Spinal Cord Anatomy, Nerve Pathways, and Autonomic Nervous System Overview

What is the typical diameter of the adult spinal cord?

About three-fourths of an inch.

How long is the adult spinal cord?

About 16-18 inches.

Where does the spinal cord extend from and terminate?

It extends from the medulla oblongata and terminates at the inferior border of the L1 vertebra.

How many parts is the spinal cord divided into?

Five parts: cervical, thoracic, lumbar, sacral, and coccygeal.

How many pairs of cervical spinal nerves are there?

Eight pairs.

How many pairs of thoracic spinal nerves are there?

Twelve pairs.

How many pairs of lumbar spinal nerves are there?

Five pairs.

How many pairs of sacral spinal nerves are there?

Five pairs.

How many pairs of coccygeal spinal nerves are there?

One pair.

What is the cauda equina?

The collection of nerve roots from the more inferior spinal nerves (L2-L5, S1-S5, and Co1) that extend inferiorly.

What are the spinal cord meninges?

Protective membranes that encapsulate the spinal cord, continuous with cranial meninges.

What is the innermost membrane of the spinal cord meninges?

The pia mater.

What space contains cerebrospinal fluid?

The subarachnoid space.

What is the tough, outermost membrane of the spinal cord?

The dura mater.

What is the shape of the central gray matter in cross-section?

Shaped like the letter 'H' or a butterfly.

What do the anterior horns of the spinal cord primarily contain?

Cell bodies of somatic motor neurons associated with skeletal muscles.

Where are lateral horns found in the spinal cord?

Only in the T1-L2 parts.

What do the posterior horns primarily contain?

Axons of sensory neurons and cell bodies of interneurons.

What is the function of the gray commissure?

It serves as communication between the right and left sides of the gray matter.

What are the three regions of white matter in the spinal cord called?

Anterior funiculus, lateral funiculus, and posterior funiculus.

What do ascending tracts in the spinal cord conduct?

Sensory nerve signals from the spinal cord to the brain.

What do descending tracts in the spinal cord conduct?

Motor nerve signals from the brain to the spinal cord.

How many pairs of spinal nerves extend from the spinal cord?

31 pairs.

How is each spinal nerve identified?

By the first letter of the spinal cord part it extends from and a sequential number.

What are the two roots that form each spinal nerve?

Anterior (ventral) root and posterior (dorsal) root.

What type of axons does the anterior root contain?

Motor axons only.

What type of axons does the posterior root contain?

Sensory axons only.

What is the function of the anterior root?

It conducts nerve signals from the central nervous system to muscles and glands.

What is the function of the posterior root?

It conducts nerve signals from receptors to the central nervous system.

What is a reflex?

A rapid, programmed, involuntary response of muscles and glands to a stimulus.

What are the properties of reflexes?

Require a stimulus, involve few neurons, are preprogrammed, and are involuntary.

What is a reflex arc?

Includes a sensory receptor, an effector, and the neural wiring between them.

What initiates a reflex?

A stimulus activates a sensory receptor.

What is the first step in a reflex action?

A stimulus activates a sensory receptor.

What is the role of interneurons in a reflex?

They process the incoming nerve signal.

What is the difference between a spinal reflex and a cranial reflex?

A spinal reflex involves the spinal cord, while a cranial reflex involves the brain.

What is a somatic reflex?

A reflex that involves skeletal muscle.

What is a visceral reflex?

A reflex that involves cardiac muscle, smooth muscle, or glands.

What is a monosynaptic reflex?

A reflex that involves only one sensory neuron and one motor neuron.

What is a polysynaptic reflex?

A reflex that involves one or more interneurons between the sensory and motor neurons.

What is an ipsilateral reflex?

A reflex where the sensory receptor and effector are on the same side of the spinal cord.

What is a contralateral reflex?

A reflex where the sensory receptor and effector are on opposite sides of the body.

What is the difference between an innate reflex and an acquired reflex?

An innate reflex is one you are born with, while an acquired reflex develops after birth.

Why is the evaluation of reflexes important?

Abnormal reflex responses may indicate nervous system or muscle dysfunction.

What does the biceps reflex test?

Flexion of the elbow when the biceps brachii tendon is tapped.

What does the triceps reflex test?

Extension of the elbow when the triceps brachii tendon is tapped.

What does the patellar reflex test?

Extension of the knee when the patellar tendon is tapped.

What does the plantar reflex test?

Flexion of the toes when the plantar surface of the foot is stroked briskly.

How does the spinal cord develop during the embryonic period?

It extends the full length of the vertebral canal.

At what level does the spinal cord end by the end of the sixth fetal month?

At about the level of the S1 vertebra.

Where does the adult spinal cord extend to?

About the level of the L1 vertebra.

What does the somatic nervous system control?

Processes that are consciously perceived or controlled, including voluntary muscle movements.

What is the primary function of the autonomic nervous system?

Regulates processes below the conscious level, controlling visceral organs.

What are the two divisions of the autonomic nervous system?

Sympathetic division and parasympathetic division.

What is the role of the sympathetic division?

Involves 'fight or flight' responses associated with exercise and emergencies.

What is the role of the parasympathetic division?

Involves 'rest and digest' activities associated with conserving energy.

How many motor neurons does the autonomic nervous system use to reach the effector?

Two motor neurons: preganglionic and postganglionic neurons.

What neurotransmitter is released by preganglionic neurons in the autonomic nervous system?

Acetylcholine.

What neurotransmitter is released by postganglionic neurons in the sympathetic division?

Norepinephrine.

What are cholinergic neurons?

Cells that release acetylcholine, including all autonomic preganglionic neurons.

What are adrenergic neurons?

Cells that release norepinephrine, primarily found in sympathetic ganglia.

What are the two major types of cholinergic receptors?

Nicotinic and muscarinic receptors.

Where are nicotinic receptors found?

On all ganglionic neurons, adrenal medulla cells, and skeletal muscle cells at the neuromuscular junction.

What effect do muscarinic receptors have on gastrointestinal smooth muscle?

Stimulate contraction when acetylcholine binds to them.

What is the function of alpha adrenergic receptors?

Typically stimulated by norepinephrine.

What is the function of beta adrenergic receptors?

Typically inhibited by norepinephrine.

What is dual innervation?

When an organ receives input from both the sympathetic and parasympathetic divisions.

What is an example of antagonistic effects between the sympathetic and parasympathetic divisions?

Sympathetic division increases heart rate, while parasympathetic division decreases it.

What is an example of cooperative effects between the two divisions?

In male sexual function, parasympathetic activity causes penile erection, and sympathetic activity causes ejaculation.

What type of axons do parasympathetic preganglionic neurons have?

Long preganglionic axons and short postganglionic axons.

What type of axons do sympathetic preganglionic neurons have?

Short preganglionic axons and long postganglionic axons.

What is the location of sympathetic preganglionic neurons?

Located in the T1-L2 segments of the spinal cord.

What is the location of parasympathetic preganglionic cell bodies?

Typically located in the brainstem or S2-S4 spinal cord segments.

What is the role of acetylcholine in the somatic nervous system?

Released by a single motor neuron to stimulate skeletal muscle fibers.

What happens to blood vessels during increased sympathetic activity?

They constrict.

What happens to blood vessels during decreased sympathetic activity?

They dilate.

What is the effect of acetylcholine on cardiac muscle?

Decreases heart rate when binding to muscarinic receptors.

What are the characteristics of sympathetic ganglia?

Located close to the spinal cord with many branches in preganglionic axons.

What are the characteristics of parasympathetic ganglia?

Located close to or within the effector with few branches in preganglionic axons.