNS

The Neurons

Basic and Functional Units

• Carry electrical signals

• Organelles are in the cell body

• The dendrites, higher branched extensions that receive signals from other neurons

• The axon is typically a much longer extension that transmits signals to other cells at synapses.

• The cone-shaped base of an axon is called the axon hillock: it is part of the initial segment of the trigger zone for the membrane potential propagation.

• Most axons are covered with myelin; a fatty whitish substance wrapped around axons to facilitate impulse conduction

Types of Neuroms

• Sensory Neurons: afferent neurons conduct action potentials toward the CNS

• Interneurons: conduct action potentials from one neuron to another within the CNS

• Efferent neurons conduct action potentials away from the CNS

Structural Classification of Neurons

• Anaxomic Neuron

• Bipolar Neuron

• Pseudonipolar Neuron

• Multipolar Neuron

Processes (neurological term)

Any projection, extension, or “arm” that sticks out from the main body (the soma) of a neuron.

Anaxomic Neuron

Found in brain and sense organs

Have more than two processes, but axons cannot be distinguished from the dendrites

Bipolar Neuron

Found in special sensory organs (sight, smell & hearing)

• Bipolar neurons have two processes separated by the cell body.

Pseudounipolar Neuron

Also called a Unipolar Neuron

Pseudounipolar neurons have single elongated processes with the cell body situated to one side

Found in sensory neurons fo PNS, e.g. dorsal root ganglion

Multipolar Neuron

Multipolar neurons have more than two processes; there is a single axon and multiple dendrites

Neuroglial Cells and Tissue

Neuroglial cells + extracellular matrix within the CNS and PNS connective tissue

Neuroglial cells (glial cells) bathe and sustain neurons

Maintain the integrity of the brain and spinal cord by glueing neurons together with blood vessels to form structures

Neuroglial Cells found in the PNS:

• Satellite Cells

• Schwann Cells

Satellite Cells

• Satellite Cells/ Satellite Glial Cells - Surround neuron cell bodies of the dorsal root ganglia (sensory ganglia) in PNS; regulate the supply of O2, CO2, nutrients, and neurotransmitter levels around neurons in the ganglia.

Schwann Cells

• Schwann Cells /neurilemma cell - Surround all axons of the nerves in PNS; responsible for myelination of peripheral axons; participate in the repair process after injury; also act as phagocytes to rid a damaged nerve of deteriorating cell debris.

Neuroglial Cells found in the CNS:

• Oligodendrocytes

• Astrocytes

• Microglia

• Ependymal cells

Ependymal Cells

• Line the lumen of the ventricles of the brain (choroid plexus) and the central canal of the spinal cord

• Produce cerebrospinal fluid (CSF)

Oligodendrocytes

• Their cytoplasmic extensions wrap tightly around the CNS axons to form the myelin sheaths

Microglia

• Roaming scavenger cells (macrophage); engulf; digest, and destroy invading organisms and foreign matter; clean up the debris of dead cells following an injury.

Astrocytes

• Most abundant; shaped like stars. From the cell body, they send out projections that grip and stabilize neurons and capillaries

• Control the condition of the ECM (ground substances + protein fibers)

• Maintain the blood-brain barrier

Frontal Plane of the brain (Pictorial)

The Blood-Brain Barrier (BBB)

• Offers molecular protection and protects the brain from harmful substances released from the blood capillaries

• Is a protective mechanism that helps to ensure the brain’s environment remains stable

• Endothelial Cells of capillaries forming the tight junction with astrocytes and pericytes in the connective tissue of the brain act as a firewall to prevent most substances from directly entering the brain tissue.

Substances that can pass through the BBB

• Gases and fat-soluble substances can move across these barriers without assistance

• Glucose and amino acids pass through via regular transport processes.

Substances that can pass through the BBB

• Gases and fat-soluble substances can move across these barriers without assistance

• Glucose and amino acids pass through via regular transport processes.

Tight Junctions

A specialized cell-to-cell adhesion structure that forms a continuous, impermeable seal between adjacent cells (a lot of times the endothelial cells of the capillaries)

Foot processes of astrocytes

Wrap around the outer surfaces of capillaries

Pericytes

(A type of smooth muscle) regulate the cerebral blood flow in capillaries

• Contractile cells located on the outer surfaces of capillary endothelial cells, embedded within the capillary basement membrane. In the brain, they regulate cerebral blood flow by adjusting capillary diameter, and they control vascular formation by stabilizing and maturing new vessels. They also support the blood-brain barrier.

