Anatomy Chapter 13: Spinal cord, spinal nerves and somatic reflexes

Four principle functions of the spinal cord

1. Conduction

2. Neural Integration

3. Locomotion

4. Reflexes

Conduction

Sends sensory (incoming) and motor (outgoing) information up and down the body

Neural Integration

It processes signals from different parts of the body and sends the right response

Example: Controlling when your bladder empties

Locomotion

It has special groups of neurons called central pattern generators that help control the repeated muscle movements used in walking

Reflexes

Involuntary responses to stimuli that are vital to posture, coordination and protection from injury

Spinal Cord

A cylinder-shaped bundle of nerves that starts where the brainstem ends - at a hole in the skull called the formen magnum

Divided into four regions:

• Cervical

• Thoracic

• Lumbar

• Sacral

Meninges

What the spinal cord is wrapped in (three layers)

• Keeps it safe from from bones of the spine

Outermost to innermost:

1. Dura matter

2. Arachnoid matter

3. Pia matter

Dura matter

The tough outer layer that forms a sleeve called the dural sheath

• Its separated from the vertebrae by the epidural space, which has blood vessels, fat, and loose tissue

Arachnoid matter

The thin, transparent middle layer that loosely fits around the spinal cord

Pia matter

The delicate inner layer that closely covers the surface of the spinal cord

Subarachnoid Space

• Located under the arachnoid matter

• Filled with cerebrospinal fluid that cushions the spinal cord

Lumbar cistern

• Below the medullary cone

• Contains the cauda equina and more CSF

The spinal cord is made up of two types of nervous tissue

1. Gray Matter

2. White Matter

Gray Matter

• Very little myelin

• Where neurons process and integrate signals

• It forms an “H” or butterfly shape in the center of the spinal cord

It includes:

• Posterior (dorsal) horns on each side

• Anterior (ventral) horns on each

• Lateral horns (found between T1-L2)

• The two sides are connected by the gray commissure, which has a central canal filled with CSF and lined with ependymal cells

White Matter

• Contains lots of myelin, making it appear lighter

• Made up of axons (nerve fibers) that send signals up and down the spinal cord

• Organized into three columns (funiculi) on each side:

• Posterior

• Lateral

• Anterior

• Each column contains smaller bundles called tracts (fasciculi)

Spinal Cord Tracts

The white mater contains nerve tracts that carry information in two directions:

1. Ascending Tracts

2. Descending Tracts

Ascending Tracts

Carry sensory signals to the brain

These signals travel through 3 neurons:

• First-order neuron: From the sense organ or tissue→ spinal cord or brainstem

• Second-order neuron: From the spinal cord or brainstem

• Third-order neuron: From the thalamus → cerebral cortex

Descending Tracts

Carry motor signals from the brain to muscles and organs

• Upper motor neuron

• Lower motor neuron

All tracts on both sides of the body (bilateral)

Some tracts decussate (cross over) to the opposite side of the body - these are contralateral tracts

Upper motor neuron

Starts in the cerebral cortex or brainstem

Lower motor neuron

Starts in the spinal cord or brainstem and ends at the target muscle/organ

Nerve

Cord-like structure made up of many nerve fibers (axons) bundles together and covered by connective tissue

Layers of a nerve

1. Endoneurium- Wraps each individual fiber

2. Perineurium-Wraps a bundle of fibers - fascicle

3. Epineurium- Wraps all the fascicles together to form the entire nerve

Types of nerves

1. Sensory nerves

2. Motor nerves

3. Mixed nerves

Sensory nerves

Carry only afferent fibers, which bring signals to the brain and spinal cord (like vision & smell)

Motor nerves

Carry only efferent fibers, which sends signals from the brain and spinal cord to muscles and glands

Mixed nerves

have both sensory and motor fibers, these make up most nerves in the body

Ganglion

A cluster or neurosomas located outside the CNS

• Covered by a layer of connective tissue call the endoneurium, which continues from the nearby nerve

Posterior Root Ganglion (PRG)

• Contains sensory neuron cell bodies that send signals from the body’s sense organs (like skin and muscles) towards the spinal cord

Anterior root

Found below the ganglion, carries motor signals away from the spinal cord

Spinal Nerves Overview

There are 31 pairs of spinal nerves, most are mixed nerves

• 8 cervical (C1 - C8)

• The first cervical nerve (C1) exists between the skull and the atlas (the first vertebra)

• The others exit through the spaces between vertebrae (intervertebral foramina)

• 12 thoracic (T1 - T2)

• 5 lumbar (L1 - L5)

• 5 sacral (S1 - S5)

• 1 coccygeal (Co1)

Two roots that each spinal nerve is formed by that attach to the spinal cord

1. Posterior (dorsal) root

2. Anterior (ventral) root

Posterior (dorsal) root

Brings sensory information into the spinal cord

Anterior (ventral) root

Carries motor signals out of the spinal cord

After the spinal nerve exits the intervertebral foramen, it divided into three branches:

• Anterior ramus- Serves the front sides of the body and limbs

• Posterior ramus- Serves the back muscles and skin

• Meningeal branch- Reenters the spinal canal to serve the meninges and spinal cord

• The anterior branch also gives off communicating rami, which connect to the sympathetic nervous system

Thoracic region

The anterior ramus forms intercostal nerves, which go to the skin and muscles between rids

Plexuses

Nerve networks that anterior rami form in other regions

Main Plexuses:

1. Cervical (C1-C5)- Neck & diaphragm

2. Brachial (C5-T1)- Shoulders & arms

3. Lumbar (L1-L4)- Lower back and front of the legs

4. Sacral (L4,L5,S1,S4)- Pelvis, buttocks, and back of the legs

Reflex

A quick, involuntary stereotyped reaction of glands or muscle to stimulation

Properties:

• Require Stimulation- only happen in response to sensory input

• Quick- few or no interneurons, which means less delay

• Involuntary- can’t control or easily stop them

• Stereotypes- they happen the same way every time

Visceral reflexes

Control organ function

Example: When stretch receptors in your digestive tract cause smooth muscles to contract and push food along (peristalsis)

Somatic relfexes

Involve skeletal muscles (voluntary muscles)

Example: The knee-jerk reflex or quickly pulling your hand away from something hot

Reflex Arc

The pathway that nerve signals follow during a reflex:

1. Somatic receptors

2. Afferent nerve fibers

3. Integrating center

4. Efferent nerve fibers

5. Effectors

Somatic receptors

In skin, muscles, or tendons

Afferent nerve fibers

Carry sensory information to the spinal cord or brainstem

Integrating Center

Process the information in the gray matter and decides if a signal should be sent out

Efferent nerve fibers

Carry motor signals from the spinal cord to muscles

Flexor and Crosses Extension Reflexes

When you step on something sharp (like glass):

1. The flexor (withdrawal) reflex makes you quickly pull your injured foot away by contracting the flexor muscles

2. At the same time, the crossed extension reflex contracts the extensor muscles in the opposite leg to keep you balanced while the injured leg is lifted