Central Nervous System

Consists of:

• Brain: Located up the cranial vault of the skull

• Spinal cord: Located in the vertebral canal

• Brain and spinal cord: Continuous with each other at the foremen magnum

CNS: The Brain

The largest and most complex part of the nervous system. It oversees various aspects of physiology, including sensation and perception, movement, and cognition.

Consists of four divisions, each distinct in the type of input it receives and where it sends its output:

• Cerebrum

• Diencephalon

• Cerebellum

• Brainstem

Cerebrum

Conscious thought, voluntary movement, sensation, language, memory and emotion

Diencephalon

Comprises of thalamus and hypothalamus

Relay and regulation- the thalamus relays sensory input to the cortex

The hypothalamus controls homeostasis, hormones, temperature, hunger, thirst and sleep.

Cerebellum

Coordinates movement, balance and posture, making motor activity smooth and accurate

Brainstem

Controls vital automatic functions- breathing, heart rate and blood pressure0 and connects the brain to the spinal cord

CNS: Spinal Cord

• Runs through the vertebral canal Brain

• Extends from the second lumbar vertebra

• Regions/ Segments: Cervicalm Thoracic, Lumbar, Sacrak & Coccygeal

• Diameter is not uniform

• Cervical enlargement: supplies the upper limbs

• Lumbar enlargement: supplies the lower limbs

• Conus medullaris- inferior end tip of the spinal cord tapered between L1 and :2

• Cauna equina (horse’s tail)- The collection of spinal nerves at the end of conus medullaris below L1 extends to the sacral region.

• Film terminale- A fibrous band extend from the conus medullaris to the periosteum of the coccyx. Helps stabilize, fixate, and buffer the distal spinal cord

(Periosteum of the coccyx is the dense connective tissue layer covering the outer surfaces of the tailbone, providing blood supply, nerve innervation, and osteopenia cells for bone growth and repair.

Arrangement of Neurons within the CNS (Gray Matter)

• Gray Matter: The dominant cell bodies aggregate (collect together) in locations found in both the brain and spinal cord, with dendrites and unmyelinated axons.

• In the brain- gray matter forms the outer few millimeters of the cerebrum (cortex), and is also scattered in deeper regions (e.g., basal nuclei, thalamus)

• In the spinal cord- gray matter is located internally (butterfly-shaped, containing nuclei that process information.

Key Point: Gray matter= cell bodies + dendrites + unmyelinated axons.

Arrangement of Neurons within the CNS (White Matter)

White Matter: found in both the brain and spinal cord; the area is dominated by myelinated axons, which give the appearance of white color.

• Each lobe of the cerebrum contains bundles of white matter called tracts. This communication connects different parts of the brain and spinal cord.

• The spinal cord contains gray matter internally, containing nuclei that process information and tracts of white matter that shuttle this information to and from the brain.

Arrangement of Neurons within the brain

Gross anatomical structures include:

• Sulci: shallow grooves on the surface of the cerebrum

• Gyri: elevated ridges found between sulci; together increase the surface area of the brain, maximising cerebral space (fits more brain tissue) within the limited space of the skull.

• Fissures: Deep grooves found on the surface of the cerebrum

• Longitudinal fissure: a long, deep groove that separates the left and right cerebral hemispheres.

Ganglia

Singular: Ganglion= a cluster of neurons cell bodies located outside the CNS (in the PNS)

Structure- mainly cell bodies, with some synapses and support cells (satellite cells)

Function- relay and process signals between the CNS and the body.

Gray matter in the brain

• Superficial layer: cortex of the cerebrum contains

• Deep layer: basal nuclei/ basal ganglion form the small nodules of the grey matter, which are deep within the cerebrum.

White Matter in the brain

Axons of white matter are organized into bundles, either called:

• Association fibers: connect signals between adjacent gyri

• Commissural fibers: the corpus callosum connects signals between the left and right cerebral hemispheres

• Projection fibers: connect signals between the cerebral cortex and the lower brain or spinal centers

Arrangement of Neurons within the spinal cord

In cross-section, the gray matter of the spinal cord looks like the letter H or a butterfly.

Horns of gray matter:

• Dorsal horn- receives sensory impulses from the dorsal root ganglion of the PNS

• Ventral and lateral horns- carry motor impulses from the brain/ CNS

• Tracts= bundles of axons in the CNS

• Gray matter (deep in the spinal cord) = signal integration site

• White matter (tracts) encircles the gray matter.

• Tracts only transmit signals within the CNS

The Spinal Cord: Dorsal Horn

Location:

• The dorsal horn is located on the posterior (dorsal) side of the spinal cord.

• Divided into six laters of nuclei, which receive different types of sensory input.

Functions:

• The dorsal horn is a primary processing center for somatosensory information, including touch, pressure, temperature, pain, and proprioception, contributing to deep tendon reflex pathways.

• It receives inputs from primary afferent fibers via the dorsal roots of the spinal nerves and sends signals to higher brain centers and motor neurons

The spinal cord: Lateral Horn

Location:

• The lateral horns is a small projection of grey matter situated between the dorsal and ventral horns, primarily found in the thoracic and upper lumbar regions (T1- L2)

• Also known as the intermediolateral nucleus involved in sympathetic functions.

Function:

• It contains the cell bodies of preganglionic sympathetic neurons, which are part of the autonomic nervous system. These neurons transmit to the sympathetic ganglia via the ventral roots and white rami communicantes.

( White rami communicantes: connect the ventral roots to the sympathetic chain ganglia )

Spinal cord → ventral roots → white ramus communicans → sympathetic ganglion

The spinal cord: Ventral horn

Location:

• located at the anterior aspect of the spinal cord

• Prominent in regions that control somatic limb/ skeletal muscle movements, such as the cervical and lumbar enlargements.

Function:

• It contains the cell bodies of motor neurons that project their axons through the ventral roots to innervate skeletal muscle, facilitating voluntary and involuntary movements

The Peripheral Nervpus System (PNS)

Composed of mixed nerves and transmits information in two directions:

• Afferent Nerve Fibers

• Efferent Nerve Fibers

Afferent Nerve Fibers

Carry sensory nerve impulses from sensory receptors, which detect changes in the internal and external environment, to the CNS

Efferent Nerve Fibers

Convey motor nerve signals from the CNS to effectors such as muscles, organs, or glands.

The PNS: Spinal Nerves

• There are 31 pairs of mixed spinal nerves.

• These are. Emitted from the intervertebral foramens- which contains dendrites of sensory neurons and axons of motor neurons.

• Conduct impulses necessary for sensations and voluntary/ involuntary movements

The PNS: The Dorsal Root

Also known as the posterior root, it is one of the two roots that emerge from the spinal cord.
It plays a crucial role in transmitting sensory information from the body to the central
nervous system (CNS).
• The dorsal root emerges from the posterior aspect of the spinal cord and extends to the
dorsal root ganglion (DRG), which contains the cell bodies of sensory neurons.
• The impulse further moves toward the dorsal horn of the spinal cord after synapse and
then moves to the interneuron of the spinal cord within the CNS and further carries the
impulse up to the other neurons inside the brain for integration

The PNS: The Ventral Root

• Also known as the anterior root, it is a crucial component of the spinal cord's anatomy. It plays a significant role in the
  motor functions of the body by facilitating the transmission of signals from the central nervous system (CNS) to muscles and other peripheral tissues.
  • These roots are composed of efferent (motor) nerve fibers. The cell bodies of these motor neurons are located within the spinal cord's central gray matter.

Motor Output

• Somatic nervous system

• Autonomic nervous system

The somatic nervous system

The somatic nervous system is responsible for voluntary/conscious activity to the effectors, stimulating skeletal muscle.

The upper motor neurons (gray matter of the brain) of the CNS (e.g., cortex) integrate and initiate the impulse to the ventral or lateral horn and synapse with the lower motor neurons (H-shaped gray matter); the signal will be further transmitted to the effectors via the efferent nerves.

Feature	Details
Sensory input	From somatic senses (skin, skeletal muscles, joints) and special senses
Control of motor output	Voluntary control from the cerebral cortex, with contributions from basal ganglia, cerebellum, brain stem, and spinal cord
Motor pathway	A single, myelinated somatic motor neuron — cell body in the CNS, axon extends to skeletal muscle fibers
Neurotransmitter	Only Acetylcholine (ACh)
Effector	Skeletal muscle
Response	Contraction of skeletal muscle

Autonomic Nervous System

• Sympathetic nervous system

• Parasympathetic nervous system

• Enteric nervous system

Feature	Autonomic Innervation
General function	Involuntary/subconscious control to regulate the body processes of internal organs
Sensory input	Mainly from interoceptors; some from somatic senses and special senses
Control of motor output	Involuntary control from hypothalamus, limbic system, brain stem, and spinal cord; limited control from cerebral cortex. (Action potential can be stimulated by the hypothalamus)
Motor pathways	Consist of 2 motor neurons (visceral efferent fibers associated with autonomic ganglia)
1st neuron	Myelinated preganglionic neuron — cell body is in the CNS (brain or spinal cord); axon leaves the CNS to synapse with the 2nd neuron
2nd neuron	Unmyelinated postganglionic neuron — cell body is in the PNS (specifically in the autonomic ganglion); axon extends to the organ it serves
Synapse location	The preganglionic axon synapses with the postganglionic neuron at the autonomic ganglion
Neurotransmitter	Either acetylcholine (ACh) or noradrenaline (NE)
Effectors	Smooth muscle, cardiac muscle, and glands (e.g., smooth muscle of colon)
Responses	Contraction or relaxation of smooth muscle; increased or decreased rate and force of contraction of cardiac muscle; increased or decreased secretions of glands

Autonomic Nervous System — Two Divisions: Compare the effects of the Parasympathetic vs. Sympathetic divisions on each target organ.

Target Organ	Parasympathetic	Sympathetic
Pupils	Constrict	Dilate
Salivation	Stimulate	Inhibit
Airways	Constrict	Relax
Heart	Slow heartbeat	Increase heartbeat
Stomach	Stimulate activity	Inhibit activity
Liver / Gallbladder	Inhibit glucose release; Stimulate gallbladder	Stimulate glucose release; Inhibit gallbladder
Intestines	Stimulate activity	Inhibit activity
Adrenal Glands	—	Secrete epinephrine & norepinephrine
Bladder	Contract	Relax
Reproductive System	Promote erection	Promote ejaculation

Origin in CNS:

• Parasympathetic: Craniosacral (brainstem + S2–S4)

• Sympathetic: Thoracolumbar (T1–L2), via the sympathetic chain

General Mnemonic:

• Parasympathetic = "Rest and Digest"

• Sympathetic = "Fight or Flight"

Autonomic Nervous System — Overview: What is its general function?

General function:

• Involuntary / subconscious regulation of internal body processes

• Operates automatically

• Keeps the body in a state of dynamic balance

Autonomic Nervous System — Overview: how do its sensory and motor components work?

Sensory (Afferent) Nerves:

• Receive signals from visceral receptors that monitor internal conditions (e.g., blood pressure)

• Send those signals to the CNS

Motor (Efferent) Nerves:

• Convey signals from the CNS to the effectors

• Effectors = smooth muscle, cardiac muscle, and glands

Effects on target tissue:

• Excitation (increased activity), or

• Inhibition (decreased activity)

Sensory in → CNS → Motor out → excite or inhibit → balance.

The Autonomic Nervous System:
Parasympathetic Division

The parasympathetic system sends its motor signals out from two places:

1. The Brain (Cranial Portion) — Signals start in the nuclei of four cranial nerves:

   • Nerve III

   • Nerve VII

   • Nerve IX

   • Nerve X

2. The Lower Spine (Sacral Portion) — Signals start in the gray matter of the sacral region and travel out through the spinal nerves there.

How the signal travels:

• The first neuron (preganglionic) leaves the brain or spinal cord.

• It synapses with a second neuron (postganglionic).

• That second neuron sits on or very close to the target organ.

The 12 Cranial Nerves: What are their numbers, names, and functions (Sensory, Motor, Parasympathetic)?

• 12 pairs associated with the brain.

• Most are mixed nerves (sensory + motor).

• Two pairs are purely sensory (I and II).

• Numbered I–XII from anterior to posterior.

#	Name	Sensory Function	Motor Function	PS Fibers
I	Olfactory	Yes (smell)	No	No
II	Optic	Yes (vision)	No	No
III	Oculomotor	No	Yes	Yes
IV	Trochlear	No	Yes	No
V	Trigeminal	Yes (general sensation)	Yes	No
VI	Abducens	No	Yes	No
VII	Facial	Yes (taste)	Yes	Yes
VIII	Vestibulocochlear	Yes (hearing & balance)	Some	No
IX	Glossopharyngeal	Yes (taste)	Yes	Yes
X	Vagus	Yes (taste)	Yes	Yes
XI	Accessory	No	Yes	No
XII	Hypoglossal	No	Yes	No

*PS = Parasympathetic

Parasympathetic Responses

Enhances “rest-and-digest” activities

Promotes maintenance activities and conserves body energy

Paradoxical fear

Parasympathetic Responses: Enhances “rest-and-digest” activities

The parasympathetic nervous system (PNS) promotes bodily
functions that occur when the body is at rest, such as digestion and energy conservation.

• Normally dominates over sympathetic impulses: In non-stressful situations, the PANS is more active than the sympathetic nervous system (SNS), which is responsible for the “fight or flight” response.

Parasympathetic Responses: Promotes maintenance activities and conserves body energy

It helps in maintaining homeostasis by
conserving energy and promoting activities like digestion, repair, and growth.
• Illustrated in a person who relaxes and reads after a meal: When a person is relaxed, the PANS is
active, facilitating digestion and relaxation with SLUDD responses:
• Salivation: Increased saliva production to aid in digestion.
• Lacrimation: Production of tears to keep the eyes moist.
• Urination: Promotes the excretion of urine.
• Digestion: Enhances digestive processes.
• Defecation: Facilitates bowel movements.

Parasympathetic Responses: Paradoxical fear

When there is no escape route or no way to win, the body may experience a
massive activation of the PNS.
• This can lead to a sudden and overwhelming response.
• Leads to loss of control over urination and defecation: In extreme cases, this can result in
involuntary urination or defecation due to the intense activation of the PNS.

ANS: Pathway Features Comparisons

Neuron chain: how many neurons are connected in a row to carry the signal from the CNS to the target organ.

ANS: Anatomical Comparison

Division	Origin of Fibers	Length of Fibers	Location of Ganglia
Sympathetic	Thoracolumbar region of the spinal cord	Short preganglionic, long postganglionic	Close to spinal cord
Parasympathetic	Brain and sacral spinal cord (craniosacral)	Long preganglionic, short postganglionic	In visceral effector organs

The Autonomic Nervous System: Sympathetic Division

Origin:

• Thoracic and first two lumbar segments of the spinal cord (T1–L2)

• Cell bodies are in the lateral horns of the gray matter

Path:

1. Preganglionic fiber leaves the spinal cord through the ventral root of the spinal nerve.

2. Travels down the thoracic or lumbar nerve.

3. Synapses at the sympathetic trunk / paravertebral ganglia (autonomic ganglia).

4. After the sympathetic chain:

   • Once the signal passes through the sympathetic chain, it takes different routes depending on where it's going.

   • Eventually it synapses with the second neuron (the postganglionic neuron).

   • From there, the postganglionic fiber carries the signal to the target organ.

   • These postganglionic fibers are usually much longer than the preganglionic ones.

The Sympathetic Trunk

What it is:

• Also called the paravertebral ganglia or sympathetic chain

• A series of ganglia running along both sides of the vertebral column

What it does:

• Acts as a conduit (pathway) for preganglionic fibers

• Allows those fibers to travel up and down to reach different levels of the body

• Houses the paravertebral ganglia where many preganglionic fibers synapse with their second neuron

Example mentioned:

• Celiac ganglion — one of the ganglia associated with the sympathetic trunk

Sympathetic Responses: What is the "fight or flight" response

What it is:

• Alarm reaction → "Fight or Flight"

• Prepares the body to confront or escape a threat

• Also promotes adjustments during exercise

Body changes:

Change	Purpose
Blood shunted to skeletal muscles & heart	More O₂ and nutrients for quick response
Bronchioles dilate; respiratory rate increases	Enhances oxygen intake
Liver releases glucose	Quick energy source for muscles

Triggers — "E situations":

• Emergency — accidents, natural disasters

• Embarrassment — social stressors

• Excitement — thrilling activities

• Exercise — physical exertion

Why effects last long:

• Norepinephrine (noradrenaline) lingers in the synaptic cleft at the effector

• This prolongs activation of the sympathetic response